This following page details a possible means of construction for the Ultrafade VCO PCB from Stroh Modular utilising the single function 4U panel from Loudest Warning.
The PCB appears to be currently out of stock at suppliers, but you can still get a panel / PCB combo at Synthcube here
The PCB is parallel mounted to the panel using threaded stand-offs.
As with all of my panels, this is module/panel is designed to be mounted into a case using euro-rack rails, more information on mounting can be found here.
The picture at left (and photo below) shows the panel with my own colour scheme for the banana jacks (yellow = audio in, green – audio out, red = pulse out, blue = CV in) If building to the ‘Serge’ standard, the colours will be different
This panel uses a 5mm LED, to mount this I use the Tru-Opto lens available from Rapid (in the UK), either standard or low-profile are available, the photo below shows a ‘low-profile’ clear lens. The lenses are on this page of the Rapid website, order number 55-0970 for low-profile, 55-1030 for standard (sticks out of the panel). Mouser appear to sell the same lens, going by the mfr part number (CLB300CTP for low-profile). The Mouser number is 593-3000C for low-profile. (NOTE – I’ve not ordered the Mouser ones myself, just cross-referenced the MFR part numbers). The hole on the panel is sized for the lens.
The knobs are available from Thonk, if ordering from there you require – to match the photo:
1 x Fine X-Large
2 x Fine small
5 x Bold small
If you prefer to order the knobs from Mouser you will need:
1 x Davies 1600BM, Mouser 5164-1600BM
2 x Davies 1610AA, Mouser 5164-1610AA
5 x Davies 1910CS, Mouser 5164-1910CS
Please not that the Lin FM input is inverted by design, a positive CV will drive the frequencies down etc.
There is an error in the parts list/BOM and schematic, R49 is listed as 10K, this should be 100K. SYNC will not work with R49 = 10K.
Linear FM input: In the Muffwiggler build thread, J3RK says this “…the 1M resistor on the linear FM input is a bit limiting. Something between 330K and 680K is better. (not necessary, but better) …” He is here talking about R107, adjust to your taste!
In the wiring diagram below I’ve mounted the AC coupling caps on the backs of the toggle switches (reduces panel wiring a little). If you also do this on your build note the pads to wire to, the drawing is correct, you have to wire to middle pad of RV4 (Exp FM) and RV102 (Lin FM). For coupling capacitor values J3RK suggests (again from MW build thread) “…The coupling capacitors can be anything from around 22nF up to 22uF. I recommend film or MLCC types depending on the value. (the higher end of those values, go with MLCC X7R, for the lower end, go with any type of film) The higher the value, the lower the frequency that will pass as FM. Lower values will block slow FM treating it like DC…“. I used 4.7UF on mine.
The VCO is designed for 15V operation, for 12V operation J3RK suggests:
…This does work fine on 12V, but you can make the crossfader a little more accurate and balanced on 12V with the following changes:
R23 change to 11K
R24 change to 12K
R25 change to 24.3K
R27 change to 24.3K
I’ve built mine stock and it seems to be okay on either supply voltage as is.
RV1 – VCA1 trim (waveshaper section)
RV2 – VCA2 trim (waveshaper section)
RV6 – Saw trim
INIT – Initial frequency trim
RV103 – 1V/Oct trim
RV104 – HF Trim
RV201 – Sine shape trim
RV203 – Sine shape trim
Set your tune pots to 12 o’clock. (optional, but I typically do this)
…for calibrating V/Oct, it’s really quite simple depending on what you have available to supply voltages. If you have a CV keyboard/synth or a MIDI/CV, you can just play octaves on a loop. It helps if you have one that can provide more than 5V range. If not, you can just transpose back and forth to check higher/lower octaves. Turn the 100R trim (RV103) until the lower end sounds correct, and the top end will likely be a little bit off (unless you’re lucky.) Then start adjusting the 100K HF Feedback trim (RV104) until the top end levels off, and sounds good with the lower end. You may need to bounce back and forth a bit between the two trims to get it just right. I tune by ear, but you could use a frequency counter too.
Once you have the octave response tuned, you can set the initial frequency trim (INIT) to where you want it in relation to your tuning pots. (this is why I put them to 12 o’clock)
3 INCH VARIANT DETAILS FOLLOW, BUILD NOTES ARE THE SAME AS ABOVE.