TGM amp goes 'tubey'

[Bigun]

I know some people don't like this kind of messing around with perfectly good SS amplifiers but it was too tempting to pass-up - and so I'd like to recommend you give it a try too

There has been much chat over the benefits of tubes for their sound. An infamous paper by Russell Hamm was one of the early attempts to explain it in terms of the harmonic profile.

Here's an extract for a 2 stage triode amplifier. Key things to note:

Low Power: the output starts off mostly as H3 at low power with H2 and H4 close behind.

Medium Power: Overall distortion heavily dominated by H2 then a strong H3 followed by H4;H5 and above remain small.

High Power: dominated by H2, some H4 with H3 falling off. H5 and above remain small.

[Bigun]

I wondered how to achieve this with my TGM amplifier. The first version TGM1 used a resistor to feed some of the output to the top of the LTP resulting in some self-modulation of the signal which creates plenty of H2 and some H3 [TGM Amplifier ?]. But the results became muddy at higher power. Too little injected distortion and it wasn't noticeable, too much and it was nasty.

TGM2 uses a CFP for the LTP and doesn't respond to self-modulation [TGM2 amplifier]. So I created some asymmetry in the feedback loop by taking a little feedback from the emitter of an output device instead of at the junction of the two output emitter resistors.

TGM2 was a little less musical than I had hoped so perhaps this change will help.

[Bigun]

Simulated results that Hamm would appreciate:

I've built this version and it sounds very good indeed. The bass is strong, the highs are clear. The mids are very smooth, if a little darker and overall it's one of the first SS amps that I've heard that has zero traces of fatigue.

[keantoken]

Cool. How did you produce that distortion graph? And I'd also like to see this paper if possible...

What about high order even harmonics though? It seems to me that high order harmonics would be produced during switching. Nevermind, this would surely show up in harmonics up to the seventh...

- keantoken

[Bigun]

The distortion graph ? - just throw the answers from spice into a spreadsheet

The infamous Hamm paper is here (and it's contested by some people of course so don't take it as gospel):

http://www.amplificadores.com.br/Tex...istors%20I.pdf

Yup there will be higher order harmonics, not just because it's Class AB but because it has gnf. It's a compromise of course, but sounds good and easy to try.

[Bigun]

FFT at 6V p-p into 8R

[keantoken]

Very nice, Bigun! You discovered a cheap modification while I was off in the corner messing the NTP. The NTP has its merits, but I'm swayed. This should serve as a nice modification to many class AB amplifiers existing already.

- keantoken

[Bigun]

The TGM tubey approach can only add distortion so it's limited to amps that are low THD to begin with. It helps out with Class AB that rely on a reasonable amount of gnf to keep their output stages under control and it sounds very good.

Your NTP is probably a better approach because it generates a good profile with low distortion to start with.

[keantoken]

Bigun, I've tried several times to reply to your post but for some reason I simply have not found an appropriate response.

Let's just say your blameless statement has thrown my brain into deep saturation negotiating and cataloging what things would change were we to switch between NTP and LTP. For me at least, it is not a trivial change.

What I can say is that I suspect net THD might decrease and I also think the spectrum would be more benign. I think the only way to know for sure is to simulate or build, since it depends on the implementation...

Could you post simulation files?

- keantoken

[AKSA]

NTP has more H2 and H3 distortion than an LTP. Overall, the thd of the NTP, I have found, is higher, though more benign.

It is better at high amplitude than the LTP, which because of the S shaped transfer function tends to compress yielding high order odd harmonics.

Anthony, does this resolve your cerebral dilemma?

Hugh