Audiolab 8000A amp , 1984

[N_Cook]

Anyone familiar with the protection/ op relay cct?
Would seem to monitor the junction of 2 resistors fed from each channel
output.
Pink, Rch, 47K
Purple, Lch, 56K
Why unequal? all inside looks original
Monitors for any sustained DC, clipping or excessive ac or all 3?

The pair of 1M resistors off the outputs are feedback into the amps via IC1

 

[Mark Zacharias]

Anyone familiar with the protection/ op relay cct?
Would seem to monitor the junction of 2 resistors fed from each channel
output.
Pink, Rch, 47K
Purple, Lch, 56K
Why unequal? all inside looks original
Monitors for any sustained DC, clipping or excessive ac or all 3?

The pair of 1M resistors off the outputs are feedback into the amps via
IC1

Can't say this necessarily applies, but Yamaha uses different value sensing
resistors so that in the unlikely event of opposite DC offsets at both amp
channels' outputs, the protection circuit won't be "fooled".
If you had +50 volts at one amp output, and -50 volts at the other, the
protection circuit would not trigger, and you'd have two blown speakers.
Maybe this logic applies here.

Mark Z.

 

[N_Cook]

Can't say this necessarily applies, but Yamaha uses different value sensing
resistors so that in the unlikely event of opposite DC offsets at both amp
channels' outputs, the protection circuit won't be "fooled".
If you had +50 volts at one amp output, and -50 volts at the other, the
protection circuit would not trigger, and you'd have two blown speakers.
Maybe this logic applies here.

Mark Z.

I'd not thought of that, it makes perfect logical sense now

 

[N_Cook]

Can't say this necessarily applies, but Yamaha uses different value sensing
resistors so that in the unlikely event of opposite DC offsets at both amp
channels' outputs, the protection circuit won't be "fooled".
If you had +50 volts at one amp output, and -50 volts at the other, the
protection circuit would not trigger, and you'd have two blown speakers.
Maybe this logic applies here.

Mark Z.

With this amp +/-44V so 4Vdc maximum available in that worse case, 3.1V if
following impedance is 100K and still 1.1V if 10K impedance