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Thoughts on this little oddity, anyone ...?
Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has two
completely separate and identical power amps, one of which was running very hot very quickly. No schematics, but problem was quickly traced to an open circuit resistor, by comparison ohms checks between the two amps. In order to replace the resistor, it was necessary to remove the offending ("B") power amp to get to the board underside. The power rails and ribbon from the mixer desk section were on removable connectors, but the 4 pole speakon connector was bolted to the rear panel and the wires were soldered directly to the board. When I came to unsolder these wires from the socket, I noticed that the "B" channel one was wired the opposite way round to the "A" channel one. I noted this down to check on later. When the amp was tested ok, I bolted it back in, and reconnected the speakon socket the same as it was when I unsoldered it. The wires lay exactly where they originally were, and looked absolutely factory-original, even down to the one which went to the uppermost connector tag, having a rubber sleeve on it, exactly the same as on the "A" channel. Except the two channels were definitely wired arse-about-face to one another. On both channels' speakons, the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel "A" the ground wire was connected to the "+" pair, and on the "B" channel, the ground was on the "-" pair. I fired it up again, and put a sine wave in, then hooked up two channels of my 'scope, one to the "A" output and one to the "B", both with the same polarity, and was surprised to find that the two signals were completely anti-phase, which meant that with the reversed connections to the speakon sockets, they would be back in phase again to the outside world. I then went back to the power amps' front ends near where the ribbon connector was, and found the same thing. At the same point on each amp, the signals were in opposite phase to one another. I then spotted three little 3 pin header blocks, with one pin pair on each designated "B", and the other pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to the power of' position. On the B amp, these three jumpers were set to "B", and on the A amp they were set to "A", so clearly, these jumpers reverse the phase of the signal coming in from the desk section. The two amps then process the signals in antiphase to one another right to the final output, where the 'correct' phase relationship is again restored, by wiring the speakon connector 'wrongly'. I have puzzled over this, and can think of no good reason for doing it that way. I did wonder at one point if it was anything to do with being able to make the amps bridgeable, but I downloaded a copy of the user manual, and there is a dire warning paragraph towards the end, specifically saying that under no circumstances should any attempt be made to operate the amps in bridge. It also warns against making any connections between the two amps or any speakers connected to them, although in actual fact, the 'ground' side of each output is truly ground, and is common to both amps, the whole (dual) power supply, and all metalwork. What am I missing here ? Not of any real consequence, as the unit is mended and back in its original condition, but for academic interest, it would be good to understand the designer's thinking. Arfa |
Thoughts on this little oddity, anyone ...?
Arfa Daily wrote: Had an Allen ? Heath PA20-CP 2 x 500 watt powered mixer in today. Has two completely separate and identical power amps, one of which was running very hot very quickly. No schematics, but problem was quickly traced to an open circuit resistor, by comparison ohms checks between the two amps. In order to replace the resistor, it was necessary to remove the offending ("B") power amp to get to the board underside. The power rails and ribbon from the mixer desk section were on removable connectors, but the 4 pole speakon connector was bolted to the rear panel and the wires were soldered directly to the board. When I came to unsolder these wires from the socket, I noticed that the "B" channel one was wired the opposite way round to the "A" channel one. I noted this down to check on later. When the amp was tested ok, I bolted it back in, and reconnected the speakon socket the same as it was when I unsoldered it. The wires lay exactly where they originally were, and looked absolutely factory-original, even down to the one which went to the uppermost connector tag, having a rubber sleeve on it, exactly the same as on the "A" channel. Except the two channels were definitely wired arse-about-face to one another. On both channels' speakons, the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel "A" the ground wire was connected to the "+" pair, and on the "B" channel, the ground was on the "-" pair. I fired it up again, and put a sine wave in, then hooked up two channels of my 'scope, one to the "A" output and one to the "B", both with the same polarity, and was surprised to find that the two signals were completely anti-phase, which meant that with the reversed connections to the speakon sockets, they would be back in phase again to the outside world. I then went back to the power amps' front ends near where the ribbon connector was, and found the same thing. At the same point on each amp, the signals were in opposite phase to one another. I then spotted three little 3 pin header blocks, with one pin pair on each designated "B", and the other pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to the power of' position. On the B amp, these three jumpers were set to "B", and on the A amp they were set to "A", so clearly, these jumpers reverse the phase of the signal coming in from the desk section. The two amps then process the signals in antiphase to one another right to the final output, where the 'correct' phase relationship is again restored, by wiring the speakon connector 'wrongly'. I have puzzled over this, and can think of no good reason for doing it that way. I did wonder at one point if it was anything to do with being able to make the amps bridgeable, but I downloaded a copy of the user manual, and there is a dire warning paragraph towards the end, specifically saying that under no circumstances should any attempt be made to operate the amps in bridge. It also warns against making any connections between the two amps or any speakers connected to them, although in actual fact, the 'ground' side of each output is truly ground, and is common to both amps, the whole (dual) power supply, and all metalwork. What am I missing here ? Not of any real consequence, as the unit is mended and back in its original condition, but for academic interest, it would be good to understand the designer's thinking. If both amps are fed the same signal, it reduces the maximum current requirements for both the + and - rails. That also lowers the current in the system ground, reducing crosstalk & possible ground loops. This is done in some RF circuits, as well. -- You can't have a sense of humor, if you have no sense. |
Thoughts on this little oddity, anyone ...?
Arfa Daily wrote:
Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has two completely separate and identical power amps, one of which was running very hot very quickly. No schematics, but problem was quickly traced to an open circuit resistor, by comparison ohms checks between the two amps. In order to replace the resistor, it was necessary to remove the offending ("B") power amp to get to the board underside. The power rails and ribbon from the mixer desk section were on removable connectors, but the 4 pole speakon connector was bolted to the rear panel and the wires were soldered directly to the board. When I came to unsolder these wires from the socket, I noticed that the "B" channel one was wired the opposite way round to the "A" channel one. I noted this down to check on later. When the amp was tested ok, I bolted it back in, and reconnected the speakon socket the same as it was when I unsoldered it. The wires lay exactly where they originally were, and looked absolutely factory-original, even down to the one which went to the uppermost connector tag, having a rubber sleeve on it, exactly the same as on the "A" channel. Except the two channels were definitely wired arse-about-face to one another. On both channels' speakons, the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel "A" the ground wire was connected to the "+" pair, and on the "B" channel, the ground was on the "-" pair. I fired it up again, and put a sine wave in, then hooked up two channels of my 'scope, one to the "A" output and one to the "B", both with the same polarity, and was surprised to find that the two signals were completely anti-phase, which meant that with the reversed connections to the speakon sockets, they would be back in phase again to the outside world. I then went back to the power amps' front ends near where the ribbon connector was, and found the same thing. At the same point on each amp, the signals were in opposite phase to one another. I then spotted three little 3 pin header blocks, with one pin pair on each designated "B", and the other pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to the power of' position. On the B amp, these three jumpers were set to "B", and on the A amp they were set to "A", so clearly, these jumpers reverse the phase of the signal coming in from the desk section. The two amps then process the signals in antiphase to one another right to the final output, where the 'correct' phase relationship is again restored, by wiring the speakon connector 'wrongly'. I have puzzled over this, and can think of no good reason for doing it that way. I did wonder at one point if it was anything to do with being able to make the amps bridgeable, but I downloaded a copy of the user manual, and there is a dire warning paragraph towards the end, specifically saying that under no circumstances should any attempt be made to operate the amps in bridge. It also warns against making any connections between the two amps or any speakers connected to them, although in actual fact, the 'ground' side of each output is truly ground, and is common to both amps, the whole (dual) power supply, and all metalwork. What am I missing here ? Not of any real consequence, as the unit is mended and back in its original condition, but for academic interest, it would be good to understand the designer's thinking. Arfa are these amps run from one power supply? |
Thoughts on this little oddity, anyone ...?
