Design review of my new discrete audio buffer

[Lars Nielsen]

Hi all,

I have been working for some years now, on a new audio buffer, for use in preamps and d/a converters. It is going to be af complete DIY project, posted on my website.

All of the selected solutions have been tested in listning tests and measered with serious equipment.

Before I continue with the final PCB layout, I would like to invite you all, to give comments, tips and ideas, to the design..

See schematic here: http://www.diyhifi.dk/1A.pdf

Best Regards
Lars

[Lars Nielsen]

No comments..

Really?

[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.

[bear]

Specs?

Real world vs. SPICE plots?

A look at the square wave?

I dunno...

As Count Basie said "...if it sounds good it IS good..."

_-_-bear

[aparatusonitus]

How about output noise filter instead single R57 at integrator input...also you have to protect integrator opamp J-fet input against the rails.

[Lars Nielsen]

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.

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.

The 680nF is selected because of high capacity at best price, and the 221K resistor is selected to have minimum influence on dc offset error that will appear because of the input current in LF412.

The servo takes about 6-7 seconds to get the output below 0,1mV offset, so in real life very precise. 20Hz is below -120dB.

I have been trying to use other type of opamps, but then I see some unstability in the first servo buffer where no parallel resister are on the feedback capacitor..

But do you think it would be better to have differing filter values?

PS: Tonight i will post some prototype pics on my website..

/Lars

[Lars Nielsen]

Originally Posted by bear

Specs?

Real world vs. SPICE plots?

A look at the square wave?

I dunno...

As Count Basie said "...if it sounds good it IS good..."

_-_-bear

Well.. the Spice plots are very similar to real life, the bandwith is a little bit lower in real.. I think it is something about the transistors..

I have mostly been doing spice work when i made the base work arround temparature stability in the stage..

The square wave.. i will work on a plot of that, but with a bandwith of 0,05 to 5Mhz i would expect to see af perfect picture..

/Lars

[Lars Nielsen]

Originally Posted by aparatusonitus

How about output noise filter instead single R57 at integrator input...also you have to protect integrator opamp J-fet input against the rails.

I do not understand what you mean with output filter?

Yes.. very good point, the opamp needs to be protectet.. I can see that maximum input voltage is 15V.

2 pcs. of 1n4148 across C29 would do the job, and they would actually also make the DC servo faster at startup, because the rail would not charge C29 at ramp up.

Another solutions would på 2 pcs. of zenerdiodes of 15V, but I am not happy with the zener at that point in the circuit.. What do you think?

[DF96]

Does the output settle monotonically to around zero volts, or does it oscillate? The simple way to loop stability is to have no more than two roll-offs. Failing that, have one dominant roll-off, with the others have frequencies much lower/higher (for a DC servo, higher). Failing that, do a full stability analysis. Having a string of similar roll-offs is about the worst thing to do!

"20Hz is below -120dB" means what? I assume this is the result of simulating/calculating the DC servo in isolation, as it is too precise to have been measured. Look at 1-5Hz; this is where the trouble may be. You need to consider phase as well as amplitude.

[Lars Nielsen]

Originally Posted by DF96

Does the output settle monotonically to around zero volts, or does it oscillate? The simple way to loop stability is to have no more than two roll-offs. Failing that, have one dominant roll-off, with the others have frequencies much lower/higher (for a DC servo, higher). Failing that, do a full stability analysis. Having a string of similar roll-offs is about the worst thing to do!

"20Hz is below -120dB" means what? I assume this is the result of simulating/calculating the DC servo in isolation, as it is too precise to have been measured. Look at 1-5Hz; this is where the trouble may be. You need to consider phase as well as amplitude.

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...

I think that it is the right way to do it? If I make the servo so fast, that it doesn't make the swing, the servo will have influence on the sound..

Yes, the 120dB is the calculated value of the servo filter.. Yes the gain of the servo is high at 1hz.. But is the way it adjust okay?