Here is some modding information on the Behringer TD-3 clone of the famous Roland TB303. This was my first proper collaboration with other synth nerds (especially Nordcore, who transferred all schematics).
More comprehensive audio impressions can be heard on my TD3 mods souncloud playlist.
Schematics and full modding guide
The most recent version of the comprehensive Td-3 modding guide is here (mistakenly dated January 2019), yet discussions are ongoing on gearslutz and sequencer.de, where many modders give positive feedback on our “rubber mod” and others. If you click on the schematics below, you open a larger PNG file which allows for zooming in.
303 Sound Tweaks (visual guides below)
In several fora TD-3 users express their hype about the sound of that plastic box, yet some OG 303 owners/owners of attentive ears also mentioned that there are some minor differences between these two bassline synthesizers. With some experimenting (thanks, el-folie!) and theoretical input from Nordcore, we quickly found a set of tweaks that can get you closer. As far as I can see, this would be:
- lower cutoff (even lower than with internal trimmer): 20k resistor in parallel with R44
- more hollow square: replace R189 (22k) with trimmer/ resistor and trimmer (the range should go to around 33k)
- hollow square: the TD’s square wave does not seem to be as hollow sounding as that of the Tb303, so you can adjust the value of R189, which regulates the pulsewidth of square; sweet spot is around 30k, so best use a 20k resistor and 15k trimmer (or just a 47k trimmer and solder less)
- soften VCA attack: place 220nf capacitor on the solder pads of C51 (unpopulated on PCB)
- increase bounce/ “rubber mod”: by placing a 1uf cap between the top of D22 (anode) and ground (cathode) reduces the staccato vibe of the TD-3’s VCA release
- VCA crunch: not a “have to” but desirable is to increase the VCA level, so that on full master volume there is some pleasant crunch, which also gives your accent a tad more impact (Emmanuel Top, “Turkish Bazaar” stylez); either remove R106 (220k – apologies for previous typo here) entirely or, if you want the effect to be more subtle, replace it with a 1megaohm resistor. NB this gives a tad crunch, not distortion.
- PSU mod for that resonance warble: replace R92 (10ohm) with 50ohm-100ohm resistor; 100ohm already gives you some pretty “empty battery” 303 bahaviour, especially if you also use the “rubber mod,” which sucks quite some power on note onset; slightly lower values give you less warble; NB that you may need to recalibrate your VCO after
All the above-mentioned mods are worth implementing permanently, i.e. they don’t need to be on a switch, although I very much like Nordcore’s idea of doing the PSU mod on a switch using a 120ohm resisor and labelling your switch “New – 25 years of party”. 🙂
NB that in all of this you might want to consider that there are differences between individual 303s themselves (for instance, different PCB revisions), and that it might be more fruitful and fun to tweak *your own* sound. I list these mods nevertheless because I believe the TD3 benefits from them in terms of a more dynamic sound.
Visual step by step guide for 303 tweaks
In the following you find some visual step by step help for implementing the basic 303 mods under ideal type conditions on a naked PCB. NB that the values seen in these pics might deviate from the ones actually used (I just recycled some old parts for demonstration purposes) and that your actual soldering circumstances might be a tad more challenging as other parts might obstruct vision etc. Mods where parts are simply added to the circuitry are: lower cutoff, softer VCA attack and rubber mod. Mods that require replacing components are: VCA crunch, PSU mod (bit of fiddly flat and broad resistor with not much space for soldering iron) and hollow square mod). As regrads actual part values, check the full guide linked above and for general tips on working with SMD stuff, check my basics guide.
1. Lower cutoff
Lowering the TD’s cutoff per internal trimmer is a good start (all new units as of early 2020 have theirs dialled to minimum as per factory shipping – at least the five or so I had open), yet if you want lower, you can just add a resistor in parallel to R44. This means you just solder the legs of a new resistor to the sides of the existing one. I personally prefer 20 Kiloohms for the additional resistor.
