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Push to talk mic circuit

Discussion in 'Electronic Repair Modifications DIY' started by blawso, Jun 21, 2010.

  1. blawso

    blawso Guest

    Hello, I'm new to the forum and have a question for what I thought would be a simple project, but seems to be giving me a bit of grief.

    I'm trying to build a simple "push-to-talk" button for a desk mic. (ie. push the button, mic is live, let go and it mutes)
    I've done this before with a dynamic mic just by using a NC pushbutton to bypass a 50+ db pad.
    But now I'm trying to do the same with a small condenser, and I'm having popping issues with the phantom power.
    Is there some kind of coupling cap configuration that will allows the mic to be powered only when the button is pressed without the audible pop.
    Perhaps someone can point me to a circuit that will accomplish this??
    Thanks in advance.
  2. MrEase

    MrEase Active Member

    This really depends on the condensor mic. Is it balanced out, is it an electret or "true" condensor etc. If you could post a schematic of what you have it would help enormously.
  3. Boswell

    Boswell Moderator Distinguished Member

    Use the NC contacts on the switch to short pins 2 and 2 on the XLR connector. Doing that will mute the mic and should not affect the phantom power.
  4. blawso

    blawso Guest

    Thanks for the replies.
    To be exact, the mic is an Audio Technica Pro 49Q gooseneck style, balanced condensor.
    I wasn't sure if there was an effect on the phantom by shorting pins 2 to 3, but I tried it anyway. Again, it's going to be shorted continuously until the button is pushed momentarily. At any rate, there were still pops when I pushed the button. Sometimes quite loud. I'll try to get a schematic posted later tonight. If I can get the shorted pin 2 & 3 version to be quiet, I'd be happy to do it that way.
  5. RemyRAD

    RemyRAD Guest

    You are not going to be able to effectively mute a condenser microphone at the front end of your preamp. You must keep the microphone energized with phantom power 100% of the time. This phantom power cannot be shorted out even post breakout resisters. You can however mute the microphone from within the preamp. There is no sane reason to try to enable a push to talk at the output of a condenser microphone. Sure, you can do it if you are not using phantom power but, something like an electret self polarized condenser microphone. These frequently only require a 1 1/2 V battery for operation. But the desire is still rather dumb. And what are you plugging this microphone into? You can mute the output of a preamp silently. You generally can't mute the output of a microphone silently if it's a condenser microphone. If you can make more sense we can be more help.

    I want to be able to smoke my cigarettes while filling my gas tank without any popping sound. How do I do that?
    Mx. Remy Ann David
  6. Laurend

    Laurend Active Member

    Shorting pin 2 and 3 mutes any mic before preamp without producing any pop even with a phatom power. Both pins are at the same voltage.
  7. RemyRAD

    RemyRAD Guest

    That is absolutely incorrect. If you interrupt the Phantom power to the microphone and then reapply it, it'll cause a huge noise. That was blatantly bad information above. You want to mute the microphone post-preamp not pre-preamp. This is not a dynamic mic and cannot be treated as such.

    Geez and French yet
    Mx. Remy Ann David
  8. Laurend

    Laurend Active Member

    I've used this trick for years to allow clients censorship when I was sound engineer for a convention center. Nobody never noticed any huge noise in the 10 KW PA system. With that solution the mic preamp is still powered. Only the audio is muted. For sure it's not the best way to operate from a technical point of view. Controling the pre-amp is the correct solution. But we're in a push-to-talk design which must stay simple and effective. Shorting pins 2 and 3 (not 1) is simple and effective.
  9. Boswell

    Boswell Moderator Distinguished Member

    Sorry, Remy, the information is absolutely correct. XLR pins 2 and 3 of a mic that has a true balanced output are at the same d.c. potential whatever the state of the phantom power, so shorting them together has no effect on the powering. I suggested this method in my post above out of experience with using an XLR M-F adaptor with a shorting switch I made many years ago, and it works fine as a mute for all the mics I have tried, both phantom-powered and not.