Arfa Daily wrote in message
... Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has two completely separate and identical power amps, one of which was running very hot very quickly. No schematics, but problem was quickly traced to an open circuit resistor, by comparison ohms checks between the two amps. In order to replace the resistor, it was necessary to remove the offending ("B") power amp to get to the board underside. The power rails and ribbon from the mixer desk section were on removable connectors, but the 4 pole speakon connector was bolted to the rear panel and the wires were soldered directly to the board. When I came to unsolder these wires from the socket, I noticed that the "B" channel one was wired the opposite way round to the "A" channel one. I noted this down to check on later. When the amp was tested ok, I bolted it back in, and reconnected the speakon socket the same as it was when I unsoldered it. The wires lay exactly where they originally were, and looked absolutely factory-original, even down to the one which went to the uppermost connector tag, having a rubber sleeve on it, exactly the same as on the "A" channel. Except the two channels were definitely wired arse-about-face to one another. On both channels' speakons, the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel "A" the ground wire was connected to the "+" pair, and on the "B" channel, the ground was on the "-" pair. I fired it up again, and put a sine wave in, then hooked up two channels of my 'scope, one to the "A" output and one to the "B", both with the same polarity, and was surprised to find that the two signals were completely anti-phase, which meant that with the reversed connections to the speakon sockets, they would be back in phase again to the outside world. I then went back to the power amps' front ends near where the ribbon connector was, and found the same thing. At the same point on each amp, the signals were in opposite phase to one another. I then spotted three little 3 pin header blocks, with one pin pair on each designated "B", and the other pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to the power of' position. On the B amp, these three jumpers were set to "B", and on the A amp they were set to "A", so clearly, these jumpers reverse the phase of the signal coming in from the desk section. The two amps then process the signals in antiphase to one another right to the final output, where the 'correct' phase relationship is again restored, by wiring the speakon connector 'wrongly'. I have puzzled over this, and can think of no good reason for doing it that way. I did wonder at one point if it was anything to do with being able to make the amps bridgeable, but I downloaded a copy of the user manual, and there is a dire warning paragraph towards the end, specifically saying that under no circumstances should any attempt be made to operate the amps in bridge. It also warns against making any connections between the two amps or any speakers connected to them, although in actual fact, the 'ground' side of each output is truly ground, and is common to both amps, the whole (dual) power supply, and all metalwork. What am I missing here ? Not of any real consequence, as the unit is mended and back in its original condition, but for academic interest, it would be good to understand the designer's thinking. Arfa Antiphase amps discussed here a year or so ago re Yamaha Stagepas and/or another amp not A&H, I forget the make. I think ther concensus view was to balance up power draw on ps + and - rails for deep near impulse audio output |
Thoughts on this little oddity, anyone ...?
"Arfa Daily" What am I missing here ? Not of any real consequence, as the unit is mended and back in its original condition, but for academic interest, it would be good to understand the designer's thinking. ** The idea is sometimes known as the "OOPS" system - for Out Of Phase Stereo. It balances the load on the common PSU at any point in time, reducing voltage drop and electro cap ripple voltage. Bob Carver used the idea in his famous M400a " Carver Cube" amp, et alia. http://www.oaktreevintage.com/web_ph...eo_Amp_web.jpg http://www.doebbe.com/hifi/pics/Carver_M400_ret_3s.jpg Fancy A&H picking up on it ..... .... Phil |
Thoughts on this little oddity, anyone ...?
Yamaha EMX5016 mixer amp and Stagepas range and Lab Gruppen amps use
antiphase/ out of phase arangement |
Thoughts on this little oddity, anyone ...?
"Arfa Daily" wrote in message
... Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has two completely separate and identical power amps, one of which was running very hot very quickly. No schematics, but problem was quickly traced to an open circuit resistor, by comparison ohms checks between the two amps. In order to replace the resistor, it was necessary to remove the offending ("B") power amp to get to the board underside. The power rails and ribbon from the mixer desk section were on removable connectors, but the 4 pole speakon connector was bolted to the rear panel and the wires were soldered directly to the board. When I came to unsolder these wires from the socket, I noticed that the "B" channel one was wired the opposite way round to the "A" channel one. I noted this down to check on later. When the amp was tested ok, I bolted it back in, and reconnected the speakon socket the same as it was when I unsoldered it. The wires lay exactly where they originally were, and looked absolutely factory-original, even down to the one which went to the uppermost connector tag, having a rubber sleeve on it, exactly the same as on the "A" channel. Except the two channels were definitely wired arse-about-face to one another. On both channels' speakons, the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel "A" the ground wire was connected to the "+" pair, and on the "B" channel, the ground was on the "-" pair. I fired it up again, and put a sine wave in, then hooked up two channels of my 'scope, one to the "A" output and one to the "B", both with the same polarity, and was surprised to find that the two signals were completely anti-phase, which meant that with the reversed connections to the speakon sockets, they would be back in phase again to the outside world. I then went back to the power amps' front ends near where the ribbon connector was, and found the same thing. At the same point on each amp, the signals were in opposite phase to one another. I then spotted three little 3 pin header blocks, with one pin pair on each designated "B", and the other pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to the power of' position. On the B amp, these three jumpers were set to "B", and on the A amp they were set to "A", so clearly, these jumpers reverse the phase of the signal coming in from the desk section. The two amps then process the signals in antiphase to one another right to the final output, where the 'correct' phase relationship is again restored, by wiring the speakon connector 'wrongly'. I have puzzled over this, and can think of no good reason for doing it that way. I did wonder at one point if it was anything to do with being able to make the amps bridgeable, but I downloaded a copy of the user manual, and there is a dire warning paragraph towards the end, specifically saying that under no circumstances should any attempt be made to operate the amps in bridge. It also warns against making any connections between the two amps or any speakers connected to them, although in actual fact, the 'ground' side of each output is truly ground, and is common to both amps, the whole (dual) power supply, and all metalwork. What am I missing here ? Not of any real consequence, as the unit is mended and back in its original condition, but for academic interest, it would be good to understand the designer's thinking. Arfa Carver has also done this on some models. Not so good when someone tries to demonstrate through a common grounded speaker switcher... Mark Z. |
Thoughts on this little oddity, anyone ...?
Mark Zacharias wrote in message
.com... "Arfa Daily" wrote in message ... Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has two completely separate and identical power amps, one of which was running very hot very quickly. No schematics, but problem was quickly traced to an open circuit resistor, by comparison ohms checks between the two amps. In order to replace the resistor, it was necessary to remove the offending ("B") power amp to get to the board underside. The power rails and ribbon from the mixer desk section were on removable connectors, but the 4 pole speakon connector was bolted to the rear panel and the wires were soldered directly to the board. When I came to unsolder these wires from the socket, I noticed that the "B" channel one was wired the opposite way round to the "A" channel one. I noted this down to check on later. When the amp was tested ok, I bolted it back in, and reconnected the speakon socket the same as it was when I unsoldered it. The wires lay exactly where they originally were, and looked absolutely factory-original, even down to the one which went to the uppermost connector tag, having a rubber sleeve on it, exactly the same as on the "A" channel. Except the two channels were definitely wired arse-about-face to one another. On both channels' speakons, the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel "A" the ground wire was connected to the "+" pair, and on the "B" channel, the ground was on the "-" pair. I fired it up again, and put a sine wave in, then hooked up two channels of my 'scope, one to the "A" output and one to the "B", both with the same polarity, and was surprised to find that the two signals were completely anti-phase, which meant that with the reversed connections to the speakon sockets, they would be back in phase again to the outside world. I then went back to the power amps' front ends near where the ribbon connector was, and found the same thing. At the same point on each amp, the signals were in opposite phase to one another. I then spotted three little 3 pin header blocks, with one pin pair on each designated "B", and the other pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to the power of' position. On the B amp, these three jumpers were set to "B", and on the A amp they were set to "A", so clearly, these jumpers reverse the phase of the signal coming in from the desk section. The two amps then process the signals in antiphase to one another right to the final output, where the 'correct' phase relationship is again restored, by wiring the speakon connector 'wrongly'. I have puzzled over this, and can think of no good reason for doing it that way. I did wonder at one point if it was anything to do with being able to make the amps bridgeable, but I downloaded a copy of the user manual, and there is a dire warning paragraph towards the end, specifically saying that under no circumstances should any attempt be made to operate the amps in bridge. It also warns against making any connections between the two amps or any speakers connected to them, although in actual fact, the 'ground' side of each output is truly ground, and is common to both amps, the whole (dual) power supply, and all metalwork. What am I missing here ? Not of any real consequence, as the unit is mended and back in its original condition, but for academic interest, it would be good to understand the designer's thinking. Arfa Carver has also done this on some models. Not so good when someone tries to demonstrate through a common grounded speaker switcher... Mark Z. Someone last week asked my advice on bridging high power amps. My advise was don't - bridge inputs if you must, thinking that any fault tends to lead to spectacular failure all round, I'd forgotton about this ,antiphase, reason for not bridging |
Thoughts on this little oddity, anyone ...?
