Quantity: 1 Dual VCO
This is a circuit of my own, based on the XR-2206 VCO data sheet examples. Features:
- Coarse and fine frequency controls
- Low/High range switch
- Sine/Triangle switched output
- Square wave output
- Linear, AC-coupled FM input
- Internally normalized for cross modulation
My intended use of these VCOs is as modulation sources for linear FM of other VCOs, such as the Cynthia ZerOscillator and the Fritz Teezer. I was looking for a sine wave VCO for quite a while, but a 1U sine wave VCO in MOTM format is not commonly found. I happened across the XR-2206 in a discussion on Muff Wiggler’s forum and found this hobby store that sells a PC board.
Electronics DIY XR-2206 PC Board
Realizing that I did not need 1V/octave tracking or other high-quality music VCO features, I bought two boards and two XR-2206 chips. (Get the XR-2206 from Mouser for under $5.00 each).
After I got the chips I started breadboarding. Although I used the PC boards, I didn’t follow the original circuit but built my own. I added an FM input, set a fixed amplitude for the sine/triangle output, and selected capacitors for the low and high ranges. The XR-2206 is powered between ground and +15V, so the output is all above ground. The triangle is much larger than the sine wave. So I designed a buffer circuit, using a TL072 on a Tellun MUUB-2 PC board to provide +/-5V sine and triangle waves, and a +/-5V square wave obtained with the comparator circuit that I often use.
Here is the Dual XR-2206 Front Panel Express Panel.
Here is the XR-2206 Circuit Diagram, and the Buffer Circuit Diagram.
There is a trimpot on the XR-2206 board for the sine shape, and trimpots on the MUUB board for the sine and triangle levels. The sine/triangle switch is DPDT. One side switches the wave selection and the other switches between trimpots on the MUUB, so that the output levels of the sine and triangle can be made the same. The FM input is AC-coupled by necessity, because the initial frequency is set by the combined resistance of the two frequency pots from pin 7 of the XR-2207 to ground. DC-coupling would be tricky to do, but not necessary in this application.
The sine/triangle output of each VCO is connected behind the panel to the switching lug of the other VCO’s FM input to facilitate cross-modulation without need for patch cords or multiples.
Here’s a demo recording of the module. It starts out showing the two frequency ranges of one oscillator, using the sine wave. (The low range is 8 Hz to 400 Hz, the high range 87 Hz to 4300 Hz.) Then the second VCO is brought in. The second then modulates the first, then the first the second, and then each other simultaneously. Then both are switched to triangle and knobs are twiddled a bit.
did you use a xr2207 or just 2 xr2206
so your circuit is using 2 xr2207 instead of xr2206 like he pcb project was using by the hobby company project
I used 2 XR2206 and built on the PC board from the hobby company. You can see that in the photos. Did you click on the schematic?
http://pugix.com/synth/wp-content/uploads/2011/02/xr-2206-board.pdf
yes I did see the schematic, I was just curious to whether you could use xr2207 instead , thanks great looking boards
Hello Richard,
I’m looking for a DC-powered VCO based on the XR-2206. I also experimented with the examples in the datasheet; with no results. It’s important to emphasize that I’m using 9V DC to power the oscillator (I also tried two 9V batteries in series for ~18V DC, still no results). Based on the datasheet and the Electronics-DIY schematic, that should be good enough.
I was glad when I found your site and decided to build the VCO by your schematic—a single one, not a dual oscillator—but sadly, it still does not work. I’m thinking it could be the TL072, as I’m using the DC power source for that too, so it’s between +9V and ground. Can that be the reason why I don’t get any audio?
I checked the connections and everything seems to be in place. I’m fairly new to electronincs, please excuse me if the error is something obvious.
Hi Aaron,
You guessed it! The TL072 buffer that I designed works off +/-15 volts. The reason you get no sound is that the reference voltage at pins 5 and 2 is ground. That needs to be the center voltage. You can do one of two things:
1) Power the TL072 with two 9 volt batteries, one for the positive power at pin 8 with the negative battery terminal grounded and one for negative at pin 4 with the positive battery terminal grounded.
2) Take two 10K resistors and make a voltage divider with them between +9V and ground. Put them in series, one to +9V, the other to ground, and take 4.5 volts from their junction. Connect this to pins 2 and 5. You’ll also need a 1uf non-polar capacitor in series with RB14, the 1K on the output. And you’ll need a different voltage divider at RA14/RA15. If you use 2K2/3K3 ohms for both of those, then the square wave output should swing between 0 and +5 volts.
Aaah, I see, now it makes perfect sense! Thanks a lot for the reply and the help! I’ll try this tomorrow.
Would a voltage converter also solve the issue of the reference voltage? I have a MAX1044. I’ve just found this: https://www.sabrotone.com/wp-content/uploads/2011/01/Charge-pump.gif
Or would this completely mess up the other part of the circuit?
Can’t hurt to try the MAX1044. Option (1) above is much simpler.
Thanks!
Eventually, I’d like to power the whole circuit with a wall adapter, this is why I try to avoid using batteries.
I’ve tried to solve the issue, but it still does not work properly. However, when I was checking the board I realized that I made a serious mistake: pin 6 was connected to ground instead of 5.
I fixed that but the VCO still does not work as intended. For some reason pin 4 reads about +6 volts when connected to -9. I probably made another mistake which I haven’t yet found. The board is a bit messy now, maybe I’ll try this again from scratch in a few days time.
Hello again Richard,
Eventually I had no success with this, so I suspended the project. But not forever! I’ve started to build a dual rail modular recently, and this was on the top of my to-do list.
Yesterday I managed to finish the circuit, and it works properly! I also got some new XR2206 ICs because I had a feeling maybe those were the issue in the first place. I might try the old ones with the now working module but I’m a bit afraid I’ll kill the new ones too.
For this build I made a dual VCO with the cross-modulation behind the panel as you proposed. It’s fantastic, thank you again for everything!
Hi!
Just want to clearify what you are writing in the comments… I am building a Eurorack module with bipolar psu. Are you saying that I should connect pins 2 and 5 in the buffer schematics to VEE (-12V) instead of GND?
I have made the XR2206-circuit and it works perfect 🙂
Tnx!
Hello Howard,
I think if you’re using a dual rail PSU for powering the op-amps, you should connect pins 2 and 5 to GND as shown in the schematic.
Pin 4 goes to -12V and pin 8 goes to +12V for powering the TL072.
With my dual rail build I followed the schematics and it works as intended!
Hi Howard. No, my comments were for running on a single voltage supply. You can build the buffer circuit just like mine, but power the TL072 with with +/-12V at pins 8 and 4, like Aaron said.
Aaron & Richard: Thanks… of course it is like that… 😀 My head was not playing along when I was reading this earlier today! Tnx 🙂