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Old 12th November 2010, 05:39 PM   #21
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The point is that the servo must have low distortion since you will inject the audio from the output back into the amp.
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Old 12th November 2010, 05:42 PM   #22
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The input bias currents are so small that you won't notice them. You may choose AD8610 with 25-100 uV offset. Excellent opamp for audio and even better for a DC servo.
Well that is a nice opamp.. but also very expensive.

It is important that all the components are avaible as leaded devices, because it must be easy for people to build.
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Old 12th November 2010, 05:47 PM   #23
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The point is that the servo must have low distortion since you will inject the audio from the output back into the amp.
I think we don't agree about what a DC servo is..

No audio signal must run through the servo, only the DC must go back.. The audio signal have to be totally removed before the servo result is injected back into the amp..
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Old 12th November 2010, 06:49 PM   #24
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Lars, there is no such thing as DC. It's AC up to 100 Hz or 1 kHz. It's against nature to remove the audio signal in the DC servo.
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Old 12th November 2010, 07:14 PM   #25
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Lars, there is no such thing as DC. It's AC up to 100 Hz or 1 kHz. It's against nature to remove the audio signal in the DC servo.
I disagree.. If i but a servo on my circuit with same filter as the one you suggest, the THD will by very high.. Implementation of a servo like that will be difficult, because the servo input to output gain in the circuit most be extremly low.

If 20Hz is below -120dB, I would say that the audio signal is below the noise floor, and totally gone.
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Old 12th November 2010, 09:19 PM   #26
DF96 is offline DF96  England
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When DC servo adjust the output to zero it goes like this example:
Startup output = +1V, servo correct to -0.5V, servo correct to +0.25V, servo correct to -0.12V, servo correct to +0.06, and so on...
So the servo loop has a damped oscillation. This means that it is not far off instability, which is what I expected. You need to read up about feedback loop stability. Try running a Spice frequency response plot from 1Hz-10Hz - you will see a big peak somewhere. Or measure the actual amp.
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Old 12th November 2010, 09:45 PM   #27
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DF96 is right in the sense that you (Lars) must tune the poles carefully but it isn't so hard. Make sure the filter is 5-10 times faster than the servo. This will create a minimal hump, part of a dB. Simulation is an excellent tool here and you don't have to draw the whole design.
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Old 13th November 2010, 12:47 PM   #28
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So the servo loop has a damped oscillation. This means that it is not far off instability, which is what I expected. You need to read up about feedback loop stability. Try running a Spice frequency response plot from 1Hz-10Hz - you will see a big peak somewhere. Or measure the actual amp.

Spice plot is uploaded to my website.. scroll to the bottom DIYhifi.dk - Forside

1mHz is +50dB and 20hz is -103dB a bit worse than I rembered..

I think the response is nice, but maybe the +50dB is to much, and that is what makes the swing..
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Old 13th November 2010, 03:50 PM   #29
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That is a plot of the amplitude of the DC servo open-loop gain? Phase matters too. Can you simulate the whole amplifier? Or do a Bode plot?

50dB at 1mHz is not a problem, as this sets the DC offset error. The problem will be somewhere between 100mHz and 10Hz, where the servo has some gain but also lots of phase shift. If you plot the frequency response of the whole amplifier you will see a big peak in this region. Then you can adjust the DC servo to reduce the size of the peak.
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Old 13th November 2010, 05:26 PM   #30
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That is a plot of the amplitude of the DC servo open-loop gain? Phase matters too. Can you simulate the whole amplifier? Or do a Bode plot?

50dB at 1mHz is not a problem, as this sets the DC offset error. The problem will be somewhere between 100mHz and 10Hz, where the servo has some gain but also lots of phase shift. If you plot the frequency response of the whole amplifier you will see a big peak in this region. Then you can adjust the DC servo to reduce the size of the peak.
Servo phase added: DIYhifi.dk - Forside

I can not simulate the hole amp, because there are too many components.. but as i remember the servo input to output gain in the transistor stage is 0.8 to 1.0 depending on level.

I have made real life measerments, and the frequency response is totally flat from 0,05 hz to 5mhz..

What is a Bode plot?
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