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Old 22nd February 2012, 08:01 PM   #1
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Default Designing a Gomes i/v stage for tda1541a

Folks,
Bare with me, I am a beginner.... and I am trying to design a Gomes (supertotempole) i/v stage for a tda1541a dac. I am messing around in LTSpice, and am rather disappointed with the THD figures (2.12% !).

This is the circuit I am using, how can I improve the performance and/or simulation? (is my simulation wrong, or is the circuit wrong?)

Thanks

Screen shot 2012-02-22 at 9.57.28 PM.png
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Old 22nd February 2012, 08:19 PM   #2
Yvesm is offline Yvesm  France
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Mmmh !
But at wich output level ?
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Old 22nd February 2012, 08:51 PM   #3
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5.2 Vpp

Doesnt LTspice normalize THD figure to 1V ?
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Old 23rd February 2012, 02:57 PM   #4
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Edit: yes, i think it does... When going to FFT, it mentions "Fourier components are normalized to correspond to the time domain RMS amplitude"
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Old 23rd February 2012, 03:39 PM   #5
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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
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Old 23rd February 2012, 04:46 PM   #6
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LTSpice shows both Fourier and normalized in the same table, so no problems with that. But there are other faults in the Spice setup that makes the THD figures go wrong.

But whats the use of making a Gomez? Don´t think it will come anything good out of that.
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Old 23rd February 2012, 05:02 PM   #7
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With an acceptable working point fior the poor tubes and correct commands there there should be very low THD.

If you want to do a FFT .tran 0 200m 60m 1u might be a good place to start.
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Old 23rd February 2012, 06:59 PM   #8
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Quote:
Originally Posted by revintage View Post
With an acceptable working point fior the poor tubes and correct commands there there should be very low THD.

If you want to do a FFT .tran 0 200m 60m 1u might be a good place to start.
Thanks... I now get to 0.049%.

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.

I thought why not try it for myself, but of course I would like to get optimum performance before I decide to build it.
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Old 23rd February 2012, 08:36 PM   #9
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Here's a simple iv stage, if you dont mind transformers. The output level is a little low, but still not so low most preamps can't handle it. My pre is a moderate gain type, and using the volume potentiometer on it's high setting is only a good thing (less potentiometer in the signal).
Sorry this isn't tube, but cant resist...
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Old 23rd February 2012, 08:59 PM   #10
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Why on earth a transformer there? What would you gain? If there is DC on the DAC output it will be completely useless as the Lundahl core will saturate. With a 22ohm I/U resistor and 1541, output will be just a few multiples of 10mV. So a 1:1 on a current DAC you can live without. A 1:1 to with primary CT will work great on a balanced voltage DAC though.

Last edited by revintage; 23rd February 2012 at 09:02 PM.
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