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UK diy (uk.d-i-y) For the discussion of all topics related to diy (do-it-yourself) in the UK. All levels of experience and proficency are welcome to join in to ask questions or offer solutions. |
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#1
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Split rail PS
When building a split rail low current PS with discrete components - like
say +/- 15v - does it matter if the transformer doesn't have a centre tap on the secondary? It's just that I've seen circuits for both around. Some say to omit the centre tap connection to ground even if it has one. -- *Santa Claus has the right idea. Visit people only once a year. Dave Plowman London SW To e-mail, change noise into sound. |
#2
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Split rail PS
In article ,
Dave Plowman (News) wrote: When building a split rail low current PS with discrete components - like say +/- 15v - does it matter if the transformer doesn't have a centre tap on the secondary? It's just that I've seen circuits for both around. Some say to omit the centre tap connection to ground even if it has one. It depends on what you are trying to power. If the device simply wants a 30v supply, then the centre tap isn't needed. If the requirement is for +15, 0, -15 then you are going to have to get the 0 reference from somewhere. It can be done in the load device with suitable circuitry, but it's much easier to use the centre tap. -- From KT24 Using a RISC OS computer running v5.18 |
#3
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Split rail PS
Dave Plowman (News) wrote:
When building a split rail low current PS with discrete components - like say +/- 15v - does it matter if the transformer doesn't have a centre tap on the secondary? It's just that I've seen circuits for both around. Some say to omit the centre tap connection to ground even if it has one. I can't think how one would work without a centre tap - and presuming it doesn't just mean two secondaries with a connection where they're joined. Is it some setup where one builds a (say) 0-15-30v PSU where the 15v terminal is used as ground and can sink current as well as supply it? -- Scott |
#4
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Split rail PS
In article ,
"Dave Plowman (News)" writes: When building a split rail low current PS with discrete components - like say +/- 15v - does it matter if the transformer doesn't have a centre tap on the secondary? It's just that I've seen circuits for both around. Some say to omit the centre tap connection to ground even if it has one. Depends on the nature of the load. If the load draws exactly same current on both rails, no current will flow in the 0V connection, and in theory it's not needed. In practice, this is very unlikely. If the load can draw different currents on the two rails, then a current will flow in the 0V connection. If there's no centre-tap on the transformer, this current in the 0V rail will need to be passed to the opposite rail, wasting power in the power supply. If the difference in rail currents is small, this probably doesn't matter. If it's big, the the power supply will have to start dissipating/wasting larger amounts of power, and in that case a design which can use a centre- tapped transformer will be more efficient and run cooler. -- Andrew Gabriel [email address is not usable -- followup in the newsgroup] |
#5
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Split rail PS
On Sun, 25 Aug 2013 14:34:06 +0100, Dave Plowman (News) wrote:
When building a split rail low current PS with discrete components - like say +/- 15v - does it matter if the transformer doesn't have a centre tap on the secondary? It's just that I've seen circuits for both around. Some say to omit the centre tap connection to ground even if it has one. If your transformer has a centre tap then the best way is to use it. (bridge rectifier to give 2 full-wave rectified supplies, earth the centre tap). If it has two separate windings then you can either 1) bridge rectify them individually, making 2 isolated supplies and then connect - on one to + on the other to form a centre tap. 2) connect the windings in series (and in phase) to produce a centre tap. If it has a single winding then the best way is probably to use a power op-amp to produce a "virtual" centre tap. Output current is very limited though, and you have an extra device to heatsink. This is sometimes done for low level audio stuff as the virtual tap can be very low noise if done properly. It is also highly accurate and will always be relative to both supply rails. |
#6
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Split rail PS
On Sunday, August 25, 2013 2:34:06 PM UTC+1, Dave Plowman (News) wrote:
When building a split rail low current PS with discrete components - like I say +/- 15v - does it matter if the transformer doesn't have a centre tap I on the secondary? It's just that I've seen circuits for both around. Some I say to omit the centre tap connection to ground even if it has one. There are ways to build +- psus with no transformer CT. One of the simplest uses a single secondary winding and 2 diodes, one giving a + rail, one a -.. The downside is larger reservoirs needed. There are other schemes around too. I think you'd need to give more info about your transformer, what rails you want, and how symmetrical the load is. NT |
