Adding CV to the Electro Harmonix EH400 Mini Synthesizer

    This is a follow up to our previous look at the EH400 here. Now we're investigating methods of voltage controlling the oscillator.

    As a reminder, this is what we're up against. It's similar to a relaxation oscillator, but it's been modified to charge through a resistance labeled "keyboard".

The original oscillator circuit

Get your VCRs ready

     One approach to adding CV would be to replace the resistor with some kind of voltage-controlled resistor (VCR). These circuits are a bit uncommon though. Another difficulty would be figuring out the relationship between resistance and frequency, then replicating it in the VCR. The saw recharges quite slowly because of R2, and this impacts the relationship.

Update

    I decided to graph the resistance that corresponds to each of the 24 note (1 being the lowest, 24 being the highest). With the help of a curve fitting tool and Wolfram Alpha, I ended up with this formula: resistance(k) = 188.198 - 44.8472 * e^(0.0574576 * note)

Current affairs

    We can do away with the resistance worries by instead forcing a current into the timing capacitor C2. We could then use a much more conventional voltage controlled exponential current source, instead of a VCR. This is a circuit that's very common in synthesizers, and will give us a standard 1 volt per octave response, while also being relatively temperature stable.

    There are some voltage requirements that make the oscillator a little hard to current control though. The cap needs to swing from 2V to 4V. The typical expo source can't do this directly, but we can add a current mirror to help it along.

    Here we've added a standard expo source (actually a current sink). We've also lowered R2 to 1k to help mitigate the slow recharge time.

Expo sink and current mirror design

    This mostly works, but the tuning goes flat on the low end. I'm unsure what to blame this on, maybe the current sink. Regardless, it's back to the drawing board.

Derivation

    Part of the trouble with this oscillator is that it's too different from typical VCOs. How can we bring it more in-line without replacing it entirely? Well, we have to look at it differently. What we've been calling a "modified relaxation oscillator" can also be thought of as a variation on a schmitt trigger.

Schmitt Trigger

    The majority of VCOs have a schmitt trigger in them somewhere, and many of them are related to this schmitt + integrator design:

    By adding an integrator we can turn this into a more normal oscillator, one that lends itself to being voltage controlled. We only have to make minimal changes to the original board to accomplish this. The original timing cap needs to be removed, and the integrator needs to be connected in its place. If we want to replicate the original's sawtooth waveform, we can tap the "pulse" output and use it to reset our integrator. As a bonus, this resets much faster than the original oscillator did, and pretty much cures the slow reset problem. Now we have a fairly conventional oscillator.

Sinking lower

    With our new oscillator design, the exponential current sink becomes more useful. It doesn't have to make the capacitor swing to any voltage, not directly anyway. The integrator does all the work, while presenting a relatively constant voltage to the current sink.

    Since the integrator is the one charging the cap, it does have to worry about the voltage swing. It must reach both 2V and 4V. If we run it using half of VCC as our AGND, then we'll run into a problem on reset. The reset pulse will only bring it down to 6V/2 = 3V, and not the 2V that we need. This is because the reset pulse "closes" T3 and turns the op-amp into a unity gain buffer. This effectively forces the AGND voltage (from the non-inverting input) to show up at the output.

Unity gain buffer

    We can fix this by lowering our AGND voltage to below 2V. We just need to play with the values in a voltage divider until we come up with something that's a safe margin below 2V. Our 6V supply into 33k + 15k yields 1.875V.

    Here it is all put together. Conveniently, it only requires one quad op-amp to add the expo sink, integrator, and buffers. Falstad link here

The complete CV circuit