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MFOS Synth 3: Starting the Front Panel

Pete Brown - 04 December 2011

In this post, I'm starting to build the front panel for my MFOS Sound Lab Ultimate analog synthesizer.

Over the next several months I'll be building what is definitely my most complex electronics project to date: the MFOS (Music From Outer Space) Sound Lab Ultimate, Ultimate Expander and (if Santa brings one) Sound Lab Mini-Synth Mark II, likely all in the same home-made wooden case, side by side. The Ultimate and Expander are together a 3 oscillator monophonic true analog synthesizer with filters, envelopment generator, ring modulator, sample and hold and more. You patch between the different logical modules using banana cables, so it's a bit of a self-contained modular synthesizer. The Mark II is smaller, newer, and has a few fewer features, but a sound of its own. You also patch that with banana cables, and can integrate the two. This blog post is another in the series. Previous posts include:

Ok, I lied. I wasn't going to do any more on this board, but I had a number of packages waiting for me today, including a wonderful box from Small Bear Electronics. They had the best prices for Alpha potentiometers. They were out of a couple of the audio taper ones in the larger size, so I substituted the smaller pots. If the action on those is significantly different, I may just bite the bullet and order the larger sized ones elsewhere, but I'd rather not. In the same order, I picked up an assortment of knobs to see which ones I would like the best on the synth before ordering the 82 knobs the three modules (plus the Volts/Octive calibrator) would use.

Also, to connect all those wonderful pots and the jacks and switches, I needed a bunch of wire. All my hookup wire is either high (thin) gauge solid wire, or low (thick) gauge stranded. I needed a bunch of 24 or 26 gauge stranded so it would remain flexible, and as many colors as I could dig up. I'm not a fan of working with stranded, but when you have something that's going to flex, it really helps to keep things from breaking.  I ordered all that from All Electronics. I also needed a little thin coax for some of the more sensitive wiring, as recommended by Ray Wilson (owner of MFOS).


If you're curious as to just how much wire gets used in this project, check out these builds from other folks. Also keep in mind these show only one of the three primary panels.


You can see more photos on Ray's facebook group. The wiring can get a bit nuts. When you consider that the circuit board looks like this:


…and that all those X spots are wires going to the panel, you begin to see how it can quickly get out of hand. I'm considering wiring things into a few pluggable (locking) headers and going from there, but you really want every wire to be as short as you can reasonably get it, so I'm not sure. I may still do it to clean it all up a little and make it easier to work on the whole thing. In fact, if I have those headers in there, I can keep the wires shorter anyway, because I wouldn't require the slack to be able to work on the board. hmmm.

At this point, I'd like to mention how great Ray from MFOS has been. I've only been into electronics for the past year, and this is my first large kit where I had to source all the parts. In the past, the kits (like my two SammichSID kits and the MeeBlip) came with all the parts included and with very detailed step-by-step instructions. That's simply not practical with a kit this large. Ray has been great answering my questions, many of which clearly show that I fall into that disclaimer on the top of his page where he doesn't recommend these boards for electronics newbies :)

In any case, I'll investigate doing something about that wiring, as the other builds I've seen just make me cringe with that pile of spaghetti in there. Nothing wrong with that, especially if you're keeping the runs as short as possible, but it makes my eyes hurt.


Small Bear had the best prices by far on the pots, so I ordered a bunch of them from there. I originally wanted to go with all nice conductive plastic Bourns pots, but at around $10 each at normal prices, no way. Together, the four projects (Ultimate, Ultimate Expander, V/O Calibrator, and Mini Synth Mark II) use 82 potentiometers. 82 x $10 might as well equal double.PositiveInfinity as it's not going to happen.

Unfortunately, small bear was out of the audio taper pots in the larger 24mm size. They had them in the smaller 16mm size though, so I used those.


The shaft of the 16mm pots doesn't quite fill the panel hole, so aligning them (and keeping them aligned over time) is a bit fiddly. However, I actually prefer the feel of those pots in use to the 24mm. That's something to keep in mind for future projects. I've heard the 16mm pots don't last as long and/or aren't as consistent, but that just smells like FUD to me.

Near the top of the panel, you can also see a bag of plastic caps for the 16mm pots. I'll install those after the soldering is done - they'll help keep any dust out of the pots. I'm not sure why they don't make any for the 24mm pots. Perhaps the 16mm are more prone to dust (perhaps the source of the FUD?)

Turns out people do make covers for 24mm potentiometers. Here's one, but holy crap that's way over priced, unless it's for like 100 of them. Here' I see another list with the same ones, but $0.95 each. That's a bit better, but still more than I'd expect them to be. I may just have to resort to a bit of tape or something as the price of the seal plus the original pot price starts getting high enough to count, or I might just say forget it, and make sure I have some pot cleaner hanging around 5 years from now :). (BTW, turns out tubesandmore.com (Antique Electronic Supply) is also a great source for knobs, pots, and other pieces. Reasonable prices too.


You can't have pots without knobs. I auditioned a number of control knobs purchased from Small Bear Electronics, as well as several I had on hand. No matter how I photograph them, no matter what light I use in here, I can't get the violet to come out as the knobs and jacks all end up looking blue. Seems like a poor spot in the digital camera CCD sensitivity, or maybe I need to switch to different lights or photograph this guy outside. These look more blue, but they're actually a violet color.


The first photo shows the color of the jacks and knobs a little better, but still not spot on. The second photo shows the color of everything else correctly.

You can see from these photos, that the knobs sit at various heights above the panel as well.


I haven't yet made up my mind. My wife and children are partial to the violet knobs (the ones that are the same color as the banana jacks, not the more purple ones). I was really hoping the three large knobs on the top left would work, as they match my Moog Slim Phatty, but they're really too big and they actually make the pots hard to turn, even with space around them. The smaller ones next to them are nice, but they're too short and therefore difficult to use. My wife also likes the aluminum colored ones on the right. I really don't care for the looks of the black skirted ones with the large position line in white. In general, all the large 1" knobs on the board are simply way too big, and the knobs that are only 1/2" tall are way too small. In terms of usability as well as aesthetics, I'm torn between the black knurled aluminum ones on the right, and the violet ones on the bottom left. However, I would have preferred a skirted knob like on my Moog, as I hate to see washers and nuts under them.

Here are my Moog and Roland racks for comparison:


The smaller skirted Moog knobs are actually a little longer than the small ones I tried out, giving your fingers more to purchase. Before I commit to a set, I'm going to make sure those aren't something I can find either at Small Bear or elsewhere. I think the Synth Pointer #4 at Small Bear may be what I had in mind. Looks like one more round of samples (good thing I can use these on other projects!) before I make a final decision. Since I'll be away most of the week, today's a good day to put in an order. Done. :)

Next Up

I'll post about the switches I ended up using in an upcoming post. I had to make a few simple substitutions that anyone else building this synth will likely have to make. In the photos above, I don't yet have those switches in place as I hadn't yet made the decision to swap out. More on that next time.

posted by Pete Brown on Sunday, December 4, 2011
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1 comment for “MFOS Synth 3: Starting the Front Panel”

  1. CarlDsays:
    Pete - While I'm not a musician, I am a long time electronics hobbiest and engineer, so I've enjoyed your journal of your adventures in the electronics hobby. If I had an ounce of musical inclination, I'd be right there with you on building an analog synth - I've always had a fascination with them and I'm sure I'd get a kick out of playing with the end product, even if the results weren't very close to musical. Looking forward to your reports as you work through this project!

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