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In Reply to: RE: SE 2A3 biasing : non-linear vs. linear biasing ...graphs posted by drlowmu on March 30, 2014 at 18:53:52
Jeff,
The THD will be lower at a lower power level, regardless of the 2A3 type.
I will run the 2.5K graphs again from 1/4W to 1W and send you the data.
DT 667
Follow Ups:
DT 667,
Just 1/4 Watt would be fine !!
Jeff Medwin
check your email.
can you post it?
dave
1/4 and 1W.
1/4W spectrum
2nd harmonic : -43.24dB
3rd harmonic : -45.47dB
4th harmonic : -76.57dB
5th harmonic : -68.39dB
The EML or NOS American brands would probably measure a lot better.
cool thanks!
I find it interesting (and a bit uncomforting) that the 2nd and 4th dropped disproportionately to the 3rd and 5th as you lowered power.
it makes me want to ask for the 1/16W levels and see if your SE has a PP spectra.
dave
I can post the data at 1/16th of a watt.
Is this some terrible problem lurking in my circuit?
The Hammond 125ESE are not the greatest OPT's and mine are 2003 versions that had the quality control issues.
It appears as if you have some "distortion cancellation" going on.
Your 1W graphs have the typical SE spectra when a single device dominates the spectra. Your 1/4W plots appear as if another device (driver or output transformer) is introducing a similar spectra and since it is 180 degrees out of phase with the 2A3 the distortions "cancel" like in a PP amp. Since the two distortion spectra that are interacting can never be identical, you will see a decrease in even order products only at certain levels.
dave
(I put "distortion cancellation" in quotes since it is the generally accepted term but the distortion doesn't really cancel but sums to higher order products)
I would bet the 12ax7 or 7b4 would measure lower distortion at 1 watt and especially 1/4 watt.
Thanks for posting DT. This is cool stuff.
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