2 Microphones to 1 input

Hello,
I would like to build a passive box that would allow me to combine the output of 2 Dynamic microphones to feed 1 Mic preamp input only.
This is handy sometimes when I don't have enough channels on a Console or in the Multicore cable that feeds the console.
I would like to use this to combine the 2 snare microphones, Top and Bottom.
The box would have switches for polarity reversal on each mic input, and I would like to have a variable pad for the snare bottom mic.

Can someone please recommend me what variable pad should I use for this application? Can I use a dual gang pot for the variable pad?

Also how should I combine the mics, just parallel the two microphones to the output or do I need any summing resistors?

thank you so much

The important thing is that the load on each microphone remains the same. You therefore need a constant impedance variable pad and a constant impedance mix. (google is your friend).

Cheers

Ian

Summing resistors at microphone level means (IMHO): noise!

There are a lot of ways to do this the easiest of which is a simple "Y" cable with a phase reverse for your snare scenario to a transformer summing array to an active buffer. The problem of course is level in any passive combiner scenario changing the volume relationship is usually not available. My suggestion is get your hands on a small mixer and sub-mix the signals then send it down the snake, this kills two birds as it also brings the mic level up to line level and as an added bonus if the little mixer has phantom capability you can use a condenser.

Whoops said:

Hello,
I would like to build a passive box that would allow me to combine the output of 2 Dynamic microphones to feed 1 Mic preamp input only.
This is handy sometimes when I don't have enough channels on a Console or in the Multicore cable that feed the console.
I would like to use this to combine the 2 snare microphones, Top and Bottom.
The box would have switches for phase reversal on each mic input, and I would like to have a variable pad for the snare bottom mic.

Can someone please recommend me what variable pad should I use for this application? Can I use a dual gang pot for the variable pad?

Also how should I combine the mics, just parallel the two microphones to the output or do I need any summing resistors?

thank you so much

Click to expand...

OK, let's write down the problem.
In order not to alter the sound of your microphones, they must see an impedance of about 1-2k. In order to satisfy your preamp's requirements, the resulting impedance presented to it should be as low as possible, hopefully about 500 ohms.
All the parallel variations won't do.
In order to satisfy these contraints, you may want to use a series connection; see attached schemo.

abbey road d enfer said:

Whoops said:

Hello,
I would like to build a passive box that would allow me to combine the output of 2 Dynamic microphones to feed 1 Mic preamp input only.
This is handy sometimes when I don't have enough channels on a Console or in the Multicore cable that feed the console.
I would like to use this to combine the 2 snare microphones, Top and Bottom.
The box would have switches for phase reversal on each mic input, and I would like to have a variable pad for the snare bottom mic.

Can someone please recommend me what variable pad should I use for this application? Can I use a dual gang pot for the variable pad?

Also how should I combine the mics, just parallel the two microphones to the output or do I need any summing resistors?

thank you so much

Click to expand...

OK, let's write down the problem.
In order not to alter the sound of your microphones, they must see an impedance of about 1-2k. In order to satisfy your preamp's requirements, the resulting impedance presented to it should be as low as possible, hopefully about 500 ohms.
All the parallel variations won't do.
In order to satisfy these contraints, you may want to use a series connection; see attached schemo.

Click to expand...

Hi Thank you so much for your knowledge.
I tried to understand the schematic, but Im really not sure I understand how can this work. Also I even didn't know that you could connect 2 microphones in series and that the signal from both would be summed.

But your schematic seems simple enough and I will do as you suggested, thank you so much

> Summing resistors at microphone level means (IMHO): noise!

Yes; but OP specified "snare top and bottom" which probably means close-micced and HIGH level (unless he works with Philly Joe Jones). Anyway we "always" noise-gate the snares to cut leakage from other drums.

> I even didn't know that you could connect 2 microphones on series and that the signal from both would be summed.

Take two 1.5V batteries. Connect them in series. They "sum" to 3V.

Same works for AC/Audio and for microphones which have NO other connections (such as Phantom).

PRR said:

> Summing resistors at microphone level means (IMHO): noise!

Yes; but OP specified "snare top and bottom" which probably means close-micced and HIGH level (unless he works with Philly Joe Jones). Anyway we "always" noise-gate the snares to cut leakage from other drums.

