O-Pad and O-hms Law (or is it Thangevin's)
« on: October 17, 2007, 08:57:56 PM »
I can't seem to wrap my mind around the correct calcualtions for this.  Say this is for a Mic pre with huge padding; Do you calculate like it's a U with the output side series and shunt making the Attenuation?

Thinking as I wright, it seems the output would see the first shunt in parallel with the series and mic shunt and vice cersa for the mic side.   :?

sidenote: I would use Balou's book if there weren't typos in that section.


O-Pad and O-hms Law (or is it Thangevin's)
« Reply #1 on: October 17, 2007, 09:06:31 PM »
Charlie, I'm not sure I understand the question the way you've phrased it. But since you're talking about a mic pad, which generally means an input impedance of 1.5K-2K and an output impedance of 150-200 ohms... For attenuation greater than 20dB, make the shunt resistor at the output = 150 (or 200) and the series resistors big enough to give the desired value of attenuation--in other words, design the U-pad in the usual way. Then add the shunt resistor at the input (turning the U into an O-pad) to lower the impedance seen by the mic to 1.5-2K.

R = (Actual Z * Desired Z) / Actual Z - Desired Z
where Actual Z equals the series resistors plus the output shunt, and R equals the input shunt.

O-Pad and O-hms Law (or is it Thangevin's)
« Reply #2 on: October 17, 2007, 11:07:20 PM »
Thanks Dave.  It's hard communicating sh-t I don't understand correctly esp. if I'm thinking Thengevin's is involved.  I think u understood though...

OK basically I have 3.6K series on the U IIRC.  So I just slap a 2K shunt on the mic side and I should get the same atten?

Wait I posted before seing that formula...l et me calc...

O-Pad and O-hms Law (or is it Thangevin's)
« Reply #3 on: October 17, 2007, 11:13:39 PM »
OK for 3.6 series on each side and 200 on shunt I get 2.74K.  Sound right?


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