Synth Project - Concept

Modular Synth

So, I watched I Dream of Wires and I thought I'd build a nice analogue modular synth. I'd like to practice my electronics, and I thought a modular synth might be a nice project that would take us step by step through analogue electronics. The nice thing about it being modular is that each module can be separated from the rest of the system so that the focus can be entirely on the functionality of each module and not a complicated juggling act of balancing circuits operations. I thought I'd blog this (due to several request).

To start this we need a standard to stick by, so that all the modules can operate happily in a common environment.

System Design

Looking at analogue synths, the top end of the market is eurorack, for which I think the Doepfer stuff looks pretty cool, so I thought Id model my system on their. (Why invent a new standard when one already exists, plus there is a lot to be said for compatibility these days!)

Here is an example of the kind of think i'm going for:

So first things first, I need a supplier for the case and the module face plates. I found that the Doepfer stuff all runs on a 3U 19" rack. Schroff are a company that supply this kinda stuff, they also have an ebay store.

Power

Very helpfully, Doepfer provide a lot of information about the basic spec of their system which allowed me to get the information required to build the system environment (or, power supply and chassis)

This diagram specifies +5, +12 and -12 power bar. I've ordered a PSU that'll take 230v AC and kick out +24v DC and give me 3 amps. I don't think I'll need more than 3 amps for this project, but If I find things are going screwy later because I don't have enough juice, it's a simple change to make.

To get the power I want I'm going to have to use a DC-DC converter and some Voltage Regulators. I found a converter on RS that'll take +24 and kick out +15 and -15. It's called a TEL32423 and is quite expensive, but I can't see another way of getting negative voltage onto my system. (I'm guessing the negative will be important for when I try and generate sine waves).

For each connector I will need a +12 VReg and a -12 Vreg. (most vregs need an overhead of a few volts to work, which is why I need a 24V PSU stepped down to 12V), and because I want each module to be correctly isolated, I think I need the vregs for each unit in the bus. Instead of building this power isolation on each board I want to keep it on the power bus, because it makes sense to keep functionality organised and together. So I've ordered a bunch of them. The connectors on this standard bus are 10 way "IDC" connectors, so I have ordered a bunch of plugs, sockets and cables also.

The CV and Gate on the bus are a lovely idea to potentially remove a little bit of the patching from the final front panel. Electrically there is nothing going on here, this is just a bus that allows modules to communicate with other modules without using patch leads. CV is designed for the control voltage (to control pitch) and Gate is designed for the 'On/Off' signal, although these can be used for anything. To isolate these there should be jumpers on the board so this can be configured depending on the system. I am not even sure how useful this will be, but as we are adhering to the standard it doesn't do any harm to do things properly, it might even come in handy later, who knows.

So with all the components on the way, next we will turn to the breadboard to check that we are getting the voltages that we expect before building the power bus board.