This Audio Mixer Is A Eurorack

Music making and DJing have both become arts predominantly pursued in a computer, as the mighty USB interface has subsumed audio, MIDI, and even DJ turntable interface controllers. There was a time though when an indispensable part of any aspiring performer’s equipment would have been an analog mixer, a device for buffering and combining multiple analog audio signals into a single whole. A mixer is still a useful device though, and [Sam Kent] has produced a very nice one that takes the form of a set of Eurorack modules made from PCB material. There are two types of modules, the main channel module which you can think of as the master module, and a series of isolator modules that handle the individual inputs.

Mixer preferences are as individual as each user, so for example where we’d expect sliders he’s used rotary potentiometers, and for us placing the master channel on the left-hand side is unfamiliar. But that’s the beauty of a modular design, there’s nothing to stop anyone building one of these to simply configure it as they wish. We notice that for a mixer described as for DJs there’s no RIAA preamp for the turntable fans, but it’s not impossible to fix with an off-board preamp. Otherwise, we like it and have a sudden hankering for it to be 1992 again with a pair of Technics SL1200s and a room full of people.

Designing a mixer, even a simple one, isn’t easy. Our own [Lewin Day] wrote a retrospective of his experiences with one.

14 thoughts on “This Audio Mixer Is A Eurorack

  1. I like the modular approach. Most mixers try to make every channel work for any kind of input, or they have only so many channels that can accept mic inputs, with the rest being line-level only. By making it modular, you can throw together just what mixer you need for a given job. I’m sure that sub-mix boards could also be made, for those situations requiring sub mixes.

    As an example, a DJ may only need one mic preamp, but also want two RIAA-equalized phono inputs, which would use a different module.

    However, as Jenny points out, some of us would really rather have sliders for the level of each channel, but once you have a slider in place, the Eurorack form factor doesn’t have a whole lot of room left for other things. Which also explains why there are only pin headers rather than XLR jacks for the inputs.

    So while I’m sure this met the maker’s requirements, if I were to do something like this, I would throw out the Eurorack spec and use panels closer to the size that individual channels have on a typical commercial mixer. But instead of having a chassis they plug into, they would each plug into the side of another module, maybe using threaded standoffs to stack them together. the common output buses would be daisy-chained from one module to the next, so the whole stack is only as wide as the number of modules you need for the job. 3d-printed housings would protect the back sides of the modules and provide a front panel separate from the PCB, and these would also be designed to maybe snap together? Modules for power (including a battery-powered version), channel front ends, sub-mixes, main mix, power, effects, even USB output modules… The possibilities!

    1. Way way back we had in club some Soudcraft DJ mixer that was modular. I don’t remember the model, but according to pictures it might be Souncraft D-Mix 1000. One of nice features of it was that I could fix it live, without interrupting the party. If I remember correct, it had three different input modules, mic, phono and line, and one output module.

    1. No. eurorack defines only the vertical size, 19″ the horizontal and the vertical are in “units”. there are many 19″ eurorack frames, but you can end up with having a gap on the edge of 6mm or so. depends on what you put in them. het nuts are in a rail, but have no fixed horizontal pitch and are adjustable. but of course wikipedia and qwant or google give a way better explanation.

    2. Eurorack is in part based on the rackmount standard, in that a row of modules fits into 3U, and there are rail/case/power supply jobs meant to fit into a rack. (The common 84HP width case is this size; modules are sized in HP — Horizontal Pitch — where 1HP=⅕”.) My understanding is that when Doepfer created their A-100 series that became the basis for Eurorack, they built based on Vector rails and the Eurocard form factor, which derived from the 19″ rack standard.

      But beyond the height matching, Eurorack case manufacturers haven’t felt compelled to keep to the 19″ width; there are cases out there both narrower and wider, so you’ll see 48HP “lunchbox” cases for small, dedicated systems and a few that run to 126HP wide.

  2. Is US$30 (Mouser) a sensible price to pay for the DC-DC converter in the power supply that produces the split rails? Without a center-tap transformer, how dangerous is it to use two (identical) laptop power bricks to get a split supply?

    1. No, not really. What some audio devices do is use an AC wall wart, and ground one side, then use half-wave rectifiers for positive and negative supplies, which they filter and run through linear regulators. For low-power devices, this works fine, because while half-wave rectifiers need higher capacitance for their filtering, this can still be a pair of pretty cheap capacitors.

      Here’s an example:

    2. Also, you can’t really depend on laptop power supplies being isolated from the AC line, especially those that have three-prong (i.e., grounded) power cords. Many have small capacitors between the neutral input and the ground side of the output. You’re better off with the ubiquitous 12V switch-mode wall warts used for things like external hard drives and other peripherals, or at least supplies with two-wire cords. But still, using two mains-powered supplies is kind of asking for trouble. Ask yourself, how will my circuit behave if one power supply is unplugged? And by the way, this is a concern any time you’re using multiple power supplies, even if they share a common power cord.

      Another way of getting split supplies from a single supply is by using a DC wall wart that supplies more than the total voltage you need, using a voltage divider to create a virtual ground between its positive and negative outputs, then using separate positive and negative linear regulators (e.g., 7805 for +5V, 7905 for -5V if you need +/– 5V) to get the split supply. Ideally, the load should use the same amount of current on the + and – supplies, which allows the virtual ground’s resistors to be reasonably high values, so they don’t have to dissipate much power. Otherwise you have to use different sizes for the two resistors. This approach is a bigger challenge if you are looking for +/- 15V, because you’ll need something like a 36V supply, but these can be found.

      But I still recommend the half-wave bipolar rectifier I mentioned in the previous comment, if you can find an AC wall wart that gives you enough voltage – I’ve used this one myself, and got very good results.

  3. You may want to consider a better device than the TL074. It was a decent part in the mid 80s and along with its smaller brothers 071 and 072 has been extensively used in semi pro mixers and gear back then, but it’s not considered low noise anymore by today’s standards. The transition from vinyl and needle to CD and DACs forced the industry to rethink what quiet meant.

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