Gritty noise on panner and oscillator mix volume pots

While testing, I heard a lot of digital sounding noise on the output when turning the volume or pan pots. I first suspected it to be digital noise from the midi connection.

After extensive testing, I've concluded that this is not noise per se, instead it has two causes:
- The midi input is stepped, each pot has max 128 positions
- The CV is linear, so changes are more audible when the volume is low

I tried adding filtering of the output CVs, e.g. making the change take more time even if the midi said to change things instantly. This worked fairly well but when turning the volume pot slowly I could still hear stepping at the lower end.

This is because the steps are of equal size, but audio loudness is exponential. A one unit change when the volume is low is much larger and audible than a one unit change when the volume is high.

I have already added support for response curves, changing to an "exp_1" curve took care of most of the stepping.

As for the filtering, a "max 64 units change per update" gave a good result. 256 is too much. 

Right now the filtering happens at the DAC update, it should be moved to the midi side as the rest of the matrix works as it should.

A simple headphones amp from yusynth.net

To test the output of my synth, I've built and tested the headphone amp found on yusynth.net:

https://yusynth.net/Modular/EN/MIXOUT/index.html

I had no BC327 transistors, so I replaced both the BC337 and BC327 with 2N3904 and 2N3906. Also, as I was unsure of the ratings of my resistors, I used two 330Ohm in place of the 150Ohm 1/2W, and two 220Ohm in place of the 82Ohm 1/2W. The output seems to have unity gain, way too high for my headphones, so I placed a resistor voltage divider plus an op amp buffer in front of the volume pot. I used 100k and 10k resistors for an attenuation from +/-5V to +/-0.45V. The max volume is still very high but at least more controllable.

Oh, and I forgot the electrolytic caps before the volume pots, I will add those and try again.

Juno 60 pre-filter level control

The Juno 60 mixes several waveforms before the filter, and it has a clever way of controlling the sub oscillator volume. As the sub oscillator output is a square wave, the wave is considered a binary on/off signal. It is connected to the base of a transistor, which when turned on and off either sinks its collector to ground or leaves it as it is.

To control the wave volume, the collector input is connected to the panel potentiometer (through a series of buffers/mux/demuxes, but that doesn't really matter, the principle is the same). The wave's amplitude (or "on value") will then equal the level of the panel potentiometer, though negative as the potentiometer value is inverted by IC17 on panel board A. The voltage is passed through an analog switch, IC16, which in turn is controlled by the Sub oscillator on/off switch, to completely disable the sub oscillator (incidentally, this feature has been removed from the Juno 106, here the potentiometer voltage is connected directly to the transistor collector).

Interestingly, the same arrangement exists for the pulse wave, only this time the analog switch connects the transistor to -15V instead of to a variable voltage. If one would like a controllable pulse wave volume, it would likely be easy to inject the control signal here.

As for the saw wave, it also has a switch and a transistor connected to it. However, as the saw wave is not binary, it cannot be controlled the same way. Instead, the wave is connected to the collector, and the transistor base is connected to the analog switch on panel board A. The wave is then sunk to ground if the transistor is on. In other words, the saw wave has no pre-filter volume control, only an on/off switch.

The noise input has its own volume control on panel board A (all voices use the same noise generator).

Oh, and by the way - all waveforms have a range of 0 to maximum -15V (the datasheet says the saw wave is 12V p.p, I have not checked the others but they have to be about the same to be mixable - the sub oscillator level for example originates as a 0 to +5V voltage but is inverted and amplified by IC17, and the pulse wave level is always 0 to -15V). They are fed through a 10uF non-polar capacitor (C5) just before the filter, which probably centers them around 0V.