The AAC Class B Amplifier

Discussion in 'The Projects Forum' started by RayInMS, Apr 26, 2013.

  1. RayInMS

    Thread Starter Member

    Dec 12, 2012
    89
    1
    I'm itching for a new project. I'm thinking about building the audio amplifier project presented in the AAC experiments (http://www.allaboutcircuits.com/vol_6/chpt_6/10.html). I have a couple of questions, though...

    1. If you've built one, did it perform as expected?
    2. Would this be a good way for my wife to use her iPod without headphones?
    3. The crossover distortion is a concern...how bad are we talkin' here?

    Thanks. :)
     
  2. TheComet

    Member

    Mar 11, 2013
    88
    12
    I checked it out, and there's one thing I just don't understand. Shouldn't there be a diode and resistor on the top side of that circuit as well? Like this?

    [​IMG]

    The comment on the page doesn't do a good job in explaining why there is only one diode there.

    Apart from that, the circuit looks fine and should function fine. Give it a shot!

    TheComet
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,028
    3,237
    1. Depends upon what you mean by "expected". It's an interesting project to learn the basics of a small audio amp, but its performance is likely just mediocre.
    2. I wouldn't recommend it for that purpose.
    3. Likely noticeable in the sound, particularly at low signal levels. If you use two diodes as TheComet suggested, making it into a Class AB amp, the bias current through the output transistors could become excessive.

    If you want something that sounds good, I recommend using an IC audio amp. They are relatively simple to use and have good performance. If you are going to operate from a battery you might want to consider some of the Class-D (switching) amps available, such as from TI, that are much more efficient then Class B or AB amps.
     
  4. patricktoday

    Member

    Feb 12, 2013
    157
    42
    Build it on a solderless breadboard then you can try it out first. The only issue would be the TO-220 transistors which probably need a heat sink and won't snap into the board. You can use 3-pin molex jacks such as these for a poor man's socket:
    http://www.allelectronics.com/make-a-store/item/CON-243/3-PIN-CONNECTOR-W/HEADER-0.1/1.html

    That said, I haven't tried this setup so I can't answer to the sonic quality. It's probably not going to sound like a Bose home theater system.
     
  5. #12

    Expert

    Nov 30, 2010
    16,321
    6,818
    It is good to thermally couple the diodes to the output transistors.
     
  6. RayInMS

    Thread Starter Member

    Dec 12, 2012
    89
    1
    Nice idea, thanks!
     
  7. RayInMS

    Thread Starter Member

    Dec 12, 2012
    89
    1
    I was planning on going with a battery. The biggest downside to the circuit was the need for negative voltage and such...large...voltage requirements. Swinging rail to rail +12/-12 is a heck of a feat for battery needs. I was wondering how heavy four 6V cells would be, LOL.

    I thought the TL082 was an audio amp, but I'm looking again now and it seems to be a general duel op amp. I guess my reading comprehension needs some serious work. :D

    Any suggestions as to a circuit using an audio IC that would run on a single supply using one or two 9V batteries AND sounded good enough for the wife to listen to her iPod?
     
  8. patricktoday

    Member

    Feb 12, 2013
    157
    42
    This could be difficult. This is to drive speakers and not headphones/earbuds, right? A 9V battery has a typical "intended" current drain of about 15mA. If you're driving an 8Ω speaker, that would represent an "allowed" voltage of no more than 0.12V RMS. That would definitely not give you the Bose home theater volume you may be looking for :p If you remember the 80s when boom boxes were popular, those usually ran off 6-8 D or C cell batteries. 8 C cell batteries would give you 12V at a total "intended" current drain of 800mA which would be the equivalent of driving an 8Ω speaker at 6.4V RMS and would allow for 5.12W of power (or 2.6W/channel). Also, a single C cell battery has more than 10X the lifespan of a 9V when drawing the same current.

    Here is some source data:
    http://www.techlib.com/reference/batteries.html
     
  9. RayInMS

    Thread Starter Member

    Dec 12, 2012
    89
    1
    I'm not looking for super-high quality. I'm thinking more along the lines of powered PC speakers. This is just a tinker project. I'm only going with one speaker (will combine stereo inputs to mono) and if the battery life is only four or five hours, that's OK. I do stuff like this to learn - and - to see if I can do it without letting the magic smoke out of the box. :)

    If there's another way, or if anybody has a better circuit (using the TL082 or generic transistors) I'd be glad to try it.
     
  10. patricktoday

    Member

    Feb 12, 2013
    157
    42
  11. RayInMS

    Thread Starter Member

    Dec 12, 2012
    89
    1
    Ahh, gotcha. I guess I'll keep looking, then. I might build that Class B amp just to see how it sounds and also for the experience. Thanks for the links.
     
  12. patricktoday

    Member

    Feb 12, 2013
    157
    42
    Make sure you post your results! :)
     
  13. RayInMS

    Thread Starter Member

    Dec 12, 2012
    89
    1
    Will do. I'm still looking for alternates. Considering, at the moment, using two 9V batteries and a floating ground for the Class B. It may not work, but hey, we learn through failure just as well as we learn through success.
     
  14. KrisBlueNZ

    Member

    Oct 17, 2012
    111
    14
    Companies like Texas Instruments (www.ti.com) and National Semiconductor (now part of Texas Instruments) make a variety of low-power audio output devices. For example the National Semiconductor "Boomer" series. These perform a lot better than the ancient LM386 and are available in stereo versions.

    Do a search on Digikey or Mouser to get an idea of what options are available.
     
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