using audio amp as a lab supply

Discussion in 'The Projects Forum' started by kubeek, Mar 2, 2012.

  1. kubeek

    Thread Starter AAC Fanatic!

    Sep 20, 2005
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    Hi,

    i was just pondering on how should I make a variable lab supply, when I had the idea that I could use one of my old amp builds I have lying around unused.

    What I got is a stereo ~300W amp running on +/- 30Vdc I think, so it should work perfectly for reasonable currents. I will have to disable the DC servo though, but that is just a minor complication..

    The only problem I face is that I would love to have it current limited. Do you have any ideas how to easily incorporate that into a typical AB amp, i.e. how would you go about controlling the input signal voltage based on the output current?
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Not sure if you mean a variable DC supply?

    An audio amp is not suitable for this purpose.

    It will have input and perhaps output capacitors.
    It will drift at DC and probably have an unacceptable offset voltage.

    I have seen audio amps pressed into service as (variable frequency) AC supplies, when configured as a power oscillator.
     
  3. t06afre

    AAC Fanatic!

    May 11, 2009
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    I once used just for fun a TDA2003 as regulator. It can be done. But I think you will be better of with say a LM723.
     
  4. kubeek

    Thread Starter AAC Fanatic!

    Sep 20, 2005
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    I built it so no problem with those
    I don't think offset matters much as I will be using it with a voltmeter. As for drift, the amp's offset is controlled by a DC servo, so I could modify that for a variable voltage, I don't think the drift of the opamp will be too high.

    Is there anything else you can think of? The frequency response of the amp could be altered, or I could add and LC filter to the output to make the response better.
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Glad it works for you. Is it a Crown amp?

    Although you did not confirm if it a DC power supply, I assume so.

    A conventional power supply regulator is basically a current follower - either a collector or emitter follower (or souce or drain) with relatively low or even unity gain.

    Your amp will have significant gain. So beware the possibility of a direct feedback from some part of a connected circuit leading to runaway.

    go well
     
  6. kubeek

    Thread Starter AAC Fanatic!

    Sep 20, 2005
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    No, it is not a crown amp :D It is a mosfet amp I made according to some article I found many years back. But after doing some research I think it will be easier to just buy an L200 regulator and use that, but this won't give me negative power rail.

    Yes I want this for DC, I started thinking about the amp because it already has a large heatsink and is stereo so I could get symmetrical rails with significant power. The only problem I see now is how to get the current limitng done..

    As for the gain, I would have to check the input stage if it would be possible to use it rail to rail or something close to that for lower gain. But I don't understand what you mean with the feedback and runaway, how would a connected circuit create feedback?
     
  7. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    You will have power dissipation problems since amps are not designed to output significant DC current, power supplies are.
     
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