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Old 11th March 2007, 12:12 AM   #1
regal is offline regal  United States
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Default sine waves on nos dac

I was running test tones from my computer to a NOS 1543 DAC. Up to 15khz sounded fine. Over that and it skews the output to a lower frequency. So 16khz sounds like ~10khz. The sound card is fine I compared it to a OS DAC which was fine.
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Old 11th March 2007, 02:42 PM   #2
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Curious. Interesting that:
22.05kHz - 16kHz = ~ 6kHz
16 kHz - 6 kHz = 10kHz.

Sum and difference products like this require significant non-linear distortion to produce. However if you have a NOS DAC with a poor to non-existant reconstruction filter you can end up pumping a huge amount of ultrasonic energy into your amp. Energy with all the alias frequencies. Quite possible that some downstream component is cheerfully creating your 10kHz tone from these. Have you looked on an oscilloscope? What happens when you go to 17kHz in? What are you listening with?
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Old 11th March 2007, 05:59 PM   #3
regal is offline regal  United States
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I have no access to a scope. This is a DAC-AH to a diy hybrid tube amp (millett.) Anything over 16khz gets distorted to a lower freqency.
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Old 11th March 2007, 10:13 PM   #4
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Quote:
Originally posted by regal
I have no access to a scope. This is a DAC-AH to a diy hybrid tube amp (millett.) Anything over 16khz gets distorted to a lower freqency.

What type of reconstruction filter do you use? Are you sure you can hear those different frequencies accurately? How about the speaker? Do you have access to a freq counter?

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Old 11th March 2007, 10:34 PM   #5
regal is offline regal  United States
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I'm using headphones and comparing the tone to a OS DAC . The DAC-AH has no filter. It is quite obvious (the skew in tone.)
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Old 11th March 2007, 10:38 PM   #6
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Quote:
Originally posted by regal
I'm using headphones and comparing the tone to a OS DAC . The DAC-AH has no filter. It is quite obvious (the skew in tone.)

Well, if you run that DAC without filter, all bets are off. The digital audio system REQUIRES both a filter before the ADC and after the DAC precisely to avoid aliasing, which is what you have.

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