ProtoTypo PT2 – GRAM
One of the first modules I bought for this Eurorack 84HP utility skiff
I’m putting together I was putting together to go with my TTSH, was a DIY kit of Ljunggren Audio‘s Roll Your Own (RYO) Penta sequencer. Upon building it I was delighted to notice that the PCB had marked solder locations on it, from which individual step gate output signals and a hold input (to pause the sequencer) can be sourced. Very modding friendly, just how I like it!
And that’s how Ljunggren Audio meant it too; they host a Penta modification page and also the Penta product thread at Muffwiggler forum has user ‘Stabilt’ (whom I assume to be among people behind RYO) suggesting a basic combined gate output circuit. I decided to use these two sources as a starting point for a gate output expander of my own, aka ProtoTypo PT2.
If you’re looking for download links to pdf schematics, they’re at the end of this post. Let’s try it this way for a change, because why not ;)
Compiling a list of features from the source designs, I ended up with the following:
- Per-step switchable combined gate output
- Gate LED brightness control
- Separate buffered outputs for each step
- Adjustable CV output range
- Toggle switch or button to pause Penta (HLD_IN)
- External overvoltage protected input to pause Penta (HLD_IN)
So adding to Stabilt‘s design there’s the individual step outputs, CV range control (adapted from Penta mod page), LED brightness control and Pause state enabled with a switch or external signal. After some iterations I ended up with this schematic:
..and a quick rundown of features for the above:
- Combined gate out: Straight copy from Stabilt minus the filtering cap for U1D. Had only 22pF at hand so used that for C1 instead. SW1 – SW5 toggle steps off/on, enabled steps are fed to R1 – R5 which form a inverting mixer with U1C .
- LED brightness: Slightly reduce value of R10 from Stabilt’s design and add trimmer R11 in series.
- Individual buffered step outs: Grab signals directly from Penta step outputs (before the step off/on switches) and run each through a non-inverting opamp buffer to designated output jack (P1 – P5).
- CV range control: SW7 is a on-off-on toggle which adds combination of trimmer R14+R16 or R15+R16 parallel to R17 (on Penta PCB). This drops resistance of R17, effectively lowering the CV output range of Penta from 5VDC either to 1V or 2V. Neat for more precise (but unquantized) adjustments. Buddy up with external offset module.
- Pause options: Following pause input jack P7, parts R12, U1A and D2 form a voltage clamp which clips the incoming signals to +5VDC and thus protects Hold input on Penta PCB. SW6 is a toggle switch to manually pause Penta.
I’m not entirely sure if my way of isolating the Pause switch signal from the external input is the best way to go about it. I did test some combinations of a CV mixer style input to boot with, but with these the on-state voltage (of the switch) was always present at the ext. in jack. Wanting to get the module done, I discarded that design and added D3 for isolation. I’m also not sure if R18 was really needed to limit current going through SW6, just sort of playing it safe there.
The LED brightness is really a convenience feature that could be omitted. I happen to have a ton of trimmers pulled from discarded devices, so I try to use them wherever I can. For some reason esp. the modern clear LEDs feel super bright to me, and I thus want to run them at a relatively low brightness to be able to see panel texts.. or get blinded!
For the panel, the first version I did was 4HP wide. However this got a bit too densely populated for my chosen panel material (oak) in that the panel would break when drilling any jack & switch holes close to panel edges. So I opted to make a 5HP wide panel instead. Add to that my 3HP PT1 build and then both combined will occupy 8HP, which is neat towards even divisions of 84HP standard enclosure width.
For the circuit boards I decided to use three different veroboards, one with circular pads for panel components and two with stripes for the other components. The other one of these stripe-coppered veroboards is for the CV range toggling resistors; this small daughterboard is soldered directly over Penta PCB. The PT2 “main” board (with opamps, see picture below) needed a bit more width than 5HP would’ve allowed, so I mounted it at an 90-degree angle against the panel component board.
For marking drill locations on the front panel I used the unpopulated panel component board as my guide, pressing a pin through the veroboard to set the marks. This is a quick DIY panel method provided that you use parts which have their solder feet & panel holes aligned to the common standard 100-mil (2.54mm) raster. Eg. if using the popular Thonkiconn jacks (like I did on my build), it’s good to keep in mind that these require about a 1mm offset from this standard raster.
The CV range I calibrated by first selecting a range with the toggle switch, then with Penta stopped to any step, I put the CV knob of that step to max and turned the trimmer for the selected range to get a precise volt reading (1V or 2V in my case). You could of course calibrate these to whatever you like, or recalculate resistor values following suggestions at the official Penta modifications page.
Continuing with the abbreviations I decided to name PT2 as ‘GRAM’, short for Gate Routing Add-on Module. It also adds neatly to ‘Penta’, just like the module :P
The most obvious short-coming with this expander module is that the step outputs each have a 100% pulse width. That is, if two adjacent steps are enabled (using switches SW1-SW5), then the combined gate output doesn’t go low state between steps. So using this output to drive any module that needs a rising edge of a pulse to trigger, it simply won’t do anything on such steps.
This can be easily fixed with a (boolean logic) ‘AND’ circuit, either adding one to PT2 or by using external logic module (eg. RYO Boolean logic modules). Fed with the combined gate output and Penta clock input, the falling edge of the clock would then switch the AND circuit output off, effectively setting the per-step gate output pulse width at 50%. Just to practice using Kicad a bit I drew a version 2 of schematic, adding a AND sub-circuit from RYO AND/NAND schematic and converting the combined gate mixer and output buffer opamps from inverting to non-inverting:
If you’re thinking about building this v2 circuit, it’s worth noting that I haven’t tested this myself. It was quite a bit of work to get v1 done, so I didn’t simply want to bother with this new version.. And there wasn’t room for the new parts on the veroboard either. I figured I’ll be anyway getting some boolean logic modules, and can then use those for the task.
One other addition I’ve come to miss lately would be to have a 2nd CV output with offset control, specifically for oscillator modules lacking root pitch adjustment. When running Penta at 1V or 2V CV out range mode you’re always targetting low notes. Having CV offset would allow keeping the range, but set the root pitch higher. But once again this is a feature which can be handled with a external module, maybe sth like this one by Stereoping. And to be honest, the current panel design simply doesn’t have free space for this addition :)