Project: Matrix Mixers


Since getting a soldering iron and Nic Collins book, I think the thing I’ve always wanted to make was a matrix mixer. Mainly because it had lots of knobs, and also because I wasn’t quite sure what it would do when used in anger. And in the end, this becamse a project of two parts...

There are two reasons it’s taken me a long time to get to it – first being that in the book, the only guide one has is a schematic, and secondly, I was scared of drilling the aluminium box.

The first reason was a tragic one, but nonetheless, I’m still a fair bit away from being able to read a schematic and know exactly what it means – all the projects up to that point in the Electronic Handmade Music have at least one helpful picture, so I just acted afraid and ignored it. When I eventually decided to bite the bullet, the additional moral support came from a pic on Concrete Dog's blog, which showed the guts of a larger matrix mixer, which helped interpret the schematic. And obviously, once that courage had been imbibed, the text in the book was actually very clear on what you had to do. Lesson – read slowly, fear less...

The second one was another fear of the unknown – in my head, these boxes would require incredible skill and expensive tools (well, a drill press at least) to make the necessary incisions. I went to a friend, Jon Dickinson, who makes amazing amps and is an all round don, who was very helpful, and demonstrated that drilling is actually very easy, as the metal is quite soft. He also made clear the importance on measuring before drilling, and his tip to wrap the box in masking tape to plot the holes was a very simple and good one. Danjec 10b was drilled under guidance, using a drill press; danjec 10a was done at home, using a mix of regular drill and a hand router (note – this hurts your hand like hell after an hour of gouging!).

10a was a gift for friend, primarily a guitarist with a love of reverb and delays, so his one would use ¾” jack sockets, and 10b would be for me, and may get used with the modular, so would have 3.5mm sockets.

10a as you might have guessed came first. I was very disciplined and patient with the wiring on this one, and made sure each pair of connecting wires was carefully braided together and tinned before being attached to the potentiometers. Took an hour or so to assemble that part, and probably another half an hour to fit the pots and jacks in place. So far, so good. The first test with noises going through was ok, so tried to shut the case – at this point, I discovered the fat ¾” sockets were a tiny bit proud, and wouldn’t allow the bottom of the case to be fitted securely. After a couple of swear words, I took the sockets out again and ‘customised’ them, clipping a bit of the plastic off each socket at the bottom of the case which gave just enough clearance for the lid to go on. Then I tested again, and this time the noise was unholy from this box, in a bad way. Stumped, I messed around with some crocodile clips and discovered my elementary mistake – I hadn’t grounded the jacks. Added a solder tag to the case of the box, and flew grounding wires across, and all became clean again. Sounded lovely with a Bugbrand Weevil, delay feeding back in, and some reverb.

10b should have been more straight forward, especially as the jacks were much smaller, but I forgot the discipline of the first build in the intervening two weeks, and inadvertently joined up the wire such that each row potentiometers were in series, rather than matrix mixing. More swear words, and a quick de-solder/re-solder session, and this one was working as nicely as the first. Until, that is, one of the potentiometers ‘snapped’ – which isn’t to say that it came off, rather than it started turning through 360 degrees. Out came the guts again and a replacement in place. All good, and now the temptation is to build a bigger one...

Results Fail

I always had a suspicion that all was not 100% right with the matrix mixer - it would successfully and cleanly bring all the inputs and outputs together, but being unaware of what matrix mixing was supposed to sound like, I thought maybe the knobs don't affect the sound that much due to all the feedback going on. But having another play many moons later, this didn't stack up. If nothing else, why would you call it a mixer?

Back to the internet and the Nic Collins bible - very quickly I saw something was awry. Having sorted the initial problem with the grounds not being in place on the jacks, I'd missed a very basic trick with the potentiometers - I hadn't grounded them (see 13.6 in the NiC Collins bible!). A quick check with crocodile leads confirmed the issue, and 20 mins with the soldering iron fixed the issue. Now, you can feel/hear the changes as the balance is adjusted with the pots. Like a mixer..!

Results Pass

 Grounding test   croc lips to the rescue 
 danjec 10a   back side 
 danjec 10b   danjec 10b (knobs to follow)