It's no secret that synthesizers and electronics are both big interests of mine. One of my ultimate goals is to build my own synthesizer (or synthesizers) which sport some combination of analog and digital circuitry, and have patch management and operating system interface written in the .NET Micro Framework. I'm not sure when or if I'll get there, but the journey sure is a lot of fun :)

I got interested in synthesizers back in Jr High school, around the same time I got interested in the Commodore 64. Interestingly enough, of of the synth projects I need to finish is a small module named the Sammich SID (the little black box making the chord, lead, and bass sounds in that video) which uses two of the Commodore 64 SID chips to make really great sounds. I set that kit aside when I borked the soldering of one of the transistors. I'll clean it up soon.

Notes, Voices and Oscillators

Synthesizers are generally monophonic or polyphonic. The former, like the classic Moog synths, can play only one note at the time and so are really only good for bass, arpeggios, leads and other single-note uses. The latter can play something higher than one, typically 6-12 if analog or much higher if a modern digital synthesizer. Those are real workhorses.

One thing every synthesizer needs is at least one Oscillator to create the tone for each note being played. In short, the combination of one or more oscillators, one or more envelope generators, an amplifier and (typically) a filter, is called a "voice" (how this is wired and whether any components are shared is up to the manufacturer).

The better sounding analog synthesizers had/have two to three oscillators per voice, with the sweet spot being three plus a low-frequency oscillator. Some incredible builds have four oscillators and can recreate almost any sound (description here, but watch the video to hear the beast and just see that beautiful build).

Each oscillator produces a waveform at a frequency dictated by the note being played and any modulations or adjustments affecting the oscillator. The basics are Sine, Square (and sometimes a variable Pulse), Triangle, and Sawtooth or Ramp. Each has a very distinctive sound and shape. The variations in those shapes, and specifically what harmonics are included in what otherwise looks like a very smooth shape, is one thing that gives each brand of synthesizer a distinctive sound. Combining the output of the oscillators (in a classic subtractive or analog synthesizer - the approach is different in other types of synths) is the first step to giving the sound the shape you're looking for.

The Function Generator IC

So, if I wanted to create an ambitious eight voice three oscillator analog synthesizer, I would need (not counting LFOs or Low Frequency Oscillators), 8x3 oscillators or 24 total. That's a lot of oscillators when you consider that a purely discrete electronics oscillator circuit is not a small thing. One thing I've looked at is using some of the classic, and now difficult to find, function generator chips. One of those is the Exar XR2206 monolithic function generator IC. You can still find these around, but often through iffy sellers on ebay.

Use of a chip like that can significantly reduce the size of the circuit required for an individual oscillator. It may not give the sound I want, but I wanted to play with it to see what I could get it to do. One easy way to do that is to build out the electronics-DIY function generator kit.

The Function Generator Kit

Electronics-DIY.com sells an XR-2206 function generator kit which makes it easy to test out the Exar chip. This is a basic kit which doesn't seem to surface everything the chip can do, but does a reasonable job of letting you get a feel for the waveform generation. For $35, it does seem a bit expensive, but considering it includes an Exar chip (don't solder yours!) and all the required parts, that's not a horrible deal. Considering that the official test board from exar, which is almost impossible to find now, runs almost $100 each, this compares favorably. Also, SparkFun used to have a custom board based on the same chip, but that is also no longer available.

Some notes about the electronics-diy kit:

• The kit says 9-18v DC input. You really need > 12v (I ran at 18v) to get the best waveforms out of the chip. At lower voltages, the waveforms get heavily clipped.
• • I use a Mastech HY3003F-3 power supply for stuff like this. If you're going to play around much with circuits, I highly recommend one, or something like it (there are less expensive and smaller units that will certainly suffice.)
• The capacitors in the photo do not match those in the kit. You'll want to follow the kit instructions for correct placement. Since the WIMA capacitors are only used for one spot now, I wonder if that change isn't the reason I got less than spectacular results from the output. TBD.
• There's no IC socket included in the kit. When you order the kit, order a 16 pin DIP socket too. The Exar chips aren't cheap when you can find them, and leaving one soldered to this board just makes me feel bad. I didn't have any 16 pin sockets around (I had 14 and 20 grr!) and was impatient to try it out, so I sacrificed a chip. Oh the humanity!
• You'll need a couple jumpers for the "switch" connections. They provide two switches which you could solder to the board through wires (the lugs are too big to fit in the through holes), but that's a pain if you're only planning to use the board to play with for a short time.
• Output levels for the Sine, Triangle, and Square waveforms vary significantly. The square is really loud. This is a result of how the chip handles those, as far as I know. Oscillator circuits based on the 2206 would need to have compensation for this.
• The triangle and sine waves left something to be desired. The shapes are interesting, but there's a lot of clipping resulting in what is almost-but-not-quite a square wave in some cases. Again, I think a good oscillator circuit could prevent this by keeping values within certain bounds.
• The link in the printed instructions is incorrect. This also brings up one of my beefs with printed instructions:

If you're going to print out directions and send them to someone, convert any links in the text to short links using bit.ly or your own shortener. Remember, people have to type in the full address from the paper and often times the underscores are not visible because of the full underline. In this case, the link was incorrect anyway, making it even worse.

In the end, the kit was fun to put together and fun to play with. It's easy enough for a beginner to solder if they wish. The board is clearly marked and there aren't any tricky things to solder (except the jumper pins - I always have a hard time getting them upright and straight on a board while I solder, even using the tacking method.) The pads are reasonably tinned (not as nice as the coating on the custom board I had done, but on par with just about every other board I've built)

It took very little time to solder up the board. I soldered the resistors first, then the capacitors, then the dip switch and the pin headers. I finished up with the audio jack and the three pots. I tried to just sit the 2206 chip in the pin holes without soldering, but as you would expect, the connection wasn't good enough for consistent results. So, the last thing I did was solder that guy in there. I do have 16 pin sockets on order from Mouser for the next board I build. :)

To test, I hooked up the oscilloscope probe to the "counter" signal pad (I later soldered a header on to this so the clip would stop popping off), and fed the audio output into my MOTU 828 into my computer. That way I could both hear and see the results at the same time, and I could also record the audio for the video. Speaking of which, here's a short video showing the board. During the video, I changed the jumpers and adjusted both the coarse and fine frequency pots, as well as the amplitude pot.

In addition to the video, here are a couple snags from the oscilloscope.

Another way you could test out the chip is to build out a full Exar 2206-based oscillator circuit like the TH-102. This is a fair bit more work and will probably run a little more than the Electronics DIY board once you add up all the components, but the end result may be more to your liking. I'm ordering one of these boards for my next project. And yes, I'll be socketing those Exar chips :) Here's hoping the chips I ordered on ebay aren't counterfeit - always a risk when ordering ICs from overseas.