DIY 4 Channel Stereo Mixer

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Hey folks...

I know next to nothing about building circuits, but I have figured some things out....

What I am trying to do is create a 4 channel stereo mixer that can be controlled by a computer. I have a computer in my car that has replaced my stock head unit. I now need a way to switch between all of the possible inputs (HD Radio, Sirius Radio, Computer audio, AUX input), with the possibility of mixing more than one source together.... Think of gps navigation instructions on the driver's side, but the rest of the speakers pump music.

I have a microcontroller that I"m using for some other things that should be able to control the mixing.

I don't think I have a need for tone control, EQ, or anything fancy... I just want to be able to control the volume and balance of 4 inputs that have a single output. The 4 inputs were listed before and are line level. The output is also line level.

A friend has pointed me to using LTSpice to model a couple of circuits, and I think I have found something that could work..... but it would be nice if I could get a design that is somewhat proven. Just guessing on an opamp doesn't seem right.

Things that I suspect I will need are:
various resistors & capacitors..... any specific types?
digital potentiometers
opamps..... LM833 or NE5532 or just a general purpose one?
power supply for opamps.

Since this is for an automotive environment, I will need to worry about my power supply. I will have access to a conditioned 12v source from the car pc's power supply. But I'm pretty sure it's +12v and ground only.... no +- 12v. Hence the need for the opamps Vcc and Vee.

Any suggestions? or would almost anything work? I am not an audiophile, but I'd like the audio to sound as close to the inputs as possible.

Thanks for any feedback...

diy mixers

Look at this link, particularly the 3rd schematic of the actual mixer part. I think 5532's with their 14 pin package are passe', I'm using ST33078 dual in line 8 pin package right now. 33078 seems to interchange completely with JRC4560 delivered in my CS800s amp. They were operating fine at +- 2.5 V power voltage in my disco mixer, but when the radio coming in was bigger than that they clipped, and turning the radio way down increased the hiss on it a lot, so I boosted them up to +- 7.4 V with 1n5344 diodes as regulators. If you build a power supply for your car regulating to +-5.1 v (a common zener size) then that should work fine. The signal ground is inbetween the line and the triangle side of the two zeners. I've got two 22 ohm resistors in series with and outside my two 5 watt zeners, use higher resistance if you use lower watt zeners or none if you use LM320-5 regulators etc.
After a substantual war on hum, I don't here any more hiss or hum on the RA88a with 3 input sliders enabled, mag phone, cd player, and FM radio. So if I were you I would first try without any automatic control except on/off. The car power won't hum, but you will need hash filters coming in to protect against switching transiients from the starter turning off, etc. MOV's are too high in clamp voltage. A 5 ohm resistor and a 20V high power zener may do the trick.
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I saw that thread.... but all the comments seemed like there was much more to it than just that schematic... just switch in a different opamp instead of the tl072... if i remember right.... put in the ST33078?

I have built a regulated 5v supply using a 7805.... but I could probably pull something out of the power supply built exactly for car computing.... I'll look into it some more...

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