Weird optocoupler in guitar amplifier

[TMaxElectronics]

I just got an old guitar amplifier for repair and noticed that it drew a lot of current, even with the amplifier ICs desoldered (~200mA at 50V AC). Feeling around the circuit board I noticed that there were two optocouplers with resistors next to them that were VERY hot (I still have a little outline of them on my finger ).

I partially reversed the circuitry and it came out to something like this, except that the amp only has one opto which, I suspect, might have a photoresistor to make the thing work in both polarities (LTSpice didn't have any so I just put two transistor ones in parallel):

U1 is an LM3886 in the amplifier.

Now I'm wondering what the hell that circuit may be doing... maybe compensation of some non-linear aspect of speakers? Has anybody ever seen something like this before or got any idea what they might be there for?
It seems very wasteful and risky to run these at such a high power level (according to spice they have 100mA flowing through them), and I'm wondering if they might not be normal optos, and just behave like this because they died together with the driver ICs. The amp is a Behringer ACX1000 and ~20 years old so they might well be something antique

[davekni]

Yes, I expect it is compression, using a ZnSe photo-resistor.  They used to make such opto-resistor devices with incandescent bulbs coupled to ZnSe sensors.  I think I still have one or two in my ancient junk.  In high school (early 1970s) I was intrigued by compression amplifiers.  Designed and built several small ones back then, including one using a ZnSe photo-resistor.  The unit you have appears to be newer, likely an LED coupled to ZnSe photo-resistor.  Otherwise there would be no need for the bridge rectifier feeding the opto.

[TMaxElectronics]

Very interesting. Never expected that in the output stage of the amp, as there is an effects board before that, which has a dsp and stuff on it. Also that circuit has no adjustments or anything on it.
Whenever I used an audio compressor it would be in very specific circumstances and not just ctrl+a -> compress

Thanks for the tip

Oh and regarding the high heat dissipation: is that something I should be concerned about? I guess it worked fine for 20 years and seems to have no issues now (apart from the blown output drivers) so it should be fine right?
Maybe those are still the old school incandescents after all, cant really measure that properly in circuit with those diodes across it, but with the high power draw and heat dissipation I'd expect that any led would be fried in minutes.

[davekni]

I can't say much about the heat.  The little incandescent/ZnSe couplers I have are around 1W or a bit less.  An ohm-meter across the coupler input should tell.  Most meters use low voltage for ohms so that diodes don't conduct.  An LED wouldn't conduct, but an incandescent bulb will.  The diodes make me think it's an LED, in which case it shouldn't be dissipating that much power.