Designing a Gomes i/v stage for tda1541a

[revintage]

It´s no problem to add the DN2540-model.

[ThorstenL]

Hi,

Originally Posted by studiostevus

I did some more simulations, and got THD down to 0.000543%.

Don't overdo it on cancellation. It sounds worse and becomes extremely sensitive to tube variation.

Originally Posted by studiostevus

-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?

Unbypassed resistors add noise. That is one of the reasons neither CD-77/777 or DP-777 have very low noise.

Originally Posted by studiostevus

-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?

Gomes always uses signal from lower anode, not upper cathode.

As you know from the CD-77 it uses an ECC81 in the first stage and a 5687 in the second.

So we are clearly doing some more, as Doctor Who likes to call it, "jiggery pokery"...

Ciao T

[studiostevus]

Originally Posted by revintage

It´s no problem to add the DN2540-model.

Simulated the CCS with DN2540, both straight and cascoded... don't get to THD specs as with the tube...

[revintage]

You still don´t get it about simulations?

1 The .four is not to trust!

2 THD is uninteresting, harmonic spectra is!

But if you believe Spice is correct, the spectra of the Gomes is not nice as the higher overtones doesn´t diminish the higher they get. How it is IRL I don´t know but probably better.

And as I said to get a better picture you must try many different tubemodels. Why not try what T used in his design, a ECC81.

In a world were the amp with the lowest THD sounded best, it would be easy to choose.

[ThorstenL]

Hi,

Originally Posted by revintage

But if you believe Spice is correct, the spectra of the Gomes is not nice as the higher overtones doesn´t diminish the higher they get. How it is IRL I don´t know but probably better.

Attached a real Gomes stage, outputting 2V RMS into 10K. Note that this one deliberately did not have any aggressive distortion cancellation applied. Plus the ECC81 is a less linear Valve than the 12AY7.

As we can see, 2nd HD is around 0.3%, 3rd is 0.003%, 4th is MIA, 5th is 0.002% & 6th and higher at 0.0005% or lower. The spike above 10KHz is from the environment, not the test gear, maybe a CRT Monitor leaking in.

We can cancel the 2nd HD a lot more, but 3rd and 5th are then higher and the HD becomes more tube sample dependent. And it does not sound as good.

Using 6072A/12AY7 lowers 2nd HD appreciably, as it is basically a linearised and "audio grade" ECC81. Other ECC81 analogs, such as 5965 also give lower 2nd HD.

Ciao T

[revintage]

Thanks Thorsten,
As I suspected. Not at all as a simmed FFT of a Gomes.

[ThorstenL]

Hi,

Originally Posted by the_manta

I hope you know John Broskie's issue about the gomes configuration: Gomez Vs XPP Amplifier

I hope you know that much of what John does is theoretcal/simulated only, that he massively overemphasises PSRR over any other considerations and rarely if references what he does to evidence of what is audible?

I had twice the occasion of rescuing projects a friend and DIY Kit Vendor had based on stuff posted at that site which simply did not work properly... I ended up completely redesigning the projects as the original circuits neither worked well electrically in reality nor sounded any good...

So any of his blog posts should be taken "cum grano salis magnitudinem montis".

Ciao T

[ThorstenL]

Hi,

Originally Posted by revintage

Thanks Thorsten,
As I suspected. Not at all as a simmed FFT of a Gomes.

I must add, there is a TDA1541 DAC included in this plot, running "sans un filtre numérique", so it is likely to add to some of the higher order stuff.

Here by comparison a TDA1541 with Op-Amp (LM6181 current feedback) analog stage:



This is probably about as as it gets for the TDA1541.

Ciao T

[studiostevus]

Makes you wonder why we mess around with tubes

Thorsten, what's your view on using a Mosfet CCS to replace the top left triode as suggested earlier?

Also i will change the operating point. Not comfortable to work with 400v in a Dac... Should be possible to get good results with lower voltage, also on 6072a.

[revintage]

Hey T,

It´s in the opposite direction. Your sim looks better in the higher order area than the sim. Just by looking at the fourier-analysis you can tell there is something funny going on.

Can also agree about the blogs. This also goes for the .four-sims above.