Fair use declaration
Many classic 303 mods are public knowledge and marked as such in the guide compiled by myself. 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. Projects like these are solely based on good faith and enthusiasm, and potential buyers can support this by requesting a proof of donation after purchase.
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). I’m hyped to see that so many folks are modding their TD3s. Our gearspace thread has exceeded some 120k views, and who knows what kind of awesome sonic craziness is yet to be discovered. Keep going, good people!
More comprehensive audio impressions can be heard on my TD3 mods souncloud playlist.
Massive update from this super TD modder genius – have a look at what Jennifer G is doing with an arduino and an acid bassline! nice!!
August 2021 – Info on decals and recent updates
Martin alias Gimmik just got in touch and shared some info on how he relabelled his Bass Frog version, which is based on our findings. Here’s what he wrote:
The cover was repainted in a few areas. I redesigned some bits in Illustrator, and the relabeling was done with waterslide decals. Beeing a former scale model nerd, i did some “weathering” and “aging” FX on the case – so the look follows the sound – Acid silver boxes have to look like they have been raving for the last 30 years! At the moment it is easy to find TD-3s for as little as 50,- Euros. If you interpret the TD-3 as an preasembeled DIY kit, these units are very much fun, and once modded they groooooove and sound like nothing else out there. – And if you are used to an original TB-303 you will feel at home straight away, the sequenzer works exactly the same…
So, open the quickstart guide with some PDF editor, pull up the TD3 pic to TD3 size, and you’ve got a template for writing labels! Next time I’m modding a TD I’ll surely try that waterslide business! Martin was kind enough to also point me to this decal tutorial.
Another Td3 with some extra strong visual game is this one here, sent in by Isaac (see his video also) – 808-style with oscilloscope and all!
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 gearspace 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 bottom of D22 (cathode) 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
- Resonance adjustment: in some TD3’s, resonance can be a tad too pronounced; if this is the case in yours, increase the value of R160 (10k) a little bit; in some units I replaced R160 with a 19k resistor and small trimmer (2k)
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.
NB that you don’t need to use TP1 for ground – any of the pot braces (the two thick pins holding the pots in (NOT THE THREE THINNER ONES!) do the trick too.
4. Rubber Mod
Update May 2021: recently got a Cyclone Analogic TT-303 (see modding info here) and did some A/B with the Td3. The TD3 staccato was immediately noticeable, so I went back to the rubber mod and adjusted it to 1uf between D22 cathode (bottom terminal) and ground. In previous versions I went for anode of D22, which also has an effect, but at the cathode the effect is stronger and gives me a gate length like the Bass Bot has on it’s neutral gate length setting (setting 6). I changed the info below accordingly.
Here’s an A/B with the TT303 on the right and the TD3 (first normal, then with rubber) on the left:
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 botttom 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.
Additional tweaks for softening accent
Over recent conversations with an owner of a recapped 303, we started softening accent on the TD3 too. As other sound aspects too, this is entirely a matter of taste, however. I like it when accent is a bit softer, so here’s two suggustions.
PCB backside with output jack socket facing north: wire 560nf film box capacitor bewteen 3rd pin from the right of resonance pot and ground. If not enough, wire 220nf film box capacitor between leftmost pin of accent pot and ground.
In the meantime, Egowaves’ video on why his 303 went and the TD3 stayed became quite viral, and here’s an interesting blind test A/B.
SOME FURTHER UPDATES/ADDITIONS TO THE GUIDE
Correct input impedance for 1V/Oct input
For wiriung up pitch CV input, connect IC10, PIN5 to a 2k resistor and 47k precision trimmer in series, which then goes to your CV input socket. At 47k you should get proper 1v/oct tracking, yet I found having some leeway towards 48k helpful in some units, which is why the extra 2k resistor comes in handy.
Accent trigger with positive triger/gate
When writing the guide this was low focs/priority, and the info in there was quite muddled. Luckily, some users at sequencer.de refreshed this, so here is an easy and stable version using an NPN transistor.
Higher Resonance Mod (easier access on back of PCB)
Sub Oscillator (simpler version)
As of February 2021, I’ll use a sub osc based on Neil Johnson’s design for the Jen SX1000. Although the previous Sh101-style design worked well, it is much more work to wire up and this current version uses less components too! KardiObreaks pointed me to this, and I adjusted his suggestions a bit so you don’t need a comparator at the input side of that sub circuit.
In order to get you sub traking nicely, use the square wave out from the waveform select switch and make a little voltage divider. 47k resistor wired to a 20k resistor. The junction of both goes to PIN1 of the 4024 IC and the other terminal of the 20k resistor to ground while the other terminal of the 47k resistor goes to the square wave signal of the TD. Your subosc output goes to a 100k pot (pin3), the wiper of that pot goes to the normal pin of the filter in jack socket, and pin1 goes to ground. Using the normal pin of the filter in socket means you lose sub when plugging an external signal. If you want both, chose a different input point for your sub signal.
Also, redloheb @ gearspae did this wonderful schematic on a subosc – looks better than mine!
Some more ideas by other TD3 modders
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 🙂