Single-line PTT for electret microphone system - Page 1
Forum section: electronics / projects

author: larry12193
author_rank: Newbie
timestamp: January 20, 2015, 06:04:35 pm
content: I am building a 8-person intercom type communication system with headsets that have electret microphones. I have been successful at pre-amplifying the microphone and have been getting really clean audio out, or at least clean enough for this application. The issue I am having is when I add the push-to-talk in series with my circuit.When I hit the PTT, the people listening in are heading a pop sound from the FET inside the electret mic turning on, or something like that. I have tried connecting the wire on the bottom (with the question mark next to it) to the shell of the XLR, through a resistor to earth, to a capacitor and resistor to + Mic, to a capacitor to Headphone common.I have also hooked the system up so the +Mic is on the PTT line and power is supplied over the question mark line. I thought this would keep the mic always powered and eliminate the popping, but no such luck.A side issue I am having is coupling between mic lines and headphone lines over the some 50ft of cable I have running from the headsets to the audio equipment. I've tried putting a capacitor in between the -Mic and Headphone Com lines to decouple them, but to no effect.So, essentially I am looking for recommendations on which microphone line I should be cutting and what I should connected to the question marked line in order to keep the audio from popping when I hit the PTT.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 20, 2015, 06:28:49 pm
content: You have the wiring backwards.The line you have indicated with the question mark is actually the ground from the microphone.The line you are trying to use for microphone ground is actually the PTT signal.You need to mute the microphone audio signal except when that PTT line is grounded.It would have been nice if the PTT switch were NC (normally closed).Then you could have simply used the switch to short out the mic audio with no extra parts.But since it is NO (normally open) you will likely need an extra circuit to do the muting.Could be as simple as a relay or a single BJT transistor.470 ohms seems unusually low for that application. Are you sure of that value?You did not mention what kind of cable you are using.Since the mic signal is low level and unbalanced, it is unsuitable for running long distances without being separately shielded from the headphone lines.If this is only for communication use, you could treat the headphone L/R as a differential pair (ignoring the common ground) and that might buy you better isolation from the unbalanced mic signal.

author: larry12193
author_rank: Newbie
timestamp: January 21, 2015, 07:40:04 pm
content: Richard,Thanks for responding. Your suggestions did the trick. I added a small switching circuit with a PNP transistor and a small relay to handle muting.The resistor value was from themicrophone's data sheetas part of a test circuit used to find the frequency response of the device or something like that. In the new circuit I replaced it with a higher value resistor I had sitting around and it didn't seem to change the operation of the microphone. What would be typical for this application?Also, the coupling I was getting over the cable has gone away completely with the new circuit.

author: Pjotr
author_rank: Frequent Contributor
timestamp: January 21, 2015, 07:53:11 pm
content: Quote from: larry12193 on January 21, 2015, 07:40:04 pm2200 ohm from 5V. At higher voltage those mics tend to be noisier due to higher drain-gate leakage. But I doubt it matters much in this application.

author: AlexP
author_rank: Newbie
timestamp: April 21, 2015, 09:18:39 pm
content: Wow, I think I'm currently working on the same exact project Larry was working on when he made this thread. Small world.Currently, the system technically 'works', but there's a pretty loud echo that occurs when you talk. This manifests itself in two ways:1. When your Push-to-talk is off, the mic line is not completely isolated. It seems like other headsets can still barely hear you, but very quietly. You can't hear yourself. This isn't really an issue, but it may be an important clue in finding the culprit of the problem.2. This is the bigger one. When you push the PTT key, other headsets can hear you, but you can also hear yourself. Your voice comes back to you slightly quieter, but extremely distorted. I've noticed that the problem is proportional to the gains on the headsets & microphones. I've tried tuning them to bring it to a tolerable level, but can't seem to hit that point, which is why I really want to identify the root of the problem.Some background real quick, the eight headsets connect through a module to VOIP, which let you access the channels through other headsets on the network. When using a network headset to talk to a single headset connected to the module, everything works fine, and there are no distortions or echoes. However, as soon as you plug in a second headset into the new module, the signal starts to return a corrupted echo to the transmitting user (per bullet 2).Does anyone have any experience with audio systems that might have ran into something similar before? I thought it might have been a groundloop issue, but so far none of my attempts to improve it have been effective.

