input impedance of THAT 1512 IC

Jul 04, 2010 3 Replies
T

Hey all;



I've been reviewing the THAT 1512 data sheet

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and am trying to figure out what the input impedance would be for the circuits they've given.In Fig. 4 on p. 6 for instance, would the input impedance be



1K, or 1K||1K = 500 ohms, or 2K, or something else altogether? What would it be for the T bias configuration in Fig.5 on p. 7? They also use the expressions 'differential' and 'common mode' impedance. When desgning a mic pre, which of these should be my primary concern?

Thanks


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P

"tempus fugit"

** The differential impedance is 2 kohms - this is what a typical "balanced" mic sees.

God knows what mic you have.

** 2.4kohms.

The impedance is set just by the resistors that you supply - the IC itself has figures in the many megohms.

... Phil

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T

be

Thanks Phil.

I have several mics, but the one I'm trying to work this pre with is an SM57. It's my understanding that it was originally designed to see a 600 ohm input Z. So the input Z is the sum of the 2 resistors then? Also, it's the differential, rather than the common mode input Z that I want to concern myself with (in terms of loading the mic and potentially HP filtering it with too low an input Z?)

If I may, how did you arrive at that figure?

itself

Yes I knew that - I was just having trouble seeing how to set the input Z with the resistors.

Thanks again Phil

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T

be

Forgot to ask one other question in my last post - so if I were working on an existing design using this IC, I could simply measure the reisistance between pins 2 and 3 to get the differntial input resistance (I realize I wouldn't actually have the impedance this way)?

Thanks again

itself

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