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RE: Where is IMD?

Posted by Triode_Kingdom (A) on November 18, 2021 at 08:44:20

In Reply to: RE: Where is IMD? posted by rage on November 17, 2021 at 12:21:07:

Well, that's pretty bad, and it will be audible.

Just in case you're not familiar with spectral displays of two-tone testing, below is the plot you posted. I added the titles at three frequencies and circled the big one at 7 kHz. The latter is primarily responsible for the IMD presented numerically in the readout. If you look at the 7 kHz energy, you can see it appears to have a lot of additional energy starting approximately 40 dB down. These are the 60 Hz sidebands riding on the 7 kHz sine. If you were to increase the resolution by spanning in around that region, some of that energy might be higher than -40 dBc (1%).

This seems excessive to me, even for a 6L6. If you haven't already, it would be worthwhile to feed the sound card output back to the input (without the amplifier) and look at this again, just for a sanity check. And be absolutely sure you're not overloading the card with the amplifier. I built a test box for this work, and I always monitor with a scope to be sure the level driving the card isn't too high.

If this measurement turns out to be correct, I'm not sure even adding GNFB will be sufficient. I would consider 6 dB or so to be the maximum to maintain transparency. However, that will only reduce distortion by half, to around 1%. If this was full output, I'd say that's OK, but the amp is only operating at 1W output. So, even with NFB, it will be too high IMO.

Most of the other suggestions I could make for improving this would require a higher primary Z for the OPTs. These transformers are much better suited to an EL34 if you have sufficient filament current capacity. I would also use standard RC decoupling at the driver anode, feed the UL tap only to the output tube screens, and hardwire the output cathode bypass in place.

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