Why, thanks, MAM! This little acid box sounds legit. It’s VCO setup is unlike that of the original 303 yet sounds great, the filter ladder is built around cheapo stuff and sounds great, and the final VCA uses a trustworthy LM13700 which crunches like heck when you pull up that main volume knob.
Above example is from a stock MB33 retro – nice, no? 🙂 Sadly, MAM do not exist any longer, as far as I know, but I think it makes sense for servicing/community purposes to have the schematics available, so check the ZIP file below. NB: should any former MAM folks take issue with this, do let me know!
Some particulars about this machine (in the schematics anyways) – PSU is AC, not DC, and internal voltages are (DC, of course) +12v, -12v, and +5V – so, in theory, this could easily be converted to a eurorack module… (why though?). There are three trimmers: two accessible from the outside (for scale and offset of the VCF), and one only accessible inside, placed next to the ENV amount knob, setting the VCO scaling.
MODIFICATIONS
Everyone knows the awesome circuitbenders.co.uk MAM mods, and I have no intention of demystifying them. That page/company has done a great service to synthkind, as it were, so do support them! I rather post a list of classics and some of my own concoctions, with the following pic serving as a master map (high resolution download directly below the pic:
PW for VCO SQUARE: the point marked PW in the MAP (junction of a 22k and a 47 resistor) can serve as an entrance point for either manual adjustment (50k pot set up as variable resistor between GND and this point) or CV input (best via 33k and clipped against negative voltages with a simple diode from ground). Sounds like this:
Pre filter overdrive: put a 250k or so pot across the 47K resistor marked in yellow nearby the audio input jack socket. Example with square wave:
Filter CV input: I yet need to test and record results, but the base of the PNOP transistor marked FFM above is the equivalent to the base of Q10 on the TB303. In classic terms, you’d connect a 100k resistor to that and use the other terminal of that R100k for sending in filter cutoff modulating signals. This can, of course, also serve as an entry point for stuff like VCO to FFM or VCA output to FM.
Resonance boost: MAM are nice in that they placed a 100k resistor in parallel with the resistor setting maximum resonance. Either use a switch (and 50k resistor in series) or a 50k pot and 50k resistor in series for replacing that 100k. Values of your resistors might need adjustment as per taste, so if you plan on doing this, best have some 100k, 68k, 50k, 47k, 33k ready,
VCF SLEW: Insert a 4.7uf capacitor between the middle pin of the ENV MOD knob and ground (cathode of your cap to ground) – have this on a switch…
Bass boost: classic stuff whereby the values of two capacitors at the VCA input (one passing the filter output signal, the other the resonance signal) are augmented. in the 303 , these are 10nf and are usually bumped up to 100nf. Here, they are 33nf, and I don’t like the sound of increasing that of the normal filter output at all. If anything, I’ll increase that of the resonance path to 220nf to compensate a wee bit for passband level loss.
Increase normal filter out cap only:
Increase reso path cap only:
VCA rubber: of of the more widely used mods for the Bollinger TD is to introduce some slew at the VCA gate in order to rid that machine of its staccato vibe. While the MAM is groovy enough as is, I was simply curious and tested a bit of rubber. 100nf at the cathode of the diode marked on the MAP sound cool to me. 220nf stand out as a clearly audible difference already, if you find the below example sounds like always the same…
VCA CLAB overdrive: The 100k resistor at the VCA gate can be reduced for more heft. below a certain value things start overdriving.
VCA de/crunch: the MAM is set up in. a way that main volume at max gives you internal overdrive. By altering the value of the 47k resistor marked on the MAP, you can augment (increase that resistor’s value) or diminish (decrease it’s value) that very effect. Sounds like this:
NB the real crunch/de-crunch lies close to the volume pot, but this shall remain a mystery 😉
Clean OUTPUT: teh easiest way to get an entirely clean output is to tap the middle pin of the vloume knob. On my unit I have a “normal”/clean toggle switch — sounds like this:
Manual accent/manual slide: for manual accent, pull the BASE of the transistor marked on the MAP to ground. For manual slide, pull the COLLECTOR of the corresponding transistor marked on the MAP to ground. When testing, best have a 4k7 resistor between ground and your pushbutton/switch in order to prevent accidental smoke signals communicating that you are a nerd in dire need of hugs (just ask people for hugs, that’s cheaper than zapping circuits).
Minimum decay/Accent time: Accent is the decay envelope switched to minimum. You can alter that minimum time by reducing the resistors I marked on the MAP. These are a 22k and a 47k resistor in series. Bridging either or both of them reduces the minimum decay env/accent time. Here i replaced both resistors with a single B100k potentiometer:
Sadly, the setup on the MB33 is a tad different from the OG 303, so you cannot easily mod for independent minimum decay & accent time settings… Yet better variable *both* than nothing, I guess.
Variable VCA env decay time: the 1.5M resistor marked VCA decay on the MAP is replaced by a 1M pot.
Increased VCA decay time: If you want to increase the maximum possible VCA decay time in general, then augment the value of your 1.5m resistor to something more, e.g. 2,4m or so.