Hi Arfa,
Arfa Daily Inscribed thus: Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has two completely separate and identical power amps, one of which was running very hot very quickly. No schematics, but problem was quickly traced to an open circuit resistor, by comparison ohms checks between the two amps. In order to replace the resistor, it was necessary to remove the offending ("B") power amp to get to the board underside. The power rails and ribbon from the mixer desk section were on removable connectors, but the 4 pole speakon connector was bolted to the rear panel and the wires were soldered directly to the board. When I came to unsolder these wires from the socket, I noticed that the "B" channel one was wired the opposite way round to the "A" channel one. I noted this down to check on later. When the amp was tested ok, I bolted it back in, and reconnected the speakon socket the same as it was when I unsoldered it. The wires lay exactly where they originally were, and looked absolutely factory-original, even down to the one which went to the uppermost connector tag, having a rubber sleeve on it, exactly the same as on the "A" channel. Except the two channels were definitely wired arse-about-face to one another. On both channels' speakons, the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel "A" the ground wire was connected to the "+" pair, and on the "B" channel, the ground was on the "-" pair. I fired it up again, and put a sine wave in, then hooked up two channels of my 'scope, one to the "A" output and one to the "B", both with the same polarity, and was surprised to find that the two signals were completely anti-phase, which meant that with the reversed connections to the speakon sockets, they would be back in phase again to the outside world. I then went back to the power amps' front ends near where the ribbon connector was, and found the same thing. At the same point on each amp, the signals were in opposite phase to one another. I then spotted three little 3 pin header blocks, with one pin pair on each designated "B", and the other pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to the power of' position. On the B amp, these three jumpers were set to "B", and on the A amp they were set to "A", so clearly, these jumpers reverse the phase of the signal coming in from the desk section. The two amps then process the signals in antiphase to one another right to the final output, where the 'correct' phase relationship is again restored, by wiring the speakon connector 'wrongly'. I have puzzled over this, and can think of no good reason for doing it that way. I did wonder at one point if it was anything to do with being able to make the amps bridgeable, but I downloaded a copy of the user manual, and there is a dire warning paragraph towards the end, specifically saying that under no circumstances should any attempt be made to operate the amps in bridge. It also warns against making any connections between the two amps or any speakers connected to them, although in actual fact, the 'ground' side of each output is truly ground, and is common to both amps, the whole (dual) power supply, and all metalwork. What am I missing here ? Not of any real consequence, as the unit is mended and back in its original condition, but for academic interest, it would be good to understand the designer's thinking. Arfa I'll bet that your first suspicion is right and the amps have or are being used in bridge configuration in some other gear. -- Best Regards: Baron. |
Thoughts on this little oddity, anyone ...?
In article , Baron wrote:
What am I missing here ? Not of any real consequence, as the unit is mended and back in its original condition, but for academic interest, it would be good to understand the designer's thinking. Arfa I'll bet that your first suspicion is right and the amps have or are being used in bridge configuration in some other gear. There's another possibility, which I saw mentioned a few years ago. By configuring the amp so that the two channels are operating in opposition to one another, you may increase the amp's ability to handle short periods of very high output (brief peaks) slightly. The reasoning is this: most of the time, high-amplitude signals are going to be in phase between the two channels. If the two channels are operating with the same polarity, they're both going to be drawing on the same power-supply rail during these peaks (i.e. both will be + or both will - at any given moment). This sudden load will tend to draw down the power-supply bypass cap for that rail, and/or will result in a large amount of current flowing through one side of the power supply rectifier chain. The other rail's supply capacitor won't have any load on it at that moment, and its rectifier won't be carrying much if any current. On the other hand, if you invert the audio signal coming into one channel, and then wire up the output of that channel to the speakers "backwards", you end up with a situation in which the two channels are usually drawing power from _opposite_ power-supply rails. This will decrease the worst-case sag in the rail voltages, and also tend to reduce and even out the current flow in the rectifiers (and reduce voltage drop and heating slightly). As a result, an amp which was designed with a "not-so-stiff" power supply system, may be able to deliver a slightly higher amount of peak power before it begins to clip and distort. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
Thoughts on this little oddity, anyone ...?
Dave Platt Inscribed thus:
In article , Baron wrote: What am I missing here ? Not of any real consequence, as the unit is mended and back in its original condition, but for academic interest, it would be good to understand the designer's thinking. Arfa I'll bet that your first suspicion is right and the amps have or are being used in bridge configuration in some other gear. There's another possibility, which I saw mentioned a few years ago. By configuring the amp so that the two channels are operating in opposition to one another, you may increase the amp's ability to handle short periods of very high output (brief peaks) slightly. The reasoning is this: most of the time, high-amplitude signals are going to be in phase between the two channels. If the two channels are operating with the same polarity, they're both going to be drawing on the same power-supply rail during these peaks (i.e. both will be + or both will - at any given moment). This sudden load will tend to draw down the power-supply bypass cap for that rail, and/or will result in a large amount of current flowing through one side of the power supply rectifier chain. The other rail's supply capacitor won't have any load on it at that moment, and its rectifier won't be carrying much if any current. On the other hand, if you invert the audio signal coming into one channel, and then wire up the output of that channel to the speakers "backwards", you end up with a situation in which the two channels are usually drawing power from _opposite_ power-supply rails. This will decrease the worst-case sag in the rail voltages, and also tend to reduce and even out the current flow in the rectifiers (and reduce voltage drop and heating slightly). As a result, an amp which was designed with a "not-so-stiff" power supply system, may be able to deliver a slightly higher amount of peak power before it begins to clip and distort. Interesting analysis ! Though I thought Arfa said "Dual" power supplies. Unless he meant dual rails from a common transformer. -- Best Regards: Baron. |
Thoughts on this little oddity, anyone ...?
"Arfa Daily" I fired it up again, and put a sine wave in, then hooked up two channels of my 'scope, one to the "A" output and one to the "B", both with the same polarity, and was surprised to find that the two signals were completely anti-phase, ** When servicing audio, knowing the polarity of the test signal and if there is significant phase shift is important. A scope triggering off the incoming wave will it not reveal this in single channel mode - so AF used both channels. A much better way is to use the External Synch input on your scope and link it to a fixed output on your bench audio oscillator ( create on if you have to). This way, you see instantly if the signal's phase has reversed or has a large phase shift. Also, triggering will be rock steady with nearly any kind of distorted, noisy or contaminated signal on the scope screen. Try it out for a day or two - you will never go back. .... Phil |
Thoughts on this little oddity, anyone ...?
