NYD Passive Mixer Rx Calculation...please help me out

[supermatt]

Hi, it really isnt my day - i just cant figure out how to calculate the Rx resistor on NYDs schematic over here http://www.twin-x.com/groupdiy/albums/userpics/balancedmixnetwork.pdf

Is it 20k/n / 20k/n --- doesnt make sense to me.

Im really dumb today, sorry.

 

[ruffrecords]

20k/n is the bus impedance for n inputs. Work this out and call it Rb.

Then Rx = (Rb * 200) / (Rb - 200).

So if Rx works out at 1K ( for 20 inputs) then Rx = (1000* 200) / (1000 - 200) = 250 ohms.

Cheers

Ian

 

[JohnRoberts]

Rx= (20K/N)200 / (20K/N) - 200

Substitute your number channels for N in the equation.

For clarity I would add brackets to insure proper order of math operations.

Rx= ( (20K/N)200 ) / ( (20K/N) - 200 )

JR

 

[supermatt]

Thanks for the responses, so if im ending up with 263Ohm for 24 inputs im on the right track, am i?
really appreciate your help.

 

[ruffrecords]

Thanks for the responses, so if im ending up with 263Ohm for 24 inputs im on the right track, am i?
really appreciate your help.

Yup, my calculator agrees with that. A 270R would do.

Cheers

Ian

 

[supermatt]

thanks!

 

[PRR]

> im ending up with 263Ohm

For 200 ohm goal, for any reasonable number of inputs, the answer should be "somewhat more than 200 ohms".

For a Hundred inputs the answer degenerates to infinity. (20,000/100 is already 200 ohms.)

Half a Hundred inputs is already 400 Ohms, another 400 ohms added makes 200 Ohm goal.

So just throwing darts, a fourth of a hundred inputs will be significantly bigger than 200 Ohm loading, but much less than 400 ohm loading. Perhaps 250? 300?

The "200 Ohms" is not critical. Mike amps work fine 150 to 300 ohms. If the "right answer" is 263, not only is 270 "exact enough" (mike amp sees 204 ohms), 220 or 330 will work also (giving 174 or 236 seen by mike-amp). Just do both channels of a stereo mix with the same value (5% is good, 2% is now cheap).

 

[supermatt]

Thanks PRR.