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design help needed

Discussion in 'Electronic Basics' started by The unknown Posterchild, Sep 6, 2005.

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  1. I want to build a preamp for my acoustic guitar, which will have a condenser
    mic and a few transducers as input.
    I would like to be able to fine tune each input and have it go through an
    onboard mini-parametric and or graphic eq.

    Anyone have any ideas where I can find this info?

    Thanks to an earlier request, I have seen some basic opamp stuff and some
    basic FET preamps (terms which I don't really understand too well yet) but
    none appears to fit my needs.

    Thanks in advance
  2. Pooh Bear

    Pooh Bear Guest

    Do you really need a full blown parametric or graphic on the inputs ?

    A decent mixer from the better manufacturers will offer a parametric mid at
    least as well as LF, HF and an HPF.

  3. I don't really need limited knowledge is my handicap and I
    just asked for what I had read about as being the "ideal."

    LF = low frew? ie: Bass
    HF = Hi Freq? ie: Treble
    HPF = high pass filter? (no win)

    Any guidance from the bear in the red sweater would be appreciated.
  4. Jim Anable

    Jim Anable Guest

    Please don't take offense, but if you don't understand the meaning of
    FET and opamp, aren't you biting off a bit too much?

    There are plenty of good commercial units out there. You don't also
    built your car from the ground up, do you?

    Personally, I'd go with a decent Fishman setup. Most standard acoustic
    electrics will have at least three bands of tone, many have four. If
    you still think you need a parametric, take a hard look at your amp and
    speakers to see if they aren't the real problem. If not, throw a
    parametric in the effects loop (or in line with recording, if that's the

    If you are looking for parametric because of feedback, some preamps have
    a built in notch filter (or you can get a feedback eliminator).
  5. Pooh Bear

    Pooh Bear Guest

    Yes - correct on all counts !

    And don't forget the mid freq control ( variable frequency ). So you get 3 EQ
    sections and a filter.

    Here's one I made earlier......

    Let me know if that's of any interest.
    Wasn't that actually Rupert the Bear ?

    Yup.... ;-)

  6. tempus fugit

    tempus fugit Guest

    Pooh you designed that mixer?

    What did you use for micpres - discrete or opamp?
  7. Pooh Bear

    Pooh Bear Guest

    A bit of both.

    The 'front end' is a low noise long tailed pair type configuration with variable
    gain ( the 'pot' is connected inter-emitter ) followed by a fixed gain
    differential op-amp stage.

    It's not actually changed very much since I first came across it in 1979. I've
    tinkered with it a bit but this circuit and its various derivatives is still
    widely used throughout the pro-audio industry. The main change since 1979 has
    been reducing the noise level slightly.

    A couple of companies make an integrated version that costs a bomb compared to
    doing it with discretes and an op-amp.

  8. tempus fugit

    tempus fugit Guest

    These would be SSM2017 or 2019?
    Actually, they're very inexpensive, because no one ever seems to carry them,
    so I end up having to contact AD directly for samples.

    The million dollar question: Does the version with the discrete front end
    sound different or better than the IC version?

  9. Pooh Bear

    Pooh Bear Guest

    Yes - and there's also a similar part from That. And indeed a TI / Burr Brown
    part, the INA103 IIRC. Yup. And there's the older INA217 too.

    As long as you can get way with it. ;-)
    Since I've never had occasion to use the integrated parts I really don't know. I
    really doubt it there's very much difference though. The integrated parts make
    life simple for those who never designed with discretes. ;-)

    I actually have TI / BB's PGA2500 digitally controlled gain mic amp in the
    office right now. That's a new development that's likely to be popular.

    There's much talk about the sound of mci pres. My own suspicion is that most of
    the 'routine' ones like my own designs are very alike in their perfromance.

    To be honest I just tend to hone the tech figures as opposed to trying to find
    fault with the sound.