    What I think is happening with the Audio Technica Pro 49Q is that it may not have a true balanced output. Some electret mics that use PP (or an internal battery) to power the FET buffer just drive the +ve output (pin 2) and decouple pin 3 directly to a.c. ground. The current for powering the buffer is taken from pin 2 via that pin's 6K8 resistor, causing the expected potential drop and therefore a d.c. imbalance with pin 3. A configuration of this sort will indeed cause clicks and pops when used with the shorting switch, but the signal will nevertheless be muted and the phantom power unaffected. Without taking one apart, it's not easy to tell if this is how the 49Q is configured, but for the switch to work silently, it would need some experimentation with load resistors from pin 3 to ground (around 33K - 47K) to balance the potentials at pins 2 and 3.
  10. RemyRAD

    RemyRAD Guest

    I'm sorry I must vehemently disagree with you. While this might be fine for line level sources, it certainly isn't for phantom powering a microphone. If you short terminals 2 & 3 the microphone preamp might make a quiet exit. However on reapplying phantom power to any condenser microphone especially those that require a true 48 V polarization voltage, the microphone will turn itself back on with an ugly thump. So I'm really surprised at you Boswell? This is a highly unprofessional question solved by a highly unprofessional solution. You don't mute condenser microphones by shorting out their power. Maybe you can with the cheap condenser microphones you've all used? But not with ones such as U 87's 414 and the like. I've never heard one of these microphones turn on quietly before, ever.

    You might consider keeping the phantom, leaving the Phantom power applied to said microphone and interrupting the secondary of the transformer it is loaded into. That makes much more sense. That way the microphone is always powered on & always loaded into a source of stable impedance. The original question is just clueless foolishness and that's why this proper answer was rebutted numerous times. If you're going to screw up, you might as well screw up the correct way.

    Push to scream!
    Mx. Remy Ann David
  11. Laurend

    Laurend Active Member

    Reminder for RemyRAD:
    - pin 1: ground 0V
    - pin 2: signal + and + 48V phantom power supply
    - pin 3: signal - and + 48V phantom power supply
    So shorting 2 and 3 doesn't short the power supply. So the pre-amp built in the mic is still powered. So there is no ugly thump. so...
    The only situation where the shorting can't be applied is for T phantom power which doesn't exist anymore for serious microphones.
  12. MrEase

    MrEase Active Member

    While I'm sure RemyRAD will spot her error when she reads the previous post I doubt very much that this will solve the original problem. When the OP first mentioned a goose neck condensor mic I was immediately wary that it might well be an electret mic and also not have a true balanced output. I would suggest that the OP reads Boswells suggestion as I fully expect he is correct that the mic in use has a pseudo balanced out. That is really why I asked for details in the first place!

    While you may be able to balance the voltages on pins 2 and 3 in this case by doing as Boswell suggested, you may also need to fit a small trimmer to the pull down to fine tune the output. As the output signal will be at a low level any tiny imbalance will present a POP to the pre-amp input when using a simple shorting switch and the trimmer should give you the best chance of success. No guarantee though!

    In this case RemyRAD may well be ultimately correct in that a mod to the internal mic or pre-amp circuit may be needed.
  13. blawso

    blawso Guest

    I want to thank all who replied to my question... except one.

    Yes I understand that what I'm attempting falls under the "quick and cheap fix" category. And yes, I was hoping for a quick and cheap solution. Yet there is a method to the seeming madness. I appreciate those of you who gave me useful and knowledgeable answers, minus the editorial comments.

    Boswell, you are probably right on. I will certainly try a little more experimentation with the load resisters. As mentioned before, I have used the shorting method before with perfect results. This is a different situation.

    To Ms. RemyRAD, I have to say: nice way to welcome a new voice to the community! Really, "clueless foolishness?" Sorry to have wasted your time, but no one forced you to reply. I've always heard there is no such thing as stupid questions, just stupid answers. Ms. Remy, you have proved that to be correct.
  14. RemyRAD

    RemyRAD Guest

    You know, we are not all beginners here except for maybe one? There is no problem shorting out phantom power. There is a problem with reapplying the Phantom power to the microphone. The preamp in the microphone has to recharge. The process of reapplying the power will cause you a thump. But maybe you just need a few thumps on the head to understand this?

