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Original Message

RE: Modified Futterman OTL amplifier

Posted by cellai@space.it on June 26, 2008 at 11:45:57:

A professional reader has kindly simulated (SPICE model)the harmonic distortion of the output stage of the proposed OTL scheme WITHOUT FEEDBACK (open loop) and the result is:
Fundamental (1 KHz) - 0 dB
2nd harmonic - -25 dB
3rd harmonic - -30 dB
4th harmonic - -45 dB
5th harmonic - -65 dB
With FEEDBACK applied (as in the schematic) the calculated improvement should be of the order of 17 dB (that is 2nd harmonic -42 dB, 3rd harmonic -47 dB and so on).
As the professional reader explained to me (in a private mail) the open loop presence of even harmonics depends on non ideal push-pull operation. Potential DIY audiophiles may wish to improve further the design and to cancel even harmonics (in open loop, before feedback application) by reducing the imbalance of the drive signal of the output stage (for a deeper discussion on the topic and on noise reduction techniques see http://www.tubecad.com/articles_2001/totem-pole_output_stage_psrr/Totem-Pole_Output_Stage.pdf, particularly the last page).