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Original Message

Measured Harmonic Distortion Modified Futterman OTL amplifier

Posted by cellai@space.it on July 18, 2008 at 08:27:21:




This is the measured output spectrum with a low level 1 KHz tone applied at the OTL input. The OTL output level is the maximum possible without saturating the A/D converter of my PC audio card.

For measurements I have used the free SW (Visual Analyzer) available at: http://www.sillanumsoft.org/
Visual Analyser is a complete professional real time (FREE) software that transform your PC in a complete set of instruments (no new hardware necessary, you can use the Sound Card of your PC).

The corresponding harmonic distortion of the OTL amplifier is lower than 0.1%.

In order to reduce the even harmonics (due to SEPP gain imbalance) a 5.4 Ohm fixed compensating resistor has been added to the schematic (see my previous mail on the subject).

The measured signal has been picked up in parallel to my speakers and I can assure that carrying out the measurement with a strong 1 KHz signal in your ears is not pleasant at all. I suppose this is the reason why dummy load resistors are used instead of real speakers.

In my opinion this measurement demonstrates that it is possible to obtain a reasonably good performance (with the proposed schematic) without resorting to the more complex 'true modified Futterman' that requires additional tubes and a 450 V additional voltage rail.

For sake of completeness I have also measured the output spectrum with no signal applied (see my next post).