small signal NPN transitor for muting microphone

"RR" schreef in bericht news:dr0pr4$g5t$ snipped-for-privacy@nwrdmz02.dmz.ncs.ea.ibs-infra.bt.com...

What is that diode doing?

Or, why use a transistor, diode and resistor? Just put the switch in series with the microphone.

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Thanks, Frank.
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Hello Frank

Thank you very much for the reply.

The switch is part of the microphone. About five meters away and will pick up some noise. Actually the switch is part of the microphone. Two extra wire comes out of the microphone.

My apology for no explanation for the diode. I have now uploaded second image to explain why I need diode (Diodes).

I hope this new image will do some explanation.

My main worry is the diode. I think having this diode in pararal with base-emmiter might reduce ON resistance of the transistor. In any case my another worry is that, will transitor offer low ON resistance at very very low current? Some quick search on transistor database gave me some basic answer about ON resistance, But it only specifed at 3mA.

Thank you very much for your help.

"RR" schreef in bericht news:dr0uc7$ev7$ snipped-for-privacy@nwrdmz01.dmz.ncs.ea.ibs-infra.bt.com...

wire

Please do not top post, put your replies on the bottom.

Okay, I saw your new schematic. If you are going to mute the output, what is the point of muting the input?

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Thanks, Frank.
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I'm surprised that it's working for you. I would expect it to do nothing at all for the negative-going part of the wave (since a bipolar transistor only conducts current in one direction), and to do a crappy job of the positive-going part of the wave (since the transistor can't make the voltage between its collector and emitter go all the way to zero, only down to the saturation voltage of 0.4V or so, which is greater than the signal from a dynamic mic).

In fact, I'm sufficiently surprised that I wonder whether either (1) your mic is actually a condensor mic (in which case, what's happening is you're switching off the phantom power), or (2) your testing is invalid (for instance, perhaps you're also muting the preamp output and therefore you're not noticing that the mic itself is not getting muted).

Also, it's been a while since I saw a 600 ohm dynamic mic; they're mostly

150 ohm. And it's been a while since I saw a low-impedance (be it 600 or 150) dynamic mic that didn't have a balanced output. But you're only showing one leg. Maybe the "preamp" you show is the in-microphone electronics?

I can't help but think that if you told us more about what's going on, folks here would be able to do a better job of helping with your circuit.

"Walter Harley" "RR"

** Huh ??

It's a bog standard, audio muting circuit !!

** Not true. ** Absolute rot.

A transistor's "Vce sat" is not some conduction threshold voltage !!!

Typical small signal transistors have an "on" RESISTANCE ( when saturated ) of a few ohms.

Easily attenuate the signal from a 600 mic by 50 dB.

** Drivel.

** Plenty of 600 ohm dynamic mics still around.

** The mic capsule has two wires - ergo, it can be used balanced or unbalanced. ** More drivel.

** Some folk here can indeed give help with circuits - but not Walter Harley.

.......... Phil

So what is the "negative-going part of the wave"? Looks like only a positive voltage is being modulated.

"Si Ballenger"

** Err - sound pressure waves go positive AND negative !

The signal from a dynamic mic is AC, with an average value of zero.

........ Phil

The schematic would be more clear if it read from left to right, instead of right to left as it presently does. If I understand what the OP is trying to do, the microphone is the signal source; normally, the microphone drives the preamp, and the OP is attempting to mute it by shorting its output to ground via the transistor.

If the microphone is indeed a dynamic (coil) type, as indicated on the schematic, then its output is AC with very little DC component, with magnitude on the order of 10mV.

You're right that I'm wrong, though. It would mute the negative-going part of the wave, if the signal were strong enough. Indeed, it would mute it regardless of the base voltage, because of the collector-base diode being forward-biased. It's horribly distorted, though.

"Walter Harley"

** It will MUTE the ENTIRE wave.

Build the cct if you still doubt that !!!!!

** Completely wrong.

When the switch is open, the transistor has a very low *resistance* from C to E - smoothly muting the audio signal.

When the switch is closed, the transistor is biased off.

........ Phil

Hello Frank,

Thanks again, Now I know what Top post are! sorry about that.

Original equipment (Bus/Motor coach - Amplifier) had/has voice activated muting circuit. Which I found totally useless. Was very simple circuit with

2nd half of the op-amp + one transistor + one 330uf capacitor etc. This voice activated circuit muted input and output. So I have no choice. Also CD player is connected to power amp. Pre amp also connects to the same point on the circuit. (simple register job!) If I don't mute pre amp output then there is noticeable increases in background noise.

Thanks

Then it's almost trivial. Get a mic with a "push-to-talk" switch. :-)

Cheers! Rich

Walter Harley wrote: ...

...

Bipolar transistors can conduct in both directions, althogh modern ones do not do it very well and have very low reverse Hfe (and breakdown voltage). The reverse Hfe may be less than unity. Decades ago symmetrical bipolar transistors were made for this type of application (choppers).

I am surprised it works though because even when the switch is closed the forward biased diode will still allow a fairly high Vbe and allow the transistor to conduct.

kevin

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my

basic

what

with

CD

on

Okay, probably better leave it that way then. I suppose your circuit is all right then, but I would use schottkey diodes that have a lower forward voltage, for your wired-and logic, because normal diodes like 1N4148 have a forward voltage of 0.6V which seems a bit high for this arrangement.

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Thanks, Frank.
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There are similar ccts in Studer tape (B62 and A80RC decks) for switching the EQ circuits around. I dont have the manuals with me, but ISTR they used the transistors "upside down" !

martin

"Kevin White"

** The forward voltage of a 1N4148 diode is circa 100mV less than the Vbe a typical small signal transistor.

Verify with your DMM on "diode test" any time.

........ Phil

Phil Allison wrote: ...

620mV. A 2N3904 collector current at that VBE is about 0.2mA and would be highly sensitive to any temperature difference between the diode and the transistor . Also because of the large ratio between forward and reverse Hfe this could also cause significant distortion.

It may not prevent operation but is not desirable.

kevin

Hello Frank,

Thank you very much for your time and advise. I will do that. I will also do some more experiment as per my new drawing! sorry about the quality of image. Please look at if you find time.

Once again thanks for all the help.

Raju

"RR" schreef in bericht news:dr3msl$km9$ snipped-for-privacy@nwrdmz01.dmz.ncs.ea.ibs-infra.bt.com...

forward

have

I would not bother to split the 4K7 in two resistors. Replace the

1N4148 with a BAT86 or something, and hope for the best ;)

--
Thanks, Frank.
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Hello Frank, Got the message! Raju