Variable Z Mic Preamps

Discussion in 'rec.audio.pro' started by R. Foote, Aug 25, 2003.

  1. R. Foote

    R. Foote Guest

    How is variable impedance being implemented on some of the mic pres
    (Vipre, Digital MPA etc)

    I would guess it would be a dual resistance to ground, but it would
    have to be on the inboard side of the transformer (or capacitors in a
    non-transformer design).

    It seems that all the pres have some set impedance just inside the dc
    blocking like 10k or so.

    What range of impedance is appropriate?
  2. Scott Dorsey

    Scott Dorsey Guest

    R. Foote <footnote@hayfork.net> wrote:
    >How is variable impedance being implemented on some of the mic pres
    >(Vipre, Digital MPA etc)


    Some are using multitap transformers. Some are using shunt resistors.

    >I would guess it would be a dual resistance to ground, but it would
    >have to be on the inboard side of the transformer (or capacitors in a
    >non-transformer design).


    Shunt resistors are not a good thing from a S/N standpoint, so if you
    are going transformerless, a transformer with a multitap primary is
    the way to go.

    >It seems that all the pres have some set impedance just inside the dc
    >blocking like 10k or so.


    Often much lower. Often the input stage is fairly low Z to begin with
    in a transformerless type.
    --scott


    --
    "C'est un Nagra. C'est suisse, et tres, tres precis."
  3. R. Foote

    R. Foote Guest

    Thanks Scott.
    I guess if I can find a multi-tapped primary Jensen transformer I will
    try refitting my diff amp input with that. (It has coupling capacitors
    now) Right now, the amps inputs are at 10k, but the transformer will
    probably want it's own specific secondary impedance? I wonder if
    Jensen supplies that info when you but a transformer.

    Obviously, the multi tap transformer is the way to go.

    Roger Foote
  4. Scott Dorsey

    Scott Dorsey Guest

    R. Foote <footnote@hayfork.net> wrote:
    >I guess if I can find a multi-tapped primary Jensen transformer I will
    >try refitting my diff amp input with that. (It has coupling capacitors
    >now) Right now, the amps inputs are at 10k, but the transformer will
    >probably want it's own specific secondary impedance? I wonder if
    >Jensen supplies that info when you but a transformer.


    Yes. Look at the Jensen web site. Each of the transformers basically want
    to see a particular load impedance, and in the case of the input transformers,
    if the load impedance isn't right, they will ring. Output transformers are
    usually designed to work into a wider range of load impedances, at the expense
    of efficiency and noise performance, because that's a lot more important in
    that application.

    >Obviously, the multi tap transformer is the way to go.


    If you like the sound of transformers.
    --scott
    --
    "C'est un Nagra. C'est suisse, et tres, tres precis."
  5. Mike Rivers

    Mike Rivers Guest

    In article <be237e31.0308250811.4980979c@posting.google.com> footnote@hayfork.net writes:

    > How is variable impedance being implemented on some of the mic pres
    > (Vipre, Digital MPA etc)


    I the case of the ViPre there's switch that selects taps on the input
    transformer from about 50 ohms up to about 3K I think. In the case of
    the MPA, I don't know. Some simply use a resistor, which also has an
    effect on the sound of the mic - a different effect than a transformer
    tap, but an effect none the less. And that's what it's all about -
    changing something and listening to see if you like it.



    --
    I'm really Mike Rivers - (mrivers@d-and-d.com)
  6. R. Foote

    R. Foote Guest

    Well I found a transformer (JT-MB-CPCA) that seems suited to work with
    my preamp which has a differential input structure. Right now the diff
    amp is at 20k impedance and this transformer wants 3k, so it would be
    an easy thing to replace the 2- 10k resistors with 1.5k resistors...

    I wonder if there is any benefit going with a single ended transformer
    tied to an AD743 for instance versus retro-fitting this existing
    differential amp?

    Of course this is not going to be a variable Z pre, but that seems a
    little tough to get parts for, and I can't just mess with the
    secondary loading of the transformer without risk of ringing.

    Anyway I just want to say thanks for taking the time to help me out on
    this Scott!

    Roger Foote
  7. Scott Dorsey

    Scott Dorsey Guest

    R. Foote <footnote@hayfork.net> wrote:
    >Well I found a transformer (JT-MB-CPCA) that seems suited to work with
    >my preamp which has a differential input structure. Right now the diff
    >amp is at 20k impedance and this transformer wants 3k, so it would be
    >an easy thing to replace the 2- 10k resistors with 1.5k resistors...


    Sounds reasonable. Run a square wave through and see what it looks like
    on the other end. Set the load capacitance and resistance so you get a
    nice-looking 1 KC square wave on the scope.

