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From: tempus fugit on 4 Jul 2010 16:40
Hey all;

I've been reviewing the THAT 1512 data sheet
(http://www.thatcorp.com/datashts/THAT_1510-1512_Datasheet.pdf) 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


From: Phil Allison on 4 Jul 2010 22:26

"tempus fugit"
>
> I've been reviewing the THAT 1512 data sheet
> (http://www.thatcorp.com/datashts/THAT_1510-1512_Datasheet.pdf) 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?

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

God knows what mic you have.


> What would it
> be for the T bias configuration in Fig.5 on p. 7?

** 2.4kohms.

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


.... Phil


From: tempus fugit on 4 Jul 2010 23:21

"Phil Allison" <phil_a(a)tpg.com.au> wrote in message
news:89cu82Fg8uU1(a)mid.individual.net...
>
> "tempus fugit"
> >
> > I've been reviewing the THAT 1512 data sheet
> > (http://www.thatcorp.com/datashts/THAT_1510-1512_Datasheet.pdf) 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?
>
> ** The differential impedance is 2 kohms - this is what a typical
> "balanced" mic sees.
>
> God knows what mic you have.

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?)


> > What would it
> > be for the T bias configuration in Fig.5 on p. 7?
>
> ** 2.4kohms.

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


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

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

Thanks again Phil



From: tempus fugit on 4 Jul 2010 23:24

"Phil Allison" <phil_a(a)tpg.com.au> wrote in message
news:89cu82Fg8uU1(a)mid.individual.net...
>
> "tempus fugit"
> >
> > I've been reviewing the THAT 1512 data sheet
> > (http://www.thatcorp.com/datashts/THAT_1510-1512_Datasheet.pdf) 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?
>
> ** The differential impedance is 2 kohms - this is what a typical
> "balanced" mic sees.
>

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


> > What would it
> > be for the T bias configuration in Fig.5 on p. 7?
>
> ** 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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