Asymmetrical waveforms !

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izzy wizzy
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#76 Re: Asymmetrical waveforms !

Post by izzy wizzy »

Trouble with many comparisons of PP vs SET fail to address the significant difference between the designs. Most PP designs are more complicated than simple triodes in the output, have feedback around a transformer, use some "not fantastic" ways of phase splitting and so some questionable ways of getting from single ended to the 2 phases needed to drive the finals.

I hear the arguments for the cathodyne and yes the same current derives the two phases over equal resistors but practice shows there is deviation especially nearing the higher frequencies. The paraphrase is odd in that one signal passes though an inverting stage to derive the other phase. Many think the "best sounding" or more like a SET but balance must be interesting to manage. The LTP needs a good high Z in the tail so is really another assymetric device. Witness many having differing load resistors per phase. They all make PP amps sound different.

Then there's a transformer. Modern ones can be surprisingly good. I repeated a test Lynn Olson did to see how good the two phases hang together using a Lissajous figure and it didn't tilt or start to open up until well above 50kHz, more like 70kHz so that's phase accurate performance that's hard to beat and I think sounds that way. There's less confusion and "other signal riding along" that is to me much of the issue with PP. A kind of fog that inhabits the music. All other things being equal of course; use only triodes, no ultralinear, no feedback around OPT.

The Gizmo link Nick posted is pretty much the same recipe I use, largely gotten from Lynn Olson.

My preference is for PP or differential circuits. No doubt I have a bias present here so read into that accordingly ;)
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#77 Re: Asymmetrical waveforms !

Post by IslandPink »

Your bias is correct, though :)
That's the way to do it, and ps. the Sowter 3575 is a great little transformer.
The Aurora I built had the best bass of any amp I've built, I've go close with SE and a shunt-reg supply, but the PP output transformer is always going to go lower and better ( Steve S, I'm not selling the Bud Purvine Litz PP outputs just yet ! )
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#78 Re: Asymmetrical waveforms !

Post by steve s »

Steve S, I'm not selling the Bud Purvine Litz PP outputs just yet !
Thats a shame Mark
After all that's been said a pp px4 must be built ! Just to compare

I'm sure if I put into practice all I've read these last few days it would sound pretty decent
Tx's either side of the o/p stage and extremely well balanced seems the key
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#79 Re: Asymmetrical waveforms !

Post by Max N »

I have made some progress with my PP experiments, I will try to update my thread over the weekend. Where I am now addresses many of the issues covered in this thread.
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#80 Re: Asymmetrical waveforms !

Post by Neal »

Im glad Nick bought up the issue of the phase splitter. In all my messing about with PP amp output and input stages the splitter had by far the biggest influence on sound all things being equal. The best solution I found, for me, apart from using transformers was to use a long tailed pair with ccs ensuring it remained balanced at high frequencies…
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#81 Re: Asymmetrical waveforms !

Post by IslandPink »

steve s wrote: Sat May 28, 2022 12:45 am
Steve S, I'm not selling the Bud Purvine Litz PP outputs just yet !
Thats a shame Mark
After all that's been said a pp px4 must be built ! Just to compare
You can borrow them until I build a 'Tabor' though ... ?
"Once you find out ... the Circumstances ; then you can go out"
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#82 Re: Asymmetrical waveforms !

Post by steve s »

IslandPink wrote: Sat May 28, 2022 12:45 pm
steve s wrote: Sat May 28, 2022 12:45 am
Steve S, I'm not selling the Bud Purvine Litz PP outputs just yet !
Thats a shame Mark
After all that's been said a pp px4 must be built ! Just to compare
You can borrow them until I build a 'Tabor' though ... ?
Thanks Mark, that's really good of you,
It would be winter again before I have time though.
I do have a pair of Chinese el34 pp transformers, but I doubt they would a fair comparison to the tributes.
So if I get chance in the meantime I have something to use
The tube manual is quite like a telephone book. The number of it perfect. It is useful to make it possible to speak with a girl. But we can't see her beautiful face from the telephone number
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#83 Re: Asymmetrical waveforms !

Post by steve s »

My intention when starting the thread was to try and understand the differences between push pull amplifiers and what I'm used to
The big thing for me is if you understand the weak points in any thing, you can then improve on it.

If you think there's no weak points, there's no scope for improvement and, unfortunately, that is the the view of many ( not here by the way)

I would love to build better amplifiers than I do presently, we all would ?

But push pull is far more complex than I thought when I started this thread.
I was looking for loopholes, things that have been missed.
But as in se, it appears its all known, just the weak points are not fully acknowledged.

But from a personal perspective, if you can get everything working with absolute accurately, I think there could be an improvement in quality compared to what's possible with a realitively simple se amp.
But every aspect of it would need some thorough checks that are outside the 'normal' scope of amplifier checks.
Just starting with getting the valves balanced, seems a key point,
And checking that balance across the frequencies in operation.

And thanks again all for the varied contributions ... so far !
The tube manual is quite like a telephone book. The number of it perfect. It is useful to make it possible to speak with a girl. But we can't see her beautiful face from the telephone number
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#84 Re: Asymmetrical waveforms !

Post by JamesD »

Problems of Class AB Push Pull.

