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Phantom powered preamp

Discussion in 'Audio' started by (*steve*), May 2, 2018.

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  1. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I'm after a preamp which runs from phantom power.

    I have 2 microphones, one of which requires phantom power, the other which won't work with phantom power turned on. The mixer turns on and off phantom power simultaneously for all mic inputs :)

    The microphone which doesn't like phantom power could also do with 10dB of gain.

    My thought was initially to just block phantom power with a few caps and resistors. However, it seems sensible to use the phantom power to good effect, and a gain stage would help me.

    I've found it surprisingly difficult to locate a schematic for a phantom powered preamp. Surely such a thing can't be too hard can it?

    The microphones have balanced outputs and the phantom power is 48V.
     
  2. bushtech

    bushtech

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    Sep 13, 2016
  3. solo2racr

    solo2racr

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    Aug 21, 2013
    One thing to keep in mind is the amperage available through the mixer for Phantom power and that required for a preamp. Normally, a mic that requires Phantom power need very little, so the supply is minimal. I would check the specs of the mixer first to see if there will be enough there to power a preamp.
     
  4. GerryG

    GerryG

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    Mar 6, 2018
    Another point past those made already on current, is the possibility of injecting your new powered device's current and noise surges into your microphone that then gets amplified many times. Just a bad idea all the way around it would seem to me. That mic power needs to be as clean and pure as you can get, if you want a good noise floor on the mic it goes to. It could even cause cross channel feed or feed from what ever that attached circuit is drawing from it. The weaker that phantom power is the worse this problem would be. This is kind of like trying to pick out the right car to build an airplane from. Maybe just not the right thing to be doing.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I thank people for their responses.

    I do have the equipment to measure noise, frequency response, the, gain, etc., so I won't be flying blind here. :)

    Also, the equipment providing the phantom power is professional equipment, and as I said, I already use the phantom power for one microphone. It's not something that I lashed together from sealing wax and string.

    I've found a few ideas, ranging from the simple

    imgext.php.gif

    To the more complex

    ECM8000mic.jpg

    And even including phantom power valve preamps!

    MSH-4v2_Condenser_Mic-2.GIF

    I'm somewhat surprised you can get the 10mA or so required for the valve's heater, but there you go.

    Obviously these circuits will require some modification to ensure that DC doesn't get to the mic and to maintain appropriate biasing, but I think I can manage that.
     
    darren adcock likes this.
  6. WHONOES

    WHONOES

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    May 20, 2017
    Why not use a low power opamp? To get rid of the excess PSU volts you could put a low current zener in series with the opamp supply lead and then bypass the amp with some 10μF and 100nf capacitors. I have done similar and it works OK.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    yeah, I'm looking at an NE5534 with a differential input as the gain stage driving a phase splitter (using a jfet) to drive the differential output (a Schoeps circuit). I'll be using a TL071 (probably) to create a 0V rail.

    Another alternative is to create my own differential input stage (and I have some dual transistors I could use for that) and directly drive the output from that. I can get by without a zero volt rail for that.

    I'll probably play around breadboarding some ideas.
     
  8. WHONOES

    WHONOES

    905
    225
    May 20, 2017
    I think the NE5534 with the phantom 0V is the simplest.
     
  9. WHONOES

    WHONOES

    905
    225
    May 20, 2017
    Be interested to know what you come up with.
     
  10. WHONOES

    WHONOES

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    May 20, 2017
    Had a quick play last night. Came up with the circuit attached. It is single ended and has (in simulation) a 10MHz bandwidth. If that is to much you could put a miller capacitor round the FET.
    Distortion for 17mV Pk - Pk is less than 0.001% at a gain of just over 10dB.
    If it is a bit too complicated, the FET on its own works quite well but distortion goes up to about 0.2%.
     

    Attached Files:

  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    That won't work with phantom power though.

    The input and output are balanced, power (+ve) is present on the two outputs, with the shield being the negative.
     
  12. Marrie

    Marrie

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    Jul 11, 2017
    There is a very good chance your mic cable or mic is wired wrong. I have seen this many times.
    On three of my own cables and once on a mic. ... otherwise your passive mic would work.
    I am as sure as any human can be; one is wired wrong.

