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.

Gate Booster board all populated.

Gate Booster board all populated..

Populated Gate Booster board with Midimplant installed.

.. and with Midimplant installed.

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!

Onboarding time

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:

MTA100 with its pins pulled out slightly

First pull out the MTA100 pins slightly..

Solder component feet to the pins..

..then solder leftover component feet to the pins..

..and solder the connector to Gate Booster board.

..push the pins back in and use them to solder the connector to Gate Booster board.

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.

Gate Booster on main board

Gate Booster on main 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).

Wiring

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:

Wire number 1

Wire number 1: Gate1 out from Midimplant to normalled pin of external gate in jack on main board (via the gate header).

Wire number 2

Wire number 2: Boosted gate out signal to Gate selector switch on main board.

Wire number 3

Wire number 3: Boosted trigger out signal to normalled pin of external trigger in jack on main board.

Wire number 4. Also cut a foil on main board (circled).

Wire number 4: Tip pin of external gate in to gate in on the Booster. Also the connection between gate jack & selector switch needs to be cut (circled).

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.

Wire number n+1.

Wire number n+1. Red/white twisted pair is MIDI in, flat cable is CV2/Gate2 out.

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:

The solder side of CV/Gate buffer board version 1

The solder side of (faulty) CV/Gate buffer board version 1. Multiple jack board traces pending to be cut.

CV/Gate out buffer board version 1 with faulty CV1 out.

Not much to see on the flipside of the board, other than the part type; TL074 quad opamp.

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!

External CV jack tip pin bent to allow cabling

Prep work for version 2 of circuit: External CV jack tip pin bent horizontally to allow tip/normal operation..

..and with the correct CV cabling.

..and PCB solder pad insulated + updated cabling to switch which source feeds the internal CV bus.

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!

Previous Posts In Series

  1. A TTSHshshstart of sorts
  2. Road case design
  3. On the case
  4. On the case… AGAIN
  5. Loudspeaker baffle mod
Advertisements

Tags: , , , , , , , , , , , ,

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: