microphone headphone monitor base on cmoy

[severthee]

good day,

i recently built a headphone amp for 2 mic using the  simple cmoy design. i split the left and right input into xlr 1 and xlr 2.  each has their own 10k pot. i dont know how to wire them balanced so i just shorted ground and cold of the xlr for now.

. i added a spdt switch right next to the xlr input hot wires , to switch between split mono (hard panned to left or right) or stereo(dual mono or mixed signal?). the stereo mode is a bit weird, because the levels of both mic are affected.

i moved the spdt switch to the headphone output. while it was doing what it was supposed to do in stereo(mixed signal on both left and righ ear) levels of xlr 1 and 2 are independently controlled. the sound was dull, lots of noise, distortion. i am not sure how to describe it. out of phase perhaps?

should i move the spdt swich right after the 10k pot to achieve what i want? both xlr signals on both left and right ear, but volume is independent from each other.

did some reading and found a thread here where it  was suggested to invert the cmoy to be a 2 channel summing mixer. how do i go about this?
does that mean i direct xlr 1 to input A + of the opamp and the xlr 2 to input B- ??
or xlr1 goes to input A+ and xlr2 to he input A-?

lastly, one of the xlr needs to be send thru, so i did a parallel wire to the xlr out. i realized that there will be a long xlr going to the PA. I was wondering how to wire that xlr to the opamp as a balanced wiring (hot, cold, ground) because right now i shorted the ground and cold pins to test and make the mic headphone amp to work.

looking forward to any suggestions. thanks in advance!

p.s. im using the same exact schematic for cmoy, no change in values. i just change the input to two xlr. using a TL072 for now but it needs 18v to work properly. so im gonna change that to jrc4556 once i sorted out my problem

[PRR]

When you combine two signals, you usually MUST use resistors to mix them. Otherwise the two sources fight.

No microphone preamp? I would not think the Cmoy had enough gain to be audible direct from mike.

[samhay]

NE5532 is a better choice for headphone amplifier than TL072 or 4558. They are all happy running on 18V.

[severthee]

Thanks for the suggestion. I will check out ne5532.

@PRR, yes , theres no preamp. Surprisingly it has enough gain, im using it for custom in ear monitor.

I was able to mix both signals now by connecting signals AFTER the pot. The pots are still affecting each other anywhere from 7 - 9 oclock of the pot(where 7oclock is the pot fully turned off ccw), but anywhere above 9oclock, they (left and right input) are totally independent.

What ive yet figured out is how to convert one of the inputs into a Balance line (+, - , gnd)

[PRR]

There are many variant "Cmoy"s. I hope you can recognize your build in this rather fancy build.



Key points:

10K resistors off the Volume pots before the mono switch/strap will reduce interaction.

For "balanced" input you need differential amplifiers(*) on the input(s). Here's a basic diff-amp with Cmoy power arrangements.
(*) Or a transformer, but good ones are expensive and hum-prone.

[severthee]

@PRR thanks for the schematic! that made alot of sense. with the resistor in series, the pot shouldnt be shunting the other pot to the ground.

regarding the differential amp on the left, looks like i need another opamp for that.
i thought i can just duplicate, the 10k pot, c2 and r2 then connect it to the negative Input A of the opamp U1A.

unfortunately i dont have a space for another opamp since i started with a veroboard and with a boss size hammond enclosure. while it seems like the enclosure is big enough, i have 3 xlr jacks that i punched on one side, and theyre taking up too much space. (xlr1, xlr2, and another xlr parallel to xlr1 for send thru that needs to be balanced)