"Michael A. Terrell" wrote in message m... Arfa Daily wrote: Had an Allen ? Heath PA20-CP 2 x 500 watt powered mixer in today. Has two completely separate and identical power amps, one of which was running very hot very quickly. No schematics, but problem was quickly traced to an open circuit resistor, by comparison ohms checks between the two amps. In order to replace the resistor, it was necessary to remove the offending ("B") power amp to get to the board underside. The power rails and ribbon from the mixer desk section were on removable connectors, but the 4 pole speakon connector was bolted to the rear panel and the wires were soldered directly to the board. When I came to unsolder these wires from the socket, I noticed that the "B" channel one was wired the opposite way round to the "A" channel one. I noted this down to check on later. When the amp was tested ok, I bolted it back in, and reconnected the speakon socket the same as it was when I unsoldered it. The wires lay exactly where they originally were, and looked absolutely factory-original, even down to the one which went to the uppermost connector tag, having a rubber sleeve on it, exactly the same as on the "A" channel. Except the two channels were definitely wired arse-about-face to one another. On both channels' speakons, the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel "A" the ground wire was connected to the "+" pair, and on the "B" channel, the ground was on the "-" pair. I fired it up again, and put a sine wave in, then hooked up two channels of my 'scope, one to the "A" output and one to the "B", both with the same polarity, and was surprised to find that the two signals were completely anti-phase, which meant that with the reversed connections to the speakon sockets, they would be back in phase again to the outside world. I then went back to the power amps' front ends near where the ribbon connector was, and found the same thing. At the same point on each amp, the signals were in opposite phase to one another. I then spotted three little 3 pin header blocks, with one pin pair on each designated "B", and the other pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to the power of' position. On the B amp, these three jumpers were set to "B", and on the A amp they were set to "A", so clearly, these jumpers reverse the phase of the signal coming in from the desk section. The two amps then process the signals in antiphase to one another right to the final output, where the 'correct' phase relationship is again restored, by wiring the speakon connector 'wrongly'. I have puzzled over this, and can think of no good reason for doing it that way. I did wonder at one point if it was anything to do with being able to make the amps bridgeable, but I downloaded a copy of the user manual, and there is a dire warning paragraph towards the end, specifically saying that under no circumstances should any attempt be made to operate the amps in bridge. It also warns against making any connections between the two amps or any speakers connected to them, although in actual fact, the 'ground' side of each output is truly ground, and is common to both amps, the whole (dual) power supply, and all metalwork. What am I missing here ? Not of any real consequence, as the unit is mended and back in its original condition, but for academic interest, it would be good to understand the designer's thinking. If both amps are fed the same signal, it reduces the maximum current requirements for both the + and - rails. That also lowers the current in the system ground, reducing crosstalk & possible ground loops. This is done in some RF circuits, as well. -- You can't have a sense of humor, if you have no sense. Thanks Michael. Good thoughts. Arfa |
Thoughts on this little oddity, anyone ...?
"Cydrome Leader" wrote in message ... Arfa Daily wrote: Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has two completely separate and identical power amps, one of which was running very hot very quickly. No schematics, but problem was quickly traced to an open circuit resistor, by comparison ohms checks between the two amps. In order to replace the resistor, it was necessary to remove the offending ("B") power amp to get to the board underside. The power rails and ribbon from the mixer desk section were on removable connectors, but the 4 pole speakon connector was bolted to the rear panel and the wires were soldered directly to the board. When I came to unsolder these wires from the socket, I noticed that the "B" channel one was wired the opposite way round to the "A" channel one. I noted this down to check on later. When the amp was tested ok, I bolted it back in, and reconnected the speakon socket the same as it was when I unsoldered it. The wires lay exactly where they originally were, and looked absolutely factory-original, even down to the one which went to the uppermost connector tag, having a rubber sleeve on it, exactly the same as on the "A" channel. Except the two channels were definitely wired arse-about-face to one another. On both channels' speakons, the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel "A" the ground wire was connected to the "+" pair, and on the "B" channel, the ground was on the "-" pair. I fired it up again, and put a sine wave in, then hooked up two channels of my 'scope, one to the "A" output and one to the "B", both with the same polarity, and was surprised to find that the two signals were completely anti-phase, which meant that with the reversed connections to the speakon sockets, they would be back in phase again to the outside world. I then went back to the power amps' front ends near where the ribbon connector was, and found the same thing. At the same point on each amp, the signals were in opposite phase to one another. I then spotted three little 3 pin header blocks, with one pin pair on each designated "B", and the other pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to the power of' position. On the B amp, these three jumpers were set to "B", and on the A amp they were set to "A", so clearly, these jumpers reverse the phase of the signal coming in from the desk section. The two amps then process the signals in antiphase to one another right to the final output, where the 'correct' phase relationship is again restored, by wiring the speakon connector 'wrongly'. I have puzzled over this, and can think of no good reason for doing it that way. I did wonder at one point if it was anything to do with being able to make the amps bridgeable, but I downloaded a copy of the user manual, and there is a dire warning paragraph towards the end, specifically saying that under no circumstances should any attempt be made to operate the amps in bridge. It also warns against making any connections between the two amps or any speakers connected to them, although in actual fact, the 'ground' side of each output is truly ground, and is common to both amps, the whole (dual) power supply, and all metalwork. What am I missing here ? Not of any real consequence, as the unit is mended and back in its original condition, but for academic interest, it would be good to understand the designer's thinking. Arfa are these amps run from one power supply? Sort of. It's one bloody great torroidal tranny, for sure, but it looked as though there was enough filter caps on the board for it to be otherwise two independant supplies. Certainly, two sets of + / - / gnd cables came out of the psu for the two amps, but I must admit that I did not look at its design closely enough to see if there was two sets of secondary windings, and two rectifiers. The ground was certainly common to both amps, and one leg of the output on each amp was that same ground. Arfa |
Thoughts on this little oddity, anyone ...?
"Phil Allison" wrote in message ... "Arfa Daily" What am I missing here ? Not of any real consequence, as the unit is mended and back in its original condition, but for academic interest, it would be good to understand the designer's thinking. ** The idea is sometimes known as the "OOPS" system - for Out Of Phase Stereo. It balances the load on the common PSU at any point in time, reducing voltage drop and electro cap ripple voltage. Bob Carver used the idea in his famous M400a " Carver Cube" amp, et alia. http://www.oaktreevintage.com/web_ph...eo_Amp_web.jpg http://www.doebbe.com/hifi/pics/Carver_M400_ret_3s.jpg Fancy A&H picking up on it ..... ... Phil Thanks Phil. That seems to be the concensus that's beginning to form here. Arfa |
Thoughts on this little oddity, anyone ...?
