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transistors in amplifier ear

Discussion in 'Electronic Design' started by pil, Jul 10, 2004.

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  1. pil

    pil Guest

  2. Norm Dresner

    Norm Dresner Guest

  3. pil

    pil Guest

    BC 547
    BC 337
  4. Just use the 3904s all through (first substitute) or 2222s mebe. Ought
    to do the trick, I'd have thought. Without thinking too deeply about
    it, of course.
    I stand to be corrected, as always.
  5. Norm Dresner

    Norm Dresner Guest

    2N3904's are fine for Vceo and Ft. Their Hfe(min) is also similar to the
    two BC's, but falls short of the BC's max by a factor of 2. It's not all
    that obvious from a quick glance why two different transistors were used in
    the original but I'd certainly try 2N3904's in both places and see what


  6. To answer that question in general, you need to understand the circuit. No
    two transistors are identical; what's important is that you use transistors
    that are matched, or in some cases better, on the important parameters. I
    don't know if you'll find someone on these groups who wants to go to the
    trouble to understand your circuit for you - really, that's your own job.
    But in a low-voltage audio circuit like this one, the parameters you most
    likely care about are small-signal current gain (beta), maximum collector
    current (Ic), and noise. Any transistors that meet or exceed the
    specifications of the original transistors will do fine.

    By the way, I notice that the rest of the parts seem to be wildly
    overspecified. For instance, they specify 63V capacitors, in a circuit
    powered by a 1.5V battery. So probably the transistors they used were
    whatever happened to be in their parts bin at the time, and anything you use
    will also be fine.
  7. CFoley1064

    CFoley1064 Guest

    Subject: transistors in amplifier ear
    Hi, pil. Q1 through Q3 can be substituted with the 2N3904s. Your primary
    problem is with Q4. If you plug in or unplug the earphone while the amp is on,
    you will momentarily short out the 9V battery with the transistor.

    A 9V transistor battery can sometimes crank out the better part of an amp when
    shorted. You might need a higher Ic(max) than you'll get on any of your TO-92
    transistors, and high gain isn't as important on the last stage. From memory,
    I believe the 2N2222 has the highest Ic(max), and if I had to, I'd go with that
    for Q4. If you promise not to hot plug the earphone, it will do the job.

    Choose wisely, and watch the pinouts on the transistors. By the way, you might
    want to look at the NTE replacement semiconductor website if you're not sure.
    Just plug in your part number, and if they have a cross reference, they'll show
    the specs on the their part number (which usually is as good or better than
    your part, so don't automatically assume your part is that good) and the pinout
    (which is always the same as your part).$$Search?OpenForm

    Good luck
  8. I read in that CFoley1064 <>
    The battery is 1.5 V and plugging or unplugging the headphones can't
    short it out.
  9. Indeed. I wonder which planet Mr. Foley is reading this.
    But there's another problem more related to the schematic.. I don't
    see how this circuit can work as shown because Q3 doesn't have any
    effective base bias. It can't derive any positive feed from anywhere
    to enable it to go into conduction.
    But there again I prolly just made another stoopid mistake. Can anyone
  10. Tam/WB2TT

    Tam/WB2TT Guest

    The description *seems* to imply that Q3 is biased from the rectified AC. A
    little too cute for my taste. I wonder if there is some micro power op amp
    that could have been used instead. He claims something like 7.5 ma Icc; not
    particularly outstanding.

  11. Are you saying Q3 is acting as a limiter in this instance, then? That
    would certainly make more sense on the face of it.
  12. The other point that springs to mind is what is the anticipated signal
    level from the electret microphone at the base of Q1 for say distant
    birdsong (say 12dB down on hushed conversation).
  13. I read in that Paul Burridge
    It's turned on by the d.c. developed by the diode rectifying the audio
    signal. In fact, the diode and the e-b junction together for a 'voltage-
    doubler' rectifier, which is a useful rectifier circuit that works even
    if the a.c. input is capacitively coupled.
  14. Andrew Holme

    Andrew Holme Guest

    Q3 is the automatic level control. It keeps the peak signal amplitude
    at one diode drop. Q3 base-emitter junction conducts briefly on
    positive peaks. D1 conducts on negative peaks. Q3 collector current
    discharge C4 regulating the gain by controlling the bias point of Q1.
  15. Okay, that makes sense, thanks.
    There are a couple of missing bits of info it would be interesting to
    know: what output level is the mic expected to generate when exposed
    to low-level sounds and what's the likely impedance of one of these
    types of mic?
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