TTSH, Gate Booster mod
If you’re all new into how analog synthesizers can be controlled, you may first want to take a look at this Wiki article on CV/Gate for some jargon busting. Other than that, let’s dive right in.
Among the first electronics mod I wanted to add to my TTSH was the Gate Booster. To fully open the ADSR envelope generator of TTSH a +10VDC gate needs to be applied, thus in it’s stock config the envelope can’t be fired properly by using eg. any external gear that outputs a +5V gate. Add to the common CV/Gate specs, the TTSH envelope generator also needs a trigger signal (very short pulse) to work properly. This can be used to restart the envelope without disabling gate signal.
So what the Gate Booster does is, it converts low voltage level gate signals to higher voltage and generates the trigger signal from the gate input. The particular circuit board I bought for myself is this modified version (sold by Oshpark) with a socket for a Midimplant MIDI-to-CV/Gate converter board. Having MIDI (keyboard) input just made so much more sense than eg. building a CV keyboard like the Arp 3620.
Minor downside to buying from Oshpark is that their minimum order is three boards whereas I would only need one. But I figured I’d find some use for each eventually.. like perhaps adding one to the Odyssey would certainly make it more versatile.. Yep whatever, three boards it is then!
Stuffing the Gate Booster with parts = no-brainer, so I’ll just move right on to installation. I figured it might be actually nice to have the option to disconnect the board easily (if ever needed).. And I did need it already while I was building the synth. So on the TTSH main board there are a good number of locations for installing a extra pin header, from which GND & +/- 15VDC operating voltages can be sourced from. Headers are also great for mounting boards, and that was exactly my plan for the Gate Booster.
One minor issue was that I only settled on this approach after I had ordered all the parts for my TTSH. This meant I didn’t have standard pin headers and ended up modifying the MTA100 connectors I had bought for the build. So sth like this:
The upside of using MTAs is that unlike the standard pin headers, they lock with the mating connector and thus the board is securely fastened. Also using MTAs adds to the install height so that eg. any transistors on main board fit neatly under the Gate Booster board.
For additional support I installed a matching standoff on the edge opposite to MTA connector. The standoff attaches to mainboard with a velcro sticker (see next pic).
A crash course to external signal input connectors (jacks) on a semi-modular like the TTSH: Internally each of these jacks often has something called a tip pin and a normal pin. Tip pin will connect with the cable when one is inserted, and normal pin will connect with tip pin when no cable is inserted. So the jack works as a switch, allowing a hard-wired signal path to be disconnected by inserting a cable.
With wiring the obvious change to Fuzzbass’ wiring instructions for his original Gate Booster board is, the Midimplant needs to be factored in. On the modified version of Gate Booster board the Midimplant outputs are routed directly to a pin header. So of course if this modified board is hooked up according to instructions for original board, then the MIDI side won’t work properly.
Midimplant supplies two sets of CV/Gate outputs, so I figured I want to have pair 1 fixed to TTSH’s internal CV/Gate buses (with option to disconnect) and have both pairs patchable. Here’s how I went about it:
So how the above works is: If gate in jack doesn’t have anything plugged in, then the gate out from Midimplant is sent through the (normalled) jack, into the gate booster circuit and from there to Gate selector switch. If a cable is inserted to front panel, the gate out from Midimplant is replaced with this and boosted to acceptable levels.
Missing from the above picture is the wire for Midimplant’s configuration button. I wired that later to the same side panel where the MIDI in connector is.
Like said I also wanted to have both CV/Gate pairs patchable. There’s a set of four multiple jacks on the front panel; these are used to split a single source signal to three destinations. I figured I can have multiples as some external solution (like these cheap splitters @ Aliexpress) and mod the four jacks for CV/Gate pairs and buffer each with a opamp. First I ended up with a slightly faulty version, something like this:
Now what I totally missed with version 1 was to check how the CV jack directly above the multiples is connected. I simply assumed it’s a regular tip/normal pin kind of treat, but in fact it’s just the tip pin without normalling. In the original Arp 2600 this CV jack is used to output whatever CV is on the internal bus, and thus the same applies to stock TTSH it being a replica and all.
I rather wanted the jack to work as a input point to the CV bus; there’s already the Midimplant feeding the bus (for external MIDI gear) and each of it’s outputs are also routed to separate jacks for patching. So keeping the CV out jack in it’s stock config would just be a duplicate feature. However, using it as a external input point to bypass Midimplant’s CV output: Now that’s something I can definitely find use for!
Well worth noting is that in it’s default config, Midimplant outputs MIDI note on/off & key data on MIDI channel 1 to pair 1 and velocity of same channel to CV of pair 2 (= gate 2 disabled). If pair 2 is re-configured to receive on a MIDI channel other than 1, then also gate signal of pair 2 can be used for something.
That’s that, on to the next mod!