This is the second in my series of tiny audio accessories: a pocket-sized two channel stereo mixer. It fits in an Altoids tin and lasts approximately forever on a pair of AA batteries.
There also two output jacks; I like to send one to speakers and the other to a recorder.
If you try to build this yourself, please let me know about it!
I am not a professional electrical engineer. Use these designs at your own risk. That said, I am pretty happy with this design and it more or less works exactly how I want it to.
The provided PCB is laid out for surface-mount components. I personally think soldering surface-mount components is no harder than through-hole, but the process is quite different and it was a little intimidating at first.
Get the .brd file or order boards from OSH Park.
I wanted to use AA batteries for a low profile, but the VCA requires a minimum 8V supply, so I used a boost converter. I selected the TI/National LMR62014 mainly because it’s cheap and readily available. The ratio of R16 to R17 sets the output voltage; I’m aiming for about 9.5V to provide a little headroom. The TLE2426 splits the boosted voltage to create a virtual ground, letting us simulate a bipolar power supply.
The core of this mixer is the COOLAUDIO 2164 quad VCA, and this circuit is pretty much straight out of the datasheet.
Recall the design of a simple summing inverting amplifier:
The current through R1 is I1 = V1/R1, and the current through R2 is I2 = V2/R2. The op amp input wants to sink zero current, so the only remaining path for the current I1 + I2 is through R3. That makes Vout = -1 * (I1 + I2) * R3; if we choose R1 = R2 = R3, then Vout = V1 + V2.
The 2164 is a current amplifier, so that means we can just drop in the middle of the summing amplifier design to add a gain control. Referring back to our mixer schematic, R1 controls the original input current, the VCA scales it according to the control voltage, and the op amp pulls it back through R9 to convert the signal back to a voltage.
The control voltage is -33 mV/dB, with ground giving unity gain. R14 – VOL1 – R15 sets up a voltage divider where the wiper on the pot can tap into the middle. Ideally the gain range would be something like -60 dB of cut and up to 6 dB of gain, but I also wanted to stick to common resistor values. The actual range ends up being something like -70dB to +3 dB, which is just fine for my purposes. We can apply the same control voltage to two channels to get a proper stereo gain control. No messy dual-gang pots required!
C14 smooths out gain changes a tiny bit; this prevents pot noise from turning into audio distortion.
You could add as many VCAs as you wanted to this circuit to make more channels.
There are two output jacks and I just tied them together.
Of course, there are two unused op amps, so you could add an independent buffer for the second output. I ran into some trouble routing the board in that configuration, so I decided the lazy approach was probably good enough.
You can get almost everything from Mouser. The VCA chip, however, is pretty difficult to find. The big distributors don’t carry it, so you have to hunt around specialist distributors.
4x battery clip Mouser
4x 1/8″ audio jack Mouser
16-pin DIP socket Jameco (or, if you want to get everything from Mouser try this one; I think the footprint is the same, but this is not exact one I used)
6x 30K resistors Mouser
4x 499Ω resistors Mouser
2x 10K resistors Mouser
3x 1.5K resistors Mouser
1x 88K resistor Mouser
1x 13.3K resistor Mouser
100μ capacitor Mouser
4x 1μ capacitors Mouser
2x 0.1μ capacitors Mouser
4x 560p capacitors Mouser
4.7μ capacitor Mouser
2.2μ capacitor Mouser
330p capacitor Mouser
10μ inductor Mouser
2x 10K linear pots Mouser