#7
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Split rail PS
On 25/08/13 14:34, Dave Plowman (News) wrote:
When building a split rail low current PS with discrete components - like say +/- 15v - does it matter if the transformer doesn't have a centre tap on the secondary? It's just that I've seen circuits for both around. Some say to omit the centre tap connection to ground even if it has one. you tend to need it if the current draw is asymmetric. -- Ineptocracy (in-ep-toc-ra-cy) €“ a system of government where the least capable to lead are elected by the least capable of producing, and where the members of society least likely to sustain themselves or succeed, are rewarded with goods and services paid for by the confiscated wealth of a diminishing number of producers. |
#8
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Split rail PS
It's the PS for a fairly esoteric moving coil pickup pre-amp which uses
balanced inputs. The quiescent current at least is very similar on each rail. Basically was chasing the last residue of hum, and discovered (by accident) if I disconnected the transformer centre tap it improved. The scope showed the hum to be more on the negative rail. Changing the neg reg to a different make improved things to the point I doubt I'll better. -- *Verbs HAS to agree with their subjects * Dave Plowman London SW To e-mail, change noise into sound. |
#9
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Split rail PS
On Mon, 26 Aug 2013 00:26:06 +0100, Dave Plowman (News) wrote:
It's the PS for a fairly esoteric moving coil pickup pre-amp which uses balanced inputs. The quiescent current at least is very similar on each rail. Basically was chasing the last residue of hum, and discovered (by accident) if I disconnected the transformer centre tap it improved. The scope showed the hum to be more on the negative rail. Changing the neg reg to a different make improved things to the point I doubt I'll better. That's exactly the sort of job where a virtual ground is useful. You can get a single, nice smooth regulated supply then split it using an op-amp and 2 close tolerance resistors. Your ground is then bang on centre-rail no matter what the loadings are (within the output capability of the op- amp). |
#10
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Split rail PS
On Monday, August 26, 2013 12:26:06 AM UTC+1, Dave Plowman (News) wrote:
It's the PS for a fairly esoteric moving coil pickup pre-amp which uses balanced inputs. The quiescent current at least is very similar on each rail. Basically was chasing the last residue of hum, and discovered (by accident) if I disconnected the transformer centre tap it improved. The scope showed the hum to be more on the negative rail. Changing the neg reg to a different make improved things to the point I doubt I'll better. Sounds like a symmetrical load. Adding resistors can help deal with small imbalance. For low current loads like that, passive power rail filtering is often enough, and can often help. IME though poor grounding layout and lack of IC decoupling are as often the problem with homemade preamps. NT |
#11
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Split rail PS
In article ,
wrote: On Monday, August 26, 2013 12:26:06 AM UTC+1, Dave Plowman (News) wrote: It's the PS for a fairly esoteric moving coil pickup pre-amp which uses balanced inputs. The quiescent current at least is very similar on each rail. Basically was chasing the last residue of hum, and discovered (by accident) if I disconnected the transformer centre tap it improved. The scope showed the hum to be more on the negative rail. Changing the neg reg to a different make improved things to the point I doubt I'll better. Sounds like a symmetrical load. Adding resistors can help deal with small imbalance. For low current loads like that, passive power rail filtering is often enough, and can often help. IME though poor grounding layout and lack of IC decoupling are as often the problem with homemade preamps. Seems the small amount of hum left is coming from the actual cartridge or wiring to it within the arm. (It's an SME arm) Unplug the connector at the base of the arm and the hum goes - only a reasonable amount of hiss left, with the gain full up. Fine at any listening level. The arm assembly is mounted on wood so it's not a ground loop. Moving the arm over the deck makes no difference to the hum - so I'd guess it isn't coming from the motor. The wiring within the arm is single wires - but then the arm itself should screen them? I've checked it is grounded to my star ground point. -- *When cheese gets it's picture taken, what does it say? Dave Plowman London SW To e-mail, change noise into sound. |
#12
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Split rail PS
In article ,
