Audio amplifier output stage

Discussion in 'General Electronics Chat' started by Spence, Jan 27, 2011.

  1. Spence

    Thread Starter Member

    Apr 23, 2010
    49
    3
    I seek information or recommendations regarding the attached circuit which features parallelled push pull output (12 transistors per channel). I would like to construct a quality stereo amplifier 60-80 watts per channel.

    I'm struck by its simplicity and the fact that I have a surplus of power transistors hanging around doing nothing.

    The circuit is from an Accuphase amplifier, P650, which boasts 100 Watts at 8 Ohms with THD of 0.02%. The PDF I have show photos of the transistors but not clear enough to make-out their type. I have a power supply with plus and minus 60v rails.
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    2sc5200/2sa1944 or the new version TTC0002/TTA0002
     
  3. Spence

    Thread Starter Member

    Apr 23, 2010
    49
    3
    Many thanks for that information.

    My main question is, is this method (transistors in parallel) normal in audio power amps, or is there a better alternative.

    My next question is, will the bd911/bd912 substitute the TTC0002 etc, taking into consideration I have plus/minus 60v rail available,

    Looking at the datasheets there's not too much difference apart from the high collector voltages.

    TTC0002
    Collector-base voltage 160v
    Collector.emitter voltage 160v
    Emitter-base voltage 5v
    Collector current Ic 18A
    Base current Ib 5A
    DC current gain 80min 160 max
    (Vce 5v, Ic 1A)

    BD911
    Collector-base voltage 100v
    Collector.emitter voltage 100v
    Emitter-base voltage 5v
    Collector current Ic 15A
    Base current Ib 5A
    DC current gain 40min 250 max
    (Vce 4v, Ic 0.5A)
     
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Yes using multiple complementary pairs in parallel is quite common for higher power amps in commercial circuits.

    Yes the BD pair should suffice for +/- 60 V rails.

    One drawback for multiple pairs for amateur construction is that the pairs need to be selected for reasonable gain matching or one pair hogs all the work.
    This is easy to achieve when the transistors are bought in bulk for the production line.

    60 / 80 watts at 0.01% distortion or better used to be commonly achieved using only a single output pair so you might like to reconsider the need for so many pairs.

    A rule of thumb calc for max power achievable into speakers is P = V^2/8R watts, where V is the rail to rail voltage, so your rails will give plenty of headroom. 80 watts RMS is 71 volts peak to peak.
     
  5. ifixit

    Distinguished Member

    Nov 20, 2008
    638
    108
    Hi Spence,

    With rails of ± 60V... that is a voltage difference of 120V. So if the output should go to the rail for some reason then half of the output transistors would have 120V across them. Therefore, to be safe, choose a transistor with a max Vce of at least 2 times your rail voltage.

    Since you have lots of BD911 with Vce of 100V, it would be nice to use them. You could pick through them to match the gains and measure Vce. Many of them will likely have actual Vce breakdowns of greater than 120V, in which case use those ones for this project.

    It seems like six driver pairs is more than is required for a 100W audio amp. That's only 8.3W per transistor, which translates into a junction temperature rise of only 11.6°C. I suppose cool is good for accurate phase translation or low distortion.

    One more thing, the BD911/12 are for switching applications and may not be good for high quality audio. Our local audio experts may have something more to say on that matter.

    Regards,
    Ifixit
     
    Spence likes this.
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
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    513
    Good point.
     
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