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In Reply to: RE: Is there any real reason a DAChas not been madewith a 0.5 volt out instead of 2 or 3 volts out.??? posted by Elizabeth on May 15, 2012 at 09:42:30
Seems to me it was designed to be 2-volts out to match all the other line level sources. 0.5 volts would seem to necessitate a reduction in output voltage as most transistor circuits require a minimum voltage of around 0.7 volts to operate the transistor junctions.
Therefore, phono stages should have enough flexibility to also attain the nominal output level, even though that voltage is mostly attenuated in the line-level stage.
It is easy to design a lower output voltage (we're talking about a AC signal, not a DC one) - if you needed to use a diode in the signal path, you would use AC to send the AC Zero into the linear part of the curve and then have the ausio signal ride on that. So any voltage right down to zero is easy.
However, most circuits have noise that does not change - and in a good DAC, you want to maintain -120dB from the peak - which means it has to be darn quiet and dropping the signal out by 6dB doesn't help!
"Knowledge is knowing that a tomato is a fruit. Wisdom is knowing not to put it in a fruit salad"
I recall 2 volts being part of the original Red Book CD spec also.
All audio transistor circuits are biased well above the point where the the junction is forward biased or there would be severe distortion.
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