small signal NPN transitor for muting microphone

[RR]

I have posted a circuit at this address, please have a look.
http://www.psv.co.uk/MicMute.JPG

What I am trying to do is, mute the microphone by using the transistor as
explained in the circuit. I can un-mute it by closing the switch. All this
works fine on one unit I have modified. I need to modify about 200 units.

My question is, can you see any problem with the circuit?
BTW new parts I will be adding, are:
one 4.7K register, one Transistor, one diode and a switch.

 

[Frank Bemelman]

I have posted a circuit at this address, please have a look.
http://www.psv.co.uk/MicMute.JPG

What I am trying to do is, mute the microphone by using the transistor as
explained in the circuit. I can un-mute it by closing the switch. All this
works fine on one unit I have modified. I need to modify about 200 units.

My question is, can you see any problem with the circuit?
BTW new parts I will be adding, are:
one 4.7K register, one Transistor, one diode and a switch.

What is that diode doing?

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

 

[RR]

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).
http://www.psv.co.uk/MicMute2.JPG

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.

 

[Frank Bemelman]

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).
http://www.psv.co.uk/MicMute2.JPG

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.

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?

 

[Walter Harley]

I have posted a circuit at this address, please have a look.
http://www.psv.co.uk/MicMute.JPG

What I am trying to do is, mute the microphone by using the transistor as
explained in the circuit. I can un-mute it by closing the switch. All this
works fine on one unit I have modified. I need to modify about 200 units.

My question is, can you see any problem with the circuit?
BTW new parts I will be adding, are:
one 4.7K register, one Transistor, one diode and a switch.

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.

 

[Phil Allison]

"Walter Harley"
"RR"

I'm surprised that it's working for you.

** Huh ??

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

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),

** Not true.

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).

** 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.

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).

** Drivel.

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

** Plenty of 600 ohm dynamic mics still around.

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.

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

But you're only showing one leg. Maybe the "preamp" you show is the
in-microphone electronics?

** More drivel.

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.

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




........... Phil

 

[Si Ballenger]

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.

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

 

[Phil Allison]

"Si Ballenger"

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

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

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





......... Phil

 

[Walter Harley]

I have posted a circuit at this address, please have a look.
http://www.psv.co.uk/MicMute.JPG
[...]

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

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.

 

[Phil Allison]

"Walter Harley"

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.

** It will MUTE the ENTIRE wave.

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

Indeed, it would mute it regardless of the base voltage, because of the
collector-base diode being forward-biased. It's horribly distorted,
though.

** 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

 

[RR]

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?

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

 

[Rich Grise]

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

 

[Kevin White]

Walter Harley wrote:
....

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).

....

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

 

[Frank Bemelman]

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.

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.

 

[martin griffith]

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).
snip

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

 

[Phil Allison]

"Kevin White"

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.

** 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

 

[Kevin White]

Phil Allison wrote:
....

** 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.

From the Fairchild data sheets the 1N4148 forward voltage at 1mA is

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

 

[RR]

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.


Thanks, Frank.
(remove 'q' and '.invalid' when replying by email)

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.
http://www.psv.co.uk/MicMute3.JPG

Once again thanks for all the help.

Raju

 

[Frank Bemelman]

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.
http://www.psv.co.uk/MicMute3.JPG

I would not bother to split the 4K7 in two resistors. Replace the
1N4148 with a BAT86 or something, and hope for the best

 

[RR]

I would not bother to split the 4K7 in two resistors. Replace the
1N4148 with a BAT86 or something, and hope for the best

Hello Frank,
Got the message!
Raju