"Dave Plowman (News)" writes: Seems the small amount of hum left is coming from the actual cartridge or wiring to it within the arm. (It's an SME arm) Unplug the connector at the base of the arm and the hum goes - only a reasonable amount of hiss left, with the gain full up. Fine at any listening level. Try connecting just the ground side of the cartridge, and see if that causes any hum. Could also try fully connecting it, but with the cartridge output shorted at source. The arm assembly is mounted on wood so it's not a ground loop. Moving the arm over the deck makes no difference to the hum - so I'd guess it isn't coming from the motor. The wiring within the arm is single wires - but then the arm itself should screen them? I've checked it is grounded to my star ground point. -- Andrew Gabriel [email address is not usable -- followup in the newsgroup] |
#13
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Split rail PS
In article ,
Tim Streater wrote: The arm assembly is mounted on wood so it's not a ground loop. Moving the arm over the deck makes no difference to the hum - so I'd guess it isn't coming from the motor. The wiring within the arm is single wires - but then the arm itself should screen them? I've checked it is grounded to my star ground point. Where the wires leave the arm to enter the cartridge? I'm not sure where SME grounds the arm. There are five wires going to the connector on the base of the assembly (other end from the cartridge, as it were) - one of which is the arm ground. They all disappear up the middle of the pivot. -- *The severity of the itch is proportional to the reach * Dave Plowman London SW To e-mail, change noise into sound. |
#14
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Split rail PS
In article ,
Andrew Gabriel wrote: In article , "Dave Plowman (News)" writes: Seems the small amount of hum left is coming from the actual cartridge or wiring to it within the arm. (It's an SME arm) Unplug the connector at the base of the arm and the hum goes - only a reasonable amount of hiss left, with the gain full up. Fine at any listening level. Try connecting just the ground side of the cartridge, and see if that causes any hum. Could also try fully connecting it, but with the cartridge output shorted at source. There's no actual ground side as the pre-amp has a balanced input. The pre-amp is mounted as close to the arm as possible. The cart has an output impedance of 2 ohms so as near a short as possible. ;-) But I'll have a play. The arm assembly is mounted on wood so it's not a ground loop. Moving the arm over the deck makes no difference to the hum - so I'd guess it isn't coming from the motor. The wiring within the arm is single wires - but then the arm itself should screen them? I've checked it is grounded to my star ground point. -- *If horrific means to make horrible, does terrific mean to make terrible? Dave Plowman London SW To e-mail, change noise into sound. |
#15
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Split rail PS
In article ,
"Dave Plowman (News)" writes: In article , Andrew Gabriel wrote: In article , "Dave Plowman (News)" writes: Seems the small amount of hum left is coming from the actual cartridge or wiring to it within the arm. (It's an SME arm) Unplug the connector at the base of the arm and the hum goes - only a reasonable amount of hiss left, with the gain full up. Fine at any listening level. Try connecting just the ground side of the cartridge, and see if that causes any hum. Could also try fully connecting it, but with the cartridge output shorted at source. There's no actual ground side as the pre-amp has a balanced input. The Ah, that's whay the screening doesn't matter much. However, it opens up the possibility for some other faults around the balanced inputs having gone wrong. Check none of them have got one side shorted to ground in the wiring or connectors. It could also be that one side of a balanced input circuit has failed. pre-amp is mounted as close to the arm as possible. The cart has an output impedance of 2 ohms so as near a short as possible. ;-) But I'll have a play. Still worth testing with the outputs shorted - that might identify a faulty balanced input stage. (BTW, I expect it's the resistance which is 2 ohms - I'm surprised it's that low, but I hope the impedance is somewhat higher.) -- Andrew Gabriel [email address is not usable -- followup in the newsgroup] |
#16
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Split rail PS
On 26/08/13 14:06, Dave Plowman (News) wrote:
In article , Tim Streater wrote: The arm assembly is mounted on wood so it's not a ground loop. Moving the arm over the deck makes no difference to the hum - so I'd guess it isn't coming from the motor. The wiring within the arm is single wires - but then the arm itself should screen them? I've checked it is grounded to my star ground point. Where the wires leave the arm to enter the cartridge? I'm not sure where SME grounds the arm. There are five wires going to the connector on the base of the assembly (other end from the cartridge, as it were) - one of which is the arm ground. They all disappear up the middle of the pivot. what you SHOULD do is to ground everything at the amplifier INPUT. traditionally that means that the arm carries a separate earth for the metalwork back to the amp chassis, which should be the ONLY part of the hifi stack that is actually attached to earth. And that ONLY at the preamp input itself. in order to reserve the safety niceties, what the means is that other equipment is only grounded electronically to cases by a 100 ohm resistor if the cases are to be earthed, and that the outer sheath of the phono/DIN plugs are NOT directly connected to mais earth on peripherals. -- Ineptocracy (in-ep-toc-ra-cy) €“ a system of government where the least capable to lead are elected by the least capable of producing, and where the members of society least likely to sustain themselves or succeed, are rewarded with goods and services paid for by the confiscated wealth of a diminishing number of producers. |
