0
\$\begingroup\$

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?

\$\endgroup\$
  • 4
    \$\begingroup\$ You would probably need to make a small single op-amp mixer to sum as many microphones as you need. The output would feed into the transmitter in place of the original mic. It could be tiny as you wouldn't need volume controls on each mic as in a standard mixer. I've been told that the definition of a gentleman is someone who can play the accordion but doesn't. Is it true? \$\endgroup\$ – Transistor Feb 21 '16 at 22:09
  • \$\begingroup\$ Haha, that made me laugh! I've heard many accordion jokes but not that! Would the op amp color the sound at all? I've never used one before. You say a single one would be enough or do I need one for each microphone? If I could use three mics it would be ideal :-) \$\endgroup\$ – jason Feb 22 '16 at 14:24
2
\$\begingroup\$

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?

\$\endgroup\$
1
\$\begingroup\$

An electret microphone with a FET buffer can be thought of as a variable current sink. A single microphone is normally powered through a resistor that gives it enough current so that there is a DC bias of a few volts across the FET.

The variable current creates a variable voltage across the resistor, which is then amplified further.

There's nothing wrong with connecting two such microphones in parallel, except that the total DC current required by them will cause the voltage drop across the bias resistor to be increased, possibly to the point where there is no longer enough bias voltage across the microphones themselves.

The fix for this would be to decrease the value of the bias resistor by a factor that's equal to the number of microphones. This will restore the DC bias voltage to its former value, but now the signal voltage will be reduced by the same factor, requiring more gain in the subsequent stages.

So, if you're comfortable opening up the transmitter, identifying and changing the microphone bias resistor, it should be the simplest way to achieve what you want.

\$\endgroup\$
  • \$\begingroup\$ If I didn't change the bias resistor (the transmitter case appears to be glued shut!) then would I simply have to turn up the gain on the transmitter? I have some headroom. Is there any possibility of damaging the transmitter? \$\endgroup\$ – jason Feb 22 '16 at 14:22
  • \$\begingroup\$ No, you won't damage the transmitter. But you may experience distortion in the audio signal. Go ahead and try it. \$\endgroup\$ – Dave Tweed Feb 22 '16 at 15:20
0
\$\begingroup\$

Just try it with the existing capsules. If they are the current sink type, they will not drive each other. If the voltage drops, too low, you will get distortion or nothing at all, then follow Dave Tweed's instructions to change the resistor. It would make sense to record the sound from the receiver in different mic configurations and compare the recordings to get a good idea.

\$\endgroup\$
  • \$\begingroup\$ Do you suggest that I simply try wiring them together as is? Would I do it in series or parallel? Would putting resisters on each one help? I'm currently only using about half of the available gain on the transmitter. \$\endgroup\$ – jason Feb 22 '16 at 14:19
  • \$\begingroup\$ Definitely in parallel, again assuming they are the current sink type. The most likely failure mode for this approach is that the DC voltage will be too low, so resistors would only make things worse. \$\endgroup\$ – biggvsdiccvs Feb 22 '16 at 22:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.