This isn't really a thread that starts with informative measurements. It's more of a WTF thread- something weird is happening that I can't explain. People with more clue than me may be able to shed some light. Maybe. Some background. I wanted to characterise the very audible difference between two e-stat amps, driving a pair of SR-303. One of them sounded a lot less "thin" than the other, so I broke out my gonzo measuring rig* and did some measurements. The results were weird- very uneven. I swapped the other amp in, and the results were similar- still very uneven. Like seriously uneven.. This is what I'd been getting all along. I tried controlling for various factors, changing headphones, amps and finally source. The common factor was that whenever I used the Gumby as a source, whatever it was feeding, balanced or SE, I was getting weird voids in the FR. If I used a different source, the FR looked normal. Here's a set of measures that show what's going on: You can see that when I used the 2i2 (Focusrite Scarlett 2i2) as a source, the magenta and yellow traces, they don't have that pronounced set of voids (or possibly spikes). The yellow is the 2i2 driving Sennheiser HD-25 directly, and the magenta is the 2i2 into a Stax SRM-313, driving SR-303. The input device was also the 2i2, but the "direct monitor" was not enabled. When any other combo is used including Gumby->Jot->HD25, Gumby->SRM-313->SR-303 Gumby(bal)->Octave 2->SR-303 (Measurements done in daytime, lots of domestic noise and it's hard to get a seal with Lamdas, so if it looks a bit ragged, just ignore that- it's the big trends we're looking at.) ...I get those weird results. The only factor that seems to need to be present to trigger them is using the Gumby. It looks pretty bad in the measurements, but I can't really hear this behavior. I just generated a 1kHz to 2kHz sine sweep (which covers the most drastic void on the graph) and can't hear any real inconsistencies outside of the usual perceptual frequency/loudness stuff. I tried a sweeps from 30 seconds long down to 5, and couldn't pick anything much out in any of them. So, my question is- what's causing this artifact? I only see it when I use the Gumby as a source. Something odd there. * Just a cheap home-made thing, built to OJNeg's old circuit design. I don't think specifics of its performance matter here, as it produces waveforms that look the way that you'd expect, normally, and doesn't seem to be busted. It's consistent when compared to itself.