Class B audio amplifier

Discussion in 'The Projects Forum' started by pätkis, Jun 8, 2009.

  1. pätkis

    Thread Starter Guest

    I want to built this kind amplifier. But what it is power? And can I replace those four 6V batteries to 12V battery, amplifier is going to be in a car. If that four 6V batteries can be replaced by 12V battery what components have to replace?
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
    287

    Greetings:

    To run this from a single ended power supply would require qute a bit of modification....at the very least a large coupling capacitor on the output stage. One alternative would to build TWO of these and drive them in a differential mode (bridge mode).

    With the unmodified circuit, you can get about 8 watts with an 8 ohm speaker, or 16 with a 4 ohm speaker.

    Eric
     
  3. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    The four batteries are configured to give +/- 12 volts. With a single battery, rather than a two-sided supply, you have a single-sided supply. That's one problem.

    As far as power goes the circuit you linked has a max peak to peak voltage of nearly 24 volts going into an 8 ohm load. This would be a maximum power of

    Pmax = (Vp-pmax)^2/(8*RL) = 24^2/64 = 9 watts. And actually it will be less because you won't get the entire 24 volts for signal swing. So your second problem, if you want this much power from 12p-p is to obtain a speaker or a pair of speakers in parallel equivalent to 2 ohms. This can be accomplished with two 4 ohm speakers.

    Since the op amps want a double ended supply and you have a single ended supply you have to split your 12 volts with a voltage divider consisting of two identical resistors and call the central node ground. Then capacitively couple the 2nd op amp to the class AB stage. Connect the collector of the NPN to 12 volts the collector of the PNP to ground. Be sure to use a 1000uF cap between its output and the speakers.

    By all means use a fuse. In line types work well underneath dashboards.
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    Dude don't bother building these into a car.
    You'll be better off building bridge mode amps using hybrids that are designed for 12V and low impedance (like 1 to 2 ohms) output.
    You'll get much better performance from those

    Rifaa
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    With a 13.8V supply, the max output voltage of the TL082 opamp is +11.6V when its output current is 30mA into the base of the NPN output transistor. The base-emitter voltage of the output transistor reduces the peak voltage at its emitter to only 10.6V. Then the swing into an 8 ohm speaker is only 7.4V peak-to peak which is only 0.86 Watts into 8 ohms at clipping. The very simple amplifier also has crossover distortion. The opamp does not have enough output current for the transistors to drive a 4 ohm speaker.

    If you use a bridged car radio amplifier IC instead, its output will be 14 Watts at clipping into a 4 ohm speaker. Its distortion is very low.
     
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