Enzo, I fixed the Acoustic 270, but why?

[stosostu]

Enzo, I finally got back to the Acoustic 270 amp, other peoples amps have to take priority , that we discussed about a month ago.

The schematic is similar to the 370 but without the negative feedback circuit, http://acoustic.control.free.fr/file...chematics2.pdf.

If you remember the problem was that the full 90 volts rail was sitting on the pseudo mid point, which should have been 45 volts.

After hunting through the components you suggested I finally found that R314, a 270k resistor had gone high, about 750k. I replaced it and like magic the amp sprung to life.

What I'd like to know is why that resistor going high had the effect that it did?

Also, I see that there is a DC 'balance' control. Do you know the procedure for adjusting that correctly?

Thanks for all the assistance you gave me with this, I'm sure I would have struggled without your help.

Bob

[Enzo]

Well... What the 270 lacks is the limiter circuit. it most certainly has negative feedback, in fact that was the problem, yours DIDN'T. R314 IS your negative feedback. See how it connects the output back to the input differential pair?

R314 forms a voltage divider with R312 and R313 to ground. 250k over 100k + 2.35k (two 4.7k in parallel). That means the output signal will be sampled and reduces to about 27% of the output - normally. With 750k in there, you only get about 12% of the output. The amp will be constantly trying to "correct" the low output voltage.

At 45vDC on the output, that diffy pair would see about 27% of that or about 12v - schematic says 11, so I am close. But when the feedback resistor is 750k, the feedback voltage is only about 5.27v. Let's just call it half the normal amount. OK, 11.7%, less than half. As far as the diffferential pair knows, your output is sitting at a little under 20v. That is what 5v means to it. So the differential pair tries to adjust the output to bring it up to 45. SO it raises the output voltage. But with only 11.7% coming back, even when it cranks the output all the way up to 90v, it STILL is under the 11 or 12v it wants to see. SO it is just doing its job reacting to erroneous feedback voltages.

Follow that?

DC balance? As in R304? Watch your 45v on the output. Turn the control. I bet the output rest voltage goes up and down with it. You would measure the power supply voltage - 90, 92, 95, whatever - and set the output voltage for exactly half of that. That would center your output between the power "rails" and present maximum headroom. A volt or so either way won;t matter much. If you are clipping that amp, I don;t think it matters much if one side clips a volt before the other.

Solid state amps are HEAVILY fed back. That is how they stay stable and linear.

Most solid state amps that have adjustments will have output bias current adjust (this one doesn;t), and a output DC offset adjust. (which we just described) Some fancier amps might have output current limit settings and other such.

[stosostu]

Thanks for that explanation, it does make sense to me. It is interesting that, as you described, I measured the voltage at the Q302 base of the diff pair, whilst the fault was still present, and it would start at about 5 volts and just reach 11 volts (what it should be) as the mid rail voltage hit 90 volts. It was also noticeable that the base of Q303 would be 0.8 volts at switch on, then fade to zero as the mid rail went to 90 volts, once again, just as you described. That was what eventually led me to R314, it was obvious that something was pulling that base voltage down, and it seemed it had to be something around the diff pair. Both transistors checked out so I was looking at the components around them.

Thanks again for all your help.

Bob