#17
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Split rail PS
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#18
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Split rail PS
In article ,
The Natural Philosopher wrote: On 26/08/13 14:06, Dave Plowman (News) wrote: In article , Tim Streater wrote: The arm assembly is mounted on wood so it's not a ground loop. Moving the arm over the deck makes no difference to the hum - so I'd guess it isn't coming from the motor. The wiring within the arm is single wires - but then the arm itself should screen them? I've checked it is grounded to my star ground point. Where the wires leave the arm to enter the cartridge? I'm not sure where SME grounds the arm. There are five wires going to the connector on the base of the assembly (other end from the cartridge, as it were) - one of which is the arm ground. They all disappear up the middle of the pivot. what you SHOULD do is to ground everything at the amplifier INPUT. traditionally that means that the arm carries a separate earth for the metalwork back to the amp chassis, which should be the ONLY part of the hifi stack that is actually attached to earth. And that ONLY at the preamp input itself. As I said, the arm is electrically isolated from the actual deck due to being mounted on a wood board. It appears to make no audible difference if the arm body is grounded or not. I've tried it direct to the main amp ground terminal both on its own and also grounding the PS O volts rail in order to reserve the safety niceties, what the means is that other equipment is only grounded electronically to cases by a 100 ohm resistor if the cases are to be earthed, and that the outer sheath of the phono/DIN plugs are NOT directly connected to mais earth on peripherals. It's the pick-up to on board pre-amp where the hum occurs. Pre-amp to main amp control unit is fine. -- *Aim Low, Reach Your Goals, Avoid Disappointment * Dave Plowman London SW To e-mail, change noise into sound. |
#19
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Split rail PS
On Sun, 25 Aug 2013 14:44:55 +0100, charles
wrote: In article , Dave Plowman (News) wrote: When building a split rail low current PS with discrete components - like say +/- 15v - does it matter if the transformer doesn't have a centre tap on the secondary? It's just that I've seen circuits for both around. Some say to omit the centre tap connection to ground even if it has one. It depends on what you are trying to power. If the device simply wants a 30v supply, then the centre tap isn't needed. If the requirement is for +15, 0, -15 then you are going to have to get the 0 reference from somewhere. It can be done in the load device with suitable circuitry, but it's much easier to use the centre tap. The question is a bit too genralised. What's the purpose of the PSU? What design[s] did you have in mind for the circuitry? A CT transformer may not be needed, but why not ask for some equally relevant information such as the type of mains switch or colour of the box you want to put it in? AB |
#20
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Split rail PS
In article ,
Andrew Gabriel wrote: There's no actual ground side as the pre-amp has a balanced input. The Ah, that's whay the screening doesn't matter much. However, it opens up the possibility for some other faults around the balanced inputs having gone wrong. Check none of them have got one side shorted to ground in the wiring or connectors. It could also be that one side of a balanced input circuit has failed. I'd expect to see a level reduction on that channel if it were one legged. pre-amp is mounted as close to the arm as possible. The cart has an output impedance of 2 ohms so as near a short as possible. ;-) But I'll have a play. Still worth testing with the outputs shorted - that might identify a faulty balanced input stage. Since both are the same it would be two identical faults? (BTW, I expect it's the resistance which is 2 ohms - I'm surprised it's that low, but I hope the impedance is somewhat higher.) The output *impedance* is quoted as 2 ohms by the maker. -- *Venison for dinner again? Oh deer!* Dave Plowman London SW To e-mail, change noise into sound. |
#21
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Split rail PS
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#22
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Split rail PS
In article ,
Archibald wrote: It sounds like you have a magnetic field interfering with the cartrige. Try sticking the end of the arm into a grounded empty baked bean tin. Then if it diminishes the hum see what electrics have been added lately. Normally if that's the case simply moving the arm will alter the hum. It's the same in both the workshop and living room, so it's not a local thing. The SME arm is aluminium or an alloy thereof as far as I can recall, not the best magnetic screen. If the PSU is still suspect incidentally I would be tempted to try a smooth DC supply and a couple of 15V Voltage converters. Not sure of the switching frequency though, you may be swapping 50Hz for 20kHz :-) Since there's no hum at all with the input to the pre-amp disconnected I'm fairly sure the PS is ok. -- *I couldn't repair your brakes, so I made your horn louder * Dave Plowman London SW To e-mail, change noise into sound. |
