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In Reply to: RE: RH84 type test amp: THD & THD+N @ 1 to 3 watts ... posted by dave slagle on September 28, 2016 at 19:19:06
With the power output of the amp @ 3W , driver stage distortion:
Left Channel : THD & THD+N = 13.59% 3rd harmonic down 8.5dB from 2nd
Right Channel: THD & THD+N = 12.38% 3rd harmonic down 9.6dB from 2nd
In the non-feedback SET circuits I have built in the past, driver stage distortion would be very low compared this circuit.
Comments?
Follow Ups:
Why are those numbers so high? Even without GNFB to the cathode, the driver's output shouldn't be that distorted. And if it was, those numbers would show up at the output stage. Seems like something is wrong with the measurement.
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Buy Chinese. Bury freedom.
Edits: 09/29/16
I thought that too but then not being well versed in the evils of feedback I built the circuit in spice and the numbers came very close to what dt667 measured.
here is the fft of the driver and the output @ 2.8W out.
and here is the numerical rundown of the distortion
If I remove the feedback and decrease the input value to get the same power out we get the expected low distortion behavior from the driver but the overall circuit gets worse.
To keep things complete, the other important thing that changes with the feedback is the output impedance. As built, the Zout is around 2 ohms and if the feedback is removed the Zout jumps to just over 100 ohms.
dave
I was just about to test the driver stage without the feedback resistor in place when I noticed your post.
In the past, I had tested driver stages in zero feedback SET circuits with my Virtins PC soundcard probes and got valid results.
Thanks for all the circuit modeling work.
Very informative.
dt 667
My fault, I wasn't thinking about the fact that the correction signal is connected to the anode of the driver. Dave Slagle's SPICE results make this clear; driver distortion can't be measured unless Rfb is removed. You're measuring a large percentage of the correction, not driver tube output per se.
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Buy Chinese. Bury freedom.
Edits: 09/29/16
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