Possible to modify a sub amp this way?

Discussion in 'Home Theater Projects' started by Anthony_I, Nov 11, 2003.

  1. Anthony_I

    Anthony_I Stunt Coordinator

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    is it possible (probably since it can be done by request) to modify a rythmik audio plate amp with MOSFETs instead of bipolar transistors and to upgrade the opamps??

    the reason i ask about the opamps is because i have a 12V power tap on my amp and it is only 100mA MAX. I wanted to be able to get more power out of it (more mA) and its the opamps that will burn out if i draw too much off of them. Also what other effects would i notice from having upgraded opamps?

    Thanks
     
  2. Michael R Price

    Michael R Price Screenwriter

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    If it's a subwoofer amp I wouldn't worry about changing the output transistors, even if you did it would probably require other modifications to the circuit. I wouldn't want to put in that much work. [​IMG]

    I don't quite understand why you want to upgrade the op-amps... what do those have to do with your power supply?
     
  3. Brian Bunge

    Brian Bunge Producer

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    Why didn't you just have Brian do the mod when you bought the amp?
     
  4. Anthony_I

    Anthony_I Stunt Coordinator

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    cause hes done the other mods, and i have consumed so much of the guys time that i dont want to bother him anymore.

    Ive got like 30+ emails from the guy of various topics and subjects. I feel bad [​IMG]

    but as for upgrading the opamps, I was told by brian that if i draw too much current from my 12V supply that i will burn out the opamps, so i wanted to put something a little more powerful in there so i could draw more current.
    Right now it is 12V @ 100mA but a couple of LEDs and that is maxed. I only want to add a couple of LEDs but i dont want to max it and risk burning out my opamps.
     
  5. Dave Milne

    Dave Milne Supporting Actor

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    Anthony,
    Changing opamps will have no affect on the power output of an amp. You may be able to effect a subtle sonic improvement in terms of "air" and "sweetness"... but this is a subwoofer amp, for gosh sakes.[​IMG] I would only dabble with upgraded opamps if you are using the high-pass filters in the plate amp for your mains.

    Excessive loading on the power supplies will NOT damage the op amps. They are perfectly happy running with whatever voltage you give them. Now if you try to get a larger signal swing than the supply rails will support, they will just saturate and clip the signal. No harm done... but it will sound bad. If you are concerned about loading the power supplies with LEDs (can't imagine this... LEDs draw a couple of milliamps at most), you can use an external supply (wall wart) for the LEDs and use transistor(s) to trigger on the LEDs. Incidentally what are the extra LEDs for??

    Finally, why would you want to change the output transistors. MOSFETs aren't inherently "better" than bipolars... just different. And you would need to change some of the other circuitry. If nothing else, Bipolars have a positive temperature coefficient... that is, when they get hot, they draw more current. Bipolar amplifiers have compensation elements to stabilize bias current over temperature. MOSFET outputs, having negative temperature coefficients, require entirely different compensation.

    I'd recommend leaving the amp alone or sticking with the mods recommended by Brian.
     
  6. Anthony_I

    Anthony_I Stunt Coordinator

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    OK MOSFETs....no go... no problem

    But as for the opamps, Like ive been trying to say, I dont want to upgrade them in hopes of getting better sound or anything.

    I was told by Brain that i should not draw more than 100mA off of my 12V TAP (This is 12V unregulated power supply that was tapped into by brian so i could power LED's) or i could burn out the opamps.
    I do not want to burn out the opamps, so i assumed that if i changed them to more powerful ones then i could run more LEDs safely without worrying about burning out my opamps.
    Ultra-Brights are about 30mA and 3.8V (Ive found a way to run them at 12V without burning them out, but thats another thread) i simply do not want to draw too much current by using too many LEDs, because i was told it would burn out the opamps.

    The LEDs will be for added effects (Ground EFX, i used a car audio sub, so i wanted to emulate the look of a car so to speak) I do not know what the specs are on the LED that is built into my speaker, but i know it is 12V since it is meant to run on car power (more likely 14.4V), and probably more than 30mA, so running that, Along with 2 other LEDs will bring the current draw ABOVE 100mA, Which i was told would burn out the opamps. I do not want to burn out the opamps.

    Dont ask me what they have to do with power supply ask brian, he is the one who put the power tap on there. Im guessing he tapped into opamps power somewhere along the line which is why they would burn out if i draw too much current from my 12V tap.

    I hope this has cleared up why i want to upgrade the opamps.
     
  7. Brian Bunge

    Brian Bunge Producer

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    Why not just run a resistor in series with the anodes of the LED's to get the current draw down?

    EDIT: I just realized my post doesn't make sense. Having said that, how is it that adding more LED's is going to increase the current requirements of the op amps?
     
  8. Dave Milne

    Dave Milne Supporting Actor

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    Ah, I see... your latest message implies that an internal op-amp is actually sourcing the 12V. Makes sense (although 100mA suggests a pretty beefy op amp!).

     
  9. Anthony_I

    Anthony_I Stunt Coordinator

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    Adding more LEDs in parallel would require more amperage right?
    But not if run in series? then it only needs more voltage? (they arent run in parallel, i was sort of lost, but Dave sort of cleared it up a bit)
     
  10. Dave Milne

    Dave Milne Supporting Actor

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  11. Anthony_I

    Anthony_I Stunt Coordinator

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    What i meant when i asked about the drop (and i think i got it now) is, you were talking about the lights werent you?
    IE if there is one light, it is running at 12V, with 2 lights, each one would be running at 6V, so there would have been a 6V drop in each light.

    Right???
     
  12. Dave Milne

    Dave Milne Supporting Actor

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    The concept of woltage drop applies to resistors, lights, relays, etc. I was using a resistor as an example, but the same theory applies to the others.

    Unfortunately, LEDs are a bit of a different animal. They typically have a constant forward voltage drop of about 0.7V per LED. If your "super brights" have 3.8V drop, they are probably actually five or six conventional LEDs in series. So, in the case of LEDs, V = I * R doesn't really apply. V is relatively constant and they just get brighter with increasing I.
     

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