#23
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Split rail PS
In article ,
Dave Plowman (News) wrote: In article , Archibald wrote: It sounds like you have a magnetic field interfering with the cartrige. Try sticking the end of the arm into a grounded empty baked bean tin. Then if it diminishes the hum see what electrics have been added lately. Normally if that's the case simply moving the arm will alter the hum. It's the same in both the workshop and living room, so it's not a local thing. The SME arm is aluminium or an alloy thereof as far as I can recall, not the best magnetic screen. If the PSU is still suspect incidentally I would be tempted to try a smooth DC supply and a couple of 15V Voltage converters. Not sure of the switching frequency though, you may be swapping 50Hz for 20kHz :-) Since there's no hum at all with the input to the pre-amp disconnected I'm fairly sure the PS is ok. is the pickup, in some way, loading the PSU? -- From KT24 Using a RISC OS computer running v5.18 |
#24
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Split rail PS
On Monday, August 26, 2013 12:56:39 PM UTC+1, Dave Plowman (News) wrote:
In article , wrote: On Monday, August 26, 2013 12:26:06 AM UTC+1, Dave Plowman (News) wrote: It's the PS for a fairly esoteric moving coil pickup pre-amp which uses balanced inputs. The quiescent current at least is very similar on each rail. Basically was chasing the last residue of hum, and discovered (by accident) if I disconnected the transformer centre tap it improved. The scope showed the hum to be more on the negative rail.. Changing the neg reg to a different make improved things to the point I doubt I'll better. Sounds like a symmetrical load. Adding resistors can help deal with small imbalance. For low current loads like that, passive power rail filtering is often enough, and can often help. IME though poor grounding layout and lack of IC decoupling are as often the problem with homemade preamps. Seems the small amount of hum left is coming from the actual cartridge or wiring to it within the arm. (It's an SME arm) Unplug the connector at the base of the arm and the hum goes - only a reasonable amount of hiss left, with the gain full up. Fine at any listening level. The arm assembly is mounted on wood so it's not a ground loop. Moving the arm over the deck makes no difference to the hum - so I'd guess it isn't coming from the motor. The wiring within the arm is single wires - but then the arm itself should screen them? I've checked it is grounded to my star ground point. Well, I don't know the solution in this case. But I know one thing. A low output pickup, as all modern ones are, with 2 ohm impedance is giong to be a major nightmare to keep quiet. Such a system will need a far better standard of screening than usual for 50k pickups. I can only think of one reason to go with 2 ohms, using a ribbon transducer.. Why do that with mc? I am wondering what the shell of the pickup is connected to. With 50k pickups it goes to one of the ground output pins, with a balanced setup it would need separate connection to amp ground. Does the arm provide for that? I forget what your screening arrangements are, but I'd want to check they're watertight all over. All cables need a dense screen, maybe foil and braid, audio plugs should be screened, and the leads to the cartridge in the headshell should be screened somehow - perhaps a little copper foil cover. If absolutely all attempts fail, you might just convert the place to dc mains NT |
#25
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Split rail PS
In article , Dave Plowman (News)
scribeth thus In article , The Natural Philosopher wrote: On 26/08/13 14:06, Dave Plowman (News) wrote: In article , Tim Streater wrote: The arm assembly is mounted on wood so it's not a ground loop. Moving the arm over the deck makes no difference to the hum - so I'd guess it isn't coming from the motor. The wiring within the arm is single wires - but then the arm itself should screen them? I've checked it is grounded to my star ground point. Where the wires leave the arm to enter the cartridge? I'm not sure where SME grounds the arm. There are five wires going to the connector on the base of the assembly (other end from the cartridge, as it were) - one of which is the arm ground. They all disappear up the middle of the pivot. what you SHOULD do is to ground everything at the amplifier INPUT. traditionally that means that the arm carries a separate earth for the metalwork back to the amp chassis, which should be the ONLY part of the hifi stack that is actually attached to earth. And that ONLY at the preamp input itself. As I said, the arm is electrically isolated from the actual deck due to being mounted on a wood board. It appears to make no audible difference if the arm body is grounded or not. I've tried it direct to the main amp ground terminal both on its own and also grounding the PS O volts rail in order to reserve the safety niceties, what the means is that other equipment is only