D1 clone input voltage for pcm1794/6 dac? - diyAudio
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Old 2nd April 2015, 09:31 AM   #1
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Default D1 clone input voltage for pcm1794/6 dac?

Hi,
Having read many threads on D1 clones I am now deeply confused. Threads by different highly knowledgeable folk contradict each other on the voltage to set a D1 clone at for pcm 1794 dac.
As the data sheet states "output logic level 2.4v" for 1794, I thought set D1 input to 2.4vbefore connecting. Other posts with tested circuits have set this to 0v for the 1794. Am I reading the data sheet wrong? Please help.
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Old 2nd April 2015, 12:17 PM   #2
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Ideally, all analog signal current output pins of the PCM1794x series DAC chips are set at a quiescent voltage level (that with no A.C. signal) of zero volts by the connected I/V circuit, which in your case is a Pass D1 clone. As a practical matter, the quiescent D.C. voltage can be a few tenths of a volt above zero without significant performance penalty, however, I suggest that you not permit that voltage to be set at a negative level by the D1 even by a little. It would be better to err on the sightly positive side of zero.

Take care in the trimming of the D1 input voltage to zero volts. While the PCM179x series likely features output protection diodes, those wouldn't be able to handle significant offset current being forced through them should the I/V circuit present a significant input offset voltage for long. I recommend including external protection diodes to each analog output pin if you're going to utilize a a manually trimmed D1 circuit, else you may risk damaging the DAC chip. An alternative is to provide a means to readily electrically disconnect the D1 from the DAC chip outputs while the D1 input voltage level is being trimmed to zero volts. After reconnection, the D1 input voltage may require a final small adjustment. While not as safe as including external protection diodes, it would probably be safe enough for DIY application.

That 2.4V logic level spec. has nothing to do with the analog outputs.
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Last edited by Ken Newton; 2nd April 2015 at 12:44 PM.
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Old 2nd April 2015, 03:41 PM   #3
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Wow, thanks Ken.
I now understand perfectly. I really appreciate your sharing your expertise. Looks like I'm not the only person to have misunderstood the data sheet.
Thanks again.
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Old 2nd April 2015, 05:56 PM   #4
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You're welcome.
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