Self-loopback of recording interface (TASCAM US-144) -
Except the SNR of TASCAM US-144 is actually around 102-103dB; it is bottlenecked by its analogue out. Strange, I know.
High distortion values, probably from the mic preamp. Fortunately we're only looking at noise today.
Now, an explanation of all the candidates -
- DAC out (no volume control available), software volume control used, input of TASCAM US-144 @ max
- Preamp front headphone connector out, software volume control used, input of TASCAM US-144 @ max
- Preamp at volume required for 1dB, software volume control @ max. TASCAM input @ max
- Preamp @ maximum volume, software @ maximum volume, TASCAM input adjusted to get 1dB
Detailed results here:
Where you put your volume control - RMAA results
As you can see, even though my DAC and it's preamp stage is capable of over 100dB SNR (and possibly bottlenecked by the recording interface), if the input to the preamp is low volume noise gets amplified along with the tiny signal and kills SNR.
The second and third tests are to compare between using software to reduce to volume entering the preamp and using the potentiometer to achieve the same thing. As you can see, there is not much difference. The bottleneck is not the quantization noise that occurs at low digital volumes, but the huge 50Hz hum from the transformer plus other noises.
The first test is to test the typical scenario where one chooses to control volume at the digital source. For this, I put WDM volume @ 25/100 and master @ 6/100 in order to give -1dB with the TASCAM @ max volume. If those scales are linear, I would've lost around 6-7 bits of resolution and added some more noise, but the leftover signal can still theoretically do better than 81dB, which is full of mains hum.
So conclusion - do not output volumes too low from your source, or risk having noises from inside the source dominate.
Unfortunately, sometimes the default volumes of DACs are too loud, I have to put my Windows' volume @ 25/100 and 8-16/100 most of the time, even with my amp @ 9 o'clock.
But does that really matter?
Maybe, not really.
W A R N I N G !
W A R N I N G !
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