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Old 23rd February 2012, 09:34 PM   #11
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Well that reminds me :-) the dac is balanced! Lol.
I did this mod a long time ago and forgot that little detail.
Works great. ( the CDplayer is one of those higher-end Sony with the weight that goes on top of the cd, cant remember which model number).
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Old 24th February 2012, 08:28 AM   #12
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Quote:
Originally Posted by the_manta View Post
Why exactly a Gomes stage ?
Without any kind of feedback or servo circuits (like a css) you will of course get high distortion. It is really not complicated.
In terms of math I think nobody here can tell you how to improve the circuit you posted. Even it looks simple, it is yet to complicated. You just have to do more simulating. I hope you know John Broskie's issue about the gomes configuration: Gomez Vs XPP Amplifier
I have tried several output stages, both discrete and tubes. As I said, I would like to try this one due to the fact it's used in one of the best sounding tda1541a implementations known to man...

Are there any ways to further optimize the performance?
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Old 27th February 2012, 04:53 PM   #13
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I did some more simulations, and got THD down to 0.000543%.

-removed the Rk bypass cap. Of course I lose gain, but I dont really need it... what else do I lose by removing the cap? Slew rate?
-different operating point
-voltage divider between the left triodes. I can't really rationalize why this is beneficial though, but it simulates better... The top-right triode now gets a out-of-phase signal which is smaller in amplitude.... maybe better match with the bottom-right triode plus its plate resistor?

Screen shot 2012-02-27 at 6.44.15 PM.png
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Old 27th February 2012, 08:41 PM   #14
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If you are looking for extremely low distortion and still maximum gain you should exchange the upper left triode for a depletion MOSFET biased for equal current through both input and output pair. To keep Zout as low as possible you can exchange the bottom resistor/cap for diodes.

Just as much Gomes as the one you propose but even better.

Also note that the one your propose is the one with highest uneven harmonics and THD in my sims.

Actually I don´t think the .four is reliable.

Last edited by revintage; 27th February 2012 at 09:01 PM.
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Old 27th February 2012, 11:53 PM   #15
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Originally Posted by studiostevus View Post
As for the reason why to use this stage, a very successful commercial player (AMR CD-77) uses this topology, and it's designer T. Loesch (active on this forum) has been promoting the circuit for some time here.
Can you please link Thorsten's discussions about this topology?
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Old 28th February 2012, 06:07 AM   #16
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Hey S,
Just noted your .four is useless. Add the Spice-directive .options plotwinsize=0 to get going.
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Old 28th February 2012, 06:17 AM   #17
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Honestly, I know even less of LTspice than I know of EE. So thanks for your comment... I need it.

I added it, same THD performance while simulating.
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Old 28th February 2012, 06:39 AM   #18
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The .four isn´t reliable.

A SRPP, that I dislike, sims a lot better(even if the harmless h2 is somewhat higher) as all the higher hamonics are a lot lower.

This indicates you can forget about these sims.

Could be something with the tubemodel. Try many different like E88CC etc. to get a better picture.
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Old 28th February 2012, 09:30 AM   #19
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Quote:
Originally Posted by Joshua_G View Post
Can you please link Thorsten's discussions about this topology?
Google is also YOUR friend...

Building the ultimate NOS DAC using TDA1541A

decoupling TDA1541A
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Old 28th February 2012, 03:21 PM   #20
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Quote:
Originally Posted by revintage View Post
If you are looking for extremely low distortion and still maximum gain you should exchange the upper left triode for a depletion MOSFET biased for equal current through both input and output pair. To keep Zout as low as possible you can exchange the bottom resistor/cap for diodes.

Just as much Gomes as the one you propose but even better.

Also note that the one your propose is the one with highest uneven harmonics and THD in my sims.

Actually I don´t think the .four is reliable.
Do you have any suitable depletion MOSFET models that work in LTspice? Cant get the supertex library to work. Also not sure whether in contains MOSFETs that can handle this voltage...
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