Preferably straight after the transformer.
Good point. When you play a soft low-frequency sine wave, you should see the noise ride on top of the sine wave when you look with an oscilloscope. If you manage to get it to trigger, or maybe with a one shot measurement, you can estimate the quasi peak-peak noise.
I'm curious if there is any relation with your ranking.
in that sense, I do agree with poly sinc lp - also I used that a long time. I've been asking because in my experience it's a calm and 'natural' filter, so I would chose that with less well tuned systems..
Playing a low level sine wav what I find is that with 3' long cable from dac output to transformer input, at the transformer input it looks just about like the unfiltered output of the dac (which is to say pulses at RF). Coming out of the transformer, scope vertical amplifier noise floor and ambient EMI/RFI dominate what can be seen. Only thing I have seen so far coming out of the transformer is some line level audio when I play music.
Marcel asked for after the transformer.
EDIT: I think the reason for wanting to take a look is to see whether differences reported subjective/discrimination of modulator and or upsampling filter sound could be attributed to, and or correlated with differences in noise visible on a scope. In retrospect, maybe not very likely. The differences found in reproduction of space/sound-stage were pretty subtle in comparison to the 8-bit resolution of a digital scope.
EDIT: I think the reason for wanting to take a look is to see whether differences reported subjective/discrimination of modulator and or upsampling filter sound could be attributed to, and or correlated with differences in noise visible on a scope. In retrospect, maybe not very likely. The differences found in reproduction of space/sound-stage were pretty subtle in comparison to the 8-bit resolution of a digital scope.
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Just tried them myself. The 'long' version sounded a little thin on male vocals in the midrange, and a little off putting in some way (maybe slightly mechanical/unnatural) . OTOH 'xla' is kind of interesting. More musical and unusual clarity of reverberations, would be my initial impression. Would like to have some other people listen to it as well before saying more.
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Any half-decent soundcard is able to measure the out-of-band noise up to fs or even up to 96kHz.
I would expect quantization noise to dominate from a hundred kilohertz or so to a few megahertz, but if I understand you correctly, you see no difference between DAC connected or probe shorted even there. Thanks for trying!