Any pointers on generating cv/gate?

[ljudsystem]

Hi guys

I've been experimenting with simple synth circuits lite the atari punk console etc. I found a cheap "animal sound" keyboard at a flea market and was thinking of turning it into a cv/gate keyboard. Is there a somewhat simple way to do this? (like a 555 toy organ circuit?)

I want to drive two simple 9v square/triangle wave oscillators to experiment with FM and AM synthesis.

[Mark Hammer]

Generating a gate is a LOT easier than generating a CV that corresponds to the pitch reasonably well.  Just how well do you need the CV to track pitch?  Or is this more or less in the realm of noise generation?

[ljudsystem]

oh, definitely more in the realm of noise generation.

[Mark Hammer]

Well, try here: https://www.diystompboxes.com/smfforum/index.php?topic=51441.0

or here: https://www.diystompboxes.com/smfforum/index.php?topic=67831.0

Here's a very recent Youtube of someone demonstrating what he did with the Penfold "Guitar Tracker".  Not especially impressive, but you said you were leaning towards noise generation.

[ljudsystem]

I think you misunderstood me. I'm not looking to convert my guitar signal to CV. I want to generate gate and pitch with a toy keyboard. Sorry not really a stompbox question I guess.

[Mark Hammer]

No problem.  In fact,since you're talking about a keyboard, that means MANY fewer problems!

There are innumerable "keyboard control" circuits around.  Most will be analog and use a resistor ladder (usually 1% 100R resistors), and the overwhelming majority will be mono/single-note.  There ARE a few duophonic analog synths from the golden era, whose schematics are available, but my guess is that you may well end up wanting to go digital, via Arduino or similar.  And while I have not delved into it extensively, I gather there are plenty of posted routines and accompanying interface diagrams out there.

Best place to start looking is muffwiggler ( https://www.muffwiggler.com/forum/ ) and electro-music.com

[ElectricDruid]

+1 what Mark said.

You've got two choices, classic analog, or keyboard scanning.

The classic analog way is a long chain of resistors with each key connected to one junction in the chain. Pressing a key connects one junction to the output and provides a voltage. Stick a op-amp buffer on the end to avoid loading the resistor string and you're pretty much done.
The later-analog-era solution was to connect the keys up into a diode matrix and then scan them with a microprocessor. The micro would then produce the control voltage using a DAC of some sort. This DAC was often very simple - binary weighted resistors were used in the TB303, for example. The modern equivalent would be an Arduino Nano and a MCP4821 DAC.

[PRR]

> a cheap "animal sound" keyboard at a flea market



??

[ljudsystem]

This one actually



I found the schematic for the Gakken SX-150 so I'll think ill try and clone it and change the ribbon for a diode ladder.

[ljudsystem]

So I tried a resistor ladder with a buffer. This is what I've got on the breadboard now, but the output voltage at the CV point doesn't change when I press the keys, it stays around 1.5V.

[11-90-an]

Don't you think 100 ohms to be too low of a value...?

Why don't you try diodes?

[ljudsystem]

Well, Mark wrote that 100r was the usual value..

Would common 1N4148 diodes work?

[Mark Hammer]

http://musicfromouterspace.com/analogsynth_new/THE_CAVE/KBD%20Controller%20Old%20Design%20I/Analog%20Synth%20Keyboard%20Schematic.htm

[ljudsystem]

Thanks Mark... that looks pretty complicated

My resistor ladder works, I get different voltages at the opamp input when I press the keys but on the opamp output the voltage stays the same. The point of a buffer is that the output voltage is the same as the input, isn't it?

I've only built AUDIO buffers before, but this is a DC buffer, right? Is there a difference?

[ElectricDruid]

There's a circuit very much like that in R.A.Penfold's "Electronic Synthesiser Construction".

The differences are that it doesn't have the two 1M resistors (which *will* hopelessly screw up the tuning) and it does have a 2Meg pot and a 470nF to ground to provide portamento. The other important difference is that it runs on a +/-12V supply, which means it doesn't need to worry about the op-amp output range. That's an issue for your circuit since the op-amp will start to clip before the voltage gets down to the bottom or up to the top. It could be avoided if you're happy to settle for an output range of 2V -> 7V or so. Just put some bigger resistors at the top and bottom of the chain to offset things a bit.

[ljudsystem]

Thanks Druid, I scrapped the opamp buffer and replaced it with a transistor, now I get the same V on the emitter and the base. AND the voltage changes when i play the keyboard.