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Hi Allen,Your graph of harmonic distortions for the PP-1C is most impressive: it looks to me like a 'perfect' harmonics spectrum! However, I was quite surprised to see the second and fourth harmonics in there, given that it's an all-balanced topology and I would expect them to cancel out. I'm sure the sound is all the better for it but, if I may ask, how did they arise?
Follow Ups:
...it was pure blind luck!The even order harmonics should cancel out in such a quasi differential design, but the amp as measured obviously had not well matched anode R's in the first stage.
If the whole thing is very well balanced up all the even order H's fall out leaving just the odd's as shown
Regards, Allen
Thanks, Allen.
I wonder if he's using the old westrex trick. More info found by searching for Lynn's "Karna" amp.It's a really wierd trick, where you unbalance a balanced stage. That is, turn a differential pair into two separate SE stages. Yet inputs and outputs are still coupled with balanced trannys. Theoretically this puts harmonics back in line. Something about cancelling both even and odd harmonics.
I haven't quite figured out why it works. My first impression is that it just seems wrong. Perhaps the beauty of it is counter-intuitive.
jh
The WE harmonic equaliser works by deliberately increasing the second harmonic on the input of the final stage, which increases production of third harmonic by intermodulation with the fundamental.Third harmonic produced this way is antiphase with third harmonic produced by the curvature of the V/T characteristics so if just the right amount of second is used the two sources of third cancel. Since equal amounts of second are added to each side the PP action will tend to cancel it as well so the end result is a reduction of both second and third.
I suspect that the second and fourth harmonic in Allen's PP1 are produced by intermodulation between fundamental and third harmonic in the driver stage. I don't think Allen uses WEHE.
Mark Kelly
Thanks for that fascinating information. How is "deliberately increasing the second harmonic on the input of the final stage" achieved, since the input of the final stage has to have come from a balanced stage? I wonder if the stage befoe that would do? It would be easier to arrange. I guess it wouldn't matter if you overdid the 2nd harmonic distortion - it would just sound like SE ;)
If you look at the current balances in a transformer it is the differential action which cancels the even harmonics. Since the centre tap currents are effectively common mode they contain the harmonics. The WEHE uses and arrangement of resistors and capacitors on the centre taps of the output and driver transformers to transfer some of the current from the output to the driver.What I've not been able to work out is how they calculated the ratios required for optimal cancellation. They may have done it by trial and error on the bench or there may be some maths missing from the literature I have seen.
Mark Kelly
Oh, I see. I wasn't thinking about transformers, because Allen uses a long tail pair splitter, made up of a pair of triode cascodes. Looking at his schematic, it seems balanced.
That's why PP is not the way to assure a "proper" harmonic structure. Well, that's been my experience, at least.Kurt
...that such a harmonic structure is optimum.To me, distortion is just that - and I like it to be as low as possible.
And yes - that spectrum pix is due to dumb luck - as explained above.
Regards, Allen
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