> I even didn't know that you could connect 2 microphones on series and that the signal from both would be summed.

Take two 1.5V batteries. Connect them in series. They "sum" to 3V.

Same works for AC/Audio and for microphones which have NO other connections (such as Phantom).

Click to expand...

Thanks, will work with this schematic and build the box and let you know the results

thank you so much

abbey road d enfer said:

Whoops said:

Hello,
I would like to build a passive box that would allow me to combine the output of 2 Dynamic microphones to feed 1 Mic preamp input only.
This is handy sometimes when I don't have enough channels on a Console or in the Multicore cable that feed the console.
I would like to use this to combine the 2 snare microphones, Top and Bottom.
The box would have switches for phase reversal on each mic input, and I would like to have a variable pad for the snare bottom mic.

Can someone please recommend me what variable pad should I use for this application? Can I use a dual gang pot for the variable pad?

Also how should I combine the mics, just parallel the two microphones to the output or do I need any summing resistors?

thank you so much

Click to expand...

OK, let's write down the problem.
In order not to alter the sound of your microphones, they must see an impedance of about 1-2k. In order to satisfy your preamp's requirements, the resulting impedance presented to it should be as low as possible, hopefully about 500 ohms.
All the parallel variations won't do.
In order to satisfy these contraints, you may want to use a series connection; see attached schemo.

Click to expand...

Hello,
tested the circuit today it works perfectly. Thank you so much.

I just added a DPDT switch to invert phase on the Top snare mic.

Just to be safe before testing with phantom,
will this circuit work with phantom power applied in case I use it with condenser microphones?
Is it safe to use it with Dynamic mics when the Phantom power is on by default?

I'm just afraid that both sides of the capsule don't receive the same voltage

thanks

Dynamics with phantom is ok, no problem there, they will just float around with no problem.

If you have two condensers you need to provide the current for both, I don't know how is your final circuit but you may need to implement something to make it work properly. Usually nothing will break, the resistors on the phantom will limit the current to safe levels, you may be a little short in headroom if there's not enough current for two mics.

Using a condenser and a dynamic for the same source is quite hard, dynamic peaks voltage are when the diaphragm is travelling fastest, at the mid point of the journey. Condensers peaks are when the diaphragm is standing on the extremes of the travel. That gives a 90º phase shift between both, so not so easy to work around just connecting them in parallel. I remember an AT I think it was which made something about this and build a dual mic, one dyn, one cond. I've used a combination of both on some sources but using one to each side in a stereo mix. Strange stereo effect, I like it for some voices or guitars. When they go to mono they sum not perfectly, so less than 6dB boost which is quite good so it doesn't jump out of the mix so badly as a perfectly centred signal does.

JS

Years ago a scratched out a design to sum 2 or 3 dynamic mics into  a single input channel. I would use the phantom power to bias up 2 or 3 LTP with relative gain trims, and sundry per input features (polarity, mute, etc).

After soaking it in alcohol for a while I determined that it was just not a commercial stand alone product since new mixers with more channels are very price competitive.

Interesting to explore to learn more about how mics behave, but beyond that just get a bigger mixer.

JR

Whoops said:

Just to be safe before testing with phantom,
will this circuit work with phantom power applied in case I use it with condenser microphones?

Click to expand...

With one dynamic and one condenser, it should work although one may object to the fact taht one leg of the condenser mic will receive a marginally different voltage that the other, and that there is DC current flowing into the dynamic. The current is usually low enough to feel comfortable with it (typically less than 2mA). However, some mics may draw more current (5mA per leg); that may alter the sound and sensitivity of the dynamic mic.

Is it safe to use it with Dynamic mics when the Phantom power is on by default?

Click to expand...

Yes; dynamic mics "float".

I'm just afraid that both sides of the capsule don't receive the same voltage

Click to expand...

Whatever the unbalance of phantom voltage, the internal voltages of the mic are unaffected (within limits).

Finally, a minor issue is that the overall CMRR will be somewhat degraded; it should not be noticeable in snare drum applications.

Thank you for the explanations.

It's excellent for my needs, wanted something like this for a long time.

Here is the setup I did for tests.

And this will be the final thing: