Electrical – Combining two phantom power microphone capsules

audiocondenser-microphonemicrophone

I just bought a two channel wireless system for my accordion. It is two separate battery powered transmitters transmitting to a single dual channel receiver. It's good that I can have one microphone for the left hand (basses) and one for the right hand (piano keyboard). I'm using condenser microphone capsules that came with a different devicebut it seems to work and the transmitters appear to be giving out phantom power (although a little low at only 4v measured by multi meter).

My problem is that the spread of the microphone for the right hand is not enough. To avoid feedback I need it close to the keyboard but when I do that it only picks up the five or six notes closest to it. I want to combine (cheaply!) another microphone capsule into the transmitter for the right hand so I can have two pointed at different parts of the keyboard. I don't want to buy a third transmitter.

I understand from reading online that this is a bad idea? People have asked this question and been told that one microphone will drive the other, etc etc, but would it really be that bad? Admittedly the quality of the six notes that ARE being picked up currently seems to be very good! However, I would give up some quality if it allowed the rest of the notes to be heard too!
Any thoughts?

Best Answer

The AKG PT40 Pro manual, page 17, states

You can use the PT 40 PRO bodypack transmitter with both dynamic microphones and condenser microphones operating on a supply voltage of approx. 4 V. You may also connect an electric guitar, electric bass, or remote keyboard.

This is good as you now have several options. Note that the guitar input has a mini-XLR connector so you need their MKG L guitar cable to connect.

Condenser microphones

On page 19 we read:

Audio input pinout:

Pin 1: shield

Pin 2: audio inphase (+)

Pin 3: supply voltage

A 4-V positive supply voltage for condenser microphones is available on pin 3.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Condenser microphone bias circuit.

Your mic should have a circuit something like that of Figure 1. R1 provides the required bias current. Since the manual says that 4 V is available on pin 3 that suggests that the resistor is in the mic capsule. In this case you could parallel several complete capsule circuits, connecting them all together at the right hand side of C1 and feeding the combined signal to your transmitter input. I'm not sure if they'd interfere with each other.

Mini-mixer

schematic

simulate this circuit

Figure 2. Mixer.

Figure 2 is a simple inverting summing op-amp mixer. The output is the sum of the inputs and the circuit configuration prevents interference between the inputs.

  • The gain is set to 1 (actually -1 as it's inverting) by the ratio of each input resistor to R4, the feedback resistor.
  • No capacitors are shown on the inputs as it is assumed that these exist already on the microphones.
  • The '+' input of the op-amp is biased to half supply, 4.5 V, to allow the signal to swing high and low. The output is biased to the same voltage so a decoupling capacitor is used to block DC getting to the next stage.

If using this circuit I suggest you feed to the guitar input. If required you could increase the gain of this circuit by increasing the value of R4 to 50 or 100k.

Why am I helping an accordion player?

Related Topic