Electronics Forums > Carbon microphone amplitude

# Carbon microphone amplitude

Rune D. Jørgensen
Guest
Posts: n/a

 02-26-2006, 09:50 PM
Hi.

I'm building a microphone amplifier and filter for a communication system
for my motorcycle. I'm filtering away frequencies below 1KHz to suppress
wind noise.

I have a microphone that has a resistance of 1.8K ohm, so I guess it's a
carbon. I have read somewhere that I should have approximately 1mA running
through it. Is that correct and what kind of amplitude should I expect out
of the microphone?

--
Rune D. Jørgensen

ehsjr
Guest
Posts: n/a

 02-27-2006, 05:01 AM
Rune D. Jørgensen wrote:
> Hi.
>
> I'm building a microphone amplifier and filter for a communication system
> for my motorcycle. I'm filtering away frequencies below 1KHz to suppress
> wind noise.
>
> I have a microphone that has a resistance of 1.8K ohm, so I guess it's a
> carbon. I have read somewhere that I should have approximately 1mA running
> through it. Is that correct and what kind of amplitude should I expect out
> of the microphone?
>

If you have 1 mA current running through 1.8K ohms
resistance, you will have a voltage across the resistance
of .001 * 1800 or 1.8 volts.

As to what you read (the 1 mA) and what you guess about
the mike, who can say? We'd be guessing, based on your

Ed

Walter Harley
Guest
Posts: n/a

 02-27-2006, 07:33 AM
"ehsjr" <(E-Mail Removed)> wrote in message
news:mIvMf.381\$dj2.321@trndny04...
> Rune D. Jørgensen wrote:
>> Hi.
>>
>> I'm building a microphone amplifier and filter for a communication system
>> for my motorcycle. I'm filtering away frequencies below 1KHz to suppress
>> wind noise.
>>
>> I have a microphone that has a resistance of 1.8K ohm, so I guess it's a
>> carbon. I have read somewhere that I should have approximately 1mA
>> running through it. Is that correct and what kind of amplitude should I
>> expect out of the microphone?
>>

>
> If you have 1 mA current running through 1.8K ohms
> resistance, you will have a voltage across the resistance
> of .001 * 1800 or 1.8 volts.
>
> As to what you read (the 1 mA) and what you guess about
> the mike, who can say? We'd be guessing, based on your
> guess. Facts would be helpful!

What AC amplitude is typical, for speech into a carbon mic element?

Rune D. Jørgensen
Guest
Posts: n/a

 02-27-2006, 07:45 AM
Det var smuk og solrig dag da ehsjr skrev
news:mIvMf.381\$dj2.321@trndny04 i sci.electronics.basics:

>> I have a microphone that has a resistance of 1.8K ohm, so I guess
>> it's a carbon. I have read somewhere that I should have approximately
>> 1mA running through it. Is that correct and what kind of amplitude
>> should I expect out of the microphone?
>>

>
> If you have 1 mA current running through 1.8K ohms
> resistance, you will have a voltage across the resistance
> of .001 * 1800 or 1.8 volts.

I know ohm's law...

> As to what you read (the 1 mA) and what you guess about
> the mike, who can say? We'd be guessing, based on your
> guess. Facts would be helpful!

Facts would be helpful, indeed. But I don't have the facts. If I did I
If I had an oscilloscope I could just measure it, but I haven't got one.

There must be some rule of thumb, as to what current should run through the
microphone, and what signal amplitude to expect. Is it 10mV, a 100mV or a
1V?

--
Rune D. Jørgensen

Rune D. Jørgensen
Guest
Posts: n/a

 02-27-2006, 07:49 AM
Det var smuk og solrig dag da Walter Harley skrev
news:(E-Mail Removed) i
sci.electronics.basics:

>> As to what you read (the 1 mA) and what you guess about
>> the mike, who can say? We'd be guessing, based on your
>> guess. Facts would be helpful!

>
>
>
> What AC amplitude is typical, for speech into a carbon mic element?

You're spot on :-) It's of course the signal amplitude, I need to know.

--
Rune D. Jørgensen
Electronics engineer, but mainly into digital design.

Don Bowey
Guest
Posts: n/a

 02-27-2006, 04:43 PM
On 2/26/06 1:50 PM, in article
Xns9776E8563A47Dblattersplatterlatte@130.225.247.9 0, "Rune D. Jørgensen"
<RUNE_dahl@hotmailREMOVE_THIS.com> wrote:

> Hi.
>
> I'm building a microphone amplifier and filter for a communication system
> for my motorcycle. I'm filtering away frequencies below 1KHz to suppress
> wind noise.
>
> I have a microphone that has a resistance of 1.8K ohm, so I guess it's a
> carbon. I have read somewhere that I should have approximately 1mA running
> through it. Is that correct and what kind of amplitude should I expect out
> of the microphone?

I don't believe that is a carbon mike. I may not be up-to-date on them, but
in my experience carbon mikes tend to have a resistance of 200 Ohms or so.
In practice the current could be 10 milliamps, more or less, and the
resulting signal from a good quality unit might be as much as 0.5V with a
circuit voltage of about 4 to 6V.

Don

Dan Akers
Guest
Posts: n/a

 02-27-2006, 05:42 PM
Rune*D.*Jørgensen wrote;
"I'm building a microphone amplifier and filter for a communication
system for my motorcycle. I'm filtering away frequencies below 1KHz to
suppress wind noise.
I have a microphone that has a resistance of 1.8K ohm, so I guess it's a
carbon. I have read somewhere that I should have approximately 1mA
running through it. Is that correct and what kind of amplitude should I
expect out of the microphone?"
______________________________________
Re;
Assuming it is a carbon microphone and you don't have the make and model
#. There is no current/sound pressure thumb-rule, that I know of, that
would cover all carbon mikes. Although the characteristic resistance for
yours may be 1.8kohm, the voltage amplitude produced for a given sound
pressure with 1 mA excitation is highly dependent on the surface area
and frequency response of the diaphragm, the particular density response
of the carbon pack, and the characteristic input impedance of the
electrical load on the microphone and it's power supply. Without these
particulars you are just guessing, as are we, and you will probably need
an oscilloscope to determine the electrical response to voice in your
planned circuit.

Dan Akers

ehsjr
Guest
Posts: n/a

 02-28-2006, 05:06 AM
Rune D. Jørgensen wrote:
> Det var smuk og solrig dag da Walter Harley skrev
> news:(E-Mail Removed) i
> sci.electronics.basics:
>
>
>>>As to what you read (the 1 mA) and what you guess about
>>>the mike, who can say? We'd be guessing, based on your

>>
>>
>>
>>What AC amplitude is typical, for speech into a carbon mic element?

>
>
> You're spot on :-) It's of course the signal amplitude, I need to know.
>
>

You guys may not be aware of how a carbon mike works.
You seem to think that the mike generates a voltage.
It doesn't. A voltage must be supplied to it. Sound
varies the pressure on the carbon, which causes its
resistance to vary. That causes the current through
the mike to vary.

What is the typical signal amplitude of a carbon mike?
Meaningless question. A carbon mike is not a source.
Without knowing the source voltage one cannot say what
the amplitude will be. Likewise, without knowing the load
impedance, one cannot say what the amplitude will be.
The signal (and by that I assume you mean the voltage
drop across the mike) is a result of the current through
the resistance of the carbon. That current depends on
source voltage and load impedance, as well as mike
impedance at a given audio level & frequency.

