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Marantz 1122 DC bias adjustment

Traverso

Traverso

Active Member
Hello,

I am here with a superb integrated from Marantz 1122 dc.
After pots cleaning and relay change, it sounds nicely.

However it runs a bit warm, which is ok but I liked to check offset and bias.
Offset was easy to check and adjust.
What I do not know is the bias current value and how to adjust it.
I expect around 25 mA the right value, to be confirmed, meaning somme 8 to 10 mV on power transistor emitter.

Thanks a lot for the feedback


Alain
 
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I don’t recall there being a bias adjustment for this model definitely the 1180dc has no bias adjustment suggest checking the service manual which would explain the correct procedure if any
 
Thanks a lot for the feedback

I checked the SM but nothing is indicated there for the bias, neither the value nor the how to proceed.
 
There is no potentiometer or rheostat for power amp bias adjustment. If the heat sinks remain warm, but not hot to the touch when the amp is idling (not playing music) then the 'bias' is probably OK.

You could measure the voltage drop across each emitter resistor and then use Ohms Law to calculate the current though each transistor output pair.
The thermisters , Q745 and Q747 for one channel and Q746 and 748 for the other, ultimately control the current flowing through the output transistors. The 300DC operates much the same way.

This may be of help.

Tom
 
Hello Tom,

Thanks a lot for the feedback, my concern is in what range is the value in mA for the bias.

Alain
 
If your amp meets or exceeds the manufacturer's specification for THD then I think the bias current is sufficient. (0.03% at 1kHz, 75W /channel, 8Ω)
My thinking is still: If the heat sinks remain warm, but not hot to the touch when the amp is idling (not playing music) then the 'bias' is probably OK.
Many class AB power amps have idle current in the realm of 25mA per transistor pair. YMMV
Tom
 
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