[sdiy] This circuit actually works?

[Glen]

At 12:22 AM 5/15/03 , harrybissell wrote:

>> I should mention that the microphone is a low impedance dynamic type. The
>> shield has been tied to one of the signal lines coming from the microphone
>> and connected to ground. I know this isn't a "proper" balanced input
>> circuit, but it does work.
>
>Why have a 'balanced' input with a single ended source... you'd have to
convert
>to
>differential signal first, then back again.

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?


>> I also realize that the output is DC coupled,
>> and there is indeed a tiny amount of DC offset present, but my computer's
>> sound card doesn't seem to mind at all. (I might experiment next with
>> nulling out the offset, but I want to try and avoid a coupling cap for this
>> pre-amp.) I also realize the output signal is inverted, but it doesn't
>> matter for my purposes.
>
>The sound card VERY likely has the cap you wanted to avoid. You're
>probably fine.

Yeah, I figured the sound card probably had a cap on its input, but I
didn't want to add to that with another one in the pre-amp.


>> Without an input resistor, how do I calculate (not measure) the gain of
>> this pre-amp?
>
>hmmm... the input is probably part resistive, part inductive. I'll pass on the
>equasion (hej Magnus ???).   You probably do not have flat frequency
>response here because of the inductive part. A series resistor in the input
>would flatten the response and lose some gain.

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.

I also wonder if there is any possible danger for the microphone,
especially if I try lower values for the feedback resistor?


later,
Glen