Ed

Walter Harley
Guest
Posts: n/a

 02-28-2006, 06:11 AM
"ehsjr" <(E-Mail Removed)> wrote in message
news:dTQMf.7608\$UN1.4908@trndny08...
> You guys may not be aware of how a carbon mike works.
> You seem to think that the mike generates a voltage.
> It doesn't. A voltage must be supplied to it. Sound
> varies the pressure on the carbon, which causes its
> resistance to vary. That causes the current through
> the mike to vary.
>
> What is the typical signal amplitude of a carbon mike?
> Meaningless question. A carbon mike is not a source.
> Without knowing the source voltage one cannot say what
> the amplitude will be. Likewise, without knowing the load
> impedance, one cannot say what the amplitude will be.
> The signal (and by that I assume you mean the voltage
> drop across the mike) is a result of the current through
> the resistance of the carbon. That current depends on
> source voltage and load impedance, as well as mike
> impedance at a given audio level & frequency.

Ed, the OP asked about how much AC voltage is generated across the mic if he
runs a DC current through it and presents it with an acoustic signal. Since
a carbon mic behaves like a variable resistor, that is an entirely
meaningful question: if a DC current is applied to a variable resistor, the
voltage across the resistor will vary. The unanswered part is "how variable
is the resistor"? Does it vary by 1% before clippping, or 10%, or what?
Does the resistance increase, or decrease, or both, compared to nominal?

Or, you could answer a different question, that still could reasonably be
presumed to be helpful: what sort of circuit might one typically and
usefully employ, to develop an electrical signal from a carbon mic?

Dan Akers suggested that there's just no way to know. But I suspect that
knowing the answer for "some" specific carbon mic would be more helpful than
knowing nothing at all - even if it's an order of magnitude away from this
one, it's a starting point.

Ban
Guest
Posts: n/a

 02-28-2006, 05:16 PM
Walter Harley wrote:
> "ehsjr" <(E-Mail Removed)> wrote in message
> news:dTQMf.7608\$UN1.4908@trndny08...
>> You guys may not be aware of how a carbon mike works.
>> You seem to think that the mike generates a voltage.
>> It doesn't. A voltage must be supplied to it. Sound
>> varies the pressure on the carbon, which causes its
>> resistance to vary. That causes the current through
>> the mike to vary.
>>
>> What is the typical signal amplitude of a carbon mike?
>> Meaningless question. A carbon mike is not a source.
>> Without knowing the source voltage one cannot say what
>> the amplitude will be. Likewise, without knowing the load
>> impedance, one cannot say what the amplitude will be.
>> The signal (and by that I assume you mean the voltage
>> drop across the mike) is a result of the current through
>> the resistance of the carbon. That current depends on
>> source voltage and load impedance, as well as mike
>> impedance at a given audio level & frequency.

>
> Ed, the OP asked about how much AC voltage is generated across the
> mic if he runs a DC current through it and presents it with an
> acoustic signal. Since a carbon mic behaves like a variable
> resistor, that is an entirely meaningful question: if a DC current is
> applied to a variable resistor, the voltage across the resistor will
> vary. The unanswered part is "how variable is the resistor"? Does
> it vary by 1% before clippping, or 10%, or what? Does the resistance
> increase, or decrease, or both, compared to nominal?
> Or, you could answer a different question, that still could
> reasonably be presumed to be helpful: what sort of circuit might one
> typically and usefully employ, to develop an electrical signal from a
> carbon mic?
> Dan Akers suggested that there's just no way to know. But I suspect
> that knowing the answer for "some" specific carbon mic would be more
> helpful than knowing nothing at all - even if it's an order of
> magnitude away from this one, it's a starting point.

A carbon capsule is likely to be big and went after the advent of electret
mikes into the garbage bin and no sane person will use this technology for
other then close voice pickup and drving a horn speaker directly though a
12V battery, a megaphone without amp. When the OP measured some resistance,
it doesn't mean it is only resistive, but might be an Electret mike which
have about 0.5mA bias current and might on a multimeter show up as some
obscure value.
If the OP would also measure with the test leads crossed the same resistance
it is certainly not an electret mike, exept those with 3 pins where there is
a resistor between supply and output.
--
ciao Ban
Apricale, Italy

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