Does anyone have any FFT plots from real Class-AB tube amps? - Page 6 - diyAudio
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Old 13th May 2010, 12:27 AM   #51
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Quote:
Originally Posted by smoking-amp View Post
Hmm, I'm not seeing any combining of feedback signals in Schades schematic thru the driver xfmr. Although the two sides of the OT should nominally be the same anyway. What seems to be the main problem to me is the disparity in gm when handing over control from one tube to the other. Schade's biasing graphs seem to address that for smoothest net gm.
Oh, so right. The feedback signals are subtracted separately from a common drive voltage. There is no feedback current in the transformer.

So if the drive signals are mirrors of each other, the main issue must be the gm bump, which can be addressed by selecting an op point for best composite gm.

I wonder if that's all that's happening here. What happens if the idle current is reduced? Is there a minimum distortion point?
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Old 13th May 2010, 02:41 AM   #52
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Michael,
Is it safe to assume that even without local feedback (only GNFB), there would be an optimal bias setpoint for minimum distortion.

I was playing around with THD vs. Bias Current when the amp is in AB. I have only really measured THD with bias >60mA on a KT88. At 80mA I found that the THD increased slightly from 70mA. In addition, as expected, max power decreased as a result of reduced b+ (due to PT load).
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Old 13th May 2010, 06:09 AM   #53
teemuk is offline teemuk  Finland
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In addition to turning off compression of the simulation data and all the other tips mentioned previously, you need to set the simulation's time frame such that it starts and stops saving data at the wave's zero crossing point. Otherwise, LTspice will interprete the "partial" wave as distortion.

A good reference is also plotting the FFT of the actual input, which in SPICE software should be ideal. Very often it's not.
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