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Micpre of Graham

Discussion in 'Electronic Design' started by Ban, Mar 20, 2007.

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  1. Ban

    Ban Guest

    The schematic
    http://rapidshare.com/files/21272377/mic_amp_2.jpg
    looks pretty simple, but it still needs work. I found a couple of gotchas:
    1. When you switch on the phantom power the Vbe of the transistors gets
    reversed momentarily(+17V instead of -0.7V), degrading beta and Vos. This
    will slowly destroy the input devices. This happens always in normal
    operation with or without a mike.
    2. The power supply rejection is very poor(-20dB) especially at higher
    frequencies. Here current sources might improve the situation. A lot of
    additional filtering is also needed.
    3. When saturating the opamps will return to normal operation in a staggered
    way, creating spikes in the O/P signal.
    4. The offset voltage varies with the gain, making it sensitive to
    variations in gain setting.
    I have attached a link to a commercial product, just to show that the art of
    making a good preamp is not *that* simple.
    http://rapidshare.com/files/21831341/mic_pre_02.png
     
  2. Jim Thompson

    Jim Thompson Guest

    And what's so marvelous about that design?

    Looks like two pounds of shit in a one pound bag to me ;-)

    (That's a "country boy" colloquialism ;-)

    ...Jim Thompson
     
  3. Ban

    Ban Guest

    Well, it avoids that big electrolytic cap in series with the gain pot.
    BTW in Grahams design this is too small, rolling off at 27Hz with full gain
    and 1.6Hz with low gain.
     
  4. Ban

    Ban Guest

    I think it is from Rane. Look at the offset adjust pins used for frequency
    compensation. The 5534A is IMHO better than the decompensated version(5532),
    less noise, higher slew rate and GBW, for gains above 3.
     
  5. Eeyore

    Eeyore Guest

    The phantom power is separately switched on/off. It should as you correctly
    point out be of controlled rise/fall time. The same holds true for every mic amp
    I've ever seen btw.

    So you say. Can you check that figure ? The only issue I see is mismatch of the
    2 x 4k7 1% resistors and the 2 x 1k5 1% resistors. In practice these typically
    match within a batch to about 0.3%.

    I did simulate it btw. The power supply for that product has about 500uV of
    supply noise in the audio band and the 100Hz component is well down.

    I don't recall seeing that problem. Can you explain why you think that may
    happen ?

    No, there's no DC gain wrt the offset voltage. You can twiddle it more or less
    noiselessly to your heart's content !

    Would you like to run over the advantages of this ?

    It looks unnecessarily complicated to me to be honest.

    Graham
     
  6. Eeyore

    Eeyore Guest

    Where do you get the 2k5 rev log pots ? The lowest value I've ever seen is 5k.

    I did a design with a dual gang gain pot btw. It worked rather well. I should
    perhaps point out that this design was rejected on cost grounds ! I hate to
    think what most mixer companies would say about your design.

    Tell you what, you cost yours and I'll cost mine !

    Graham
     
  7. Eeyore

    Eeyore Guest

    I believe I did make a note to that effect somewhere. It really ought to be
    1000uF.

    Graham
     
  8. Hawker

    Hawker Guest

    I'm always interested in looking at others mic pre-schematics.
    Just wondering who's the one you uploaded is. The fact that they are
    using 5534s and not say 2114s or even 5532s (which test a bit better
    than a 5534) makes me wonder about it.

    thanx
    Hawker
     
  9. Hawker

    Hawker Guest

    I think I was getting confused, you are correct, sorry.
    I was thinking the 5534 was the quad, not single part. I remember the
    compensation is in the single part if I remember correctly.
    I'm not doing much analog these days. Mostly doing digital design. My
    part number memory is getting bad.

    -H
     
  10. Eeyore

    Eeyore Guest

    Can you explain how you think that happens ?

    Graham
     
  11. John Larkin

    John Larkin Guest


    Thank you. That is a wonderfully bizarre circuit.

    John
     
  12. cledus

    cledus Guest

    Because I had the day off and was fascinated by Graham's "improved mic
    preamp", I threw it into my Spice simulator and tinkered around a bit.
    Here are some of the results

    My simulator does not have built-in models for the transistor and op-amp
    that he used. So I substituted the venerable 2N2907 and LM833 parts to
    see what happened. If you can believe the simulator, the noise
    performance is impressive at around 2.5 nV/rt-Hz referred to the input.
    However, the distortion leaves a bit to be desired. At a gain of ~30
    and driving with +/-100mV pk-to-pk, the third harmonic is about -60 dBc
    at 100 Hz (~.1% THD). At 1kHz it gets better at ~-90 dBc for both the
    2nd and third harmonics. If I try substituting the LT1028 model in my
    simulator for the op amp, the circuit goes unstable. The circuit may
    depend on a slower op amp to keep it stable.

