My synthesizer concept

I want this to be a portable instrument. Therefore the size of the panels will have to be kept to a minimum. I'm not re-inventing the Moog Modular. This is going to be something different – a portable modular synthesizer. I don't know if that has been made before. Most portable analog synthesizers of old (EMS Synthi AKS etc) were not fully modular.
It goes without saying that portability means some compromises have to be made regarding knob spacing etc. Still, the ergonomics must be acceptable. The challenge is to use every potentioneter, switch and jack to its fullest and carefully consider which features to include and which to leave out. Careful, consistent layout and colour coding are used to make the panels easier to understand and use.
The panels are 3U (132 mm) high. The width is variable in steps of 10 mm. I know there is an industry standard width unit based on 1/5 inch (5.08 mm). I don't intend to mix with modules from other sources and a width in uneven millimeters only makes the design and construction of the panels more difficult.
The minimum acceptable distance between knobs is 11 mm, according to my experiments. The smallest available knobs, that fit 6 mm pots, are 13 mm in diameter. Therefore the minimum pot spacing is 24 mm. I use 25 mm horizontal and 24 mm vertical spacing.
Eventhough this is an electronic instrument, the mechanical side of things must not be neglected. Good mechanical design and construction is very important for the usability. A well thought out mechanical design also simplifies construction of the modules.
To make the panels easier to understand, the knobs, switches and jacks are colour-coded. A certain parameter always have the same colour. For instance knobs and inputs that affect frequency are always Neon red. AM inputs and amount knobs are Light yellow.

Patching

On a modular synth, the output of any function should be possible to use as input for any other function. The traditional means to achieve this is front panel jacks and patch cords.
The drawback with this system is that the patchcords tend to obscure the panel mounted controls (knobs and switches). I have used 3,5 mm front panel jacks for every useful signal output and input. In addition to this, there is an internal patching system, using thumbwheel switches. which I call Bergfothumb. This system covers the most common connections, leaving the more exotic ones to be made using patchcords. In this way the majority of the patchcords will be eliminated, minimizing the clutter on the panels. Changing thumbwheel settings also is quicker and more convenient than unplugging and plugging in patchcords. Another plus is that the patches can easily be documented by just writing down the numbers on the thumbwheels. The only major drawback to this system is cost and some additional internal wiring.

This is the Bergfothumb bus. To the left is the 18-pin connector which carry the signal that goes to the dual inputs on each module. With one thumbwheel for each input, you can select any signal from the first nine lines and then mix that with any signal from the last nine lines.
The shorter connectors to the right is the envelope bus. This is used on all modules that have an input for envelope signals. You can select between nine such signals.
The signals that are available on the busses are connected to the strips in the front. By soldering a wire here, that signal becomes available on all modules that are connected to the bus. Currently the following signals are used on the busses:

Input bus

        Input A    Input B

0:    reserved    reseved
1:    VCO1        VCO2
2:    VCO3        VCO4
3:    VCO mix    Noise
4:    Wavefold   Wavemultiplier
5:    VCF Korg   VCF Moog
6:    VCA1        VCA2
7:    VCA3        VCA4
8:    vacant     vacant
9:    vacant     vacant

Envelope bus

0:    reserved
1:    EG1
2:    EG2
3:    EG3
4:    EG4
5:    LFO1A
6:    LFO2A
7:    LFO3A
8:    LFO4A
9:    vacant

Jacks

There is an ongoing debate wether 3,5 mm jacks are any good or if only 1/4 jacks are reliable enough. I have used 3,5 mm stereo jacks and connected the two poles in parallel. This way, the probability for bad contact should decrease by a large factor. I also use metal jack plugs with steel spring strain relief. The cords are unshielded, 4 mm diameter laboratory wire. So far this system has worked perfectly. Note though, that 3,5 mm stereo jacks are not fully compatible with 3,5 mm mono jacks and plugs. You should standardize on one make of jacks, if possible.

VCAs

Where a VCA is frequently used, there should be a dedicated VCA on the module. This means slightly more VCAs will have to be constructed, but VCAs are cheap and fairly easy to build. Compared to (fewer) separate VCAs, the cost, work and panel space required should be about the same. This approach improves usability. There are less modules to patch in and the signal path is easier to follow. For unusual patches a couple of separate VCAs are of course available too.

Modulation knobs

All modulation knobs have both positive and negative settings. No modulation is at the 12 o'clock position. Turning the knob anticlockwise adds negative modulation and turning it clockwise adds positive modulation. This way the modulation sources do not need to have both negative and positive outputs. And no separate inverters are needed, which saves patching time and panel space.

Signal inputs

All modules with signal inputs have two mixable inputs. For each input there is a rotary pot that controls amount and a thumbwheel switch that selects among the ten most used input sources. A plug inserted in the panel input jack overrides the thumbwheel selection.

Signals

All modulation signals are calibrated to 10 volt peak to peak amplitude (-5 to + 5 volts) or 0 to10 volts if they are unipolar.