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.
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.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...
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:
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