    If on the other hand you can keep the Phantom power applied to the microphone and keep it loaded with 1200 ohms, then you can short the microphone input tube preamp without shorting out the power to the microphone. This will require at least a 2 PDT switch. Then since it will be obnoxious sounding with ambience on followed by ambience off, that could also be obnoxious sounding. A better technique would be what I created for NBC radio. Yes it involves a relay. Yes it involves 2 optical devices similar to what's found in optical compressors. One to softly lower the volume and the other to softly raise the volume. Along with timing capacitors to work out the proper fade in fade out points. Then you have an effective soft switch.

    Greet a newcomer newbie who likes to correct people with over 40 years experience. Well you must be the smarter one?? You came here for free advice and have obtained such. Correcting us just proves your inexperience. Many professionals have numerous different ways of obtaining the same outcome. You didn't like my answer because it wasn't the answer you wanted. You know better. So why even ask the question?

    Happy thumping
    Mx. Remy Ann David
  15. MrEase

    MrEase Active Member

    I'm sorry Remy but it's clear the penny hasn't dropped for you when you keep harping on about shorting out the phantom power! Not one post has suggested shorting the phantom power..... there is absolutely no issue with re-applying power when it has never been removed!

    The suggestions so far are quite correct as shorting pins 2 and 3 short out the differential signal without shorting the common mode phantom power.

    You are not the only one with over 40 years experience around here and the tone of your posts is quite insulting and not just to the OP. What you really need to do is actually understand the suggestion before hurling these insults as you are not doing yourself or this forum any favours.
  16. RemyRAD

    RemyRAD Guest

    Every time you short pin 2 & 3, it doesn't hurt the Phantom power. It doesn't hurt the microphone. Reestablishing power to a condenser microphone is a noisy prospect. So when pin 2 & 3 are shorted, the microphone is not powered up. It is seeing no voltage since the voltage cancels out. And so tell me how many people here have plugged in condenser microphones with the volume up? You know what kind of sound that makes. The same will hold true with the pushbutton. I wouldn't keep responding if I didn't know it wasn't correct. Unfortunately I also know you can't fix stupid. So have your fun telling this poor person that everything will be fine shorting 2 & 3 together. I just happen to know better and you don't.

    I like accepting insults from clueless people
    Mx. Remy Ann David
  17. MrEase

    MrEase Active Member

    Quite wrong! While the differential signal is shorted, the phantom power is only shorted to itself and NOT to ground. The microphone circuits still have power.

    Again wrong. The differential signal cancels out by being shorted (which is what we want to mute the mic.) but the common mode is still there (see above).

    Yes I do.

    No it won't!

    I am not prepared to trade insults with someone who has not yet realised their error. Nor will I crow when you realise your error.
  18. Laurend

    Laurend Active Member

    Dear Remy I'm very sorry. I've been in the audio business for only 30 years now. I'm not only a live and recording sound engineer, but also a designer for both analog and DSP. I'm not throlling this forum so I ask for some respect.

    I suggest you to apply the scientific way. Just run the experience I've suggested, and revise your theories according to the experimental results. I'm sure you will understand your ridiculous situation as forum moderator.

    I make tens of mistakes a day but I always try to learn from them. You shouldn't hesitate to follow that way.
  19. MrEase

    MrEase Active Member


    I'm attempting to attach a pdf schematic of a typical mic output circuit and phantom power source as you seem to understand schematics. Please explain exactly how the power to the Mic pre-amp (across C1) will be affected by throwing the switch.

    I contest with you that there will be NO DC current through the switch (either on or off!) therefore there will not be a uV of difference across C1. This is clearly not the same as unplugging the mic when C1 will discharge through the pre amp circuit.

    What is patently clear though is that any differential signal will be shorted and hence the mic muted when SW1 is closed.

    Attached Files:

  20. RemyRAD

    RemyRAD Guest

    That is a single circuit that may in fact work. But most microphone transformers do not have a center tap to ground. That is only a single instance. So I might be half right or half wrong and you may be half right or half wrong. That's what's fabulous about audio. There are many ways to accomplish many of the same things differently. So this works for the poster, terrific. Not all microphones nor their inputs are all created equally.

    Mx. Remy Ann David

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