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Post subject: Mixing resistors AB763 vs Bassman
Posted: Wed May 29, 2019 5:55 pm
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Hi all. Question regarding mixing resistors. The 220K mixing resistors on all AB763 amplifiers follow the coupling caps before the PI, while on all the Bassman amps the mixing resistors are before the coupling cap(s). Is there some sonic reason for this, or does it make any difference at all? Thanks!


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Post subject: Re: Mixing resistors AB763 vs Bassman
Posted: Sun Jun 02, 2019 4:56 am
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I see these resistors after de PI not before.


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Post subject: Re: Mixing resistors AB763 vs Bassman
Posted: Sun Jun 02, 2019 7:51 am
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The mixing resistors on the AB763 are simply to mix AC signals into one.
The mixing resistors on the AA270 Bassman are in the mixed signal domain with AC signals and DC plate voltages present.
These amps were designed to use both inputs at the same time. The cathodes of the channels are tied together to help blend the two channels too.
The differences in the AA270 are to reduce the gain of the Bass channel when it is pushed hard. It does it by lowering the DC plate voltage of the entire network of resistors that feed V1 and V2b and V3 plates.
V3 has a negative feedback loop via the 470K resistor. This lowers the gain of both channels when either channel is driven hard.
V1 and V2a are run by another circuit designed to lower gain. The 27K 1watt resistor coupled to the 8uf lowers the plate voltage by 50 volts. This works as a starvation compressor. When either channel is driven hard, the demand for plate current starves the plate supply. This lowers the output and compresses the signal.
When recapping this 8uF cap, don't increase the value, or the starvation design is rendered useless.


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