One of the problems with Class AB PP amplifiers is at low signal level (i.e. near the zero crossing point), where both devices are operating at the same time in Class A, this low level signal is then subjected to the problems of two stages output adding together compared to a single stages output where it has transitioned to one side only operating in class B - this effectively double the transconductance in the class a region compared to the class b region... There is also the problem of two stages operating in parallel (if you believe that is a problem). For OPT based PP stages there is the problem that at low level and across the zero level signal transition the transfer curve of the transformer is heavily non linear compared to its large signal operating region. Its also quite common for Class AB PP amplifiers to have a distortion profile with level where as the signal ( and hence output) decreases the distortion level increases - the opposite trend to most natural phenomena and the opposite trend to SE Class A amplifiers

If you are aware of these problems they can be mitigated to a degree - greater or lessor degree

SE in class a has its own problems of course as has been mentioned throughout the thread

These all contribute to the difference sound and experience that we have with PP and SE amplifiers - to say nothing about how different loudspeakers/ audio systems respond to the two amplifier types...
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#85 Re: Asymmetrical waveforms !

Post by jack »

Completely concur with James' and other comments - we are not trying to produce a "straight piece of wire with gain" - use a Doug Self "Blameless" if you want close to that.

It's the harmonics and other distortions/non-linearities that give warmth and colour. We're just trying to get warmth & colour that we like! (and maybe understand)
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#86 Re: Asymmetrical waveforms !

Post by Max N »

I decided it would make more sense to post here rather than in my thread (I will still try to update that at some point).
Output transformers - I wasn’t really making much progress until I invested in decent transformers. I took a punt on a pair of Monolith Magnetics BA-14s (amorphous core). They were better than the Lundahls I had been using. Nothing against Lundahls, I had been using a pair of ll1663s, small, standard cores, not a fair comparison. The Monoliths have better extension at both ends, and amorphous cores do seem to be more transparent.
With the better transformers in place, I could now start to hear the effect of other changes.
I’ve tried various phase-splitters, and decided to avoid the issue by using a DAC with balanced output.
For unbalanced sources I’ve had good results using input transformers with this arrangement:
https://www.jensen-transformers.com/wp- ... /as060.pdf
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#87 Re: Asymmetrical waveforms !

Post by Max N »

I also prefer negative grid bias in the output stage, and have had good results with the Tent/vanderveen bias module. This pretty much eliminates any DC in the OPT, and no need for cathode resistors or capacitors.
The bias module can only sink/source 1.5 mA, but I buffer them with mosfet source followers (I use Pete Millett’s A2 buffer boards). This allows for large grid resistors and small, high quality coupling caps.
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#88 Re: Asymmetrical waveforms !

Post by Max N »

I’m using 12SN7s as the voltage amplifiers, with active loads. Almost zero distortion in balanced configuration. The relatively high output impedance doesn’t matter with the buffers.

I think that sums up where I am at now. I wouldn’t claim any of this is better than other people’s choices. The only thing I would really hang my hat on is the improvement I heard with the better OPTs.

Hope that gives you some ideas Steve :D
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#89 Re: Asymmetrical waveforms !

Post by IslandPink »

jack wrote: Sat May 28, 2022 6:31 pm Completely concur with James' and other comments - we are not trying to produce a "straight piece of wire with gain" - use a Doug Self "Blameless" if you want close to that.
It's the harmonics and other distortions/non-linearities that give warmth and colour. We're just trying to get warmth & colour that we like! (and maybe understand)
I don't agree with you myself and I don't think Nick would agree either.
I would say most of my system has the aim of being low distortion, and the dev. stages over the years have only brought out colour and often 'warmth' in the material, not the opposite.
I just put in some even lower distortion speaker cables and all the stuff like colour/tone, body, presence etc got better, more feelings of real stuff in the room.
The only part of my system with some distortion engineered-in is the power amp, where I find the typical SE descending distortion profile does lend some charm to vocals.

So, what's this Doug Self amp then ?
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#90 Re: Asymmetrical waveforms !

Post by jack »

IslandPink wrote: Sat May 28, 2022 8:44 pm
jack wrote: Sat May 28, 2022 6:31 pm Completely concur with James' and other comments - we are not trying to produce a "straight piece of wire with gain" - use a Doug Self "Blameless" if you want close to that.
It's the harmonics and other distortions/non-linearities that give warmth and colour. We're just trying to get warmth & colour that we like! (and maybe understand)
I don't agree with you myself and I don't think Nick would agree either.
I would say most of my system has the aim of being low distortion, and the dev. stages over the years have only brought out colour and often 'warmth' in the material, not the opposite.
I just put in some even lower distortion speaker cables and all the stuff like colour/tone, body, presence etc got better, more feelings of real stuff in the room.
The only part of my system with some distortion engineered-in is the power amp, where I find the typical SE descending distortion profile does lend some charm to vocals.

So, what's this Doug Self amp then ?
I assume that this is a wind-up, but if not (and for those that haven't come across the Blameless Amplifiers yet...):

Just look up "Blameless" amps. It all stems from an article he wrote in 1997 "Distortion In Power Amplifiers", which laid out his reasoning on the sources of every type of distortion in a power amplifier (he addresses 8 different ones). In the conclusion, he produced a design that address all the points he raised and which, if correctly constructed, had a vanishingly low distortion (typically 0.0005% THD @ 1kHz) and thus, in a system, it would be "blameless": http://douglas-self.com/ampins/dipa/dipa.htm and expanded in his brilliant book, "Audio Power Amplifier Design Handbook", - I have the 5th ed from 2009 and it's pretty similar.

You can buy boards from: http://www.signaltransfer.freeuk.com/ - these are Doug Self approved boards & layouts...

Note: I like that he refers to his tri-modal amps, when in class A as "Winter Mode" and class B as "Summer mode".
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