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MFOS Synth 5: Trimmers and some capacitors in place

Pete Brown - 18 December 2011

The next step was to solder yet more components to the board. I soldered the trimmer potentiometers as well as a number of the capacitors.

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:

Trim pots

The MFOS synthesizer uses multi-turn cermet trim pots to make tuning the various parameters possible. I picked them up from Tayda Electronics. These are used throughout the synthesizer to enable calibration.


These are +/- 10% Cermet pots with a top-mounted multi-turn (25 I think) adjustment screw. Not for this board, but for the others, I recall running into a case where I could only find the side-mounted adjustment screw versions for at least one of the values. Those will work too, but the adjustment screw may be more difficult to access.



Pay attention to the build notes at the MFOS site. Ray has a few substitutions you may wish to make, specifically swapping out some of the Tantalum capacitors for aluminum electrolytic ones of the same value.


One thing I didn't like about the board layout was that several of the spots for the aluminum electrolytics were sized for a much larger capacitor with 5mm leads. The smaller sized ones (of the correct voltage and capacitance) work fine, but they stick up from the board a bit rather than sitting flat. For grins, I tried finding 5mm lead versions on Mouser, but those are almost impossible to find. My guess is that Ray had much larger values originally spec'd or something.


I ordered some additional capacitors from Mouser. It turns out I was missing a few values. Those should hopefully arrive either just before or just after Christmas. Once those are in, I'll be able to calibrate the three oscillators. That's where the real fun begins :)

Other work

Based on feedback from the last post, I also replaced the doubled-up 1/8W 3M resistors with single ones. Doubling up the resistors in parallel actually decreased the resistance, and it turns out the board won't need the full 1/4" watt ones anyway (although those might be more heat stable). I'll keep an eye on them in case heat becomes an issue.

Also, after pricing out the headers I'd need to make this board easily disconnected from the panel, I decided to just pass and instead wire the front panel directly to the board like everyone else. I will, however, end up using coax for the mixer section as Ray recommended in his MFOS Sound Lab Mini Synth Mark II build.

posted by Pete Brown on Sunday, December 18, 2011
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1 comment for “MFOS Synth 5: Trimmers and some capacitors in place”

  1. Jacksays:
    Is there a specific reason you've selected these trimpots? would a single turn <a href="http://www.resistorguide.com/trimpot/">trimpot</a> also do? i'm a bit low on budget...

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