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roender 17th September 2007 11:53 AM

DC blocking cap in feedback path or DC servo?
Hello all,
Based on your experience, aka sound quality, what is better in a power amp? A DC servo or an DC blocking cap in series with resistor to ground feedback network?
The amp has jfet LTP and don't implement an input capacitor

AndrewT 17th September 2007 06:31 PM

What stops/blocks source offset?

Left to it's own devices the source offset could dominate what happens to the power amp.

KSTR 17th September 2007 08:33 PM

A blocking cap in the NFB reduces DC gain to 1, a servo reduces it to virtuelly zero and no matter where the DC comes from. Plus the NFB cap is fully in the signal path (it is connected to the non-inverting input), if it has considerable inductance this will show up in the bode plot. And if it has considerable DA and sees a rather high impedance, this really becomes an issue, soundwise. I'd say a DC-servo will have more of a "not there"-quality than a cap, if it is designed carefully. Drawbacks are cost and complexity, though.

- Klaus

AndrewT 17th September 2007 09:22 PM


and no matter where the DC comes from.
that's the bit we have to disagree on.
Compensating the DC conditions inside the power amp for external DC applied through the input is very likely to upset the carefully arranged balance and matching that we all strive for.
This is very likely to be audible.

KSTR 17th September 2007 09:26 PM

It just raises/lowers the CM voltage of the input diff (assuming the servo feedback goes to there, otherwise you might be right, of course). As long as we are talking mVolts and not Volts I don't see much of a problem.

traderbam 17th September 2007 09:44 PM

roender, what amount of dc offset at the output is acceptable to you? And what is the open-loop dc gain of your circuit?

These answers affect your choice. The sound quality will depend upon the specific implementation of either method. A capacitor will add some distortion but so will a dc servo.

Can we assume the input signal will be dc-free?

Conrad Hoffman 17th September 2007 09:50 PM

The traditional passive method invariably involves a large electrolytic in the feedback network. If you believe electrolytics are bad, than that's the worst place to put one. Any non-linearlty in the feedback network ends up in the signal. With the servo, you can arrange things to use a reasonable value polywhatever or even Teflon. Downside is complexity, stability, cost, reliability, and probably other stuff. My amps to date have used the big electrolytic, but the servo is awfully appealing. I have trouble imagining that a properly designed servo wouldn't outperform a plain cap, but the devil is in the details. "Properly designed" might be a bit more involved than first suspected.

AndrewT 17th September 2007 10:02 PM


Originally posted by Conrad Hoffman
............ "Properly designed" might be a bit more involved than first suspected.
read Tom's thread and Peranders' views for more insight to the difficulties of avoiding audibility.

I have heard that servos are bad for sound quality, now that I see how easy it is to get it wrong I am beginning to understand the complexity of the task, even with the experts guiding me.

Gordy 18th September 2007 04:47 AM

Assuming no dc from the source, then good basic design and initial adjustment plus a servo would seem the rational approach to minimising output offset.

The key to servo design seems to be finding a way to inject the signal outside of the main signal path.

This method seems to work:

(Note to self: must read servo related threads...)

Lazy Cat 18th September 2007 04:54 AM

DC servo is an active part and its role is to act completely independent from amp audio function. That means all audible frequencies above 1 Hz should be outfiltered and input/output impedance of DC servo unit should be properly designed. If all these conditions are accomplished than you have to choose OP amp with some crucial characteristics to make DC servo correct, that means to servo output of the amp to exactly 0mV. All corrections of well designed DC servo unit must be unaudible and unmeasurable in all amp characteristics. If you look to major well known amp producers they all have DC servo units implementated in their amps and for good reason because all input/feedback capacitors colorate the sound of the amp no matter of the type used. Different type of capacitors are sometimes used to intentionaly corect the sound of the amp with their colorations. In my opinion ther should be no capacitor in signal/feedback path of amplifier :whazzat:

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