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Improved mic amp posted on Rapidshare

Discussion in 'Electronic Design' started by Eeyore, Mar 16, 2007.

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  1. Eeyore

    Eeyore Guest

  2. tempus fugit

    tempus fugit Guest

    Thanks posting your design Graham. You obviously actually work in this
    field, so I was hoping I could ask some questions regarding your design; or
    design in general.

    How would this design compare with one of the all-in-one ICs (like,2877,SSM2019,00.html)? Would they 'sound'
    the same? I ask this for 2 reasons - one, I'm using a micpre that I made
    using that IC, and two, if there performance ans sound is close, wouldn't it
    be easier and cheaper just to use the IC? Is there something better about
    the discrete approach?
    I'm sure you're aware of much higher priced micpres like Focusrite, etc. Is
    their design so radically different (and more expensive to implement)from
    the type of design you have here, or is the difference in cost more due to
    Along that same line, I've never had the $ to be able to use one of those
    hi-end pres, so I can't make a direct comparison myself, but is there
    something in the design of those really expensive micpres that makes them
    more "musical" or appealing to the ears of musicians (I ask this because you
    work in the field and so assume that you've heard some of these others, or
    done some comparisons yourself)?

    Thanks again
  3. Fred Bloggs

    Fred Bloggs Guest

    Finally got it- interesting, but I don't see how the operating points of
    the pnp's are well defined.
  4. Eeyore

    Eeyore Guest

    Their bases are close to ground. Ic (well Ie) is predominantly set by the 4k7 tail
    resistors which in turn sets the collectors' DC operating point at V- + (Ic*1k5)

  5. Fred Bloggs

    Fred Bloggs Guest

    That much is obvious but your differential feedback only forces equality
    and not any particular Ic,Q for the pnp's. So where is the magic I have
    missed? Did you ever build that circuit?
  6. Eeyore

    Eeyore Guest

    Is Ie not good enough for you ? The emitters are connected to -17V by 4k7 and 0V
    by 6k8. That makes the tail current set by V = -17/(6k8/6k8+4k7) = -10.05V
    divided by 6k8 in parallel with 4k7 = 2k78. Ie = 10.05/2k78 = 3.62 mA.

    That specific one with all those values ? No. It had swiched resistor values to
    set the gain in a rather perverse change from the norm. I did another one with
    some differnt values long ago though and yet another in 1988 with a rather more
    interesting feedback arrangement. Problem ?

  7. Fred Bloggs

    Fred Bloggs Guest

    Are we looking at the same circuit? You have the 4k7 going from the
    emitter /node/ to V+. That only sets the current into the /node/ which
    is also drained by the 6.8K ckt, it does not set Ie for the pnp.

    I thought so...
  8. Fred Bloggs

    Fred Bloggs Guest

    I see where you're coming from now. First we contrive a circuit
    configuration with indeterminate operating point, then we force it to
    behave by our desires. This is a very enchanting mix of mystery,
    confusion, and emotional investment.
  9. You mean +17V, not -17V? Where's Vbe in your calculation?
  10. Eeyore

    Eeyore Guest

    The other end of the 6k8 is at 0V.

    Can't you do the Norton Thevenin thing ? I've *already* posted the answer btw.

  11. Eeyore

    Eeyore Guest

    AT LAST !!! Wonders will never cease.

    Only in your mind.

    I believe the phrase you're looking for is "negative feedback" !

  12. Eeyore

    Eeyore Guest

    Sorry. Mea Culpa. You're right of course.

    I ignored it for simplification.

    You're correct of course.

    It's effectively (16.4/4.7k) + (-0.6/6k8). That should do the Norton Thevenin
    thing in one hit.

    = 3.40 mA - 88uA. = ~ 3.3 mA.

  13. Jim Thompson

    Jim Thompson Guest

    Your levitation not working today, Fred ?:)

    ...Jim Thompson
  14. Fred Bloggs

    Fred Bloggs Guest

    Apparently Graham is unaware of the fact that transistors in the active
    region develop a Vce. His circuit, which he admits never existed in
    hardware, is indeterminate.
  15. Eeyore

    Eeyore Guest

    You mean the pot was replaced by fixed resistors ?

    Bwahahahahahaha Why don't you build it and come back and say that with a
    straight face ?

    The fact is that you've been 'out-thunk' by a mere audio practicioner.

  16. Fred Bloggs

    Fred Bloggs Guest

    Get real, Graham, those NJM's are railed or something, but you have no
    clue what the circuit bias is. You did sucker me in for a few
    milliseconds thinking you might have had something, but your remarks
    have since lost you that status. From now on we will all be
    ....errr...skeptical of giving you serious consideration.
  17. Eeyore

    Eeyore Guest

    Nope. That circuit works a treat.
    What you mean is that *you* can't do a DC analysis !

    How about you build the circuit ?

  18. Fred Bartoli

    Fred Bartoli Guest

    Fred Bloggs a écrit :
    in fact this circuit will work, but I'd like to hear Graham's
    explanation :)

    Details please, instead of 'did you heard of feedback'.
  19. Eeyore

    Eeyore Guest

    It *does* work and is widely used.

    Simplistically, the discrete input devices provide low-noise gain to 'front-end'
    IC1. IC2 simply inverts the signal so that differential feedback can be applied
    to the emitters.

    How much explanation do you need ?

  20. YD

    YD Guest

    Late at night, by candle light, Fred Bartoli
    This tip-toeing around the issue is getting boring fast. Everyone
    giving "hints" to everyone else and expecting them to come up with a
    complete analysis. I suppose it's because no one really is sure and is
    afraid of making a fool of himself. That, or hopes the other will make
    a mistake and get something to jump on. This is no way to have a
    technical discussion. Too many divas in here.

    - YD.
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