Archive for September, 2016

Tiny battery-powered 2×2 active mixer

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.

Design notes

Power supply

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.

Output stage

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.

Parts list

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
Switch 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)


TLE2426 rail splitter Mouser
LMR62014 boost converter Mouser
COOLAUDIO 2164 VCA Small Bear
TLV2374 op amp Mouser


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


Indicator LED Mouser
Schottky diode Mouser

Portable battery-powered mic preamp

I am SUPER INTO the current trend of tiny musical instruments (OP-1, Volca series, Pocket Piano, etc). I’ve built a few accessories to complement my collection. Some people at the Operator-1 forum have expressed interest, so I thought I’d share the designs. I hope to provide enough information that you can make your own.

In this case, I wanted to use a pro microphone with the OP-1. So I designed a small, battery-powered mic preamp with an 1/8″ output.



I am not a professional electrical engineer. Use these designs at your own risk.

The design has no phantom power, and therefore will only work with dynamic microphones.

I had a bit of a brainfart when I designed the output stage, and the result is that it won’t work plugged into a balanced input. The preamp works great with the OP-1, but you might get funny results if you plug it into a pro mixer or such.

The provided PCB uses some surface-mount components, which require a different soldering technique from through-hole electronics. If you can do through-hole soldering, you can probably learn surface-mount, but it is different. I highly recommend using a hot air tool, but it is theoretically possible to do it with a plain old soldering iron as well.


Click for full size:



I shared the project on OSH Park, where you can download the .brd file or order PCBs.

Design notes

I can’t really take any credit for this design as it’s more or less straight out of the INA217 datasheet.

Power supply

The TLE2426 is a nifty IC for converting a single voltage into a bipolar power supply. It finds the midpoint voltage of its two inputs, and then you can pretend that’s ground. Your 9V battery then looks like a +/- 4.5V bipolar supply.

D1 is for reverse protection. D2 is a power indicator. If you use the same LED I did, it’s SUPER bright, so feel free to replace R5 with a slightly bigger resistor. You can swap in your favorite color of LED instead, just adjust R5 to match. C5 is for smoothing the power supply. This circuit is not very power hungry, so you don’t need much; 47μ is probably plenty. I happen to have a giant bag of small 100μ caps so that’s what I use.


This is a totally standard application of the INA217 preamp.

R6 is the gain potentiometer; it should have a reverse-log (also called reverse audio) taper for smooth control. Unfortunately, it’s backward: fully clockwise is minimum gain, fully counterclockwise is maximum gain. I could not find a suitable arrangement that gave me both a nice gain curve and conventional operation, so I chose the gain curve.

R2 sets the maximum gain at about 60 dB.

Output stage

There’s a 1/4″ and 1/8″ output, each of which has its own op-amp buffer. Even though the schematic says 4227, you should use a rail-to-rail op amp like the TI 2374 instead. (This is a case where I didn’t really know how to use Eagle yet.)

It was kinda dumb of me to give the tip and ring of the 1/4″ output independent buffers with the same signal. This is really confusing to a balanced input. I never bothered to fix this, because I mainly just connect the 1/8″ output to a stereo line input and that works great.

Parts list

You can get almost everything from Mouser. There are a couple random things that I got from other suppliers, mainly because they matched the parts that happened to be in my Eagle library. Feel free to substitute, just check the data sheets carefully.


9V battery clip Digi-Key
8-pin DIP socket Jameco
Neutrik XLR/TRS combo jack Mouser
1/4″ output jack Sparkfun
1/8″ output jack Mouser
Switch Mouser


TLE2426 rail splitter Mouser
TLV2374 op amp Mouser
INA217 instrumentation amplifier Mouser


10K reverse log pot Mouser
2x 2.2K 0603 resistors Mouser
10 ohm 0603 resistor Mouser
1.5K 0603 resistor Mouser
4x 1μ capacitors Mouser
100μ capacitor Mouser


Indicator LED Mouser
1N4148 diode Mouser

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