"Mark Zacharias" wrote in message .com... "Arfa Daily" wrote in message ... Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has two completely separate and identical power amps, one of which was running very hot very quickly. No schematics, but problem was quickly traced to an open circuit resistor, by comparison ohms checks between the two amps. In order to replace the resistor, it was necessary to remove the offending ("B") power amp to get to the board underside. The power rails and ribbon from the mixer desk section were on removable connectors, but the 4 pole speakon connector was bolted to the rear panel and the wires were soldered directly to the board. When I came to unsolder these wires from the socket, I noticed that the "B" channel one was wired the opposite way round to the "A" channel one. I noted this down to check on later. When the amp was tested ok, I bolted it back in, and reconnected the speakon socket the same as it was when I unsoldered it. The wires lay exactly where they originally were, and looked absolutely factory-original, even down to the one which went to the uppermost connector tag, having a rubber sleeve on it, exactly the same as on the "A" channel. Except the two channels were definitely wired arse-about-face to one another. On both channels' speakons, the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel "A" the ground wire was connected to the "+" pair, and on the "B" channel, the ground was on the "-" pair. I fired it up again, and put a sine wave in, then hooked up two channels of my 'scope, one to the "A" output and one to the "B", both with the same polarity, and was surprised to find that the two signals were completely anti-phase, which meant that with the reversed connections to the speakon sockets, they would be back in phase again to the outside world. I then went back to the power amps' front ends near where the ribbon connector was, and found the same thing. At the same point on each amp, the signals were in opposite phase to one another. I then spotted three little 3 pin header blocks, with one pin pair on each designated "B", and the other pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to the power of' position. On the B amp, these three jumpers were set to "B", and on the A amp they were set to "A", so clearly, these jumpers reverse the phase of the signal coming in from the desk section. The two amps then process the signals in antiphase to one another right to the final output, where the 'correct' phase relationship is again restored, by wiring the speakon connector 'wrongly'. I have puzzled over this, and can think of no good reason for doing it that way. I did wonder at one point if it was anything to do with being able to make the amps bridgeable, but I downloaded a copy of the user manual, and there is a dire warning paragraph towards the end, specifically saying that under no circumstances should any attempt be made to operate the amps in bridge. It also warns against making any connections between the two amps or any speakers connected to them, although in actual fact, the 'ground' side of each output is truly ground, and is common to both amps, the whole (dual) power supply, and all metalwork. What am I missing here ? Not of any real consequence, as the unit is mended and back in its original condition, but for academic interest, it would be good to understand the designer's thinking. Arfa Carver has also done this on some models. Not so good when someone tries to demonstrate through a common grounded speaker switcher... Mark Z. No, I guess it wouldn't be ... :-\ Arfa |
Thoughts on this little oddity, anyone ...?
"Dave Platt" wrote in message ... In article , Baron wrote: What am I missing here ? Not of any real consequence, as the unit is mended and back in its original condition, but for academic interest, it would be good to understand the designer's thinking. Arfa I'll bet that your first suspicion is right and the amps have or are being used in bridge configuration in some other gear. There's another possibility, which I saw mentioned a few years ago. By configuring the amp so that the two channels are operating in opposition to one another, you may increase the amp's ability to handle short periods of very high output (brief peaks) slightly. The reasoning is this: most of the time, high-amplitude signals are going to be in phase between the two channels. If the two channels are operating with the same polarity, they're both going to be drawing on the same power-supply rail during these peaks (i.e. both will be + or both will - at any given moment). This sudden load will tend to draw down the power-supply bypass cap for that rail, and/or will result in a large amount of current flowing through one side of the power supply rectifier chain. The other rail's supply capacitor won't have any load on it at that moment, and its rectifier won't be carrying much if any current. On the other hand, if you invert the audio signal coming into one channel, and then wire up the output of that channel to the speakers "backwards", you end up with a situation in which the two channels are usually drawing power from _opposite_ power-supply rails. This will decrease the worst-case sag in the rail voltages, and also tend to reduce and even out the current flow in the rectifiers (and reduce voltage drop and heating slightly). As a result, an amp which was designed with a "not-so-stiff" power supply system, may be able to deliver a slightly higher amount of peak power before it begins to clip and distort. -- Dave Platt Thanks Dave. This seems to be what everyone is thinking, and makes a lot of sense when you stand back and think about it Arfa |
Thoughts on this little oddity, anyone ...?
"Phil Allison" wrote in message ... "Arfa Daily" I fired it up again, and put a sine wave in, then hooked up two channels of my 'scope, one to the "A" output and one to the "B", both with the same polarity, and was surprised to find that the two signals were completely anti-phase, ** When servicing audio, knowing the polarity of the test signal and if there is significant phase shift is important. A scope triggering off the incoming wave will it not reveal this in single channel mode - so AF used both channels. A much better way is to use the External Synch input on your scope and link it to a fixed output on your bench audio oscillator ( create on if you have to). This way, you see instantly if the signal's phase has reversed or has a large phase shift. Also, triggering will be rock steady with nearly any kind of distorted, noisy or contaminated signal on the scope screen. Try it out for a day or two - you will never go back. ... Phil Interesting. Are you squaring up the generator's sine output, to give a good sharp transition for syncing the scope ? I was looking at the phase reversals on this particular unit, by using 2 of the channels on the scope, and using internal auto trigger derived from CH 1. The sync circuitry on this Hitachi scope is very good, and will produce a perfectly stable trigger point from virtually any waveform, no matter how complex or scruffy, but I can see where it would be useful to be able to have a fixed known trigger point, and from there, be able to look at relative phasing between amps, and phase shift within an amp, using a single scope channel. Arfa |
Thoughts on this little oddity, anyone ...?
"N_Cook" wrote in message
... Mark Zacharias wrote in message .com... "Arfa Daily" wrote in message ... Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has two completely separate and identical power amps, one of which was running very hot very quickly. No schematics, but problem was quickly traced to an open circuit resistor, by comparison ohms checks between the two amps. In order to replace the resistor, it was necessary to remove the offending ("B") power amp to get to the board underside. The power rails and ribbon from the mixer desk section were on removable connectors, but the 4 pole speakon connector was bolted to the rear panel and the wires were soldered directly to the board. When I came to unsolder these wires from the socket, I noticed that the "B" channel one was wired the opposite way round to the "A" channel one. I noted this down to check on later. When the amp was tested ok, I bolted it back in, and reconnected the speakon socket the same as it was when I unsoldered it. The wires lay exactly where they originally were, and looked absolutely factory-original, even down to the one which went to the uppermost connector tag, having a rubber sleeve on it, exactly the same as on the "A" channel. Except the two channels were definitely wired arse-about-face to one another. On both channels' speakons, the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel "A" the ground wire was connected to the "+" pair, and on the "B" channel, the ground was on the "-" pair. I fired it up again, and put a sine wave in, then hooked up two channels of my 'scope, one to the "A" output and one to the "B", both with the same polarity, and was surprised to find that the two signals were completely anti-phase, which meant that with the reversed connections to the speakon sockets, they would be back in phase again to the outside world. I then went back to the power amps' front ends near where the ribbon connector was, and found the same thing. At the same point on each amp, the signals were in opposite phase to one another. I then spotted three little 3 pin header blocks, with one pin pair on each designated "B", and the other pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to the power of' position. On the B amp, these three jumpers were set to "B", and on the A amp they were set to "A", so clearly, these jumpers reverse the phase of the signal coming in from the desk section. The two amps then process the signals in antiphase to one another right to the final output, where the 'correct' phase relationship is again restored, by wiring the speakon connector 'wrongly'. I have puzzled over this, and can think of no good reason for doing it that way. I did wonder at one point if it was anything to do with being able to make the amps bridgeable, but I downloaded a copy of the user manual, and there is a dire warning paragraph towards the end, specifically saying that under no circumstances should any attempt be made to operate the amps in bridge. It also warns against making any connections between the two amps or any speakers connected to them, although in actual fact, the 'ground' side of each output is truly ground, and is common to both amps, the whole (dual) power supply, and all metalwork. What am I missing here ? Not of any real consequence, as the unit is mended and back in its original condition, but for academic interest, it would be good to understand the designer's thinking. Arfa Carver has also done this on some models. Not so good when someone tries to demonstrate through a common grounded speaker switcher... Mark Z. Someone last week asked my advice on bridging high power amps. My advise was don't - bridge inputs if you must, thinking that any fault tends to lead to spectacular failure all round, I'd forgotton about this ,antiphase, reason for not bridging Such amps are naturally adaptable for bridging. Mark Z. |
Thoughts on this little oddity, anyone ...?
