My unbalanced line mixer design needs something...

[thefunkybeat]

So, a buddy of mine does music production for video games and has a lot of synths and sound modules. He asked me to build him a summing amplifier.

He wants to sum all the signals from all his synths and samplers and things into one stereo pair. Since he's controlling volume and panning via MIDI sequences, he doesn't need ANY controls at all; just a lot of stereo inputs that get blended into one stereo output.

So, I've decided to build him an unbalanced stereo summing bus with 16 stereo inputs. (He doesn't need that many, but I wanted to make sure he has some extra inputs for when he buys more sound modules later.) I don't have an electrical engineering education myself, but I'm pretty good with schematics and putting things together. After some research, I found this:
MIXER 6+ channel in English

I figured I'd take the unbalanced stereo input schematic, multiply that 16 times, skip the EQ section and go right to the summing amp. So far my plans look like this:


... but... I feel like I'm missing something... My intuition is telling me I need a cap or a resistor or both on each Left and Right output of each unbalanced stereo line in circuit... or maybe just a couple of caps on the Left and Right inputs of the master summing amp?

What am I missing here?

[Speedskater]

That circuit is not going to work at all. The output of each input IC will try to drive the output terminal of all the other input IC's. The output impedance of an IC is just over 0 Ohms so all the IC's are driving almost dead shorts. Normally a summing bus is connected to the negative input and each signal has it's own input resistor (maybe 1K to 10K)

[thefunkybeat]

Originally Posted by Speedskater

That circuit is not going to work at all. The output of each input IC will try to drive the output terminal of all the other input IC's. The output impedance of an IC is just over 0 Ohms so all the IC's are driving almost dead shorts. Normally a summing bus is connected to the negative input and each signal has it's own input resistor (maybe 1K to 10K)

Well, I'm sure some parts will work, but others won't. So, you're saying I need a resistor on the Left and Right outputs of each channel huh? ... and maybe rework the summing amp a little? I'll do some more research on that summing amps using the inverting input thing... I've seen that too.

Anybody else have suggestions? tips? solutions?

Thanks!

[wahab]

You can use an inverting amp to sum the signals
through a resistor for each channel, but with 16 inputs,
the noise gain will be very high.

[zdavesf]

read this and subtract what you need.

High Quality Sound Mixer

This project can become time consuming very quickly. input and output buffers could both be improved. if your buddy is looking for a relatively cheap low noise mixer and wants it fast i would suggest maybe looking at Musician's Friend | Your Online Music Instrument & Pro Audio Store | Best Prices, Great Service

I do not want to discourage any DIY project but Realize if you are going with a simple design it will take some R&D to get it tweaked. Best of luck. Like mentioned before use a inverting opamp configuration as a summing buss... i believe most commercial mixers use 4K7 feedback resistors (or there abouts) due to lowest noise. usually no more then 4 signals per mixer (ie 4 groups of 4 mixed together with another mixer).

Dave