Push to Talk Box

Discussion in 'The Projects Forum' started by card9inal, Aug 25, 2012.

  1. card9inal

    Thread Starter New Member

    Aug 25, 2012
    Hey everyone...

    Could someone help me figure out what I'm doing wrong here?

    This is a box I built for communications between the engineer and the talent for a radio show.

    The goal was to have the program audio in the headphones at all times. When the engineer presses a button, the program dims and the engineers voice (out of the mic pre) gets inserted into the headphones.

    The ring on the output comes from a modified Rolls PS16. The outputs feed Rolls PM50 headphone amps, and the ring carries 12VDC to power the amps over the TRS, carrying the audio signal on the tip. Ordinarily, the PS16 has a single input and sends that out, with power, to all 4 outputs. Since I needed each output to get it's own feed, I modified it so that it only is contributing the power, allowing the rest of the circuit to focus on the audio signals.

    That part works (though I have to go back and adjust the resistors because the level is too low when the button is pressed). However, the problem I'm having is that when you press button 1 to talk to Person A, Person B can hear the engineer faintly. The program level won't dim obviously, but I don't like the fact that the engineer's voice isn't discreet as it should be.

    How can I fix this? Is there a better circuit that does the same thing but more reliably?

  2. cedguru

    New Member

    Feb 25, 2012
    It sounds like crosstalk to me, if the eng. mike signal is faint in the other headphones... How is this wired from the control room to the guest studio? Each headphone feed needs to be a discrete shielded pair, (even if it's a multiconductor cable, eg: a snake) with the shield (drain) going to the sleeve (but, of course, on one end only). You should be able to run only one wire between the rooms for power... It sounds like crosstalk to me, if the eng. mike signal is faint in the other headphones... If this doesn't work, consider shielding audio lines to the relays also.

    Code ( (Unknown Language)):
    2. Shield--------------------------O Sleeve   2-cond shielded cable
    3. Black___________________________|        Connect shield and black on this end
    4. Red_____________________________O Tip  Leave shield disco. on ctrl room end
    5. Shield---------------------------
    7. Power___________________________O Ring on all Headphones
    Alternatively, connect the shields to studio ground on one end, and leave them floating on the headphone end.
    In most mixing boards made for radio, switching like this is done with relays. I didn't see anything wrong with your layout.
    Last edited by a moderator: Aug 26, 2012
  3. card9inal

    Thread Starter New Member

    Aug 25, 2012
    It's strange because it seems like it's more than just crosstalk. That could be the cause, but I'm not sure.

    In reality, the engineer/producer are in the same room. They are all in a broadcast booth together, but because of the ambient noise, there's no harm in the engineer/producer speaking at normal volumes into the microphone.

    I realize now that I used the wrong symbol (it was the closest one in the stencil pack I had downloaded) in that I'm not using relays, but DPDT pushbutton switches.

    You are correct that each headphone feed is getting it's own shielded pair. (It's actually some of the very best snake cable I've ever used, flexible Mogami stuff.)

    The shield is connected on both ends of the pair, as I think it has to be because it's the ground and reference for both the audio on the tip and the 12VDC on the ring. Am I missing something about that?

    Before I introduced the talkback circuitry for each individual person, the Rolls PS16 does exactly the same thing but splits the same input to 4 outputs, whereas I'm giving each person their own feed with talkback. I don't believe I can only use 2 conductors on one side of the cable.

    I could shield the lines to the switches I suppose, but do I just connect those to ground, as every other shield is? What do you recommend I use to wrap them with?