"Arfa Daily" "Phil Allison" "Arfa Daily" I fired it up again, and put a sine wave in, then hooked up two channels of my 'scope, one to the "A" output and one to the "B", both with the same polarity, and was surprised to find that the two signals were completely anti-phase, ** When servicing audio, knowing the polarity of the test signal and if there is significant phase shift is important. A scope triggering off the incoming wave will it not reveal this in single channel mode - so AF used both channels. A much better way is to use the External Synch input on your scope and link it to a fixed output on your bench audio oscillator ( create on if you have to). This way, you see instantly if the signal's phase has reversed or has a large phase shift. Also, triggering will be rock steady with nearly any kind of distorted, noisy or contaminated signal on the scope screen. Try it out for a day or two - you will never go back. Interesting. Are you squaring up the generator's sine output, to give a good sharp transition for syncing the scope ? ** FFS - that processing is inside any decent scope. Supply it with 0.5volt rms sine wave and you are away. The sync circuitry on this Hitachi scope is very good, and will produce a perfectly stable trigger point from virtually any waveform, no matter how complex or scruffy, ** Stable maybe, phase locked to the audio generator NO. can see where it would be useful to be able to have a fixed known trigger point, and from there, be able to look at relative phasing between amps, and phase shift within an amp, using a single scope channel. ** Most scope have an internal /external switch you can flick anytime. If you ever deal with tape recorders or tape echos, there is a big difference with a play back signal. .... Phil |
Thoughts on this little oddity, anyone ...?
"Arfa Daily" "Dave Platt" As a result, an amp which was designed with a "not-so-stiff" power supply system, may be able to deliver a slightly higher amount of peak power before it begins to clip and distort. Dave Platt Thanks Dave. This seems to be what everyone is thinking, and makes a lot of sense when you stand back and think about it ** Dave is wrong. The only time the OOPS idea increases power is when the test frequency is low, less than 120Hz. This is why Bob Carver did it - to reduce by half LF ripple on the PS voltage at frequencies between 20Hz and 120 Hz to meet his published power specs. .... Phil |
Thoughts on this little oddity, anyone ...?
Arfa Daily wrote:
"Cydrome Leader" wrote in message ... Arfa Daily wrote: Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has two completely separate and identical power amps, one of which was running very hot very quickly. No schematics, but problem was quickly traced to an open circuit resistor, by comparison ohms checks between the two amps. In order to replace the resistor, it was necessary to remove the offending ("B") power amp to get to the board underside. The power rails and ribbon from the mixer desk section were on removable connectors, but the 4 pole speakon connector was bolted to the rear panel and the wires were soldered directly to the board. When I came to unsolder these wires from the socket, I noticed that the "B" channel one was wired the opposite way round to the "A" channel one. I noted this down to check on later. When the amp was tested ok, I bolted it back in, and reconnected the speakon socket the same as it was when I unsoldered it. The wires lay exactly where they originally were, and looked absolutely factory-original, even down to the one which went to the uppermost connector tag, having a rubber sleeve on it, exactly the same as on the "A" channel. Except the two channels were definitely wired arse-about-face to one another. On both channels' speakons, the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel "A" the ground wire was connected to the "+" pair, and on the "B" channel, the ground was on the "-" pair. I fired it up again, and put a sine wave in, then hooked up two channels of my 'scope, one to the "A" output and one to the "B", both with the same polarity, and was surprised to find that the two signals were completely anti-phase, which meant that with the reversed connections to the speakon sockets, they would be back in phase again to the outside world. I then went back to the power amps' front ends near where the ribbon connector was, and found the same thing. At the same point on each amp, the signals were in opposite phase to one another. I then spotted three little 3 pin header blocks, with one pin pair on each designated "B", and the other pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to the power of' position. On the B amp, these three jumpers were set to "B", and on the A amp they were set to "A", so clearly, these jumpers reverse the phase of the signal coming in from the desk section. The two amps then process the signals in antiphase to one another right to the final output, where the 'correct' phase relationship is again restored, by wiring the speakon connector 'wrongly'. I have puzzled over this, and can think of no good reason for doing it that way. I did wonder at one point if it was anything to do with being able to make the amps bridgeable, but I downloaded a copy of the user manual, and there is a dire warning paragraph towards the end, specifically saying that under no circumstances should any attempt be made to operate the amps in bridge. It also warns against making any connections between the two amps or any speakers connected to them, although in actual fact, the 'ground' side of each output is truly ground, and is common to both amps, the whole (dual) power supply, and all metalwork. What am I missing here ? Not of any real consequence, as the unit is mended and back in its original condition, but for academic interest, it would be good to understand the designer's thinking. Arfa are these amps run from one power supply? Sort of. It's one bloody great torroidal tranny, for sure, but it looked as though there was enough filter caps on the board for it to be otherwise two independant supplies. Certainly, two sets of + / - / gnd cables came out of the psu for the two amps, but I must admit that I did not look at its design closely enough to see if there was two sets of secondary windings, and two rectifiers. The ground was certainly common to both amps, and one leg of the output on each amp was that same ground. Arfa It looks like others answered what I sort of suspected where they wanted to try to balance out the + and - rails when the audio peaks. I've never actually seen this before though. That reminds me- does anybody unnderstood what Carver's "power steering" was supposed to be or mean in their amps? |
Thoughts on this little oddity, anyone ...?
"Cydrome Leader" That reminds me- does anybody unnderstood what Carver's "power steering" was supposed to be or mean in their amps? ** Bob Carver was an expert at coming up with high falutin' names for various clever dodges he used to save weight and cost. His " Magnetic Field " amps were nothing of the kind. They simply employed a triac based voltage regulator in the AC supply to improve the regulation factor of a ridiculously small iron transformer. In his M400 and PM-2 designs, these puny transformers would emit cracking noises and *smoke* if you ever bench tested them at rated power for more than 60 seconds. No thermal shut down was fitted to save the trannies either. From the blurbs on "Power Steering" it applies to 5.1 channel amps and allows one audio channel to dominate the load on PSU at any given time. Likely a simple boost inhibit circuit that allows the use of a puny AC transformer again. .... Phil |
Thoughts on this little oddity, anyone ...?
"Cydrome Leader" wrote in message
... Arfa Daily wrote: "Cydrome Leader" wrote in message ... Arfa Daily wrote: Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has two completely separate and identical power amps, one of which was running very hot very quickly. No schematics, but problem was quickly traced to an open circuit resistor, by comparison ohms checks between the two amps. In order to replace the resistor, it was necessary to remove the offending ("B") power amp to get to the board underside. The power rails and ribbon from the mixer desk section were on removable connectors, but the 4 pole speakon connector was bolted to the rear panel and the wires were soldered directly to the board. When I came to unsolder these wires from the socket, I noticed that the "B" channel one was wired the opposite way round to the "A" channel one. I noted this down to check on later. When the amp was tested ok, I bolted it back in, and reconnected the speakon socket the same as it was when I unsoldered it. The wires lay exactly where they originally were, and looked absolutely factory-original, even down to the one which went to the uppermost connector tag, having a rubber sleeve on it, exactly the same as on the "A" channel. Except the two channels were definitely wired arse-about-face to one another. On both channels' speakons, the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel "A" the ground wire was connected to the "+" pair, and on the "B" channel, the ground was on the "-" pair. I fired it up again, and put a sine wave in, then hooked up two channels of my 'scope, one to the "A" output and one to the "B", both with the same polarity, and was surprised to find that the two signals were completely anti-phase, which meant that with the reversed connections to the speakon sockets, they would be back in phase again to the outside world. I then went back to the power amps' front ends near where the ribbon connector was, and found the same thing. At the same point on each amp, the signals were in opposite phase to one another. I then spotted three little 3 pin header blocks, with one pin pair on each designated "B", and the other pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to the power of' position. On the B amp, these three jumpers were set to "B", and on the A amp they were set to "A", so clearly, these jumpers reverse the phase of the signal coming in from the desk section. The two amps then process the signals in antiphase to one another right to the final output, where the 'correct' phase relationship is again restored, by wiring the speakon connector 'wrongly'. I have puzzled over this, and can think of no good reason for doing it that way. I did wonder at one point if it was anything to do with being able to make the amps bridgeable, but I downloaded a copy of the user manual, and there is a dire warning paragraph towards the end, specifically saying that under no circumstances should any attempt be made to operate the amps in bridge. It also warns against making any connections between the two amps or any speakers connected to them, although in actual fact, the 'ground' side of each output is truly ground, and is common to both amps, the whole (dual) power supply, and all metalwork. What am I missing here ? Not of any real consequence, as the unit is mended and back in its original condition, but for academic interest, it would be good to understand the designer's thinking. Arfa are these amps run from one power supply? Sort of. It's one bloody great torroidal tranny, for sure, but it looked as though there was enough filter caps on the board for it to be otherwise two independant supplies. Certainly, two sets of + / - / gnd cables came out of the psu for the two amps, but I must admit that I did not look at its design closely enough to see if there was two sets of secondary windings, and two rectifiers. The ground was certainly common to both amps, and one leg of the output on each amp was that same ground. Arfa It looks like others answered what I sort of suspected where they wanted to try to balance out the + and - rails when the audio peaks. I've never actually seen this before though. That reminds me- does anybody unnderstood what Carver's "power steering" was supposed to be or mean in their amps? Probably refers to "rail-switching" where higher voltage power supply rails are switched in on demand. (during musical peaks). Has several advantages including being able to get away with a smaller power transformer and smaller heat sinks. Reduces power dissipation at idle and at low power. It's said that such amps can sound nasty - not sure I've ever noticed that. Mark Z. |
Thoughts on this little oddity, anyone ...?