grounded electronically to cases by a 100 ohm resistor if the cases are to be earthed, and that the outer sheath of the phono/DIN plugs are NOT directly connected to mais earth on peripherals. It's the pick-up to on board pre-amp where the hum occurs. Pre-amp to main amp control unit is fine. Right.. You say this amp is a "balanced" input is that transformers or electronically balanced?. Either way what happens when you simply short out the inputs with a total short then say 10 ohms then 100, any reduction or rise in the "hum" level?. Do that at the input terminals presumably XLR or similar then repeat the same on the cartridge leads on the SME arm where the cart goes. Of course each lead from each pickup coil left and right that is, is connected via two isolated from any ground leads is it not?.. -- Tony Sayer |
#26
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Split rail PS
In article ,
wrote: Well, I don't know the solution in this case. But I know one thing. A low output pickup, as all modern ones are, with 2 ohm impedance is giong to be a major nightmare to keep quiet. Such a system will need a far better standard of screening than usual for 50k pickups. I used it before with the supplied transformer which went at the amp end. There was some hum - but about par for the course with any pickup. Better than some, like the Decca FFSS. I can only think of one reason to go with 2 ohms, using a ribbon transducer. Why do that with mc? I suppose the more the number of turns, the greater the mass. I am wondering what the shell of the pickup is connected to. With 50k pickups it goes to one of the ground output pins, with a balanced setup it would need separate connection to amp ground. Does the arm provide for that? The shell simply grounds to the arm. There is a separate ground wire which comes out of the arm. I dunno where that is connected inside the arm - but adding another makes no difference. I forget what your screening arrangements are, but I'd want to check they're watertight all over. All cables need a dense screen, maybe foil and braid, audio plugs should be screened, and the leads to the cartridge in the headshell should be screened somehow - perhaps a little copper foil cover. The leads within the shell are unscreened. But if they were picking up the hum I'd expect moving the pickup around to alter the hum? -- *El nino made me do it Dave Plowman London SW To e-mail, change noise into sound. |
#27
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Split rail PS
In article ,
tony sayer wrote: It's the pick-up to on board pre-amp where the hum occurs. Pre-amp to main amp control unit is fine. Right.. You say this amp is a "balanced" input is that transformers or electronically balanced?. Electronic. Uses an SSM 2017 at the front end. Either way what happens when you simply short out the inputs with a total short then say 10 ohms then 100, any reduction or rise in the "hum" level?. Do that at the input terminals presumably XLR or similar then repeat the same on the cartridge leads on the SME arm where the cart goes. No XLRs. The pre-amp board is mounted only a few inches from the pickup arm. The SME arm has a plug and socket connector, which is hard wired to the PCB. Unplug that, and the hum totally disappears. No need to provide a dummy load. Of course each lead from each pickup coil left and right that is, is connected via two isolated from any ground leads is it not?.. Each coil goes to a balanced input. No other connection. What I might try is removing the internal wiring to the arm, and see if it is twisted. I suspect not. -- *Speak softly and carry a cellular phone * Dave Plowman London SW To e-mail, change noise into sound. |
#28
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Split rail PS
In article ,
Dave Plowman (News) wrote: In article , wrote: Well, I don't know the solution in this case. But I know one thing. A low output pickup, as all modern ones are, with 2 ohm impedance is giong to be a major nightmare to keep quiet. Such a system will need a far better standard of screening than usual for 50k pickups. I used it before with the supplied transformer which went at the amp end. There was some hum - but about par for the course with any pickup. Better than some, like the Decca FFSS. I never had hum with my FFSS. -- From KT24 Using a RISC OS computer running v5.18 |
#29
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Split rail PS
In article , Dave Plowman (News)
scribeth thus In article , tony sayer wrote: It's the pick-up to on board pre-amp where the hum occurs. Pre-amp to main amp control unit is fine. Right.. You say this amp is a "balanced" input is that transformers or electronically balanced?. Electronic. Uses an SSM 2017 at the front end. OK... Either way what happens when you simply short out the inputs with a total short then say 10 ohms then 100, any reduction or rise in the "hum" level?. Do that at the input terminals presumably XLR or similar then repeat the same on the cartridge leads on the SME arm where the cart goes. No XLRs. The pre-amp board is mounted only a few inches from the pickup arm. The SME arm has a plug and socket connector, which is hard wired to the PCB. Unplug that, and the hum totally disappears. No need to provide a dummy load. Well in the process of elimination it would be a good idea