Has anyone built this circuit from Chapter 6?

Discussion in 'General Electronics Chat' started by phaeton, Sep 25, 2007.

  1. phaeton

    Thread Starter New Member

    Sep 25, 2007
    1
    0
    I am currently attempting to get the Class AB amplifier circuit at the bottom of this page working:

    http://www.allaboutcircuits.com/vol_6/chpt_6/10.html

    I have a feeling that if this place is as 'busy' as it looks, someone would have pointed out if the circuit is wrong. I did some searches of the forum but didn't dig anything up, so I'll assume it is correct until corrected.

    In any event, I'm getting no output at all from the circuit, except for a little 'tick' noise in the speaker when i turn the power on. The signal I'm attempting to amplify is the naked output of a bass guitar- should be around 100-200mV in amplitude. The speaker(s) are a pair of 4ohm 6x9s that I put in a cabinet, and I can switch them out to be one (4ohm) or two (8ohm series). I'm using 3 'mildly depleted' 9V batteries for the power, and I measure 23-25V across them, depending on how long they've sat since I last tried it. Right now, I read 23.08V powering the circuit which should be close enough (I presume).

    Probing at the transistors I see:

    TIP42:
    Base: .007V
    Collector: 0.00V
    Emitter: .474V

    TIP3055:
    Base: .013V
    Collector: 22.93V
    Emitter: .474V

    ....and at the TL072 (circuit specs a TL082, but I have TL072s):

    VCC-: 0.00V
    VCC+: 23.08V

    Out1: 1.37V
    -IN1: 1.37V
    +IN1: 1.06V

    +IN2: 0.00V
    -IN2: .474V
    Out2: 22.35V

    '1' is the first opamp half (closest to the input) '2' is the second opamp half (closest to the transistors). I've never used an opamp this way before (I usually create inverting amplifiers) but tying the -IN1 input to 1OUT looks odd. I've double checked all my connections, I've even torn the whole circuit out of the breadboard and rebuilt it all from scratch (twice). I've used a different TL072.

    I don't have different power transistors to use, but I built the true Class B circuit (6th from the bottom, with the disclaimer "It will amplify signal, but it will sound horrible!") and it worked, sort of. It sounded horrible indeed, but moreso than I think they meant though. The crossover distortion was there, but it had the telltale sound of it being biased way wrong- a blatty/gated sound that didn't decay linearly with the input, but dropped off abruptly with a farty sound.

    I've reassembled the AB circuit at the bottom for the fourth time now, and done all the usual checks. The problem must be there, but I just don't see it.

    Any help, pointers, flames or such is very appreciated.
     
  2. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    GND should be in the center. So VCC- should be -11.5V and VCC should be 11.5V
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If all you have is a single power supply, you can use an inexpensive LM675 power op amp to split your supply.

    Schematic is available in National Semiconductor's datasheet for that part.
    http://www.national.com
    Search for datasheet: LM675
    Look on page 2.
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    The TL074 can handle +18 and -18 for +V and -V. Try adding a fourth 9v battery to your stack and hooking things up exactly as shown in the instructions.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK, I missed the part about the three 9V batteries. :rolleyes:

    Trouble is, if the OP installed four NEW 9v batteries, he'd be running the TL074 right at the edge of specs - things tend to not last very long when run like that. Could use a couple of forward-biased diodes at either rail supply, I suppose - each would knock 0.6v off the supply voltage. Easier than using even LM317 / LM337 regulators or the LM675 idea.
     
  6. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    Phaeton,

    If you haven't yet noticed, the circuit requires plus & minus 12V. Notice the ground symbol between the 4 batteries in the schematic?

    You haven't, and can't, properly divided the voltage of your three 9V batteries evenly. So, if you remove one of the batteries and connect the center point to GND, then you will have +/- 9V (or maybe +/-8V). But, if you use four midly depleted 9v batteries, then you can easily create +/-17V (or thereabouts) which will also work.
     
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