240W class AB amplifier

Discussion in 'The Projects Forum' started by Experimentonomen, Jan 7, 2012.

  1. Experimentonomen

    Thread Starter Member

    Feb 16, 2011
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    Hello!

    Just thought i'd share my latest project, a class AB amplifier capable of pushing about 245 Watts into a 4 ohm load before clipping.

    The circuit is a basic Douglas Self "Blameless" topology but with a single transistor VAS instead of the darlington "beta enhanced" VAS used in the blameless.

    The rail voltages are about 60VDC per rail with a 800VA power transformer, so its no toy, and should NOT be attemted by someone that doesent have experience in electronics and such.

    A few pictures, the little board is a speaker dc and thermal protection board:
    http://i.imgur.com/Eybjs.jpg
    http://i.imgur.com/Y8JVR.jpg
    http://i.imgur.com/yVJNH.jpg
    http://i.imgur.com/mJDmO.jpg

    A video of the thing in operation:
    http://www.youtube.com/watch?v=E5KR4TeSXYU

    Schematic:
    http://i.imgur.com/9UU4a.png
     
    Last edited: Jan 10, 2012
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Nice.
    Pretty power for a simple thing.
     
  3. Experimentonomen

    Thread Starter Member

    Feb 16, 2011
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    On the same heatsink i am also putting a stereo amplifier that does around 50-100 watts/ch into 4 ohms, using the same circuit, it will run off a separate 300VA transfomer with 2x35VDC rails.
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    are you making an active speaker system
     
  5. Experimentonomen

    Thread Starter Member

    Feb 16, 2011
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    More like a 2.1 channel amplifier, the 245 watt mono channel will also have a active lowpass filter with selectable xo frequencies, around 50, 90 and 120 Hz and a level control.


    And beeing forced air cooled im calling it a PA style amplifier as it will be able to take more of a beating than a normal convection cooled amplifier would, like beeing able to sit at rated power all day long without overheating, and if it does get too hot, the thermal protection will just shut off the power supply.

    Im willing to bet most commercial consumer amplifiers cannot survive outputting rated power for more than a few minutes before they overheat and break down.
     
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I am way ahead of you Mate.

    I have a 3 way active Tri Amplifier I made. Been using it for like I dunno 3 years may be.
     
  7. Experimentonomen

    Thread Starter Member

    Feb 16, 2011
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    got pics ?
     
  8. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Need to open it up. It was before I joined AAC.

    Bloody thing weighs like a TON :eek:

    But, it needs a bit of dusting now. And I am thinking of remaking it a bit stronger.

    PIC's will come soon.

    Let u know.. keep visiting.
     
  9. Experimentonomen

    Thread Starter Member

    Feb 16, 2011
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    My amp will just be sub and main stereo channels.
     
  10. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Mine was
    2 X 300W rms Woofer
    2 X 100W rms Mid
    2 X 60W rms tweeter
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    Maybe your amplifier produces more power than you measured.
    Most DMMs can measure 50Hz and 60Hz voltages but are wrong (the measurement is too low) at 1kHz.
    Your 'scope should show the output p-p then divide it by 2.828 to get the RMS value.

    Your supply is 120V (but maybe it drops when the amplifier is blasting?). The amplifier has a loss of about 8V p-p. Then the output is 112V p-p which is 39.6V RMS. Then the power into 3.9 ohms (maybe its value is a little higher or lower?) is 402W.
     
  12. Experimentonomen

    Thread Starter Member

    Feb 16, 2011
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    The dmm may be very cheap but it actually shows the same voltage all way from 20Hz to 20kHz with minimum rolloff, which is suprising for a cheap dmm, especially for one thats not even true RMS.

    I noticed this last time i measured an amps output and tried different frequencies.

    At 400 watts output, the transistors would explode! According to the SOA diagram in the datasheet, im already exceeding SOA at 200 watts, i guess its rail sag that so far prevents the output transistors exploding, and the fact that i always look at the SOA curve for DC, i do this as to have a small safety margin.
     
  13. Experimentonomen

    Thread Starter Member

    Feb 16, 2011
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    Last edited: Jan 11, 2012
  14. Experimentonomen

    Thread Starter Member

    Feb 16, 2011
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  15. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Question, Q7/R17/D1 make a current sink.

    What's Q6 doing as it is already inline with the current sink, and it has a 4.7uF cap across C-E?
     
  16. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Egad !! ur work bench is worse than mine...

    But I see you got toroid..mmm I luv to get me couple of those hefty ones.
     
  17. Experimentonomen

    Thread Starter Member

    Feb 16, 2011
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    Q6 is the thermal compensation circuit to prevent thermal runaway. It's also called a bias circuit.
     
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