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Thread: 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

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    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.

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    John R. Frondelli
    dBm Pro Audio Services, New York, NY

    "Mediocre is the new 'Good' "

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    Quote 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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    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...
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    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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    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.
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    Lifetime Member Enzo's Avatar
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    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.

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    Quote 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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    Old Timer tedmich's Avatar
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    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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    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.....

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    John R. Frondelli
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    Old Timer J M Fahey's Avatar
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    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.

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    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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    Old Timer J M Fahey's Avatar
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    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 !!!!

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    Old Timer tedmich's Avatar
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    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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    1) I also thought so, but was mistaken.
    Although they are more sensitive to yellow-green (about the same as us), red leds work much better (just test it yourself)
    The explanation is counter-intuitive but logical: our sight is much less sensitive to red than to yellow-green, so red leds are *heavily* doped to be seen at all, and high efficiency ones even much more.
    They swamp the LDR with photons.
    Hands-on experiment supports theory.
    2) I live in Buenos Aires, Argentina, and pay 0.25U$S each, which qualifies as pennies to me , compared to any Vactrol or whatever.
    I buy 200 unit bags straight from the importer/wholesaler.
    I attach a sheet from his catalog.
    He is no Mouser, he sells only to shops or factories.
    In theory they shouldn't sell *me*, but after 41 years in one trade, one gets to know everybody, sort of.


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    Different opto's have different specs, resistances, characteristic curves etc etc. they are not all the same~!

    I have figured out that the Behringer amp is a ripoff of the Ampeg design used in the BA-115 and BA-210 type amps. they Ampeg schematic calls out for a Vactec VTL5C7 opto which has a very specific curve to it. I suspect that the Behringer optos do not follow this same curve and this allows the amp to clip and blow up. I would bet repairing the modules with the correct VTL5C7 opto would make them work better!

    I don't have at 5C7's on had but i have a 5C4 which doesn't quite match. it has a quicker response which may or may not be good. I will try it. and now that i know what the original part was supposed to be. I will purchase some and give them a try.

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    Old Timer J M Fahey's Avatar
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    Hi zero cool.
    You can get the real stuff here:
    Vactec VTL5C7 :: Optocouplers :: Optoelectronics :: Electronic Parts :: Banzai Music
    Good luck.

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    The 5C4 works but i discovered that what i was seeing is actually 2 different problems! the opto IS limiting the output of the amp, the clipping however is the front end clipping. I put a pot in parallel with the output of the opto so i could experiment with reducing the gain farther then the opto was bringing it down. and what i found is that while the output LEVEL would drop. it was still clipped. So something in the front end was clipping at that point. otherwise further reducing the gain should have brought the output stage out of clipping.

    I will need to study both sets of schematics to see whats going on. but the 5C4 seemed to work ok. better than the stock(bad) opto that I attempted to fix. and I would be curious to compare to the 5C7


    Zc

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    I just tried a VTL5C3 in one of these and looking at the output it limits nicely at around 32 volts.

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    Don't forget the joker g1's Avatar
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    Quote Originally Posted by Davey View Post
    I just tried a VTL5C3 in one of these and looking at the output it limits nicely at around 32 volts.
    Curious as to whether this was a NOS part, or one of the modern repro's like xvive?

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    Quote Originally Posted by g1 View Post
    Curious as to whether this was a NOS part, or one of the modern repro's like xvive?
    It was an Xvive Enzo.

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    Well, I couldn't think of a more deserving guy to have a part named after him.

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    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

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    Lifetime Member Enzo's Avatar
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    You know, not far from my old farm house was a road side rest stop, basically a toilet in an outhouse with some parking. It was named for someone, like the J. Fred Muggs rest stop. I forget the real name. I used to wonder if that was really much of an honor to have a roadside toilet named after you.

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