to try that just in case that isn't as "Balanced" as it ought to be. Then connect the pick-up by a short, well a few feet of balanced twin screened, just wrap the inner wire very lightly round the pins. That then should also be quiet at those frequencies at least then move that around to see if its inducting a magnetic field from anywhere if it is then sort that, if not then do check that the wiring of the SME arm is exactly as it ought be.. Don't earth any of the 2017 input leads from that the only earth should be a single lead from the arm back to a "tech" earth.... Of course each lead from each pickup coil left and right that is, is connected via two isolated from any ground leads is it not?.. Each coil goes to a balanced input. No other connection. What I might try is removing the internal wiring to the arm, and see if it is twisted. Shouldn't really matter.. I suspect not. -- Tony Sayer |
#30
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Split rail PS
In article ,
charles wrote: I used it before with the supplied transformer which went at the amp end. There was some hum - but about par for the course with any pickup. Better than some, like the Decca FFSS. I never had hum with my FFSS. With the usual Garrard 301? -- *Is there another word for synonym? Dave Plowman London SW To e-mail, change noise into sound. |
#31
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Split rail PS
In article ,
Dave Plowman (News) wrote: In article , charles wrote: I used it before with the supplied transformer which went at the amp end. There was some hum - but about par for the course with any pickup. Better than some, like the Decca FFSS. I never had hum with my FFSS. With the usual Garrard 301? what else? But I was told, by a man from Garrard at a Radio Show, to run a 60w lightbulb in series with the motor to reduce its magnetic field. -- From KT24 Using a RISC OS computer running v5.18 |
#32
Posted to uk.d-i-y
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Split rail PS
In article ,
charles wrote: In article , Dave Plowman (News) wrote: In article , charles wrote: I used it before with the supplied transformer which went at the amp end. There was some hum - but about par for the course with any pickup. Better than some, like the Decca FFSS. I never had hum with my FFSS. With the usual Garrard 301? what else? But I was told, by a man from Garrard at a Radio Show, to run a 60w lightbulb in series with the motor to reduce its magnetic field. It reduced but not eliminated it here. -- *If a thing is worth doing, wouldn't it have been done already? Dave Plowman London SW To e-mail, change noise into sound. |
#33
Posted to uk.d-i-y
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Split rail PS
In article ,
Dave Plowman (News) wrote: In article , charles wrote: In article , Dave Plowman (News) wrote: In article , charles wrote: I used it before with the supplied transformer which went at the amp end. There was some hum - but about par for the course with any pickup. Better than some, like the Decca FFSS. I never had hum with my FFSS. With the usual Garrard 301? what else? But I was told, by a man from Garrard at a Radio Show, to run a 60w lightbulb in series with the motor to reduce its magnetic field. It reduced but not eliminated it here. In those days, I had good hearing and I couldn't hear any. -- From KT24 Using a RISC OS computer running v5.18 |
#34
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Split rail PS
Had a bit more of a play. Disconneced the cartridge - hum goes. So it's
apparently being induced into the cartridge itself, rather than wiring. No large transformers anywhere near - and it's more or less the same on the workbench, in a different room. Next thing to try is removing the mains to the turntable, I suppose. -- *A chicken crossing the road is poultry in motion.* Dave Plowman London SW To e-mail, change noise into sound. |
#35
Posted to uk.d-i-y
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Split rail PS
In article ,
charles wrote: I never had hum with my FFSS. With the usual Garrard 301? what else? But I was told, by a man from Garrard at a Radio Show, to run a 60w lightbulb in series with the motor to reduce its magnetic field. It reduced but not eliminated it here. In those days, I had good hearing and I couldn't hear any. I'd imagine your hearing is still ok for mains hum? ;-) I wouldn't say it was audible in normal use but definitely there is you whacked the wick up while not playing a record. -- *Young at heart -- slightly older in other places Dave Plowman London SW To e-mail, change noise into sound. |
#36
Posted to uk.d-i-y
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Split rail PS
In article ,
Dave Plowman (News) wrote: In article , charles wrote: I never had hum with my FFSS. With the usual Garrard 301? what else? But I was told, by a man from Garrard at a Radio Show, to run a 60w lightbulb in series with the motor to reduce its magnetic field. It reduced but not eliminated it here. In those days, I had good hearing and I couldn't hear any. I'd imagine your hearing is still ok for mains hum? ;-) I can certainly hear it when it turns up on sound sysytems. I wouldn't say it was audible in normal use but definitely there is you whacked the wick up while not playing a record. I remember an alleged problem when RDS first came out. Went to see someone near Wrotham, but on the back of a hill. He turned up the volume on one of R3 long silent pauses to demonstrate, the next minute we were blown out of our seats by the opening chord of something. -- From KT24 Using a RISC OS computer running v5.18 |
