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How does ultrasonic noise affect the sound quality?

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#1
Except for the NOS mode, the DACs always apply a digital low-pass filter to clean the sound, especially for the DSD playback. Does that mean ultrasonic noise is audible?
Rob Watts from Chord claims that RF noise makes sound warm and bright. Is it the truth or another hype?
My personal experience is that a noisy amplifier does sound warm and bright, but such noise is not solely ultrasonic noise.
I see amirm posts ultrasonic performance for power amplifiers but not for DACs or headphone amplifiers. I'd like to know the reason.
 

DonH56

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#2
"Warm and bright"? Hmmm... In any event RF noise should only affect the sound if something in the signal chain is not properly rejecting it. If it is making significant audible changes I'd guess it is being mixed or rectified someplace and converted down to the audio (audible) range.

IMO the main reason for testing the ultrasonic performance of an amplifier is to makes sure it does not act up when presented with such an input, either from a wideband analog source, higher sampling rate DAC with high filter cutoff, or insufficiently suppressed DAC images.

FWIWFM - Don
 

tuga

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#3
"Warm and bright"? Hmmm... In any event RF noise should only affect the sound if something in the signal chain is not properly rejecting it. If it is making significant audible changes I'd guess it is being mixed or rectified someplace and converted down to the audio (audible) range.

IMO the main reason for testing the ultrasonic performance of an amplifier is to makes sure it does not act up when presented with such an input, either from a wideband analog source, higher sampling rate DAC with high filter cutoff, or insufficiently suppressed DAC images.

FWIWFM - Don

Isn't there a chance that noise-shapping (i.e. DSD) and wide-bandwidth amplifiers (i.e. Spectral) and hard-dome tweeters with 10 or 20dB peaks at 25 or 30kHz (i.e. BnW CM5) might act up? (extreme examples)
 

DonH56

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#4
Isn't there a chance that noise-shapping (i.e. DSD) and wide-bandwidth amplifiers (i.e. Spectral) and hard-dome tweeters with 10 or 20dB peaks at 25 or 30kHz (i.e. BnW CM5) might act up? (extreme examples)
Not that I can see. Noise shaping pushes the noise band well above that, and then it is filtered, so little energy reaches the output above the audio band. Noise shaping produces a rising noise response, true, but there is still a low-pass filter that rolls off the noise. DSD also requires an anti-image filter, just at a higher frequency, but again that higher frequency is well below the noise hump. Ultrasonic energy should be in the mud for any competently-designed DAC, and Amir's wideband measurements seem to agree.

I'd be more worried about the images from non-filtered "NOS" DACs that produce wideband tones and noise. Too much marketing, too much ignoring of fundamental engineering, in my mind.
 
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Not that I can see. Noise shaping pushes the noise band well above that, and then it is filtered, so little energy reaches the output above the audio band. Noise shaping produces a rising noise response, true, but there is still a low-pass filter that rolls off the noise. DSD also requires an anti-image filter, just at a higher frequency, but again that higher frequency is well below the noise hump. Ultrasonic energy should be in the mud for any competently-designed DAC, and Amir's wideband measurements seem to agree.

I'd be more worried about the images from non-filtered "NOS" DACs that produce wideband tones and noise. Too much marketing, too much ignoring of fundamental engineering, in my mind.
NOS means no digital filter but still could have an analog low-pass filter as I guess?
 
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