[sci.electronics] Desperately seeking clean mic sound

tindle@ms.uky.edu (Ken Tindle) (08/05/89)

I hate to do this, but I'm rather desperate...

I had previously posted asking for tips on building a mic preamp.  So far,
I've not found a solution.  Ordinarily I'd stumble along, and eventually
find the holy grail.  But I'm under rather strong time pressure this time
out, because I figured it'd be relatively easy to come up with a preamp.
Bzzz, wrong answer!

What I've got is a 200 ohm dynamic microphone in a headset.  I want to 
preamp the signal, and feed it via an op amp mixer and headphone driver
back to the ear.  The h.p. driver and mixer just work.  But the preamp has
a fair amount of hiss.

The power supply is clean, the shielding is OK.  I've got a schematic of a
circuit the headphone manufacturer uses, but it produces no gain.  Query:
I'm using bipolar transistors different than the manufacturer (in exact type,
the polarity and such is the same.)  Can that cause no gain?

I've built a bipolar design, similar to the manufacturer's, which did have
gain- and plenty of noise.  I've tried a LM381- in its noninverting config,
no gain.  In an inverting config, gain and hiss.  I've had a tip to try a
transformer, but I can't find one with the spec I'd need.  Another tip 
suggested a SSM2016, but I haven't the foggiest where to get one of those
jewels.  Someone else pointed out an article in QST.  Whatever that means.  
All dead ends. 

The best results have come from a 1458, doing both mixing and mic preamp
duties.  The mic input is hooked up as a diff amp with gain 100, using a
single +5V.  BTW, all I've got is +12 and +5.

So, I've got something relatively poor to use, which I might have to do
because of the time pressure.  But I know these mics are very quiet when
they're happy; I've heard 'em.  I want to be able to do that too!  It really
bothers me to do half a job.

Knew I should've stuck to digital...

Any replies welcome; very welcome indeed.  Thanks.

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INTERNET:tindle@ms.uky.edu | "I heard you." -Kirk 
BITNET:tindle@ukma.bitnet  | "He simply could not believe his ears."     
Ken Tindle - Lexington, KY | -Spock, The Trouble With Tribbles
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