Arduino out to VCA help please...

comrade_zero · Posted: Thu Sep 30, 2010 7:36 pm Post subject: Arduino out to VCA help please...

So, I am trying to use an arduino to produce control voltages for a small handful of modules.
Using the PWM out through a voltage divider and a simple RC low-pass filter I can control the pitch of a Rene Schmitz 4069 VCO. The problem is trying to gate a VCA from the arduino.
I've breadboarded the MFOS VCA from the Soundlab (lm13700 based linear VCA) as well as a linear CA3080 VCA (described in Chamberlain's Musical Applications of Microprocessors). I can wire up a pot as a voltage divider and manually control both of these circuits, so they appear to be functioning properly, but I cannot seem to get them to respond to any voltage coming off of the arduino. I've tried both the PWM and the digital outputs.
If anyone has had any experience with a similar issue, any help would be appreciated. I would like (if possible) to be able to build a software based envelope generator that could drive the amplitude of the VCA, including a "full off" at low voltages.
Thanks in advance for any help,
c_z

cbm · Joined: Oct 25, 2005 Posts: 381 Location: San Francisco

Are you sure there's a common ground between your Arduino and the VCA?

Is the filtered PWM signal buffered? If not, try a voltage follower and see what that does.

Can the VCO control the VCA?

comrade_zero · Posted: Thu Oct 07, 2010 12:22 am Post subject:

Thanks for your reply, cbm!
It seams the problem was mainly with calibrating. I got the soundlab circuit up and running.
I added a smoothing cap between the PWM out and the cv input of the VCA, and had to change one resistor (R17 in the schematic, if anyone out there is interested)... But it now responds well to the signals from the arduino.
So, I can put this one to rest for the moment.
Just out of curiosity, is anyone else out there trying a similar setup? Basically I am building an arduino based sequencer/interface for a bare-bones mono synth. So far I have the Schmitz 4069 VCO, along with the Soundlab mini-synth VCF and VCA. I'm going to attempt to get a decent 16 step pitch/gate sequencer along with some basic envelope control from the arduino. Any advice from anyone out there doing a similar project would be great.
Thanks again,
c_z

elmood · Joined: Sep 05, 2009 Posts: 22 Location: Toronto

You might want to learn how to use an actual DAC chip. I like the MCP4922 from Microchip. It's 2x 12 bit so you can get some really good resolution for tuning and filter frequency. I'm sure your Arduino can do SPI with some built-in libraries.

The DAC has an internal buffer, although I use an op amp to make its 0-5V into +/-5V which is a bit more useful, and I also use a trimmer resistor to adjust the exact V/octave span in the case of VCOs. (nicer than fudging the numbers in software)

Anyway, it's a bit more to wire up than an RC filter, but if you're doing tuning of oscillators and things it should work much better.

adamon · Posted: Thu Jan 06, 2011 11:36 am Post subject:

Hey, I'm just starting to use arduino for the same purposes comrade. I've got a couple different intentions in mind (all control signal sources).

Sounds like you got some methods worked out to get yours going, but I thought I'd share my method. I've couple my output with vactrols to drive my modules. The arduino outputs just drive the vactrols, and I can control whatever I want with the resistive half. This also gets rid of any grounding or cross-talk issues. The trick then is to figure out the scaling of the output (dealing with the brightness of the led rather than the voltage level of the output).

I'm using it for lfo's and envelopes, so I'm not sure how well it would work for precise sequence tuning, but its a fairly basic method with a low part count!

comrade_zero · Posted: Sat Jan 08, 2011 1:23 pm Post subject:

@adamon:
I actually started my experiments by playing with LED/Photoresistor combos. It's a nice way to go for envelopes (especially at lower speeds).
If you are interested in controlling a VCO though, that is not what I would recommend.
I am using a Rene Schmitz 4069 VCO (it offers saw and square out) and can get a decent three octaves out of it with the 5v from the Arduino. For that I'm using simple caps and resistors to smooth the arduino's pwm out.
Thanks for the ideas, though. Let me know how your experiments go. I am kind of disapointed that there doesn't seem to be a whole lot out there as far as ardiuno>analog synth stuff, maybe that will change...
cheers, and thanks for the help,
c_z