OK, I'll drop the audio file idea.
Today I tried bypassing the Sowter SUTs and listening to (and recording the waveforms of) an LP. Had to turn the preamp volume up all the way and there was tube rush, but the LP seemed to sound distortion-free. What’s more, the resulting waveform, though tiny and hard to magnify clearly, looked symmetrical.
Is that a clue? Could the asymmetrical waveforms I’ve been seeing and the distortion I’ve been hearing be caused by the SUTs stepping up the signal from my cartridge too much?
If so, this wouldn’t make sense: I have a cartridge (Hana ML) that puts out 0.4mV into Sowter 1990 SUTs wired 1:10. That should put the input to the Eros circuitry at the low end of its 4-6mV target range. And yet when I’ve recorded music from LPs in the past few days, the signal into my digital interface has clipped a few times. That means the Eros is putting out a signal somewhat hotter than line level, yes The badly distorted mass strings I heard the other day looked to measure 9v p-p on the oscilloscope. From what I understand, that shouldn't be too hot an output signal from the Eros.
As far as I can tell, my Eros is wired correctly. The oscilloscope measurements of my RIAA network now read as expected. I’ve checked and rechecked my wiring of the Sowters (and I double-checked that they are indeed 1990s). And yet could the AC signal into/out of the EF86s be too high? Is there a wiring mistake associated with the 6922 tube that would explain my symptoms?
I’ve swapped out all of the tubes and that makes no difference. Changed interconnects. Surely a bad AC cable couldn't cause this. Right?
I’ve been reading up on asymmetrical waveforms and, from what I’ve gleaned, DC offset can be the source but when it is the mean amplitude will be displaced from the central axis. That doesn’t seem to be the case with my asymmetrical waveforms, so I’m thinking DC offset is not behind my problem. From what I remember observing my oscilloscope while I play music through the Eros, my mean amplitude is typically –1 volt or so. And the waveforms from music I’ve recorded on Vinyl Studio look on-axis.
If I correctly understand what I’ve been reading, an asymmetrical AC waveform where the mean amplitude is on-axis (or close, as in my case), the source of the asymmetry can be high pass filters or “aggressive low-end processing.”
Is it conceivable that I’ve somehow inadvertently created a high pass filter in both channels of my Eros? What kind of miswiring would create such a high pass filter?
This is the most puzzling and frustrating problem I’ve faced in 20-25 years of tube DIY builds. And I’ve done dozens.
