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Enzo: Behringer Optocoupler info?

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  • Enzo: Behringer Optocoupler info?

    I am repairing a Behringer LPA1180 power amp module that uses an optocoupler in a neat lil anti clipping circuit. there are no markings on the opto and it is definitely dead.

    Any information or a part number for these opto's? looks to be LED/Cell based.


    Zc

  • #2
    FYI- if that gives you a huge headache, we just placed an order for those. Don't know the price (probably pretty cheap) or when we will get them, but we will have them in stock.
    John R. Frondelli
    dBm Pro Audio Services, New York, NY

    "Mediocre is the new 'Good' "

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    • #3
      Originally posted by jrfrond View Post
      FYI- if that gives you a huge headache, we just placed an order for those. Don't know the price (probably pretty cheap) or when we will get them, but we will have them in stock.

      Excellent! I looked at your website first but didn't see them listed and didn't want to bug you about them without knowing a part number. PLEASE let me know when they come in etc.

      I have several of these LPA1180 boards and all have the same failure. one mosfet dies and blows 2 resistors and the opto...or maybe the opto fails and allowing the amp to clip and this is why they are failing im not sure.

      Zc

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      • #4
        Lets do a little reverse engineering.

        here is the clip limiter circuit. the speaker output from the amp is fed into D13/D15. The Voltage dividers of R15/R19 and R16/R17 create a point approx 30V. assuming the opto has a Fv of 1.2V, when the output of the amp reaches approx 31.8V (30v+ .6V drop of D14 or D16 and LED Fv of 1.2V) the LED starts to conduct. at some point the Cell resistance drops below the 15K feedback resistor value lowering the gain.

        So, How much gain drop do we need? 6db?

        Looking at the Siliconix optos:
        Silonex Inc.: Products: Audiohm Optocouplers

        I see there is quite a range of ON resistances available. I guess what would be needed would be to find an opto whose on resistance when in parallel with the 15K feedback resistor R22 would reduce the stage gain by some amount to keep the amp out of clipping even if the input keeps getting louder...at least to a point.

        I am guessing at all this...
        Attached Files

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        • #5
          thinking about this some more. the lower the ON resistance the faster the gain reduction. but how much is too much?

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          • #6
            I did some testing today. one after examining the optos closer i learned one end has round legs and one end has square legs like a regular LED. So I took a dremel tool and carefully ground down the end with the square legs until i was able to remove the LED. I connected the LED to a AA battery and after flipping around the leads to get the polarity right the LED lit up. its a normal red LED with a clear lens.

            BUT...after carefully examining it side by side with a regular LED i learned that the polarity is reversed on these "bad" opto's.

            If you look at the attached picture. the -side or the cathode has the larger internal structure in the LED. I took a standard red LED and connected it up per the diagram and it did in fact light up so the diagram is correct but on the bad opto's If you connect it as a conventional LED as this diagram depicts it will not light. but reverse the polarity and the opto LED lights up just fine.

            SO, could an over voltage on the LED cause polarity reversal? for giggles I reassembled the opto with the LED reversed and put it back in circuit and tested it. there is just enough of the end of the LED hanging out now that i can see when the LED starts to light up and I can now see that the gain reduction circuit is kicking in!

            So either the LED's in the optos are bad from the factory or they go bad after an amp failure. or the go bad flipping polarity CAUSING amp failure!

            I measured the ON resistance of the 2 opto's i have on hand. I got 1.2K for the rebuilt opto but the end is not sealed so that may not be correct. for the other opto i measure 500 ohms which i think is more accurate. Both opto's have the polarity reversed but both came from amps that had blown outputs so neither may be right.

            off resistance is higher then my fluke 189 can measure so its way up in the megaohm range.

            When JR gets some opto's in i will order some and do some further testing.
            Attached Files

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            • #7
              I "think" this part here would be a good replacement....


