Design review of my new discrete audio buffer

[peranders]

Originally Posted by DF96

Is the DC servo stable? It appears to have a large number of low pass filters with similar time constants - exactly the conditions for loop instability. I would expect at least a gain peak at a few Hz, or maybe oscillation.

You should have max two poles in the DC servo loop and if they suffiently apart you will get a microscopic hump in the frequency response. Good work Lars, but why don't you make the schematics in colour? Easier to read.

Oops, forgot to read following posts. When I look closely I see that there are two integrators. Only one is needed.

[peranders]

Originally Posted by Lars Nielsen

I have been working very much with the DC servo, because i have experienced that many of the typical servoes in other constructions have influence on the sound quality.. and THD.

/Lars

Lars, the DC servo opamp should be a good one and the cut off frequency shouldn't be too low. 1-5 Hz is recommended (by me) because if you have a lower frequency you will have fluctuations due to temperature shifts. The DC servo opamp must be good up to 1 kHz at least since you have it in the audio path.

[peranders]

A DC servo can look like this. The example is a non-inverting circtuit and an inverting one is even easier.

[Lars Nielsen]

Originally Posted by peranders

You should have max two poles in the DC servo loop and if they suffiently apart you will get a microscopic hump in the frequency response. Good work Lars, but why don't you make the schematics in colour? Easier to read.

Oops, forgot to read following posts. When I look closely I see that there are two integrators. Only one is needed.

Thanks.. Schematics in colour.. hmm I thougt that schematics were suposed to be boring..

Yes.. there are two opamps.. The LF412 is a dual opamp, so it is there anyway.. But if I remove the second opamp, will the filter not be able to control the current generator, and the filter will also be loaded, so bigger caps and smaller resistors will be needed..

At the beginning I started with the filter of C26/C27 at the input of U1B, but then will the input leak current of the opamp have influence on the overall DC precision.

[Lars Nielsen]

Originally Posted by peranders

Lars, the DC servo opamp should be a good one and the cut off frequency shouldn't be too low. 1-5 Hz is recommended (by me) because if you have a lower frequency you will have fluctuations due to temperature shifts. The DC servo opamp must be good up to 1 kHz at least since you have it in the audio path.

You are right about the temparature drift can have an influence when the servo is slow.. but hopefully most stages are so stable that this not can be an issue.

Up to 1Khz? Why? The signal most never be able to go back through the servo, that would make af large amount of THD.

[Lars Nielsen]

Originally Posted by peranders

A DC servo can look like this. The example is a non-inverting circtuit and an inverting one is even easier.

Well.. It is basicly the same circuit as mine.. and I also started with this more simple servo.. But listening tests showed me that much more filtering is needed. My transistor stage can run without a servo, so it is easy to check..

In my tests I discovered that a 3 pole of 221k x 680nF is not enough.. it can be heard.. the 4 pole could not be heard, but to be absolutly sure I inserted the 5.

[Lars Nielsen]

Some pictures of the prototypes is now on my website: DIYhifi.dk - Forside

[peranders]

Originally Posted by Lars Nielsen

Up to 1Khz? Why? The signal most never be able to go back through the servo, that would make af large amount of THD.

You have full signal at 1-5 Hz and 10 Hz 20 dB lower, 100 Hz 40 dB etc. so at 1 kHz you will have a tiny amount of the signal left and at 10 kHz you are near the noise level. Why don't use LTSpice and do some probing at the opamp output?

[peranders]

Originally Posted by Lars Nielsen

At the beginning I started with the filter of C26/C27 at the input of U1B, but then will the input leak current of the opamp have influence on the overall DC precision.

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.

[Lars Nielsen]

Originally Posted by peranders

You have full signal at 1-5 Hz and 10 Hz 20 dB lower, 100 Hz 40 dB etc. so at 1 kHz you will have a tiny amount of the signal left and at 10 kHz you are near the noise level. Why don't use LTSpice and do some probing at the opamp output?

If the servo only lower 100hz to -40dB it would be a direct global negative feedback, which is out of phase..

I am not sure I see your piont here..?