O-Pad and O-hms Law (or is it Thangevin's)

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guavatone

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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.
 
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.
 
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...
 

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