Author Topic: Weird optocoupler in guitar amplifier  (Read 465 times)

Offline TMaxElectronics

  • High Voltage Engineer
  • ****
  • Posts: 216
  • Karma: +12/-0
    • View Profile
    • My random (and very empty) electronics blog
Weird optocoupler in guitar amplifier
« on: January 20, 2021, 07:02:04 PM »
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 :P).

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 ;)

Offline davekni

  • High Voltage Expert
  • ******
  • Posts: 1293
  • Karma: +62/-0
  • Physicist, engineer (electronic), and hobbiest
    • View Profile
Re: Weird optocoupler in guitar amplifier
« Reply #1 on: January 20, 2021, 08:24:54 PM »
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.
David Knierim

Offline TMaxElectronics

  • High Voltage Engineer
  • ****
  • Posts: 216
  • Karma: +12/-0
    • View Profile
    • My random (and very empty) electronics blog
Re: Weird optocoupler in guitar amplifier
« Reply #2 on: January 20, 2021, 11:19:41 PM »
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 :P

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.

Offline davekni

  • High Voltage Expert
  • ******
  • Posts: 1293
  • Karma: +62/-0
  • Physicist, engineer (electronic), and hobbiest
    • View Profile
Re: Weird optocoupler in guitar amplifier
« Reply #3 on: January 20, 2021, 11:51:48 PM »
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.
David Knierim

High Voltage Forum

Re: Weird optocoupler in guitar amplifier
« Reply #3 on: January 20, 2021, 11:51:48 PM »

 


* Recent Topics and Posts

post prodc9 interrupter plexi case
[Dual Resonant Solid State Tesla coils (DRSSTC)]
costas_p
September 24, 2021, 07:14:53 PM
post Re: Idea for QCW DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Uspring
September 24, 2021, 05:37:56 PM
post Re: Tesla plate
[General Chat]
Benbmw
September 24, 2021, 05:20:01 PM
post Re: Idea for QCW DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Netzpfuscher
September 24, 2021, 07:26:30 AM
post Tesla plate
[General Chat]
Duane B
September 24, 2021, 01:54:49 AM
post Re: Idea for QCW DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
September 23, 2021, 09:08:23 PM
post Re: Idea for QCW DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Uspring
September 23, 2021, 07:09:23 PM
post Re: Idea for QCW DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
September 23, 2021, 05:46:38 PM
post Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
[Solid State Tesla Coils (SSTC)]
AFreshLad
September 23, 2021, 11:12:25 AM
post Re: Idea for QCW DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Netzpfuscher
September 23, 2021, 08:49:51 AM
post Re: Idea for QCW DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
September 23, 2021, 06:15:34 AM
post Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
[Solid State Tesla Coils (SSTC)]
davekni
September 23, 2021, 05:48:40 AM
post Re: Idea for QCW DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Uspring
September 22, 2021, 07:21:12 PM
post Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
[Solid State Tesla Coils (SSTC)]
AFreshLad
September 22, 2021, 10:41:20 AM
post Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
[Solid State Tesla Coils (SSTC)]
davekni
September 22, 2021, 07:07:48 AM
post Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
[Solid State Tesla Coils (SSTC)]
AFreshLad
September 22, 2021, 05:26:40 AM
post Re: Measuring TC secondary parameters
[General Chat]
MRMILSTAR
September 22, 2021, 05:14:30 AM
post Measuring TC secondary parameters
[General Chat]
Duane B
September 22, 2021, 05:01:22 AM
post Re: Load resistance and discharge terminal for my 14-stage CW multiplier
[Voltage Multipliers]
MRMILSTAR
September 21, 2021, 10:33:34 PM
post Re: Futurist's DRSSTC 0.5
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Mads Barnkob
September 21, 2021, 07:21:50 PM
post Re: Load resistance and discharge terminal for my 14-stage CW multiplier
[Voltage Multipliers]
klugesmith
September 21, 2021, 05:24:35 PM
post Re: BrOdin coil - High power Big sparks!
[Dual Resonant Solid State Tesla coils (DRSSTC)]
futurist
September 21, 2021, 04:18:47 PM
post Load resistance and discharge terminal for my 14-stage CW multiplier
[Voltage Multipliers]
MRMILSTAR
September 21, 2021, 06:30:44 AM
post Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
[Solid State Tesla Coils (SSTC)]
davekni
September 21, 2021, 06:04:52 AM
post Re: BrOdin coil - High power Big sparks!
[Dual Resonant Solid State Tesla coils (DRSSTC)]
fh89
September 21, 2021, 03:56:28 AM
post Re: Idea for QCW DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
johnnyzoo
September 20, 2021, 09:24:10 PM
post Re: BrOdin coil - High power Big sparks!
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Uspring
September 20, 2021, 05:07:43 PM
post Re: Large Rectifier Tube
[General Chat]
Benbmw
September 20, 2021, 04:12:27 PM
post Re: Proper phase lead adjustment at the input
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Chrisader
September 20, 2021, 10:40:33 AM
post Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
[Solid State Tesla Coils (SSTC)]
AFreshLad
September 20, 2021, 10:39:59 AM
post Re: BrOdin coil - High power Big sparks!
[Dual Resonant Solid State Tesla coils (DRSSTC)]
futurist
September 20, 2021, 10:38:31 AM
post Re: Large Rectifier Tube
[General Chat]
klugesmith
September 20, 2021, 07:07:55 AM
post Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
[Solid State Tesla Coils (SSTC)]
AFreshLad
September 20, 2021, 06:03:57 AM
post Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
[Solid State Tesla Coils (SSTC)]
davekni
September 20, 2021, 05:31:57 AM
post Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
[Solid State Tesla Coils (SSTC)]
AFreshLad
September 20, 2021, 04:59:17 AM
post Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
[Solid State Tesla Coils (SSTC)]
AFreshLad
September 20, 2021, 04:28:03 AM
post Re: Idea for QCW DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
September 20, 2021, 12:53:39 AM
post Re: Idea for QCW DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
johnnyzoo
September 19, 2021, 11:09:37 PM
post Re: Idea for QCW DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
AstRii
September 19, 2021, 09:47:14 PM
post Re: Idea for QCW DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
September 19, 2021, 08:28:05 PM
post Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
[Solid State Tesla Coils (SSTC)]
davekni
September 19, 2021, 08:19:01 PM
post Idea for QCW DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
AstRii
September 19, 2021, 07:58:09 PM
post Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
[Solid State Tesla Coils (SSTC)]
AFreshLad
September 19, 2021, 03:16:40 PM
post Re: Driverless MOSFET SSTC
[Beginners]
AstRii
September 19, 2021, 01:32:47 PM
post Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
[Solid State Tesla Coils (SSTC)]
AFreshLad
September 19, 2021, 10:15:24 AM
post Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
[Solid State Tesla Coils (SSTC)]
Magneticitist
September 19, 2021, 06:32:18 AM
post Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
[Solid State Tesla Coils (SSTC)]
davekni
September 19, 2021, 06:06:44 AM
post Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
[Solid State Tesla Coils (SSTC)]
AFreshLad
September 19, 2021, 04:14:18 AM
post Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
[Solid State Tesla Coils (SSTC)]
Magneticitist
September 18, 2021, 11:33:37 PM
post Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
[Solid State Tesla Coils (SSTC)]
AstRii
September 18, 2021, 10:42:45 PM

Sitemap 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 
SimplePortal 2.3.6 © 2008-2014, SimplePortal