Looking for a small dual audio mixer, I found this kit from Transient Modules.

The 2M is two unity gain mixers in parallel. Each mixer has level controls for the 3 inputs and also for the output. As its design has been specially focused on audio applications, all the input potentiometers are audio-taper type while the output ones are linear. Low distortion / high quality audio operational amplifiers have been selected in order to achieve clean audio mixing. It is also DC-coupled so it can perform CV signal mixing as well.

This is a manufacturer’s kit that comes with all needed parts. The op amps are Texas Instruments LME49720, a rather expensive ($3.00 ea.) audio op amp. No schematic was supplied, but it is easy to read it right off the PC board. I sketched the basic schematic for the photo above. It is totally straightforward. In fact it’s almost identical to a mixer I designed and built in MOTM format. The only quibble I have with the design is that the output attenuator pot just hangs off the end of the 1K series resistor, making it function passively. I’d like it better if the output attenuator had been situated in front of the output amp; then the output impedance would be constant at all levels.

Assembly

The downloaded build instructions are great, giving step-by-step instructions and lots of photos. This is a perfect kit for beginning DIY. I found one mistake in the instructions. In step 10, it calls for the female header to be installed, but the male header goes here. The photos helped avoid a mistake. I soldered in the ICs, as I usually do, for better reliability. I used organic, water-washable solder for the whole component board.

A cautionary note: A very small standoff is used to secure the pot board to the front panel, in Step 19. It says, “Use your hands…”, and this is good advice. The screw thread is very fine and could easily be stripped by using too much torque.

An interesting feature of the module is that you choose during assembly how you want jack normalling to work. I chose option 3, which normals the left side input jacks to the right side, exactly like I did it on my MOTM sized DIY mixer.

This is the top of the component board, which ends up inside the sandwich. The power header is cleverly located between the board to minimize the module depth. I left out one resistor, seen in lower right corner, since it was not used in my chosen normalizing scheme.