Marantz CD94 DAC circuit question

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I am about to modify my Marantz CD94 (not MkII). I already bypassed the SAA7320B/P and installed DIP sockets for the opamps. I just quick replaced the NE5534s to OPA134s for now. The next step will be to get rid of the second opamp, that is an unity gain filter. For the I/V opamp I intend to use OPA627A and I want to try biasing it to Class A. Studying the schematics I came across something interesting: there is a resistor at the output of the DAC. It is connected to an +5.6V Zener diode through some resistors, so I thought it was for supplying the 2mA idle bias to the TDA1541A chip. However, the calculated current is only about 0.6mA. What could be the real purpose of these resistors? I attach the part of the DAC that I am talking about.
 

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Extreme_Boky

Member
2003-12-07 11:57 am
To bias DAC out and keep (close to) 0 V DC at OP amp inputs.

The unity gain filter is also a buffer.... you may need it...

OPA627A output stage is ALREADY biased in class A. What you intend to do will shift the output symmetry and introduce uneven clipping...

Cheers,
Boky
 
Extreme_Boky said:
To bias DAC out and keep (close to) 0 V DC at OP amp inputs.

The unity gain filter is also a buffer.... you may need it...

OPA627A output stage is ALREADY biased in class A. What you intend to do will shift the output symmetry and introduce uneven clipping...

Cheers,
Boky
Shouldn't that be 2mA? That would need about 2.8k from the 5.6V zener voltage, or a FET CCS. In the actual circuit there is 2.7k + (10uF capacitor to GND) + 4.7k + (2 x 1.21k) = 9.82k. This makes 0.57mA. Also part of the output AC current flows through the 2.7k resistor, which seems a waste. Not to mention the 10uF capacitor that comes into the picture at low frequencies.

I read about biasing the OPA627 in class A in another thread and there was a consesus that it is a good thing to do:

http://www.diyaudio.com/forums/showthread.php?threadid=38984
 
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