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Copying Op Amps to Make Mic Amps

Discussion in 'Electronic Design' started by D from BC, Mar 17, 2007.

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  1. D from BC

    D from BC Guest

    Trade offs....
    I'm aware of:
    noise
    gain
    PSRR
    CMRR
    distortion
    dynamic range
    stability (DC bias and feedback)
    power efficiency
    PCB size
    cost

    How much more can there be?
    D from BC
     
  2. Phil Allison

    Phil Allison Guest


    ** This is an unsupported opinion typical of a " sound engineer ".

    Mostly, these dudes live a fantasy world of their own invention.

    It sure as hell ain't the real world.

    And they do not deal in fact.




    ........ Phil
     
  3. D from BC

    D from BC Guest

    Transistors are like atoms and IC's are like molecules.
    D from BC
     
  4. Phil Allison

    Phil Allison Guest

    "tempus fugit"


    ** You're not using a condenser mic - then?




    ....... Phil
     
  5. tempus fugit

    tempus fugit Guest

    I think this is a point that Graham has been trying to make. For a musician
    or sound engineer, there is a certain sound to micpres, etc., and maybe
    that's why his design was successful. In technical terms, I suppose it's
    probably some lack of quality (I'm not saying this about Graham's design
    BTW) in the design that imparts a difference in tone to whatever's being
    recorded. This explains why you see Neve channel strips going for a couple
    grand on eBay, even though Rupert himself admits that today's designs are
    far better. This may also help explain why the high end boutique micpres can
    fetch a similar price, even though there couldn't possibly be enough spent
    on actual components or R+D or whatever to justify the cost. Maybe Phil's
    right and the engineer types are deluded, but they seem convinced that there
    is a difference, and maybe there is - this mysterious tonal quality that
    certain designs impart. Whether it is better for the sound or not is a
    matter of opinion and personal taste. I'm a musician myself, and what I
    record is largely classical guitar, which requires a very low noise micpre,
    which the SSM2017 is able to deliver. The noise contributed by the micpre
    itself, with whatever resistors or other noisy passives I have in there is
    miniscule, and for all intents inaudible, so this one works great for me.
    I've not had the fortune to A/B it with a Focusrite to see if the latter has
    a better 'sound', although a local studio owner has invited me to bring my
    micpre and go head-to-head with his Focusrite. Maybe I'll take him up on it
    some day, but I figure that my playing itself needs more work than my
    recorded sound anyway. ....
     
  6. D from BC

    D from BC Guest

    Welll...I have some audiophoolery for yah... :)
    The mic amp difference is... the distortion.
    (Here comes the tomatoes... :) )
    Some distortion sounds nice.
    But it's gotta be the right flavour of distortion..
    Something to do with even harmonic content sounding better than odd
    harmonic content..
    (A gross example is the Tony Iommi (Black Sabbath) guitar sound. But
    much more subtle in a mic amp.)

    I keep showering but the audiophoolery doesn't wash off.... :)

    More scientifically I'd get an Audio Analyzer from:
    http://ap.com/
    And test the mic amps for a difference.
    It's rentable....
    D from BC
     
  7. Fred Bartoli

    Fred Bartoli Guest

    Eeyore a écrit :
    The LT1028/1128 has 850pV/rtHz for a diff pair. That's 600pV/rtHz per
    transistor. Pretty close.

    The 1/f corner is at 3.5Hz typ 100% tested and current noise is 1pA/rtHz.

    Offset is 20uV typ, 120uV max over temp with 0.2uV/°C typ. and you have
    bias cancellation (Ib=+/-25nA typ, 90max, and +/-40nA typ over temp) for
    the same price.

    Ok, not all that is relevent to audio, but now try to beat that in
    discrete designs *for the same price*.
     
  8. Eeyore

    Eeyore Guest

    But it's *not* the same circuit. As has been shown, you can do it with one
    op-amp or two. The one op-amp solution is attractive where price is the main
    driver.

    When that mic amp IC gets to 50 cents I'll be interested.

    Graham
     
  9. Eeyore

    Eeyore Guest

    How do you make a true differential amplifier with one LT1028 ? The classic
    instrumentation amp configuration will imtroduce the noise of 2 of them so
    you're already up to 1.2nV/sqrt Hz.

    Never mind the damn *expense* !

    Graham
     
  10. krw

    krw Guest

    Have you ever priced Monster Cables? Oxygen-free connectors?

    ....or oxygen free gunpowder? ;-)))
     
  11. Indeed. Some rare sense from Phil here.

    My take, the proliferation of designs is simple a meme replication and
    variation for its own replication advantage, like as in religion. There is
    no basis in ether as to any superiority, they just multiply as as does any
    other virus bcause thats what they do.
     
  12. Eeyore

    Eeyore Guest

    Well no.

    The most recent design I posted is a significant improvement over typical
    previous examples in every respect.

    Graham
     
  13. Eeyore

    Eeyore Guest

    Oh there's plenty of money in it at some levels but the commodity 'prosumer' gear
    isn't it.

    Graham
     
  14. Eeyore

    Eeyore Guest

    Condenser microphones. Not dynamics (the majority).
    Only in part and you still want the hum rejection of the differential input.

    Graham
     
  15. Eeyore

    Eeyore Guest

    It still has inferior performance and costs more. Plus you're tied to a single
    source.

    Graham
     
  16. Eeyore

    Eeyore Guest

    He's alluding to the fact that condenser mics have higher outputs than dynamics
    and as a result, input noise is less of a problem with them.

    Graham
     
  17. John  Larkin

    John Larkin Guest

    That's one of the many problems with audio: there's not much money in
    it.

    John
     
  18. John  Larkin

    John Larkin Guest

    Please clarify one point. Your mic amp applies phantom power to the
    microphone, so isn't the real amplifier out there?

    John
     
  19. John  Larkin

    John Larkin Guest

    For your application, an INA103 would work.

    John
     
  20. tempus fugit

    tempus fugit Guest

    I'm using 2 actually. Why, does that make a difference for noise level in
    the way it interacts with the preamp?
     
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