              Silonex - NSL-32SR3 - Optocouplers / Optoisolators - Optoelectronics & Lighting - Allied Electronics

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              • #8
                Looks like the opto is home grown at Behr. The amp part list calls it as a subassembly then lists the 3mm LED as W03335RSC-6S, and the optosensor as a KE-50910. They also list the plastic tubing and the two kinds of epoxy they built it with, black and clear, not to mention even a particular nozzle to squirt the epoxy with.
                Education is what you're left with after you have forgotten what you have learned.

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                • #9
                  Originally posted by Enzo View Post
                  Looks like the opto is home grown at Behr. The amp part list calls it as a subassembly then lists the 3mm LED as W03335RSC-6S, and the optosensor as a KE-50910. They also list the plastic tubing and the two kinds of epoxy they built it with, black and clear, not to mention even a particular nozzle to squirt the epoxy with.
                  Thanks Enzo!

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                  • #10
                    Behringer went lowball and ate the labor cost; fairly typical when you consider many Mfg. have to shell for PE VTL5C1's for $6.49 each at Allied, although only $3.73 at >500 lots.



                    Fill me in here; why do people still use the LED/CdS optoicouplers? Since few (any?) are actually driving the LEDs with a true dimmer/PWM circuit, can't a decent opto/MOS SS relay work here? Hell you could even include a series resistor if you need the ON state resistance closer to the CdS cell.

                    Sharp/Clare/Panasonic SS relays are DIRT cheap and can handle +300v without breaking a sweat. The eastern europe boards are full of SS alternatives and DIY takes on the spendy PE units...

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                    • #11
                      CdS photocells have some particularly desirable characteristics in audio circuits, as well as (if you ask me) better reliability than the VacTec stuff.

                      I went and looked at the Behringer BOM just like Enzo did, and yes, it's a homebrew! Whatever.....
                      John R. Frondelli
                      dBm Pro Audio Services, New York, NY

                      "Mediocre is the new 'Good' "

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                      • #12
                        Well, I've never bought a Vactec or whatever opto-ldr, given that making one is so cheap and easy.
                        I buy 200 unit bags of 5mm LDRs for pennies, I guess they are used as an automatic brightness or contrast control in TVs or Monitors, plus 5mm extra bright diffuse red Leds plus 8 mm 5/16"? shrink tube.
                        I usually make about 10 in a couple minutes and leave them close by for future use.
                        I don't even epoxy or hot glue them any more.
                        They provide the muscle in my Bass and Keyboard amplifiers compression/limiting.
                        They would work very well on this particular amp .
                        Ah!! In that schematic the voltage drop is (2x0.7V)+1.9V=3.3V plus any extra voltage drop through the resistors to get at least a couple mA through the Led.
                        So in practice the voltage that's left to account for the Vce(sat) loss across the power and driver transistors plus the Vbe across those bases means a grand total of around 5 V, quite a tight margin.
                        These circuits in practice are best roughly designed and then bench-adjusted for the final touch.
                        I think these Leds may have been factory mounted backwards by mistake; I've seen a lot of dead Leds (open rather than shorted) but never an inverted polarity one because of overload.
                        Juan Manuel Fahey

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                        • #13
                          I have the amp working with the LED reversed but it does not limit very well. i suspect that the LED isn't as bright now. So now im curious and if/when i can buy an original Behringer part I will do some testing. this may be a large part of why these amps blow up...well besides the fact that its a Behringer built amp LOL!

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                          • #14
                            Hi zerocool
                            *Does* the Led light up now?
                            Did it when inverted?
                            One of both possibilities can't be.
                            You can test the optocoupoler by passing, say, 5 mA through the Led and measuring the LDR resistance, that will show you how much can it limit.
                            If it's in parallel with 10K and the LDR gets down to only 10K, it can compress 6dB, not much.
                            If it gets down to 1K, it may compress 20dB (a lot).
                            That's what I get with my homemade Optocouplers.
                            I use very bright, efficient red Leds.
                            Make one yourself !!!!
                            Juan Manuel Fahey

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                            • #15
                              I thought CdS LDRs were best with yellow green/ LEDs while the CdSe used red?

                              LDR sure aren't "pennies a bag" on the ebay; where do you get them?

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