MOSFET Amps using opto's

Discussion in 'General Electronics Chat' started by R!f@@, Jan 3, 2011.

  1. R!f@@

    Thread Starter AAC Fanatic!

    Apr 2, 2009
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    I am currently fighting an amp that has a MOSFET output stage.
    A Professional Music Power amp used by Music bands.
    This thing is a Chinese design, this I can tell pretty easily.

    I am having a hard time finding a fault.
    I did fix it once, but the owner took the cover and did some thing, what I dunno, he called and said fire and smoke came out form the board.

    Previously a SMD resistor was blown in the driver stage. This time another resistor was blown, replacing it is causing another to heat up.

    The topology is AB, since it uses MOSFET pairs at the output. But they are rather small for a professional grade Power amp. I mean it uses the IRF640. TO-220 packages.
    Funny thing is, this thing uses 3 opto couplers with in the driver and pre-drive stage. I have never seen any one using opto's in an audio amp.

    Does any one have any schema's of this kind, that I might refer to.....

    Problem is that this bloody thing is driving me nuts. I just can't seem to find a faulty component. I have been at it non stop for more than a night...
    I do have pictures, if any one likes to see it, just ask..Since I am fighting this thing right now I cannot edit and upload the pics.

    The reason for asking this is a funny one.
    Now I know how opto's work. But this one is a dooozy....
    I measured 18V across the input of one in circuit, which lead me to believe that the LED would have definitely be blown, so I took it out but the diode check says it's OK.

    What Gives..........:confused:
     
    Last edited: Jan 3, 2011
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
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    He took the cover off, tell him he f*cked it up and it's toast.

    And you wonder why some manufacturers put those warranty is void if seal is broken stickers on things?
     
  3. mik3

    Senior Member

    Feb 4, 2008
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    Some optos, which have a feedback path via a second LED, are used to isolate analog signals since they achieve a good linear response. Maybe this is the purpose of these optos.
     
  4. R!f@@

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    Good linear response....hmmm..

    Amps are suppose to follow the input. The bandwidth is like saying dynamic...
    I don't get it when you say linear in amps.

    It's like a linear amplifier or Logarithmic amplifier...right.
    I understand the difference between a linear pot and a log pot but I cannot get my head around the Linear amplifier part.

    Anyways...the opto's are used in FB path but what for....wouldn't a resistor suffice?

    Do any one have such a diagram I can relate to?
     
  5. kubeek

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    Sep 20, 2005
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    Like this one? :D
     
    shortbus likes this.
  6. mik3

    Senior Member

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    Have you got a circuit diagram?
     
  7. R!f@@

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    No such luck.

    If I had, I would have posted on how to repair it....no?
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Linearity is a basic requirement for audio amplification. A non-linear amplifier generates distortion, generally unwanted for high-fidelity amplification. By contrast, a controlled degree of non-linearity may be useful for some applications, such as guitar amplifiers where a particular sound quality is obtained at high drive levels.

    Opto-couplers are often used to transfer signals between circuits at very different voltage levels, where a direct connection is impossible or inconvenient. Sometimes this is necessary for electrical safety.
     
  9. t06afre

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    May 11, 2009
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    What kind of optos are used?
     
  10. R!f@@

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    Apr 2, 2009
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    @to6
    Standard Opto's.

    The Ajuster saying is true I think. Since this one uses two Supplies. One for the Output and another to boost the driver stage by 10V from both rails. The 10V adds up with the main ±70V
     
  11. bertus

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    Apr 5, 2008
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  12. R!f@@

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    I don't think it's class D.

    I say this cause this circuitry does not have complicated switching circuits. No opamps what so ever. Just SMD transistors, opto's, couple of MJE340,350's and IRF460,9640's.
    and one gigantic toroid, but tiny smoothing caps. 5000μf at most per rail. :eek:

    The best part is, there are vacant space marked for lotta smoothing caps but above is what this has.
     
  13. R!f@@

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    Well ! after going totally bonkers, with this thing I decided to bring the owner to find out where he saw smoke coming out.


    He showed some where near a connector. Decided to do what I did for the fact that all the resistors that goes up in smoke has a track going to that area. Took all the components covering tht area, nothing.

    Second took out the connector holder, covering the tracks in that area. guess what. A track arcing to a Voltage pin of the connector. No wonder I did not find any bad component.

    Three nights went by just to find a scorched pcb area.

    Scraped the area, jump the trace and good as new.

    Sometimes I hate these Chinese products. :mad:
     
  14. dmfraser

    New Member

    Jan 12, 2011
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    In AC line operated amps optos are sometimes used in the current limiting circuits where they want to sense a floating voltage to get the amp to shut down.

    As for fixing this thing, first you need to look for parts that smoked. If you dare to fire it up again, have a 100W to 200W incandescent light bulb wired in series so it does not blow up again when you think you have fixed it.

    To be honest with you, unless you have tons of experience on fixing pro amplifiers without a schematic, you are best off telling the guy it cannot be fixed and you want $90 for looking at it. When he objects offer to keep the amp (because the chassis might come in handy).

    Because really when these Chinese made amps blow they can rarely be repaired again so they last and if you get it working, chances are you will be fixing it again in a month. Both he and you are better off if he just buys another one.
     
  15. R!f@@

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    Sorry to disappoint you but I have finished it as you can see from the post 13.
    It's good as new. It blew second time cause the customer took out the cover to do who knows what.
     
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