On account of the happy occasion of modding two Basslines for this notorious musical duo here – Nerds With Acid – I wanted to share some info on these machines and some modifications too. If you do these mods, link me on socials/ write some email, that’d be sweet!
Right, a while back I scored a used Acidlab 2 locally and did some mods/ a bit of a writeup on Modwiggler but never really got round to demoing some mods properly. So here we go:
NB all modding info here is under CC-BY-SA. Don’t start with some “But 303s have been modded for ages” bs, please, and be decent and reference me if using some info here. If you have someone mod your Acidlab Bassline maffez-style request the modder to show you a proof they donated at least 5% of what you paid them to a charity helping refugees.
For now, I simply focus for now on some prosaic/ pragmatic things rather than talking capacitor meets sumida coil mysticism.
Right away, a couple of things jump at you in this TB303 clone. Instead of discrete transistors, the VCO consists of a “mystery” IC (an LM3046 – NB that identifiers on some ICS are manually scratched off, I guess in order to complicate identification), and an IC comparator. First I mistakenly thought the pulse wave does not do the proper 303 thing where it has a different width depending on the not frequency, but toying with PW mods I noticed it somewhat does. Moreover, attenuation into the VCF is stronger (still more oomph than on 303, I guess, because reducing VCO level further gives more squelch in the sound). Slide is analogue and can be modded to engage per press of a button.
VCF and VCA are also somewhat different:
In the VCF we find another LM3046 transistor array, which is convenient in terms of using less space. Since the NPNs in this array are thermally coupled (and some of them set up as a matched pair), you can expect this filter to losen up a bit more when using discrete transistors (though hole or SMD doesn’t matter as much) – at any rate I like the filter sound enough (more than on a TD3 actually). The VCA is one half of an LM13700 OTA and sounds good enough to me (the gain cell bits of a 13700 and a Roland Ba662 are practically the same, yet the buffers aren’t). The other half of the LM13700 is used for resonance control. I toyed around a bit with reso CV but meh… Here’s a schematic of the VCF/VCA section as I traced it (not verified by anyone, but still a better love story than twilight…)
What’s more? There are some smaller differences in the accent setup, and especially the impact of accent to VCA is a bit overdone to me, but that can easily be fixed 🙂
I did quite some, ands there are oodles more possible, but the following ones I liked best…
Just a note on nomenclature: for some mods you want a variable resistor (pot), for some a voltage divider (also per pot) – the latter means sometimes you need to connect pin1 of your pot to ground; the former means sometimes you need to connect the pot wiper to pot pin 3 and the junction of both then to something else. Pot pin nomenclatue as per here: https://www.digikey.at/en/articles/the-fundamentals-of-digital-potentiometers.
Here I got two nice ones:
VCO level control: replace the 340K resistor directly above the waveform select switch with a 100k pot set up as a voltage divider. Waveform output from switch goes to the pot’s right hand terminal (pot fully open), the filter input (use the terminal of the 340k resistor you removed) goes via a 1k resistor to the pot wiper, and the left hand pin (fully closed) of your pot goes to ground. This way you can dial in VCO level from zero to well overdriven.
PW control: remove the 100k resistor marked in the picture below and wire a 100k pot in (no voltage divider – just connect pin 3 with wiper and pins3 and pin1 with terminals of former resistor). If you find the minimum pulse width too narrow, insert a 10k resistor between pot PIN1 and PCB terminal.
Manual Slide trigger: slide is engaged per IC switch, and you can implement a manual switch that can then slide non-sliding notes (not the other way round though). Scroll down to the Envelope Mods section and look at the picture until you find SLD trigger marked (there’s a transistor, the top terminal of which you want to switch to ground for engaging slide). Transistor top terminal to on/off switch –> via 1k resistor to ground (actually you can ditch the resistor, but it doesn’t do any harm there either).
Here I got VCF keyfollow, filter slew, and resonance boost. FFM yeah sure, but also: plug headphones out into fffm in…
Resonance boost: find the 47k resistor side A of the LM13700 that is connected to PIN1 of the LM13700). Use an on/off witch to switch a 200k resistor (or 150k if you want more screech) in parallel to the 47K.
