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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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Audio amp circuit help?
"DaveC" wrote in message
al.net... Working on a VSP Labs MOSFET power amp: http://home.covad.net/~peninsula/Amp/VSPschem1.pdf Questions: 1. There are several adjustment pots on this PCB: "Open offset", "Short offset", and a circuit controlled by one of the TL072 op amps (in the lower center of the drawing) titled "Servo". What are these adjustments? If I was to guess, I'd say that the two former adjustments are for protection of the output stage from short circuit. Is the latter circuit for adjustment of the idle current in the output stage? 2. What does the circuit that includes a TL072 (in the lower left corner of the drawing) do? It includes a thermistor, which seems to indicate some kind of thermal protection. Does this reduce the input signal as temperature increases? Thanks, -- DaveC At a quick guess I'd go for input short-circuit/open-circuit adjustment, output overload protection, and output stage high-temp protection. Ken |
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Audio amp circuit help?
On Tue, 2 Dec 2003 10:22:45 -0800, DaveC wrote:
Working on a VSP Labs MOSFET power amp: http://home.covad.net/~peninsula/Amp/VSPschem1.pdf Questions: 1. There are several adjustment pots on this PCB: "Open offset", "Short offset", and a circuit controlled by one of the TL072 op amps (in the lower center of the drawing) titled "Servo". What are these adjustments? If I was to guess, I'd say that the two former adjustments are for protection of the output stage from short circuit. Is the latter circuit for adjustment of the idle current in the output stage? 2. What does the circuit that includes a TL072 (in the lower left corner of the drawing) do? It includes a thermistor, which seems to indicate some kind of thermal protection. Does this reduce the input signal as temperature increases? Thanks, Looks like open and short offset are just offset adjustments for the input and output stages - balance each separately? Offset implies the zero adjustment (DC level at the speaker terminals, when there is no input) There is an adjustment for BIAS that would be the idle current. Servo is confusing. The "servo" amps I'm familiar with usually include the speakers in the feedback loop. Sense the speaker sound with a transducer and apply that as feedback for the amp. An attempt to include speaker nonlinearity as feedback and correct for it. From looking at the servo circuitry . . . that looks like an automatic gain control or limiter of some sort. Looks like there is a thermostat to shut down the output stage with an over temp condition, and some LED's and thermistor to watch the temperature. Where'd you find the circuit, and what info have you got? -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
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Audio amp circuit help?
On Tue, 2 Dec 2003 10:22:45 -0800, DaveC put finger
to keyboard and composed: Working on a VSP Labs MOSFET power amp: http://home.covad.net/~peninsula/Amp/VSPschem1.pdf Questions: 1. There are several adjustment pots on this PCB: "Open offset", "Short offset", and a circuit controlled by one of the TL072 op amps (in the lower center of the drawing) titled "Servo". "Offset short" appears to be a nulling adjustment for a zero input signal. That is, the output of the differential pair should have no DC offset when the input is shorted. "Open offset" would appear to be a similar adjustment for when the inputs are disconnected. The servo TL072 appears to be sensing the presence of DC at the speaker terminals. The time constant appears to be set by C28 (0.1uF) and R75 (1M), which is of the order of 0.1sec. Perhaps the circuit is some kind of automatic bias adjustment. What are these adjustments? If I was to guess, I'd say that the two former adjustments are for protection of the output stage from short circuit. Is the latter circuit for adjustment of the idle current in the output stage? 2. What does the circuit that includes a TL072 (in the lower left corner of the drawing) do? It includes a thermistor, which seems to indicate some kind of thermal protection. Does this reduce the input signal as temperature increases? The TL072 is operating as a comparator with hysteresis. It shorts the input signal when the temperature exceeds a certain threshold. - Franc Zabkar -- Please remove one 's' from my address when replying by email. |
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Audio amp circuit help?
"default" wrote in message ... On Tue, 2 Dec 2003 10:22:45 -0800, DaveC wrote: Working on a VSP Labs MOSFET power amp: http://home.covad.net/~peninsula/Amp/VSPschem1.pdf Questions: 1. There are several adjustment pots on this PCB: "Open offset", "Short offset", and a circuit controlled by one of the TL072 op amps (in the lower center of the drawing) titled "Servo". What are these adjustments? If I was to guess, I'd say that the two former adjustments are for protection of the output stage from short circuit. Is the latter circuit for adjustment of the idle current in the output stage? 2. What does the circuit that includes a TL072 (in the lower left corner of the drawing) do? It includes a thermistor, which seems to indicate some kind of thermal protection. Does this reduce the input signal as temperature increases? Thanks, Looks like open and short offset are just offset adjustments for the input and output stages - balance each separately? Offset implies the zero adjustment (DC level at the speaker terminals, when there is no input) There is an adjustment for BIAS that would be the idle current. Servo is confusing. The "servo" amps I'm familiar with usually include the speakers in the feedback loop. Sense the speaker sound with a transducer and apply that as feedback for the amp. An attempt to include speaker nonlinearity as feedback and correct for it. From looking at the servo circuitry . . . that looks like an automatic gain control or limiter of some sort. Looks like there is a thermostat to shut down the output stage with an over temp condition, and some LED's and thermistor to watch the temperature. Where'd you find the circuit, and what info have you got? -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- The "Servo" loop is a slow integrator to keep the output average voltage near 0V. I've seen that terminology used before to indicate it is a slow out-of-bandwith-of-interest adjustment. The thermistor and comparator (note the positive feedback round the TL072) will shunt the input signal to ground by the zero Vgs resistance of Q20 (50 ohms max) when something gets too hot (probably the output heatsink). I'm with Ken on the "Open" and "Short" adjustments. There might be a link with the shutdown, in that if the previous stage has a significant offset and output resistance, or leakage current through a coupling electrolytic, you would have to tweak offsets for both cases of Q20 resistance. For example, if there were only 1uA leakage not catered for, going into or out of shutdown would produce a 3.3V step on the output. Regards Ian |
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