#37
Posted to uk.d-i-y
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Split rail PS
In article , Dave Plowman (News)
scribeth thus Had a bit more of a play. Disconneced the cartridge - hum goes. So it's apparently being induced into the cartridge itself, rather than wiring. No large transformers anywhere near - and it's more or less the same on the workbench, in a different room. Next thing to try is removing the mains to the turntable, I suppose. Dave.. seriously suggest a small value resistor connected across the inputs and that via a short bit of twin screened and see if the hum is still there or not. Then connect the cart to the same cable and move that around it shouldn't under normal conditions "hum" unless its picking that up from a transformer or motor. If so then you should be able to track that easily. In a way this is much the same as a moving coil microphone and they can and do as I'm sure you know be used on rather long leads.... -- Tony Sayer |
#38
Posted to uk.d-i-y
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Split rail PS
In article ,
tony sayer wrote: In article , Dave Plowman (News) scribeth thus Had a bit more of a play. Disconneced the cartridge - hum goes. So it's apparently being induced into the cartridge itself, rather than wiring. No large transformers anywhere near - and it's more or less the same on the workbench, in a different room. Next thing to try is removing the mains to the turntable, I suppose. Dave.. seriously suggest a small value resistor connected across the inputs and that via a short bit of twin screened and see if the hum is still there or not. Then connect the cart to the same cable and move that around it shouldn't under normal conditions "hum" unless its picking that up from a transformer or motor. If so then you should be able to track that easily. In a way this is much the same as a moving coil microphone and they can and do as I'm sure you know be used on rather long leads.... Had a play this morning before reading this. Turns out to have been caused by the transformer in the PS - even although it's a weedy 6VA and 600mm or so from the pickup. Nor does the hum change when moving the pickup around - which is why I didn't go for this before. But running the pre-amp from an external PS removes the hum totally. (the hum and noise on the output of the original PS and external one are near identical on the scope) I've ordered up a toroidal transformer and Eddystone box to put it in from CPC. Still free carriage. ;-) -- *My dog can lick anyone Dave Plowman London SW To e-mail, change noise into sound. |
#39
Posted to uk.d-i-y
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Split rail PS
In article , Dave Plowman (News)
scribeth thus In article , tony sayer wrote: In article , Dave Plowman (News) scribeth thus Had a bit more of a play. Disconneced the cartridge - hum goes. So it's apparently being induced into the cartridge itself, rather than wiring. No large transformers anywhere near - and it's more or less the same on the workbench, in a different room. Next thing to try is removing the mains to the turntable, I suppose. Dave.. seriously suggest a small value resistor connected across the inputs and that via a short bit of twin screened and see if the hum is still there or not. Then connect the cart to the same cable and move that around it shouldn't under normal conditions "hum" unless its picking that up from a transformer or motor. If so then you should be able to track that easily. In a way this is much the same as a moving coil microphone and they can and do as I'm sure you know be used on rather long leads.... Had a play this morning before reading this. Turns out to have been caused by the transformer in the PS - even although it's a weedy 6VA and 600mm or so from the pickup. Nor does the hum change when moving the pickup around - which is why I didn't go for this before. But running the pre-amp from an external PS removes the hum totally. (the hum and noise on the output of the original PS and external one are near identical on the scope) I've ordered up a toroidal transformer and Eddystone box to put it in from CPC. Still free carriage. ;-) A Toriod tranny?, best bet for hum field reduction. Make sure there no shorted turns thru it.. -- Tony Sayer |
#40
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Split rail PS
In article ,
tony sayer wrote: I've ordered up a toroidal transformer and Eddystone box to put it in from CPC. Still free carriage. ;-) A Toriod tranny?, best bet for hum field reduction. Yes - quoted at 1/10th of a 'normal' type. Make sure there no shorted turns thru it.. Quite - although since they are single hole fixing with a kit usually provided you'd have to do something very silly to achieve that. Snag is the smallest toroidal I could find is way over sized for this application. Are there any 6VA or so trannies made with low external field? -- *Life is hard; then you nap Dave Plowman London SW To e-mail, change noise into sound. |
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