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In Reply to: Harmonic Balancer? posted by Jim Hagerman on February 19, 2007 at 11:32:41:
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.
Follow Ups:
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.
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.
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