building a line level mixer

[bantam]

i am trying to build this line level mixer
http://www.forsselltech.com/8chsum_1.pdf

i have a few questions
1) what is the opa604 doing in a passive mixer? it doesn't indicate power going to it so that throws what i know about op amps on its ear
2) can i take out from the 50k log pots to the output, effectivly taking out the gain stage. id rather use one of my line amps (like my rnp)
3)what the hell is the CW mark all over the place?

thanks guys!!!!

this could be a great out of the box mixing solution for me

[housepig]

regarding #1 - I don't think it's passive; I see lots of stripped-down schematics where stuff is implied, and you don't get full info on pin connections, where the op-amp connections just show the +, -, and output, not the power and ground.

[Family Hoof]

The OPA604s are the makeup gain amps. Yes, they are active devices and do require power, but the power connections are implied and have been omitted from the schematic for simplicity's sake. The term "passive mixer" refers to voltage summing. The signal voltages from each line input are being passively combined through the resistors and then the resulting voltage is amplified to makeup for the amount lost in the resistors. It's a bit confusing because an active mixer would still require summing resistors and makeup gain, except that the op amps would be running in inverting mode, holding all the inputs at virtual ground potential. In that scenario the signals are being summed as currents. The active devices (op amps) are actually performing the summing, hence active mixer aka current summing. The proper term for mixer is summer, because that's what it's doing - adding, computing a sum of multiple signals. Read Fred's associated paper on summing to gain a better understanding.

[edanderson]

in answer to your questions:

2) yes, but you probably should replace the 50k pot with a fixed resistor. this resistor will set the output impedance of your passive network. the value depends on the number of channels you decide you need, the input impedance of the device you're feeding, and how much loss you want to take in your summing network.

have a look at this thread for more info:

http://messageboard.tapeop.com/viewtopic.php?t=28331

3) the "CW" mark indicates clockwise rotation for increasing value on the pot.

ed

[ram3n]

I've read that paper Fred wrote on summing several time... it's a very good read .