Welcome to Pete Brown's 10rem.net

First time here? If you are a developer or are interested in Microsoft tools and technology, please consider subscribing to the latest posts.

You may also be interested in my blog archives, the articles section, or some of my lab projects such as the C64 emulator written in Silverlight.

(hide this)

MFOS Synth 2: Some main synthesizer board work

Pete Brown - 02 December 2011

In this post, I'm starting to populate the main board 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:

Yesterday, I received a huge (5lbs!) box of discrete components from Mouser. If you think about the typical weight of components like resistors and capacitors, and realize the box included no solder, heat sinks, switches, potentiometers, or other heavy metal objects, you can imagine that 5lbs of electronics components is quite a bit of stuff.


Also today, I received my set of tempco (temperature compensating) resistors from Precision Resistor Company (PRC). They're not cheap at $4.50 each. However, if you want to have parts of your voltage controlled oscillator (VCO) or other temperature-sensitive components stay relatively stable after warm-up, you need these or an alternate circuit. (My Moog Slim Phatty has a different microprocessor-controlled tuning circuit which is both more complex, and more error-prone than this approach. However, when it works, it tends to be more stable)

I also have some resistors from MFOS, but because I later realized I needed some for other synth module projects, I decided to simply put in an order with PRC.

Normally, you want resistors which stay relatively stable regardless of temperature, so most are designed to have very little fluctuation (a temperature coefficient of 100ppm or 50ppm for most 1% resistors). A temperature-compensating resistor is one which is specifically manufactured to change its resistance based upon its temperature. It might have 3300ppm or 3500ppm, for example. You stick one of these down right on top of the part that gets affected by the heat so the two are at the same temperature, and then you hook the leads into an appropriate place in the circuit.

I half expected the resistors to be individually wrapped like little toffees. Ben (my 5yo son) said they look like Darth Maul's light saber - he wanted to borrow some for his action figures.


You can also see in the picture that I did a little bit of work on the front panel installing half the banana jacks and several of the switches. Because I'm still waiting for most of the parts for that, I'll cover it in a different post.

So, instead, I decided to start working on the main board.

I installed some of the IC sockets first. I've concentrated on the three oscillator sections as I intend to test them before moving forward to the rest. The sockets are machined pin, gold plated sockets as Ray recommends. With something like an analog synth, you want the best possible connections to your ICs. Soldering them directly to the board would be the best connection, but it's also the least flexible, and it's easy to destroy an IC during soldering. You can get the cheaper dual-wipe IC sockets, but the machined pin sockets are a lot kinder on the ICs if you remove them; the dual-wipe kind can often bugger up the IC pins something awful. The machined pin sockets are supposed to also have better conductivity. The downside is the cost of the machined sockets compared to the dual wipe kind - they're significantly more expensive. Buying in quantity can help bring that cost down.


You can see the three oscillators called out in this diagram from the MFOS site. They are located at the left side. Each oscillator is identical. Also, each oscillator will need to be tuned independently on the scope. I have a 5v/octave test board from MFOS that I plan to use to help me out with that.


I also installed a number of the small signal diodes. You can see two of them in this photo


I eventually installed all of them and decided that when doing a particular component value, I'd just do all of the board, but at least focus on the components that are in the three oscillators.

Here's the result of the 100k resistor install, which is an exception as there are none in the oscillators. However, as the most numerous resistor, I had to just go and plop them all in.


Here's the board with all the 100k resistors, all the IC sockets, and all the small signal diodes in place. I'm following one of Ray's latest modifications which exchanges a couple of the 100k resistors for some smaller values, so a couple of the slots at the bottom right, which would normally have 100k, will have different values.


I used 1% resistors simply because when buying in bulk, they're just about the same price as 5% ($4 for 200 1% resistors 50ppm from Mouser in most cases). If I'm going to stock up on resistors for various projects, I'm going to stock up on 1%, not 5%. For this project, I'll only have a few 5% ones at all. (Psst: I happen to like the look of the blue 1% resistors too, but don't tell anyone I said that)

That's probably all I'll do on this before I head to VSLive. I do actually need to prep my presentations :). When I return, the potentiometers and remaining banana jacks for the panel should be in, so I'll likely continue with that in the next update.

posted by Pete Brown on Friday, December 2, 2011
filed under:        

Comment on this Post

Remember me