author: lincoln
author_rank: Regular Contributor
timestamp: April 21, 2015, 09:29:58 pm
content: You might snoop the internet for the key word "clear com body pack", I haven't needed them in a long time but they were the go to party line system format in the us, telex being the other non compatible.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 21, 2015, 10:30:12 pm
content: Quote from: AlexP on April 21, 2015, 09:18:39 pmThis is mostly a function of the part of the circuit/system that is NOT shown in this discussion.QuoteIn the circuit above, the mic line is completely isolated by the relay contacts. If there is still crosstalk, then it is happening elsewhere (beyond the circuit shown). There are probably at least a dozen variables here, none of which we have any visibility into.QuoteAgain, this is a function of what you have the headsets connected TO.QuoteIf you want any practical help or advice, you will need to provide all the details about your mystery "module", how you have everything configured, powered, wired, etc. etc. There are probably a dozen critical details not in evidence at this point.As @lincoln said, the traditional configuration for a party-line intercom system is something like the ClearCom. They still sell the original analog party-line system as well as a dozen more modern, digital systems.I actually have a clone project for the original ClearCom analog "belt-pack" on my website here:http://www.rcrowley.com/ComClone/default.htm

author: larry12193
author_rank: Newbie
timestamp: April 21, 2015, 11:06:47 pm
content: Alex, if you are my replacement then I would look into the the CAT6 cable runs being the issue. I wish the pairs were individually shielded, that would fix thefeedback issue I think.

author: AlexP
author_rank: Newbie
timestamp: April 22, 2015, 01:45:58 am
content: Quote from: Richard Crowley on April 21, 2015, 10:30:12 pmHey Richard,I read through your ClearCom project, very cool! (and cheap!) Sorry for the somewhat vague description. Here's a more detailed diagram of the current setup.The zener barriers are required as the headsets are for use in a hazardous area. I tried different configurations of the setup; removing both the CAT6 and Zener barrier led to the same problem, which makes me doubt that they are the issue. Additionally, both the headphone and mic amplifiers are off the shelf rack components, which gives me some confidence in those as well. Other than the CAT6, all lines are fairly short. (1-5 ft.) Please ask if there's a specific part you'd like me to elaborate on.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 22, 2015, 02:14:49 am
content: Dunno what is a "zener barrier", perhaps because your image isn't working.

author: AlexP
author_rank: Newbie
timestamp: April 23, 2015, 06:03:36 pm
content: Hey Richard, that last picture was fairly comprehensive and showed the entire system. I tried it without both the zener barriers and the CAT6 spans and the problem persisted. These might be more relevant:In the PTT circuits, all of the 12V and grounds are tied together (not to eachother). Could this be causing an issue? Are you aware of any problems with running a differential mic signal attached to ground?

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 23, 2015, 06:15:42 pm
content: @AlexP, your images are NOT WORKING. We can't see anything but a little "missing image" icon.

author: AlexP
author_rank: Newbie
timestamp: April 23, 2015, 06:28:06 pm
content: Hmm, sorry about that. That's really strange, they're showing up for me. Do the direct links work for you?http://s2.postimg.org/3ra5myqzb/Diagram.pnghttp://s17.postimg.org/3laiaab7y/Diagram1.jpghttp://s18.postimg.org/lud0bnw0o/Diagram2.jpg

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 23, 2015, 06:37:08 pm
content: Yes, the links work. Do you have any information about those "Mic Pre-Amp" and "Headphone Amp" black boxes? The exact nature of the circuits is quite probably a critical issue here.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 24, 2015, 12:00:37 am
content: There are SEVERAL parts of the circuit scheme that could contribute to the symptoms you are describing.It isn't really possible to even begin to analyze the circuit without knowing exactly what are those black boxes.It isVERYunfortunate that the PTT switch is NO (normally open) If it were NC (normally closed) your circuit would be soooo much simpler.So much so that I would consider replacing the switches vs. using that kludge transistor/relay circuit.It is also very unfortunate that the microphone is unbalanced and requires power. While that would be no big deal for the normal deployment where it is plugged directly into an appropriate circuit, it is NOT conducive to running over long unshielded cabling.It is also not clear exactly WHICH wires you are using for the various functions.Since Cat cable is arranged in four pairs, it makes a big difference which wires you use for which function.