The MicroMix is an 8-channel stereo line mixer based on the 4580 ultra-low-noise operational amplifier. It has eight 6.3 mm stereo jack inputs and one 6.3 mm stereo jack output. Each channel has a volume control and a mono/stereo switch. The output signal level is indicated with four LEDs. The device is powered from USB-C, but a power supply is not included. There is no power switch. Built like a tank with a 190 mm × 70 mm × 40 mm steel enclosure, it is heavy for its size (385 g). Its rubber feet prevent it from sliding, even on a smooth surface.

 

A minor inconvenience, at least to me, is that the black knobs do not have a clear position marker. This makes it difficult to see a knob’s position from a distance. Similarly, the position of the mono/stereo switches is hard to see.

Note that the MicroMix also exists in 4-channel and 6-channel versions. It is an unbranded device, and it can be found rebranded.

Clear up Some Details

The description of the MicroMix tells you what it is seen from the outside. But what if we look a bit closer and take off the lid? There are two reasons to do this: 

1. The product description mentions the use of 4580 ultra-low-noise operational amplifiers. To take advantage of such op-amps, the circuit should be designed accordingly. Is it?
2. The manual mentions 6.3 mm mono sockets, but it’s a stereo mixer. What is going on here?

A Look Inside the MicroMix

After opening the MicroMix and removing the circuit board, we can inspect the in- and output sockets. These are two-channel sockets, i.e., stereo, not mono, so the manual is wrong here (as expected).
 

The board contains ten 8-pin SO-8 ICs with ‘4580’ printed on it. However, the print on them looks somewhat fishy. Usually, there would be more text on an IC of this size, like a date or batch code and a logo. Here it just says ‘4580’, in rather large characters. There is no logo or anything else. This does give the impression as if it were unbranded low-cost op-amps with ‘4580’ stamped on it. The only way to check this would be to measure the signal-to-noise level of the op-amp, but is that really worth the effort?

The Capacitors of the MicroMix

Besides the op-amps, there are many low-cost electrolytic capacitors. Reverse-engineering the circuit, we see that a signal passes through three of them from input to output.
 

The potentiometers are all low-cost 50-kΩ linear devices (B types). The 10-kΩ resistor in parallel with the wiper is probably intended to give it a pseudo-logarithmic behavior. However, as a side-effect, the input impedance now depends on the position of the potentiometer and ranges from 50 kΩ down to 8.3 kΩ.

 

The MicroMix Reverse-Engineered

The reverse-engineered schematic also reveals what the mono/stereo switch really does. Naively, you would expect the mono position to sum the left and right channels into two identical signals, but it doesn’t. When set to mono, only the left channel (the tip of the jack) is passed on to both output channels. The right input channel is simply disconnected. Therefore, when a channel is set to mono, it becomes a pure mono channel, not a “mono-ized” stereo channel. In itself, this is not a problem, but it is good to know. Stereo panning is not possible, however.

Gain

The voltage gain of each input is a bit more than +3. The total voltage gain of the mixer is approx. −10. This is a lot, meaning that the output can easily overdrive the input of the next stage to which it is connected. At the same time, it allows for connecting weak signal sources.

Switched-Mode Power Supply

The power supply of the MicroMix is a simple switched-mode design that turns the 5-V input voltage from the USB-C port into +15 V. An inverter is added to create −15 V. The switching frequency is about 400 kHz. These voltages power the op-amps and the rest of the circuit. I measured a 400-kHz, 55-mV residue on the op-amp’s supply pins.
 

Conclusion

From the above, it will be clear that the MicroMix is not the ultra-low-noise mixer it claims to be. However, this doesn’t mean that it is a bad mixer, and it will definitely be useful in many situations. An electric guitar can be connected directly, for instance, and produces a clean and strong output signal. Thanks to its high amplification, even microphones can be connected. The mixer’s compact size and strong build make it a practical device for the rambling musician as it doesn’t take up much space in a bag, and it definitely will not break easily.