    If building a pre-amp for you mic, I would start with a a JFET 2SK30A-GR. Use a batch with low gain
    if you can find one like: 2SK30A-GR, with a batch suffix of 3A if you can get it. ( 2SK30A-GR-3A )

    This will will give you a voltage gain of 3, or a little more. (remember every batch of JFET are different). That is your 10dB you are looking for. With a drain resistor of 6.8k your source resistor would be about 1.9k ohm.
    Use a trimmer for the above 1.9k resistor of say; 5k ohm because the mixer phantom resistance will also count as part of the drain resistor and bias is CRITICAL.
    Use a 10 meg. resistor from gate to ground. Use a .1uf input coupling cap. coming in to gate from the dynamic mic. You can use a 1uf cap. if you want the lowers of percussion dynamics.
     
    Last edited by a moderator: May 8, 2018
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Absolutely bizarre, but both mics work when using a different mixer. (As I found out when doing some initial tests after breadboarding stuff).

    At the time of initial testing I only had a single XLR cable with me, the one for the mic which requires phantom power.

    Having equipped myself with more cables, I find that both mics work with phantom power on using standard XLR cables.

    So I'm wondering if there's something odd about the XLR cable which came with the Vonyx CM400 that makes it incomparable with the Rode Procaster. They work fine with my Behringer UMC404HD, but not with the other 2 channel audio interface unit in using.

    Since they both work, I can live with the slightly lower levels from the Rode mic, but at some point I'd like to boost the levels slightly.

    Your idea for a circuit assumes an unbalanced input and output. For best noise performance I was wanting to preserve the balanced nature of the interface. (Edit: maybe I misunderstand what you're saying)
     
  14. Marrie

    Marrie

    2
    0
    Jul 11, 2017
    Still think it is a funny mis-wire in a cable, maybe the mic.....But
    You got me thinking... balanced input / balanced output
    I could only think of a couple ways to do it with a single 48V supply.

    Simple circuit, low noise, etc. I have not made 2 circuits to be used as
    a differential, but it should work.
    It may be wise to increase the source resistors to 10k ohm trimmer (multi-turn best),
    because the drains may see a 14k ohm or more in the total circuit.
    (I don't think it will have too much drift)

    Sorry for the colored post-it. was not thinking about looks.
     

    Attached Files:

  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,400
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    Jan 21, 2010
    Hi @Marrie, the circuit you've come up with is very similar to those that are actually used.

    I posted a few previously, but here is a trivially simple example:

    ECMMicPhantomPoweredNew.jpg

    It might not look too similar, but let's re-draw it...

    mic1.png

    The microphone (V1 and Q3) in conjunction with R4 and R5 simply produces two signals 180 degrees out of phase.

    If we assume that Q3's bias allows some current to flow through it, then (if R4 and R5 are equal) the voltage across R4 and R5 is equal. At the gate voltage is modulated by the signal, the current through Q3 changes. A change which causes the voltage across R4 to change causes an identical change in the voltage across R5. Thus the voltage (with respect to XLR 1) at the drain and source of Q3 changes by an identical amount, but in the opposite direction.

    So lets remove the microphone, and replace it with it's DC equivalent (a current sink):

    mic2a.png

    But there's now more we can do. The resistor in series with the constant current source doesn't do anything important, and neither does the capacitor across it. Well, they do do something, but nothing that is particularly important right now. And in the interests of simplicity we can also remove C1.

    mic2b.png

    OK, so let's tidy that up a bit:

    mic2c.png

    But let's remember that XLR 1 is ground, and that XLR 2 and XLR 3 have a positive DC voltage on them. That means our diagram is not drawn very conventionally. Lets put positive near the top, and ground near the bottom...

    mic2d.png

    That almost looks familiar. But let's remember that phantom power is essentially added by pulling the inputs up to around +48V via a resistor. So let's add them in...

    mic2e.png

    And if we move them slightly...

    mic2f.png

    That looks just like a differential amplifier!

    You may notice that the transistors are upside-down compared to how we would normally draw them...
     
    Last edited: May 12, 2018
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