    For fun, I attempted to simulate a plain-jane LT1028 inverter based on
    the built-in model for my Spice simulator. I'm not sure that I can
    trust the model. I could get no where close to the noise performance
    claimed in the data sheets. And the noise was orders of magnitude worse
    than Graham's circuit. Like the data sheet recommends, I used 1.8k
    feedback and 60 ohms input resistors. The noise shows around 1.75
    microVolts/rt-hz referred to the input as opposed to less than 1 nV!
    Maybe I am doing something wrong, or maybe the model is not trustworthy.
    But the distortion looks very impressive and is similar to the data
    sheet. At +/-100 mV and gain of around 30, harmonics were all
    suppressed well below 100 dBc for input freqs of 10 Hz, 100 Hz, 1 kHz
    and 20 kHz. Because of the noise discrepancy, I don't know how reliable
    these results are. But they do seem to follow the data sheet
    extrapolation at these input levels and no external load.

    Anybody know where I can get a reliable Spice model for the LT1028?

    -c
     
  13. Jim Thompson

    Jim Thompson Guest

    I got ~960nV/rt-Hz
    I suspect you made a mis-entry.
    From LTC ?:)

    ...Jim Thompson
     
  14. cledus

    cledus Guest

    It would appear that my LTC Spice libraries are not very accurate for
    noise simulation. I ran through a bunch of noise simulations for just a
    simple inverter circuit (Ri=30 ohms, Rf=1k). Here are some results:

    OP27A (AD model) 3.9nV/rt-Hz @ 1kHz (spec sheet says 3.0 nV)
    OP27A (LT model) 12.4uV/rt-Hz @ 1 kHz (spec sheet says 3.0 nV)
    MAX4106 (MAX model) 2.9nV/[email protected] kHz (spec sheet says ~.85nV)
    AD797 (AD model) 1.1nv/rt-Hz @ 1 kHz (spec sheet says .9 to 1.2 nV)
    MC4558 13.1nv/rt-Hz @ 1 kHz (spec sheet says 12 nV)
    UA741 21nV/rt-Hz @ 1 kHz (spec sheet says 23 nV)
    TL071 12.5nV/rt-Hz @ 1 kHz (spec sheet says 18 nV)
    LT1028 (LT model) 1.32uV/rt-Hz @ 1 kHz (spec sheet says .85 to 1.1 nV)

    Most models other than LTC get me into the ballpark. None of the LTC
    noise simulations were even in the same city. Some Op Amps have an
    equivalent AD part. These appear to be a much more faithful
    representation of the spec sheet performance than the LTC models.
     
  15. YD

    YD Guest

    Late at night, by candle light, John Larkin
    Looks like something's wrong with the feedback connections of IC23/R88
    and IC30/R115. Maybe I'm just missing something.

    - YD.
     
  16. Fred Bloggs

    Fred Bloggs Guest

    That circuit is just /that/ simple, a simple rehash of the same tired
    old subcircuits, nothing original, all kinds of matching of discretes
    required, and the same old two channel input gain stages with those dumb
    inverse pot things.
     
  17. Phil Allison

    Phil Allison Guest

    "Eesyore"

    ** Of course, you know the colossal fool cannot.

    If the +48 volt supply is somehow snapped on, then about + 15.7 volts
    momentarily appears on each base.

    Not a reverse Vbe situation at all, as the emitters are supplied from +17.

    However, if the +48 is already on and a short is applied to the XLR input
    ( 1 to 2 or 1 to 3) the 47 uF electro cap (charged to 48 volts) discharges
    via the 4.7 ohm and 1N4148 diode into the -17 volt supply.

    Means a peak current about 6 amps ( approx 30 /4.7).

    Are those parts up to it ?



    ........ Phil
     
  18. Phil Allison

    Phil Allison Guest

    "Ban"

    ** Good pre-amp ?

    With a pair of 2.2 uF film caps ( = 1.1 uF ) in series with the mic input
    ?

    Not too good for the noise figure at or below 1kHz.



    ....... Phil
     
  19. Eeyore

    Eeyore Guest

    What exactly is dumb about the pot ?

    Graham
     
  20. Eeyore

    Eeyore Guest

    Exactly.

    I was hoping that Ban would see his error himself.

    It seems that a 1/4W 4R7 does in fact survive such torture but an 0805 won't (at
    least with repeated abuse) although for good measure I upped it to 1/2W on my
    latest design. I also put a 1N4004 in there.

    Graham
     
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