    I expect you start getting interesting colourations when a transformer is used.
    I'm sure you do in fact. It seems likely to me that the 'interesting' mic pres
    sound that way on account of their flaws.

  10. After a day of pounding my head against the wall, I think I got it done! schematic dual preamp.jpg

    2 condenser mics into separate preamps, blended by a dual gang volume
    control into a simpe tone control network, then into the volume control then

    I'd like some comments/critiques on the design.
    Thanks for your support!
  11. Pooh Bear

    Pooh Bear Guest

    That tone control is passive and won't work with linear taper pots. It's typical
    of 1950s and early 1960s thinking ! For example I've been building audio
    circuits since .... 1969 and never once done a passive tone control !

    Yeah - it's designed around the 10% taper log pots that were once common ( the
    end-stop values are a giveaway ).

    These days, 'log' pots have a15% taper So it won't be flat with the controls

    An active 'tone control' stage is much, much better and likely to be far, far
    quieter and less prone to hum - etc pickup too !

    In fact all the R values are horribly high and it simply will have trouble
    feeding any amplifier that doesn't have about 1 megohm input impedance. The
    output impedance is absurdly high !

    It'll stink. Where on earth did you find such a retarded 'circuit' btw ? The
    toob nuts weren't involved by any chance ?

  12. Jim Thompson

    Jim Thompson Guest

    I'm glad you said it ;-)

    ...Jim Thompson
  13. John Fields

    John Fields Guest

    With so much experience under your belt, it seems that you'd have
    caught the fact that there are couple of FET's in there.
    Blah, blah, blah.

    Look at the circuit again. it's not passive. You fucked up. Admit it
    or piss off. Or not, it doesn't make much difference. No matter
    what, you were wrong and all your grandstanding can't change that.
  14. tempus fugit

    tempus fugit Guest

    The integrated parts make


    Like myself
    Indeed. The late 60s Rupert Neve designs are very sought after, and even
    Rupert himself concedes that just about anything nowadays sounds better, and
    it was probably the xformers themselves that were adding most of the sought
    after sound to the pre.
  15. Pooh Bear

    Pooh Bear Guest

    Very true and indeed the modern 'copies' make a great fuss of the fact that
    they've closely copied the transformers.

    Those early transistor circuits weren't exactly terribly linear by today's
    op-amp standards either. 'Fractions of a per cent ' THD are quite normal. In
    those days 0.1% was considered really good going. Todays mic pres routinely
    deliver 0.0xx to 0.00xx and better THD.

  16. What makes it active? the N FETs? ..they are in the preamp phase...
    I admit to scouring the net and stealing what I found...
    Before today, I didn't kniow a fet from an accompli

    I am going to separate the bass and treble to separate tone controls...with
    center being neutral and less and more being left and right. Does that mean
    I need more amps on the + side of the controls?

    Bear with me...really..up till today I never tried doing this..does it show?
  17. I read in alt.binaries.schematics.electronic that Pooh Bear
    So is the input impedance.

    Oh, I see; the OP is using crystal microphones.(;-)

    And the 'balance' control is wired as a master volume control.
  18. I read in alt.binaries.schematics.electronic that The unknown
    <NrQTe.1613$>) about 'design help needed',
    John Field's comment is misleading. The **tone control** circuit is
    passive. It strongly attenuates the signal. You will end up with only a
    few tens of millivolts output from your 'preamp'
    Yes, very much so.
  19. let's see...circuits came from...

    balance + volume:

    as for resistors and caps's what the circuits said.
    I thought they were modern audio log pots...

    I am still looking for an active tone control circuit but can't seem to
    understand what I should be looking for,,,some kind of opamp circuit for

    ...and since I am not using pickups but condenser microphones, the amp
    circuit may have to change a bit.

    I don't do electronics math so I have no idea about impedance issues with
    the output

    Thanks again for the input.
  20. T N Nurse

    T N Nurse Guest

    It's a Baxandall circuit. It's fine with linear pots.
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