Mystery feedback?

[Richard Neill]

I’ve used several of these devices, and usually the result is perfect, while on the most recent amplifier I’ve tried (which happens to be a 30-year-old design), I hear the whine. When I looked at the output with a scope, (while playing a wav file of silence), I see a large amount of noise: about 200mV pk-pk at 180 kHz or so! I think that the idea here is noise-shaping (the DAC moves all the noise out of band), though I’d have expected the amplitude to be much lower.

Anyway, my *guess* is that when such a signal is fed into most amplifiers, the input is band-limited, and the RF is cut off entirely, resulting in excellent audio. However, some amplifiers don’t have a low-pass RC filter at the input, and the RF goes into some kind of diode junction (anything non-linear), which will cause mixing/aliasing of the RF down to ~ 1kHz, resulting in the observed whine.

I’m going to experiment with a simple RC filter on the phonos to see if it helps.