I have a Proton AA-1150 Power Amp. In the absence of a diagram, I started to draw one out for myself, and only just got past the input stage.
I am now intrigued by this input buffer.
Can anybody elaborate on what the purpose of this could be?
I'm guessing in part, it is to raise the input impedance of the amp - however there is also some odd thing going on with the feedback loop. I'm guessing this isn't a servo either.
As I do not yet have a schematic for the power stages, I have simply enclosed those in a block - but to summarize: the NFB appears to be fed back to the input of the second opamp, via R1 (180k), which to me appears to be positive rather than negative feedback....?
From what I have seen so far, there is some other NFB, but it does not appear to terminate in a conventioanl R/C network to ground.
If anybody could offer a rational explanation for this circuit, I would be grateful.
Also - I would be even more grateful if somebody has a copy of the full schematic, that I could have?
Many Thanks,
Tony.
I am now intrigued by this input buffer.
Can anybody elaborate on what the purpose of this could be?
I'm guessing in part, it is to raise the input impedance of the amp - however there is also some odd thing going on with the feedback loop. I'm guessing this isn't a servo either.
As I do not yet have a schematic for the power stages, I have simply enclosed those in a block - but to summarize: the NFB appears to be fed back to the input of the second opamp, via R1 (180k), which to me appears to be positive rather than negative feedback....?
From what I have seen so far, there is some other NFB, but it does not appear to terminate in a conventioanl R/C network to ground.
If anybody could offer a rational explanation for this circuit, I would be grateful.
Also - I would be even more grateful if somebody has a copy of the full schematic, that I could have?
Many Thanks,
Tony.
The first stage is a sallen-key vcvs 2nd order high pass filter, the second is a 2nd order sallen-key vcvs low pass filter which includes the power amplifier in its dc signal path if the schematic is accurate. ("pseudo" dc servo - very high dc loop gain for the PA/ac closed loop gain will be unity for the op-amp based LPF/ac gain of PA determined by its NFB loop) Offset at the amplifier output should be equivalent to the input offset of the second op-amp in the circuit.
These are to constrain the input bandwidth to something reasonable and provide an integrated dc servo as well.. Clever as long as that 10uF cap is good..
You can model these in LTspice IV and see exactly what they are doing. (Linear Tech LTSpice IV is free and available from their site for windows use - runs fine under wine in Linux as well.)
These are to constrain the input bandwidth to something reasonable and provide an integrated dc servo as well.. Clever as long as that 10uF cap is good..
You can model these in LTspice IV and see exactly what they are doing. (Linear Tech LTSpice IV is free and available from their site for windows use - runs fine under wine in Linux as well.)
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Hi Kevin,
Many thanks for the explanation.
I have posted an updated schematic, where I have now located a return path to ground for NFB.
Incidentally, that 10uF cap is a bipolar - would it be ok (and worthwhile) to replace it with a cerafine cap, but a polarised one? (Realising I would have to determine the polarity of course - although I'm guessing it's all 0V around there).
Thanks,
Tony.
Many thanks for the explanation.
I have posted an updated schematic, where I have now located a return path to ground for NFB.
Incidentally, that 10uF cap is a bipolar - would it be ok (and worthwhile) to replace it with a cerafine cap, but a polarised one? (Realising I would have to determine the polarity of course - although I'm guessing it's all 0V around there).
Thanks,
Tony.
Don't replace that cap with a polarized cerafine because over time and temperature the voltage across it might change sign. I recommend using a bi-polar (NP) Blackgate if you can get one. (Parts Connexion and Michael Percy here in North America still stock them.)
Edit: The voltage across that cap is probably very small, but not zero, and as I indicated above it may drift with time and temperature. The dc voltage present at the output of the op-amp is the error term required to correct the dc at the output of the power amplifier.
I would use at least a 16V capacitor in this location.
Edit: The voltage across that cap is probably very small, but not zero, and as I indicated above it may drift with time and temperature. The dc voltage present at the output of the op-amp is the error term required to correct the dc at the output of the power amplifier.
I would use at least a 16V capacitor in this location.
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