Make sure that the legs of your new resistor don’t touch the cutoff potentiometer or any other metal parts. Should you bend it to bring it into position, do so *very* gently (this goes for all parts you solder in), so that you don’t accidentally rip something off. No need to have this on a switch as you can always fine-tune the amount of cutoff reduction with the trimmer on the PCB.
2. Hollow Square
This is basically the process for replacing R189 with a trimmer or resistor & trimmer combination in order to adjust the pulsewidth of your TD square wave. There are visually nicer ways of fitting this, of course, but I think you get the gist. Also, if you want to have your square as close to the TB as possible, please find reference tones linked in the following section:
TB303 reference sounds for fine-tuning
- Wedok on GS was so kind as to record reference tones with his TB303 for adjusting PW, resonance and such. If you search that thread you will also find 303 patterns for checking that bounce.
- the formidable Din Sync also provide 303 recordings for download
3. Softer VCA attack AKA C51 mod
C51 is an unpopulated capacitor on the PCB – there was a plan for implementing something but then they didn’t but left the place in the final production run. Good for us! Basically all you need to do is to apply some more solder to the existing pads, bend the legs of a 220nf (non-polarizing) capacitor in shape, and solder them onto the empty pads. This has your VCA attack click less.
As seen on the right, you need to make sure there are no solder joints between the solder blobs. The lower terminal of C51 (on the right in the larger pic) is ground, which means if both legs touch, your TD goes silent (not broken but just schtum).
4. Rubber Mod
The TD-3 as per factory can have a bit of a staccato sound, which can be reduced by this mod. As someone on GS put it, the TD sounds like a fully inflated tyre, while the TB sounds like a tyre with the air let out a bit – hence, the rubber bounce moniker for this mod. Technically, this introduced a tad of slew on your VCA envelope, thus introducing a bit of a release tail, which glues note transitions more together. As this mod also affects power consumption on note on, there are some other side effects towards subtle liveliness in sound. The mod itself is easy: wire a 1uf electrolytic capacitor between the top of D22 (the positive leg of your cap goes there) and the nearest ground connection (in the case of the pics below, this is the lower leg of C51).
5. VCA level/crunch mod
VCA crunch may seem paradox vis-a-vis the VCA atack softening done by the C51 mod, yet these affect different things. The TD can be a wee bit clicky under circumstances, so C51 helps a lot with this and its potentially staccatosih sound. Increasing the VCA level via removing/exchanging R106 simply boosts the general level by means of which the VCA out hits the final opamp. What this gives you is louder in general and a bit bitier on accent.
6. PSU Mod
The TD3 responds very well to our adaptation of a classic Xoxbox mod, known as the resonace warble mod. When the power supply of the 303/Xox/Td3 is fully busy (cutoff high, reso high), this mod introduces subtle (resistors up to 100ohm) to quite audible (100ohms or 110/120ohms) liveliness and, well, wobble in its resonance. This mod is a tad trick since a) stuff is a bit in the way and b) it affects the power supply, i.e. a very essential part of the whole instrument, so patience and care is required.
Fair use declaration
As for those mods developed by Nordcore, el-folie, and myself, please note that we invested quite some time, energy and solder into this. The info here and in the guide is free as in “free to use and adapt”, yet please, if you offer any of these mods commerically, donate money to a charity that helps refugees or is dedicated to the protection of the environment.
Going nuts with your TD-3
In our guide you will find quite many options for taking your TD beyond bonkers. I will post updates and newer findings soon.
In the meantime, first time modder AC1 on sequencer.de went wild with chassis design – look at this gorgeous “Hello YellOoWw, Modded TD3, Bumble T-Bee by Det.Playback”!
Grischa posted this recently on GS and kindly agreed to have this linked here. Seems that we don’t need that Beats headphones mod after all to get some weight to this box, and ahhh, hang on, this mod also gives you battery power for about a day 🙂