amplifier KA-5090R - strange circuit in the Kenwood KA-5090R amplifier - Repair

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I'm repairing Kenwood KA-5090R
It has on one DC channel at the output ...
Interestingly, it seems that the transistors in the OK and DC path are at the output - a topic for further analysis
But this is still a small problem, because there are two operational amplifiers IC7 and IC8 in the feedback circuit.
But the circuit is strange, the amp outputs are (cascaded) shorted to ground! What is this amp supposed to do? linearize gain? but how?
I don't get it ....   
Anyone seen something like this?   

[bob91343]

It's almost as if those are circuit buffers to measure some point.  Maybe an artifical test point.  I don't know.

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This is not a virtual mass (there are such suggestions)
Or rather, it is not intended to reduce the supply voltage of operational amplifiers

[coromonadalix]

Could be a constant current sink

Since they are in the first differential tansistors pairs circuit on the amp input ....

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No .... And do you see the connection somewhere?

[m k]

I'm with Bob.

Maybe it's factory Idle Adj zero check.

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How is it regulated?
Because that's the problem that this amplifier is not connected anywhere (its output)

[m k]

It's a comparator.

Output is that resistor connected to its output.

[max-bit]

Can you explain how it works in this amp?

[m k]

It must be filtering the power or balancing it somehow.
Still, never seen any of its kind.

Upper pair.
When pin 5 goes a bit away from GND the OpAmp pin 7 flips.
Then its partner flips and pin 1 starts heating R179.
When power is on that is pretty irrelevant.
(typical input resistance of NJM4565 is 5M)
But when power is turned off the situation changes, pin 5 is 270/75k of L-Out.
Maybe then the circuit starts doing its thing.

[coromonadalix]

Check carefully the circuit  how its connected,   and  you seem  kinda close minded  to answer rapidly without checking every possibility

It is not a virtual ground, and a line goes to the muting section ...  hum  could be the mentioned 30db attenuator ??


The Ka-5010 and the KA-5020  doesn't have this attenuator and you can see the amplifier differences in the amp inputs section

[m k]

I don't get it.

Be a teacher and explain.

[floobydust]

The circuit looks like it's trying to cancel ground-currents on the PC board.  Samples R205 and inverts and drives that into ground with R179, both are 270R.
Sometimes that is a marketing gimmick, "distortion cancelling" or some wild name given to it. Does the user manual mention anything revolutionary lol.

What is strange though, the DC feedback looks +ve. R155 is also seeing output DC and some AC signal.

[max-bit]

Perhaps ... this is it.
In fact, the amplifier has quite low noise. But I haven't experimented with desoldering these (IC7. op-amps.
Interestingly, the C143 220uF / 35V capacitor was damaged (breakdown) and this caused the appearance of DC voltage on the Left channel.