building a line level mixer

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

1. The op-amp is just shown as an example of a makeup-gain stage that may be used in the mixer. Power connections to ICs are often omitted on schematics, but their presence is implied. (An amplifier will not work without power).

2. Yes, but the source impedance would be rather high. If you must use an off-board amplifier--especially one intended to be driven from a low-Z source, such as a mic preamp--use a lower-value pot, say 1K log. This will increase the mixing loss by about 10dB, but your crosstalk will improve by a roughly equal amount, and the signal will still be well within a good range for feeding a typical preamp.

Note that putting the master volume directly after the mixing network is generally not a good practice. It's better to interpose a stage of gain between them. When the first stage of gain comes after the master volume, the S/N degrades significantly as you turn down the master. Freq. response may also be affected, depending on the input characteristics of the following amplifier.

3. "CW" means "clockwise." It's the end of the pot contacted by the wiper when at full clockwise rotation--in other words, turned all the way up.

[quote author="NewYorkDave"]12. Yes, but the source impedance would be rather high. If you must use an off-board amplifier--especially one intended to be driven from a low-Z source, such as a mic preamp--use a lower-value pot, say 1K log. This will increase the mixing loss by about 10dB, but your crosstalk will improve by a roughly equal amount, and the signal will still be well within a good range for feeding a typical preamp.
[/quote]

Would the source impedence be too high for a line level input? I intend to use a outboard amp that can accpet line or mic level signals would you say that i should change the values in this case?

thanks a million, this forum is incredible :guinness:

just realized that slide potentiometers are only common in 10k , which is the opposite direction of where you suggested. Can i compensate for this by lowering a resistor value later in the signal chain?

> what is the opa604 doing in a passive mixer?

There are no mixers that don't need added amplification. Mixing is a lossy process.

So-called "active mixers" inject feedback from an amplifier output into the mixing bus. This greatly reduces crosstalk, interaction, and simplifies calculations. However the amplifier has a very hard job.

So-called "passive mixers" simply mix with resistors. In the process, a lot of power is lost. If your system had just-enough gain before you inserted a mixer, it now needs extra gain, an amplifier. However the amp does not need to do all the work of the amp in an "active mixer", and can often be an available mike preamp. Forssell's plan puts a simple good amp in the box, just another way to skin the cat.

If your lineamp has a wide range of gain, omit the 50K pots and the OPA604, take output directly from the junction of all thoss 22K resistors. One 0dBu input will come out at about -22dBu; eight equal 0dBu inputs will come out about -13dBu. (We see why Forssell picked 21dB gain plus variable loss after the mixing.) Output impedance is around 4K. If your Line Input is 10K, there is an additional 3dB loss.