Phil Allison wrote:
"Cydrome Leader" That reminds me- does anybody unnderstood what Carver's "power steering" was supposed to be or mean in their amps? ** Bob Carver was an expert at coming up with high falutin' names for various clever dodges he used to save weight and cost. His " Magnetic Field " amps were nothing of the kind. They simply employed a triac based voltage regulator in the AC supply to improve the regulation factor of a ridiculously small iron transformer. Hmm, was anybody in the 70s not obsessed with triac pre-regulation in all sorts of power supplies? In his M400 and PM-2 designs, these puny transformers would emit cracking noises and *smoke* if you ever bench tested them at rated power for more than 60 seconds. No thermal shut down was fitted to save the trannies either. nice. From the blurbs on "Power Steering" it applies to 5.1 channel amps and allows one audio channel to dominate the load on PSU at any given time. Likely a simple boost inhibit circuit that allows the use of a puny AC transformer again. I own an older 5 channel carver amp. I recall upon opening it that the power supply seemed way small for the alleged rated power across all channels. The manual said something about this power steering crap, but the block schematic didn't show anything indicating it was anything other than "you can overload a channel a little bit before it burns out or the fuse blows." It seems ok as far as amps in the early surround sound era were. My DVD+sound setup is straight from 1998 and is this complex mess of stuff running into a technics decoder and from there there are 5 RCA cables running to each channel on the carver amp. There's some goofy fiber link between the DVD player and the technics thing. Prior to this, I had a stack of multiple stereos running everything. For a crappy action movie where everything explodes, balance doesn't really matter. I'm not even sure how you'd do this these days if you didn't get one of this does everything consumer amps. |
Thoughts on this little oddity, anyone ...?
That reminds me- does anybody unnderstood what Carver's "power steering"
was supposed to be or mean in their amps? Probably refers to "rail-switching" where higher voltage power supply rails are switched in on demand. (during musical peaks). Has several advantages including being able to get away with a smaller power transformer and smaller heat sinks. Reduces power dissipation at idle and at low power. It's said that such amps can sound nasty - not sure I've ever noticed that. Mark Z. The Carver PM1.5 had both the triac and inadequate transformer, and rail switching. High rail was + and - 125v IIRC. Gareth. |
Thoughts on this little oddity, anyone ...?
"Gareth Magennis" wrote in message ... That reminds me- does anybody unnderstood what Carver's "power steering" was supposed to be or mean in their amps? Probably refers to "rail-switching" where higher voltage power supply rails are switched in on demand. (during musical peaks). Has several advantages including being able to get away with a smaller power transformer and smaller heat sinks. Reduces power dissipation at idle and at low power. It's said that such amps can sound nasty - not sure I've ever noticed that. Mark Z. The Carver PM1.5 had both the triac and inadequate transformer, and rail switching. High rail was + and - 125v IIRC. Gareth. Er, actually I seem to recall now being corrected on that figure once before by Mr Allison, and it is slightly less? 117? Gareth. |
Thoughts on this little oddity, anyone ...?
According to the manual:
"CONSTANT POWER SWITCH Set the rear panel slide switch to match the impedance of the speakers used. Use the combined impedance value if you are connecting speakers in series or parallel. Choose the nearest match if the value is not exactly 4 or 8 ohms. Note that no damage will be done if the switch is left in the wrong position. However, you may not benefit from the full capability of the amplifier. Constant power explained The typical amplifier is optimised to produce full power into 4 ohms and therefore considerably less into 8 ohms.. The PA Series features a unique facility that ensures you get full power output into either 4 or 8 ohm speakers. It does this by reconfiguring the power supply for optimum current (4 ohms), or voltage (8 ohms)." Perhaps this is why. They also have the ability through switches to delegate signal to the individual amps for mains and monitors. |
Thoughts on this little oddity, anyone ...?
"Cydrome Leader" ** Bob Carver was an expert at coming up with high falutin' names for various clever dodges he used to save weight and cost. His " Magnetic Field " amps were nothing of the kind. They simply employed a triac based voltage regulator in the AC supply to improve the regulation factor of a ridiculously small iron transformer. Hmm, was anybody in the 70s not obsessed with triac pre-regulation in all sorts of power supplies? ** What drugs are you on ? Carver amps were the virtually the only ones to use the idea and the first ( M400) came out in the early 1980s. ( Yamaha made a version and sold about 5. ) .... Phil |
Thoughts on this little oddity, anyone ...?
"Gareth Magennis" The Carver PM1.5 had both the triac and inadequate transformer, and rail switching. High rail was + and - 125v IIRC. ** No rail switching as such, just diode steering. The regulated DC rails were +/- 23, +/- 55 +/- 80 and +/- 125. .... Phil |
Thoughts on this little oddity, anyone ...?
"Klay_Anderson" wrote in message
news:14356237.137.1320865621980.JavaMail.geo-discussion-forums@prnv30... According to the manual: "CONSTANT POWER SWITCH Set the rear panel slide switch to match the impedance of the speakers used. Use the combined impedance value if you are connecting speakers in series or parallel. Choose the nearest match if the value is not exactly 4 or 8 ohms. Note that no damage will be done if the switch is left in the wrong position. However, you may not benefit from the full capability of the amplifier. Constant power explained The typical amplifier is optimised to produce full power into 4 ohms and therefore considerably less into 8 ohms. The PA Series features a unique facility that ensures you get full power output into either 4 or 8 ohm speakers. It does this by reconfiguring the power supply for optimum current (4 ohms), or voltage (8 ohms)." .Perhaps this is why. They also have the ability through switches to delegate signal to the .individual amps for mains and monitors. Huh? I would suggest Mr. Carver was doing what he was best known for; taking an unneeded or ordinary function, obfuscating or renaming it, and making it a "feature". More likely, although I have not checked the schematic, that the "impedance switch" is like those of modern surround receivers: changing the transformer output voltage downward for "4-ohm" operation, and higher for "8-ohm" operation. Like the rail-switching, this reduces quiescent power dissipation and heat losses, but also limits full power capability. Example: I ran into an older McIntosh MA-6100 the other day. I didn't remember it had a "4-ohm - 8-ohm" switch. So I tested it. In the "4-ohm" position, it was a 50-watt per channel amp. In the "8-ohm" position, it got about 90 watts at clipping. Mark Z. |
Thoughts on this little oddity, anyone ...?
