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In Reply to: RE: possibly something wrong in power supply . . . posted by dave789 on December 11, 2014 at 03:33:09
Hi,
I would not class anything below around 0.3% H2 and 0.1% H3 at full scale in a DAC as "defect" (in the light of for example the distortion in speakers or microphones), unless it is very frequency selective.
I suspect it could be something in the PSU, but this is less likely as the feedback would likely kill it. My bet is on a coupling cap.
With electrolytic's (and or certain ceramic's) you must keep the AC voltage across the cap very low, or distortion will result. If the cap is too small in value, you will get distortion only at very low frequencies, improving quickly as the frequency rises, even though there is no obvious LF rolloff.
Ciao T
At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to untolerable pain. Really, what more could we need?
Follow Ups:
There is a possibility that the power supply circuit has an active ground drive circuit, and its internal series resistance is high and non-linear due to some defect.
Then, even with the feedback in the output stage, the DAC will show nonlinearity in low frequencies? Just my rough guess.
Hi,
Active ground drive problem, maybe. I doubt it, but we are going now deep into design details. My money is on coupling Caps...
Ciao T
At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to untolerable pain. Really, what more could we need?
The harmonic distortion and IMD is low in mid and high frequencies.See the 19kHz + 20kHz IMD measurement (Pierre Verany CD track 51). The 1kHz ( 20 - 19 = 1) distortion component is below the noise level of the Behringer A/D converter. Such a low distortion is expected in the frequency range 0Hz - 20kHz from PCM1792, OPA627, and OPA604. (Ignore the spikes below 300Hz in the graph. They are noise on the A/D side.)
The roll off at 20Hz (ref. 500Hz) into about 9kOhm load is approximately 0.4 dB.
Anyway, I think the manufacturer should be able to say whether the high harmonic distortion at low frequencies is within their spec or not. They refuse to say anything like that.
Edits: 12/11/14
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