Function of this studio microphone circuit

Discussion in 'Analog & Mixed-Signal Design' started by Willen, Oct 10, 2018.

  1. Willen

    Thread Starter Member

    Nov 13, 2015
    It's said that it's related to MXL Microphone. Please look at the buttom section of the circuit. The condenser capsule has been supplied the power through the circuit. I guess it's not an oscillator. Does this rejects ripple or why? As photo of real mic, there are two board in two side, as shown in schematic.
  2. kubeek


    Sep 20, 2005
    Not sure what you´re asking. The bottom part of the circuit is an oscillator with a voltage doubler, that provides voltage bias for the mic capsule.
    Top part is a phase splitter with some gain and then two buffers to drive the symmetric line.
    Willen likes this.
  3. Willen

    Thread Starter Member

    Nov 13, 2015
    Hi kubeek,
    Thank you, I got the answer from your reply. I had simulated the circuit in LTspice but was not getting oscillation so I thought it is not an oscillator. Maybe I made some mistake in simulation.

    Since the XLR has 48V supply already then why they regulated it to 6.8V at that buttom circuit and again double the voltage ( to 12v?) ? I thought the condenser capsule need 48v but ?
  4. kubeek


    Sep 20, 2005
    Maybe the capsule needs lower voltage? Not sure now. Also I don´t see a way for the supply to be powered from the xlr. I would expect coupling caps in series with R1 and R2, and some resistors from the xlr going to the positive rail to power it. The diodes also seem to be drawn in the wrong direction (if they are replacing that power supply resistor)
  5. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    The two inductors form a transformer. It won't oscillate if you simulate as separate inductors.
  6. ericgibbs


    Jan 29, 2010
    hi willen,
    On the LTS asc diagram, add the Directive K1 L1 L2 1
    Where K1 is the instance of the transformer [ K2 for another pair ... and so on]
    L1 and L2 are the Inductors
    1 is the coupling factor between L1 and L2 [ typically set for 1 = 100%]

    230Vrms mains to 9Vrms transformer example.
    AA1 10-Oct-18 15.01.gif
  7. Willen

    Thread Starter Member

    Nov 13, 2015
    If the capsule need low voltage then why need phantom power? Just for XLR technology which ignors sorrounding noise?

    I was simulating as seperate inductors. Because had I found a photo of MXL microphone board here:
    and here too.
    I thought the bottom circuit in #1 is supply side board of the mic, same as shown in the link at right side. There are two seperate inductors and two diodes etc which seems pretty similar as the circuit I am asking.
    Last edited: Oct 10, 2018
  8. DickCappels


    Aug 21, 2008
    Another way to put that is the phase splitter and emitter followers deliver a balanced signal to the XLR connector.
  9. bertus


    Apr 5, 2008

    The circuit looks like a phantom powered microphone to me.

  10. Willen

    Thread Starter Member

    Nov 13, 2015
    Yes it is. But the lower section of the circuit (as post #1) is confusing. Some are saying it's an oscillator with L1 & L2 as transformer but circuit boards has seperate inductors (as #7).
  11. ebeowulf17

    Distinguished Member

    Aug 12, 2014
    I'll admit I haven't wrapped my head around that circuit yet, but I can answer part of your question about voltage levels.

    There are two types of condenser mics. "True condensers" require an external bias voltage to charge the condenser element, whereas electret condensers have a charge permanently applied to the capsule - no external bias required.

    The bias for "true condensers" is high - 48V was a common enough number that it became the original standard for phantom power, and that's what nearly all "true condenser" mics require. Since electrets don't require a bias voltage for their capsule, they only need enough voltage to power the internal preamp circuitry. That requirement is much lower - numerous electret mics spec acceptable phantom power as low as 11V.

    Most pro audio equipment is still set up for 48V phantom power, but there are some mixers, field recorders, and power supplies that only provide 12V phantom power.

    So, it's a safe bet that the mic in question is an electret mic, and it makes sense for them to design a circuit that can run happily on the lower 12V standard (with some margin for error.)