[sdiy] This circuit actually works?
harrybissell at prodigy.net
harrybissell at prodigy.net
Thu May 15 18:46:08 CEST 2003
<snipped>
From: mclilith at charter.net
>Well, the microphone is a typical low impedance type. The 3-pin connector
>at the end of its cord has two signal lines and a separate connection for
>the shield. Wouldn't you consider this a balanced source? I thought that
>this type of microphone was normally referred to as balanced?
Yes it is balanced. You can run it single ended. I was judging from
the schematic.
>I originally had a 1K input resistor in the circuit, and I tried bypassing
>it. The gain increased noticeably, but I can't say *exactly* how much. I tried measuring levels with some recording software, and they seemed to increase by roughly 10dB, when the input resistor was bypassed. I have no idea about frequency response anomalies yet, but I'll try to check that in the near future.
>>This circuit could also be considered a current to voltage converter, and the
>>microphone
>>'sees' zero ohms impedance. I think if I had to do the math, I'd probably
>>consider this to be true
>>and start there.
>That sounds a bit confusing to me, because if the input impedance truly is
>"zero", wouldn't the microphone be shorted out and its signal would be
>reduced to nothing? I'm sure that I misunderstand something in what you said.
No... if its a current source, the current will still flow. Current in
a series loop is the same everywhere in the loop. If you pass 1mA through a zero ohm resistor, it still passes through... but there is no VOLTAGE drop you can measure.
Thats why if you scope the inverting input of your opamp you will see nothing. The voltage is zero, because the current from the microphone
is exactly opposed by the current from the 1M feedback resistor... but
real current flows in each one.
The microphone will not be harmed by this connection, because the voltage (and current) are so small that the heat generated in the
wire (it has real resistance...) is not enough to cause damage (or even
to measure, likely)
>I also wonder if there is any possible danger for the microphone,
>especially if I try lower values for the feedback resistor?
Lower values of resistor (feedback) will reduce the voltage gain only.
The current generated by the microphone will remain fixed, and the current through the feedback resistor will equal the input current in all cases. The voltage gain comes because the opamp has to generate a higher voltage at the output to make the current in the feedback resistor equal to the input current.
The only danger would be a circuit failure that might apply DC across the mic element. That could cause enough heat to fry the coil. This would be prevented by limiting the possible current (by using an series
resistor) or blocking the DC with a capacitor...
H^) harry
>later,
>Glen
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