Halving transformer voltage question

Discussion in 'General Electronics Chat' started by garyh, Dec 20, 2008.

  1. garyh

    Thread Starter New Member

    Dec 20, 2008
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    Hi all,

    I know common sense suggests that one should use the correct transformer for a desired output voltage but perhaps there is a little room for improvisation here.

    I have an audio amplifier that is not performing well anymore, I assume due to aging capacitors. I have decided to replace the circuit with a simple chip design. I would like to reuse the transformer and dc filtering caps if possible since they are the most expensive components to replace.

    My transformer gives me 40-0-40 volts. My design requires around 20V supply. Is there a safe and economical way to split the 40V supply I have into 2 20V supplies? This would also allow me to employ a dual mono design.

    Thanks,

    gary
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Your amp is likely not performing well anymore due to the degradation of the DC filtering caps.

    You might try removing your caps, and re-forming them. This involves using a DC supply voltage fed to the cap via a 20k or 30k resistor, with the DC supply voltage being ramped up very slowly while keeping the leakage current below the maximum permissable leakage rate, until the capacitor's rated voltage is reached.

    The basic formula for permissible leakage (in mA) is:
    Capacitance(in uF) x working voltage rating x 1.5 / 10000
    or 0.1mA, whichever is greater.
    For example, a 5000uF capacitor rated for 100 volts has a maximum permissible leakage of 75mA.

    If you settle for an amplifier design using much lower voltage, you'll have much lower output power available.

    If it were me, I'd try to save the amplifier.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    If you operate an amplifier with half the supply voltage of the original amplifier then the output power will be 1/4th or less.

    But the original has an 80V total supply so if you use only 20V then the output power will be very low.
     
  4. garyh

    Thread Starter New Member

    Dec 20, 2008
    4
    0
    Thanks for the replies.

    SgtWookie, the amplifier's problem is in one channel only. There is a single PS generating DC for both channels, I assume this rules out the DC filter caps. Besides, I am not equipped to perform such remedial tests.

    Audioguru, does the 1/4th formula apply if I run the primaries in series instead? If I understand correctly wiring it for 230V mains but giving it 120V will give me half the original secondary voltage. I'll end up with 20-0-20 but is this safe, and will it provide enough current? The gainclone I intend to build requires at least 160VAs. In it's original configuration (120 mains - parallel primary, 40-0-40 secondary) the amp is rated at 2 X 130W.

    thanks,

    gary
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    896
    Hi Gary,
    The absolute max supply for an LM3875 and LM3886 (used in Gain Clone amps) is plus and minus 42VDC. The LM3875 is spec'd with a plus and minus 35VDC supply and the LM3886 is spec'd with a plus and minus 28VDC supply.

    A 26V-0V-26V transformer will make plus and minus 35VDC.
    A 21V-0V-21V transformer will make plus and minus 28VDC.
    A 40V-0V-40V transformer will make plus and minus 55VDC.

    If you connect the primaries in series then your transformer will produce 20V-0V-20V which makes plus and minus 26VDC and a gain clone will have an output of 30W to 35W into 8 ohms.
     
  6. garyh

    Thread Starter New Member

    Dec 20, 2008
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    Thanks again for the help. I think 30 W into 8 Ohms is enough to rock and roll.

    Happy holidays,

    gary
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    896
    30W is enough to rock and roll when you wear the speakers on your ears.
    300W will sound twice as loud. Try 6000W then you will really rock and roll.

    Look at the Jackhammer sub-woofer speaker in Google. It weighs 369 pounds and can play 6000W RMS continuously. Its 22" cone moves 5" p-p linearily. It makes your car rock and roll.
     
  8. garyh

    Thread Starter New Member

    Dec 20, 2008
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    0
    Ah, you are clearly talking about Rock and Roll with a capital "R." And that is why you are the guru.

    Sadly, in an effort to preserve what hearing I have left, I must make efforts to favor quality over quantity. Perhaps I should aquire one of these massive speakers as a totemistic gesture, mounted on an altar in my livingroom it might give the gods of rock and roll cause to take pity on me.

    Thanks for the help. By the way, what would be involved in actively regulating 40V rails? Probably more than just coughing up the dough for the right Xformer...

    Gary
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Power amp supply rails are normally just run right off the rectifiers. Add capacitors until happy with low frequency square wave response. The transformer generally is the power limited device in the system.
     
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