VCF Keyfollow: find the tip pin of the pitch CV output socket and wire a 33k resistor to that. Then wire that resistor to pin3 of a 1M (linear) potentiometer. connect pin3 and wiper. Connect Pin1 of that pot to the point in the pic above marked “FCV” (see blue cable right next to that chrome electrolytic capacitor). This FCV point, of course, can also be used for FFM and such (beware of amounts – I’d never go under 10k resistance for an input here).
Filter slew (my beloved rubberduck mod): this turns bow wow into bwhopp. In teh pic above at the very top you see some green cable attached to a resistor just above the envelope amount knob. This 10k resistor is attached to a little smd capacitor (hope this helps finding it easier). Now, wire a 10uf electrolytic capacitor anode to that junction resistor/smd cap and to your electrolytic 10f cap cathode wire an on/off switch that, in turn, connects to ground. When switching on you create an RC envelope whopping the whoop out of the filter env. If 10uf is too much, use 4.7uf.
Bass Boost: that’s the classic low end modification whereby you increase the value of two highpass capacitors in the two VCA inputs (one for VCF out and one for the resonance path). Usually, folks just add another 100nf cap on top of the existing 10nf, and I did too. Boom.
VCA boost/crunch: yeah, the notorious “turkish bazar” grmmfwhapp sound. The VCA is different that that of a 303, yet you can get it to crunch nicely by boosting the VCA input (per switch). As per pic above (look also at the schematic on top of this page – the VCA drive point s marked there too) you see a 10k resistor working as a voltage divider to bring down the VCA input level. Remove that resistor and take an A/B switch connected as follows: middle pin to the former 10k resistor terminal that connects to the LM13700 OTA chip. the two outer pins go to 1: a 10k resistor that connects to ground and 3: a 33k resistor that goes to ground. With more resistance towards ground, you lift the VCA input level up, causing crunchalicous crunchening.
VCA Slew: thsi one is weird/cool. By switching a 560nf capacitor /film box, non polarized – polarized caps make your VCA go silent here…) between VCA control and ground, you soften the attack of the VCA considerably. (This is on non-accented notes only, and deliberately so for effect). In conjunction with keyfollow, this makes som trippy soft sounds.
Envelopes and Accent
Lots of stuff in the picture above. Let’s focus on variable envelopes first:
Variable VCA decay time: the VCA envelope is a simple decay the time of which is set by a simple 1,5M resistor. Replace this resistor by a 1M pot and enjoy your newly found pattern variatrix! Use your pot as a variable resistor here, i.e. connects wiper and pin 1 to one cable and pin3 to another – the further the wiper is from pin 3, the greater your resistance/time.
Variable Accent decay time: When accent is ON, different things happen, but the main bit is that the filter envelope is switched to it’s minimum decay time (exactly the same length when you have filter decay knob on zero). This is not much but also not nothing (imagine this being spoken in thick Austrian accent). By replacing the 68K resistor marked accent decay in the pic above with a 250k lin pot, you can change that in powerful ways – from no accent at all (where’s my note?!?) to long fwhoohhghff sounds.
Accent to VCA reduction: On the Bassline 2, I find accent hits VCA too hard, so I increased the 47k resistor involved in that particular routing to 100k, which attenuates this impact just right (IMO, anyways). NB the 47 resistor below is involved in the Accent to VCF routing.
Manual Accent trigger: check on the busy picture above and find accent trigger (bottom terminal of that single resistor between two ICs). Wire that to a switch or button (I use momentary on pushbuttons), and that button connects to ground (via a 1k resistor – again, that resistor is not needed but just for feeling good, actually). Unaccented notes can be accented now (not the other way round though).
Gate hold: (I guess in xoxbox terms this is the gate on forever thing) again, check the busy pic above and put a switch between 1k resistor that goes to ground (or ditch the resistor and go to ground directly) and the top terminal of that transistor labelled gate (on my pic). What happens is: following notes after the last played just before you hut that button cannot engage gate any longer, so the sound volume slowly fades out but pitch information is still played!
And some diddle ditty at the end – pulse width meets accent decay and vca decay – slight delay fx on demo
And, as per usual, Max does some final QC and tuna/chicken inspection, just in case…