# PTT Box



## dannyn (Oct 22, 2009)

Does anyone have a good schematic for building a push to talk box with xlr in and out?

Thank you.


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## mbenonis (Oct 23, 2009)

Not offhand, but I bet you could come up with one pretty quickly. Basically, I would recommend the switch be a DPDT switch, with the on terminal connecting the input to the output and the off terminal grounded to avoid pops and clicks. Give it a shot and see if it works!


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## Chris15 (Oct 23, 2009)

mbenonis said:


> Not offhand, but I bet you could come up with one pretty quickly. Basically, I would recommend the switch be a DPDT switch, with the on terminal connecting the input to the output and the off terminal grounded to avoid pops and clicks. Give it a shot and see if it works!



Mike Mike Mike Mike Mike...
I'd have expected better of you.
If you are going to use Mike's approach you need to do some coupling workarounds to not go destroying things when phantom power comes into the mix... Namely a diode bypassing the switch and a decoupling cap on the output side of the switch, for each of the hot and cold legs...

Or use the easier approach. Loop the signal in and through. Insert switch between pin 2 & 3 (SPST switching is all that is needed). This shorts the signal so you get no audio (assuming the CMRR of the input is reasonable) but don't stuff up any phantom...


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## mbenonis (Oct 23, 2009)

I stand corrected...I felt a bit odd about my solution but I couldn't put my finger on it. Chris15's solution of tying pins 2 and 3 together is a MUCH better one and I highly recommend it.

Recall that a balanced input stage looks for a voltage difference *between* pins two and three, and amplifies it. Thus, by shorting the pins together, there is no ac differential and thus the signal is turned off. 

Thus, don't do what I suggested above!


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## dannyn (Oct 27, 2009)

Hello guys,
Thank you for the prompt responses. I forgot that this forum does not email you when you are posting in a thread, I need to go change my settings on that one. 

I will try that and see how that works.

Again thanks.


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## Chris15 (Oct 28, 2009)

Oops, just remembered... What I described is really a push to mute circuit, so if you are using a push button to achieve PTT, you need a normally closed switch...


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## Austel91 (Oct 28, 2012)

I was wondering if this diagram looks accurate. The box works fine but there are still some minor issues that I think have to do with the fact that I am using a poorly shielded cable. I used this PTT box with a 5 pin DMX cable and found that there was a lot of feed back/ or hum especially as the volume was turned up. I plan on re-making it with a better SPDT switch as well as with a 50 ft. 5 core wire. Any suggestions would be greatly appreciated.


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## bishopthomas (Oct 29, 2012)

How many do you need? If just one I would consider buying it. Here's what I have: Sennheiser MZTX 31 PTT NX - IS-Series Gooseneck MZTX31PTT NX B&H It also happens to double as a table top mic stand.


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## Austel91 (Oct 29, 2012)

I just need one to extend comm for the length of our jibb. I looked into buying one, but it is a lot cheaper to make and I am trying get more familiar with these types of home made alterations to intercom systems/pinouts. Thanks for the link, I will probably still shop around so I can have a back up if my time becomes more limited.


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## Austel91 (Nov 3, 2012)

Thanks for the help I made this one work. I just need the write shielding on the cable I was using. I was trying to make use of old cable and it was the wrong stuff. And it seems like stanton makes their control boxes that also have a talk button installed on them. My next project is to figure out how the camera is wired with regards to intercom and talk buttons and how they interact. 


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## Chris15 (Nov 3, 2012)

A few thoughts...
Shorting your mic to mute it may end badly.
SOME systems will provide a bias voltage on the mic pins to allow the use of electret microphones.
Shorting this is not wise and is likley to produce clicks and pops as the least of its symptoms...
If one capacititively coupled the switching that would eliminate the issue.

What headsets are you using?
I'm not convinced 500R is the right pot to be using but that will depend on the headset ear impedance...

Your drawing is not bad, but generally we only expect to see straight lines on such diagrams, no diagonals, so I'd be shifting a few lines a few pixels to make straight corners and it will make it look much more professional...


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## Austel91 (Nov 4, 2012)

We're using the Telex PH-1/PH-3. And there is definitely a dirty noise when the switch is turned on or off. The impedance of the headset is 150 ohm. 


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## Austel91 (Nov 4, 2012)

My apologies the mic impedance is 150 ohm and the headset has an impedance of 300 ohm per side. And when you say capacitively couple the switch, you mean to use a cap on the talk side of the switch to prevent any voltage overages created by shorting the mic to ground. Would the correct capacitor prevent any unwanted noise (clicks, beeps, etc). I'm in the process of learning all of this, so if this doesn't make sense I apologize. 



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## pmolsonmus (Nov 6, 2012)

http://recording.org/diy-pro-audio-forum/46137-push-to-talk-mic-circuit.html 
More discussion here


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## DrPinto (Nov 6, 2012)

pmolsonmus said:


> http://recording.org/diy-pro-audio-forum/46137-push-to-talk-mic-circuit.html
> More discussion here



Very interesting thread on a really informative BB, although I did have to laugh when I noticed that their BB censored a user's location of "West Sussex, UK".


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