    >I wonder if there is any benefit going with a single ended transformer
    >tied to an AD743 for instance versus retro-fitting this existing
    >differential amp?


    Depends on the stage itself. A differential amp stage, if it's well
    designed, can give you some linearity benefits over a simple
    one-transistor input stage.

    All op-amps basically have a differential front-end going into an
    output stage (usually push-pull but sometimes single-ended).

    >Of course this is not going to be a variable Z pre, but that seems a
    >little tough to get parts for, and I can't just mess with the
    >secondary loading of the transformer without risk of ringing.


    Why not? You can see ringing on the scope. You can "select on test"
    until it looks good on the scope.

    >Anyway I just want to say thanks for taking the time to help me out on
    >this Scott!


    The Jensen applications notes have a couple nice designs for transformer
    input preamps with various technologies.
    --scott
    --
    "C'est un Nagra. C'est suisse, et tres, tres precis."
  8. R. Foote

    R. Foote Guest

    I am studying the Jensen schematics as we speak. Thanks Scott and Mike
    for the invaluable input!
  9. R. Foote

    R. Foote Guest

    Well I ordered a Sowter #3678 multi-tap mic transformer. Tapped at 50,
    200 and 600 ohms. Wish it had some higher taps (1k or so) but at least
    it has some variation.

    All of the Jensens seem to have between 1.2 and 1.4k primaries, so if
    a person was to put a resistance3 across pins 2 and 3, it would lower
    the impedance even more.. So maybe 600 ohms won't be so bad.

    I think I will try a transformerless design with variable z. I guess
    Panasonic bipolar caps would be best?
  10. Scott Dorsey

    Scott Dorsey Guest

    R. Foote <footnote@hayfork.net> wrote:
    >Well I ordered a Sowter #3678 multi-tap mic transformer. Tapped at 50,
    >200 and 600 ohms. Wish it had some higher taps (1k or so) but at least
    >it has some variation.


    You'll find the higher taps aren't all that useful, but you can call
    Sowter and they'll make you one with more, I suspect. They are very
    good about that.

    >All of the Jensens seem to have between 1.2 and 1.4k primaries, so if
    >a person was to put a resistance3 across pins 2 and 3, it would lower
    >the impedance even more.. So maybe 600 ohms won't be so bad.


    Right, but now the transformer is seeing a lower source impedance too.

    >I think I will try a transformerless design with variable z. I guess
    >Panasonic bipolar caps would be best?


    For what?
    --scott
    --
    "C'est un Nagra. C'est suisse, et tres, tres precis."
  11. R. Foote <footnote@hayfork.net> wrote:

    > Well I ordered a Sowter #3678 multi-tap mic transformer. Tapped at 50,
    > 200 and 600 ohms. Wish it had some higher taps (1k or so) but at least
    > it has some variation.


    the people who do it "right" (Great River, myself, certainly others)
    simply use a relay to strap a dual-primary transformer for series or
    parallel connection. This gives you nominally 150 or 600 ohm
    impedance.
    I make a circuitboard that handles this function along with polarity
    switching, -20dB pad, phantom power, and XLR input jacks all on a 3x3"
    2-channel board. http://www.rollmusic.com/projects/3125.shtml

    > All of the Jensens seem to have between 1.2 and 1.4k primaries, so if
    > a person was to put a resistance3 across pins 2 and 3, it would lower
    > the impedance even more.. So maybe 600 ohms won't be so bad.


    Ultimately these numbers are pretty meaningless most of the time. A
    transformer doesn't have an actual impedance of its own, only an
    impedance ratio. So the actual source and load impedances are
    dependent on the microphone, the transformer ratio, and the amplifier.
    Along with any terminating resistance and/or capacitance.

    > I think I will try a transformerless design with variable z. I guess
    > Panasonic bipolar caps would be best?


    Transformerless preamps with a variable input impedance are simply a
    bad idea if you ask me. Unles you come up with some very sophisticated
    circuit to vary the actual input impedance of the amplifier, then
    you're forced to use loading resistors which throw away precious
    mic-level signal voltage and really defeat the purpose of the whole
    concept. The reason impedance-variance works so well with transformers
    is that varying the ratio lets you trade impedance for gain without a
    noise penalty. You still have to face the limitations of high-ratio
    transformers, but if you're buying new transformers from the likes of
    Jensen, Sowter, or Lundahl then you've got a big technology advantage
    over the iron of yore which suffered phase, frequency, and distortion
    penalties in high-ratio arrangements.

    ulysses
  12. R. Foote <footnote@hayfork.net> wrote:

    > I think I will try a transformerless design with variable z. I guess
    > Panasonic bipolar caps would be best?


    What are you going to do with the bipolar caps?

    ulysses

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