2 inputs microphone combiner with balance control

[saint gillis]

Hi friends,
I need to build a box to combine 2 microphones, with a balance control between the 2 mic inputs, working in a kind of unity gain.

  How would you proceed?

 

[joaquins]

  Best way is two mic preamps and then a summing amp, you get the line level at the output, if you really need you can attenuate and get the mic level. IC mic preamps is probably the way to go. Then the summing could be passive if you don't want an extra opamp.

  If the two mics are identical and passive and you always want the same level in both you could connect them in series and probably get away with it.

JS

 

[Gene Pink]

  How would you proceed?

Me?

Probably in a butcher way, something down and dirty passive, as the attached jpeg. Note that I guessed at what might be reasonable values for the resistors and pot, tweaking may be needed, depending on source and load Z.

You did say "a kind of unity gain", right?  ;D

And it should work with phantom.

Gene

 

[saint gillis]

Thanks guys, I  forgot to mention it should be powered by phantom.
So the butcher way should work hehe, but it's a bit too brutal..

I guess using 2 preamps like that1542 draws a bit too much power for a phantom, especially if we consider that we want to also use the phantom with the 2 connected mics, and in addition I guess I also need something to make the signal balanced again..

 

[JohnRoberts]

Last century when I was designing for this (and other) market(s), I speculated about an active mic input expander for when a mixer just needed one or two more inputs.

I scratched out a design powered from the mic input's phantom voltage. Just imagine one NPN LTP per additional mic input. A variable resistance between the LTP emitters could act as a relative gain trim. The LTPs would be biased up at some fraction of the phantom voltage and collectors would all be summed together into + and - inputs . The input current signals convert back to voltage in the preamp's input resistive termination (approx 2k).

This mic extender box could include polarity swap switches, pad, mute, gain trim, etc.  I even speculated about phantom powered mics still working and they probably could but loading down the input DC operating point, while supplying larger signal swing was working in the wrong direction for optimal headroom.

I am pretty confident this "could" work, but never got as far as melting solder, because the business model was absolutely terrible... why sell a customer a $20 accessory gadget, with miniscule profit, when you could just sell them a new larger mixer.  ;D  Back then the price of new mixers were dropping so the upsell was relatively easy for dealers.

When combining identical microphones doing pretty much the same thing, passive combining (like Gene suggested) can work... for more money than it is worth (IMO) dual primary mic transformers  could combine the two mic signals. To keep mic termination impedance correct you would probably give up several dB of signal level.

JR

 

[saint gillis]

Thanks JR!
I finally have to abandon this project but your LTP idea seems to be the thing!

It is true that we usually never need microphone buffers, I guess it explains why their are so few resources on the subject...

 

[JohnRoberts]

Thanks JR!
I finally have to abandon this project but your LTP idea seems to be the thing!

It is true that we usually never need microphone buffers, I guess it explains why their are so few resources on the subject...

Years ago I actually designed and made a few JFET "Direct Injection Cords" (DICs  8) ) for studio friends.  I built a JFET LTP with bias network into a 1/4" guitar plug, the other XLR end plugged into a phantom mic input...  Presto a low noise, high input Z,  phantom powered guitar input DI/buffer... all built into a cord. 

Another gadget I never brought to market (mental masturbatiuon).

JR