Would something like this actually work?
A mixer like that will work very nicely in a Part 15 setup, either direct into the transmitter, or into the computer sound card.
I've been using a Radio Shack 32-2056, which is essentially the same as the mixer in your link, and it gives everything needed to do the job.
This circuit is an operational amplifier summer where the input voltages at J1 thru J4 as modified by R1 through R4 are added according to the values of R5 thru R8. The overall gain of the summer can be set by R9.
Try a search on "operational amplifier summer" for more detail.
Neil
Thinking further, I suddenly realize that several things need to be taken into account before claiming this will work for your situation. The input levels might need special treatment in order to give what you want.
If all the inputs are intended to run at microphone levels and all your microphones are the same, then setting R9 will be the only adjustment necessary.
Or, if all the inputs are coming from line-level sources, same thing... just adjust R9.
But if your input levels vary, perhaps a dynamic mic, a louder condenser mic, a line-level source such as a CD player, you'll need level control over each input, which can be accomplished with resistor attenuator circuits.
BUT, as a basic starting point for having a useful mixer, the circuit is good.
For individual input volume control; would that be sticking a variable resistor on R[5-8] ?
I think it would be cool to put together a simple 4-channel mixer kit. It would be cooler if the whole of part15 folks would provide some input as to how it should work.
Once the basics are done, maybe add in a "hybrid" for skype/voip or something..
No, I think to keep the mixer stable the input as it is should be kept.
I volunteer to join a committe of Part 15 folk to help design a "Made by Part 15" type audio mixer! Great project.
As I see it, there should be several possible versions, to match the various uses of different stations.
Version 1 - All line-level inputs;
Version 2 - All microphone inputs;
Version 3 - A mixture of mic and line inputs.
I don't have a software tool that allows drawing schematics and my scanner is out-of-order, so I am stuck using words.
BUT ANOTHER IDEA popped into my head that I will call VERSION 4!
Build TWO mixers... one for line level and one for microphones, then combine their outputs! It would be SMARTER and EASIER than Version 3. Maybe.
To be continued.
I have a box of 10k pots, a few 10k sliders, some momentary contact switches, and some of those amp ICs from the schematic. I will attempt the initial design seen there, and report back.
Good and a new idea popped up.
By building the mixer as shown, and setting it for line level (-10dB in/-10dB out), you could build or find some inexpensive microphone preamps for the microphone channels, which boost from mic-level (perhaps -60dB) up to line level (-10dB). Each mic pre-amp could have a trimmer to adjust its gain to particular microphones.
Carl - is there a specific method for calibrating the schematic shown to be -10dB IO?
The tools needed to set the level are an AC Voltmeter and an audio sine-wave generator.
Pick a mid-range tone like 1kHz, first set the output of the audio generator for X-Volts.
Forgive me for not knowing the exact voltage but one of our other members will come along with that information.
Once the tone is producing -10dBm, apply it to an input and attach the Volt meter to the mixer output, and set the mixer gain control for that same X-Voltage.
I will later look in my reference files, I have those numbers somewhere.
At this point we are assuming that the next stage, the input where the mixer will feed, is designed for standard -10dBm, and indeed most of the part 15 transmitters are, and sound card inputs to the computer probably are, but you can double check all the specifications to be sure.
Now pending the Voltage figure for -10dBm.
Part of the afternoon was spent sorting through my audio reference stacks, but the answer was as close as Wikipedia
http://en.wikipedia.org/wiki/Line_level
The voltage you are looking for is the P-to-P (peak-to-peak), I think. Anyone is welcome to correct me on that.
Awesomeness. Looks like I have stuff to do this weekend!