preamp output stage resistor from output to V- ? ? - diyAudio
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Old 19th October 2007, 01:59 PM   #1
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Default preamp output stage resistor from output to V- ? ?

Hello

opamps can be biased into classA , does it make any sense with a discrete output stage of a preamp ?

Bias is 7,5 mA, rails are 38V and I started with 10k, how low should I go ?
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Old 19th October 2007, 02:06 PM   #2
OliverD is offline OliverD  Germany
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It doesn't make any sense without knowing what the final stage looks like. It might be easier to modify the stage instead of adding an external current sink or resistor.
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Old 19th October 2007, 02:13 PM   #3
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It is a Pioneer C21, rather old with output coupling caps which I would like to bypass but there is no DC servo and there is 1V DC on both channels.
I have no manual but will draw a schematic from following the traces.
It should be possible to add DC offset adjustment, must not be 100% stable, some mV is ok.
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Old 19th October 2007, 02:23 PM   #4
OliverD is offline OliverD  Germany
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No problem to add a DC servo. If done properly, it will sound better than the cap.
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Old 19th October 2007, 02:55 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
a discrete output stage could already be operating in ClassA. If it is, then it does not need any extra loading to shut down half the stage and force it into ClassA.

Post the schematic with operating currents or operating voltages. We can calculate the lowest load value that keeps the stage in ClassA and from there make sensible recommendations, instead of flying blind.
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