The BS II recently came on my radar again after learning that you can turn the sub into a fully-fledged oscillator with that patchbase midi editor, and after a unit showed up locally for cheap (although with quite some smoke stains) I thought heck, why not? Funny – after a couple of years without one, hearing its sound is a bit like coming home. Also, I think I’m starting to identify a pattern in synthesizers the sound of which I like – LM13700 OTAs…
While more elaborate modifications and circuit descriptions are to follow, I’ll kick off with a rough map through the main PCB and a (rather long) list of points at which DCO and FCV signals can be tapped and CV can be sent in. As for pitch CV, however, I’m still a bit at loss (need to read up more on DCO control)/ not entirely sure if that can be had as easily.
Audio clips will follow soon too (had this thing for three days now and need to tinker more first 🙂
As for hardware, all three DCOs are identical (except the DCO2 sync circuit), which means the way it is adressed by firmware at the moment, the sub is a full oscillator on a leash.
I had a quick look at the ace schematic of the older Bass station, as mapped by the excellent Zeninstruments (http://zeninstruments.blogspot.com/2017/09/novation-bass-station-schematic.html), yet DCOs on the BS II seem to be done slightly differently. Basically, here too you have a saw core, followed by a triangle shaper (both on an op-amp), a HC4053 multiplexer, and a LM13700 one half of which does the sine shaping and the other pulse (including PWM). With this setup, you have all DCO waveforms available at all times in addition to the selected wave (switched by an HF4051).
The Ring Modulator is built around half a LM13700, and noise I yet need to properly trace. Level control for each sound source is done via half a LM1300 each (as well as the general level of the DCO mix, i.e. pre-vcf overdrive, and, of course the final distortion). This is neat since you can easily fire CV from your modular for level control and amplitude modulation.
The “Classic” Filter, as far as I understand, is a riff on the Oscar Filter, with two -12db stages that can be switched for different filter types. At the core you have LM13700s, again, op-amps for buffer and mixing purposes, and HC4051 for switching types. Resonance CV is realized per LM13700 setup. Like on the DCOs, you have all filter types available for tapping and external remixing. Still to do: cutoff spread (that’d be so ace!). BTW, here’s Sam Hoshuyama’s redraw of the oscar vcf schematic: https://houshu.at.webry.info/200803/article_1.html.
The so-called “Acid” filter is a diode ladder inspired by the TB303 filter. At the core you have your ladder (with the first capacitor in that ladder being of a smaller value, i.e. this should be closer to -18db) and one half of an LM13700 for resonance and another for volume compensation. Both of these interact (the 303 reso pot is a dual gang where you have a similar behaviour meant to compensate level drop at higher resonance), yet it’s also fun to address them independently with CV. If you address the “reso” half only, resonance sound much thinner, and if you address the “level” half only, maximum resonance will also increase but the sound remains much fuller.
On the pic above you can see how the filter out is fed back into the reso path through R269 (with R270, 330ohm, pulling the signal to ground). Toy around with these resistors and find the acid vcf can self-oscillate! Since on my unit if found the signal level of the acid vcf relatively low in comparison to the other filter, I reduced R266 by wiring a 15k resistor across to drive the acid vcf harder – with higher input level, maximum resonance decreases.
BTW, the extra caps and cable wired in by hand are a “hack” found in probably all units- at least this looks on mine like on pics seen on a modwiggler thread. I guess that’s a method of keeping voltage at the control pins of the Lm13700 stable? Anyway, pull PIN16 (the one labelled “AMP” towards ground via a 15k resistor and your Acid VCF get more bite!
The VCA is built around yet another LM13700 plus op-amp (U57-d), and the distortion I have not yet mapped.
OUTPUTS AND CV INPUTS
|Classic -12db LP
|Classic -12db BP
|Classic -12db HP
|Classic -24db LP
|Classic -24db BP
|Classic -24db HP
|Classic -12db Main
|Classic -24 Main
|FFM from VCO2
|External Input level
|Pre-VCF Mix Level
|VCF parameters — soon (need to test more first)
CV inputs take -5V to +5V, with around zero volts constituting the nominal internal maximum (yet need to measure actual span). Anything beyond zero just gives you more, which can lead to cool effects! In order to achieve an offset of -5V (which is where your internal levels are at zero), use an inverting amplifier with the positive input not tied to ground but to -5V (in socket to 100k resistor, 100k resistor between out and negative in, positive in to -5V, 4k7 resistor between out and your respective CV point).
NB internal voltage rail for synth part is -5V/+5V, so clip any CV that exceeds this (best by diode clamping).
Cutoff separation like on OSCar VCF:
The CLASSIC VCF in -24db mode is two separate -12db filters in series. Luckily, you can address the cutoff frequency of both halves independently and do cool stuff such as cutoff separation (pushing the two resonant peaks apart, like on the legendary OSCar filter) or send CV independently to each. If you like toying around, start with wiring a 100k resistor to DD3 and DD4 each and send in CV:
Cutoff separation per CV: this is the version for which I eventually settled, and this requires one inverting op-amp (I used a dual op-amp for the phase inverted sub mod, so I had one op-amp left). Wire your CV input socket to FCV1 via a 200k resistor and to the -IN of your opamp via a 100k resistor. Feedback resistor between -IN and OUT = 100k (+IN to ground), and op-amp out via 200k resistor to FCV2. (If you find the amount of separation to little in this setup, replace your 200k resistors with 150k/100k each.) What happens: if your incoming CV is zero volts all is normal, and with increasing positive CV voltage, the cutoff frequencies of both filter halves start shifting apart. Sounds like this when fed by an LFO:
Cutoff Separation Saveable and Midi-Controllable: (NB you repurpose the OSC FILTER MOD funtionality, i.e. lose the original function of that that knob when doing this mod). What you do is to decouple the connections of the OSC FILTER MOD circuit first. Remove R276 (33k) and cut the trace leading from U51, PIN12 (right hand side of IC, fourthmost PIN from bottom). Wire the right hand terminal of R111 (the one facing away from R110) to the bottom terminal on the PCB where R276 used to be. [Right hand terminal of R11 carries ca. -2V fixed voltage that thereby becomes your new inpult voltage for the Separation Knob]. Then wire U51, PIN12 to VCF1 and VCF2 as described in the section above [i.e. FVC1 via a 200k resistor and FCV2 via an inverting op-amp and 200k resistor]. The OSC FILTER MOD knob know dials in cutoff separation on -24db mode. You can control this action per midi AND save the knob settings in presets!
I like the scaling a lot since it just hits the sweet spot for doing vowel sounds. If you like your spread more pronounced, either wire your input voltage connection (bottom terminal of former R276 to U51, PIN6, which carries some -4.6V or reduce your 200k resistors to 100k.
Rewire FFM to FM from VCA Output
As you have an ACID filter, you migt as well pay tribute to the Debilfish and do this classic 303 mod. Rather than having the FM pot dial in DCO2 to filter cutoff, remove R276 (33k) and wire I58 (VCA out) via a 100k resistor to the bottom terminal of former R276.
You could extend this modification quite easily by wiring a select switch at your new FFM Input resistor and connect that switch to all sorts of ins (DCO3, VCA, ect. see connection points section above).