"Mark Zacharias" "CONSTANT POWER SWITCH Set the rear panel slide switch to match the impedance of the speakers used. Use the combined impedance value if you are connecting speakers in series or parallel. Choose the nearest match if the value is not exactly 4 or 8 ohms. Note that no damage will be done if the switch is left in the wrong position. However, you may not benefit from the full capability of the amplifier. Constant power explained The typical amplifier is optimised to produce full power into 4 ohms and therefore considerably less into 8 ohms. The PA Series features a unique facility that ensures you get full power output into either 4 or 8 ohm speakers. It does this by reconfiguring the power supply for optimum current (4 ohms), or voltage (8 ohms)." .Perhaps this is why. They also have the ability through switches to delegate signal to the .individual amps for mains and monitors. Huh? I would suggest Mr. Carver was doing what he was best known for; taking an unneeded or ordinary function, obfuscating or renaming it, and making it a "feature". ** The paragraph is from Allan and Heath. ..... Phil |
Thoughts on this little oddity, anyone ...?
"Phil Allison" wrote in message ... "Arfa Daily" "Phil Allison" "Arfa Daily" I fired it up again, and put a sine wave in, then hooked up two channels of my 'scope, one to the "A" output and one to the "B", both with the same polarity, and was surprised to find that the two signals were completely anti-phase, ** When servicing audio, knowing the polarity of the test signal and if there is significant phase shift is important. A scope triggering off the incoming wave will it not reveal this in single channel mode - so AF used both channels. A much better way is to use the External Synch input on your scope and link it to a fixed output on your bench audio oscillator ( create on if you have to). This way, you see instantly if the signal's phase has reversed or has a large phase shift. Also, triggering will be rock steady with nearly any kind of distorted, noisy or contaminated signal on the scope screen. Try it out for a day or two - you will never go back. Interesting. Are you squaring up the generator's sine output, to give a good sharp transition for syncing the scope ? ** FFS - that processing is inside any decent scope. Supply it with 0.5volt rms sine wave and you are away. I'll have to look at that on my scope, but off the top of my head, I thought that the trigger point was still adjustable on external and could be set anywhere on the trigger signal and if so, I'm not sure that I see how it helps to trigger the scope externally from the same sine wave as it would use internally on auto trigger. I can see how it would be helpful to have a fixed trigger point, virtually at the beginning of a cycle, by squaring up the generator's output, and then feeding that to the external trigger input. Arfa |
Thoughts on this little oddity, anyone ...?
"Arfa Daily" ** When servicing audio, knowing the polarity of the test signal and if there is significant phase shift is important. A scope triggering off the incoming wave will it not reveal this in single channel mode - so AF used both channels. A much better way is to use the External Synch input on your scope and link it to a fixed output on your bench audio oscillator ( create on if you have to). This way, you see instantly if the signal's phase has reversed or has a large phase shift. Also, triggering will be rock steady with nearly any kind of distorted, noisy or contaminated signal on the scope screen. Try it out for a day or two - you will never go back. Interesting. Are you squaring up the generator's sine output, to give a good sharp transition for syncing the scope ? ** FFS - that processing is inside any decent scope. Supply it with 0.5volt rms sine wave and you are away. I'll have to look at that on my scope, but off the top of my head, I thought that the trigger point was still adjustable on external and could be set anywhere on the trigger signal and if so, I'm not sure that I see how it helps to trigger the scope externally from the same sine wave as it would use internally on auto trigger. ** But it is NOT the same sine wave !!!!!!!!!!!! Various audio devices invert, phase shift, phase modulate and even time shift sine waves !!! I can see how it would be helpful to have a fixed trigger point, virtually at the beginning of a cycle, ** Best set the trigger level to a zero crossing of the generator feed so the same wave on the scope starts at a zero. I have mine so the first half cycle is positive when there is no phase shift. With external synch, the scope is always triggered and the level on the screen does not matter. FFS - TRY IT !!!!!!!!!!! .... Phil |
Thoughts on this little oddity, anyone ...?
Phil Allison wrote:
"Cydrome Leader" ** Bob Carver was an expert at coming up with high falutin' names for various clever dodges he used to save weight and cost. His " Magnetic Field " amps were nothing of the kind. They simply employed a triac based voltage regulator in the AC supply to improve the regulation factor of a ridiculously small iron transformer. Hmm, was anybody in the 70s not obsessed with triac pre-regulation in all sorts of power supplies? ** What drugs are you on ? Want some? As I said, all sorts of old power supplies were made with SCRs and preregulators before the transformer. The smaller HP/Agilent bench power supplies are still made this way. I guess you get a smaller heatsink and a little more efficiency. |
Thoughts on this little oddity, anyone ...?
"Cydrome Leader" Hmm, was anybody in the 70s not obsessed with triac pre-regulation in all sorts of power supplies? ** What drugs are you on ? Want some? As I said, all sorts of old power supplies were made with SCRs and preregulators before the transformer. ** But not in audio amplifiers !!! You PITA ****wit troll. .... Phil |
Thoughts on this little oddity, anyone ...?
Phil Allison wrote:
"Cydrome Leader" Hmm, was anybody in the 70s not obsessed with triac pre-regulation in all sorts of power supplies? ** What drugs are you on ? Want some? As I said, all sorts of old power supplies were made with SCRs and preregulators before the transformer. ** But not in audio amplifiers !!! You PITA ****wit troll. Oh goody, you're so smart, you just figured out that "all sorts of power supplies" is not a subset of audio amplifiers. When did this occur to you? |
Thoughts on this little oddity, anyone ...?
"Phil Allison" wrote in message ... "Arfa Daily" ** When servicing audio, knowing the polarity of the test signal and if there is significant phase shift is important. A scope triggering off the incoming wave will it not reveal this in single channel mode - so AF used both channels. A much better way is to use the External Synch input on your scope and link it to a fixed output on your bench audio oscillator ( create on if you have to). This way, you see instantly if the signal's phase has reversed or has a large phase shift. Also, triggering will be rock steady with nearly any kind of distorted, noisy or contaminated signal on the scope screen. Try it out for a day or two - you will never go back. Interesting. Are you squaring up the generator's sine output, to give a good sharp transition for syncing the scope ? ** FFS - that processing is inside any decent scope. Supply it with 0.5volt rms sine wave and you are away. I'll have to look at that on my scope, but off the top of my head, I thought that the trigger point was still adjustable on external and could be set anywhere on the trigger signal and if so, I'm not sure that I see how it helps to trigger the scope externally from the same sine wave as it would use internally on auto trigger. ** But it is NOT the same sine wave !!!!!!!!!!!! Various audio devices invert, phase shift, phase modulate and even time shift sine waves !!! I can see how it would be helpful to have a fixed trigger point, virtually at the beginning of a cycle, ** Best set the trigger level to a zero crossing of the generator feed so the same wave on the scope starts at a zero. I have mine so the first half cycle is positive when there is no phase shift. With external synch, the scope is always triggered and the level on the screen does not matter. FFS - TRY IT !!!!!!!!!!! ... Phil Yeah, ok. I tried it today, and I see what you're saying now. Once you've set the trigger point on the sine sample from the generator that you're driving the external trigger input from, then that's it fixed for all subsequent measurements and you should be able to see any phase shifts or inversions. When I said about squaring up the sample, I was thinking that by doing this, not only do you phase lock the scope to the original wave, you also have an always-known point that the triggering is locked to. If you did the squaring with a decent comparator, that trigger point could be fixed within a few degrees of the zero crossing for all cases, eliminating the need to twiddle the trigger level, and pick some arbitrary point anywhere on the rising or falling slopes of the sine. Arfa |
Thoughts on this little oddity, anyone ...?
The most likely reason is that it reduces the peak current drawn from each
side of the power supply. |
Thoughts on this little oddity, anyone ...?
That reminds me - does anybody understand what Carver's
"power steering" was supposed to be or mean in their amps? Whatever Bob meant it to mean. Bob was notorious for coming up with desciptions that bore little or no relationship to how the circuit actually worked. |
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