To use DC servo or not?

[jaycee]

Is it worth the extra effort of using a DC servo on an amplifier ? Is there any big advantage over using a DC blocking capacitor in the feedback network ?

I've looked at putting in a servo into my design (similar to the Leach Amp) and have messed about a bit in the simulator, but that's it. One thing that intrigues me is servo's that cancel the offset by playing with the LTP current sources.

Any comments from those far more experienced than me ?

[darkfenriz]

One thing that intrigues me is servo's that cancel the offset by playing with the LTP current sources.

Steven used one and published in thread : 'is there anybody built a non-feedback amp' or something like this.
Then it must be good solution

Is there any big advantage over using a DC blocking capacitor in the feedback network ?

Obvious one is lower value of capacitor. Caps like 1u are of much better quality than big electrolytics.

[anatech]

My thought is that it has to be better than a large electrolytic with very little bias across it. If your circuit is relatively stable you can offset the DC offset with a pot and DC potential lke they used to do it.

-Chris

[jtcc1015]

Hi all,

I believe I come across this thread 3 months too late. I am also very interested to hear about others opinions on this subject.

As matter of fact, I have found and purchased couple of this DC servo kits from a web site in Taiwan. The DIY community there has been heavily promoting this simple little circuit as a major improvement to sonic performance of any audio gear that require the use of large size (capacitance) capacitor as a mean of DC de-coupler or filter in the signal path. They also advertised that this device can be applied to power amp in order to eliminate the need of using high capacitance bipolar electrolytic caps as a low frequency filter in the feedback path or to pre-amp & DAC in order to eliminate the need of DC blocking caps on the output.

Attached please find the circuit diagram. I don't believe they are the inventor of this circuit but rather, as far as I know, the first DIY group generally implement it.

Well, I don't about pre-amp and DAC application but I would most certainly try it on my Leach Superamp. After all, these kits only costed 1/4 of the price of a very good NH-series "Black Gate" electrolytic capacitor.

So far, I have finished soldering the board and in the process of building a separate DC power supply for it because the DC rail on my power amp is too high for the kit to handle.

Will post further updates on this project soon.

[amplifierguru]

jtcc1015,

You are using OPA627 for a servo? Overkill?

that PS could easily be adapted to run from your Leach rails.

Cheers,
greg

[lineup]

I have never used a DC-offset servo.
I havent found it necessary.
With amplifiers having a gain of only 15-25 this is not often a problem.

I do not know if any degrading of performance
has been reported by a small DC-offset, say within +-0.100 Volt.

An input capacitor of good quality wouldnt hurt much.

[amplifierguru]

Hi lineup,

There are potential huge problems with DC servo's degrading the sound. Most simple servo's use a non-inverting chip whose gain reduces to unity at HF. But the cheap FET chips used have very poor PSRR at HF and, if their rails are not adequately filtered at HF the PSRR lets through the spray of even harmonics generated by AB on the supplies of the amp - and feeds them through to the input (usual).

The solution is to split the output resistor from the servo to input and put in a C to ground. Cleans it right up.

Cheers,
greg

[jtcc1015]

An input capacitor of good quality wouldnt hurt much.

lineup,

Thanks for your comment.

Please advise me, how would a input capacitor can improve the DC offset condition on the output of a Leach Power Amp when the root cause of the problem came from slight mis-match of the transistor or other commponents in the input stage??

Should I add a coupling capacitor on the input even though the original design does not require one??

amplifierguru,

Thanks for your comment. Yes, the OPA627 was a bit of overkill. The kits actually came with OPA134, but I have couple of OPA627 sitting around doing nothing, may as well use them for experimental purpose. I will even try out other OP-Amps in my stash too, such as AD797, and OPA637 etc.,

Just try to have some fun.

[peranders]

Originally posted by jaycee
Any comments from those far more experienced than me ?

I have used DC-servos in almost every design I have made and it works very good as long as you do it right.

The opamp must have good audio properties up to 1 kHz (which almost any has) and also descent DC properties. It's also an advantage to use an opamp with small input bias currents because then you will have no problems with offset coming from that source.

[peranders]

Originally posted by amplifierguru
There are potential huge problems with DC servo's degrading the sound.

I would say that there are microscopic problems with DC servo's as long as you do it right, good power supply etc.

One way to reduce the potential HF problems is to insert a filter at the servo output. By this you create a very small hunch at 100 Hz or so but 0.1-0-3 dB is bearable.
http://www.sjostromaudio.com/hifi_fi...0schema_p3.pdf The whole design consists of 6 pages.