I just built this sweet little module by Geosync Synth and will put up some pieces of info here on possible modifications. Thankfully, the nomenclature on the PCB and in the build docs follow 303 schematics, so identifying mods and tweaks should turn out to be easy.
I just started with this page (and shoukd actually do other things…), so please be a bit patient as for its expansion and some more visual aid for modifications.
Yet, first, why and who would want this module? For me, purchasing this kit from thonk was triggered by a full-on luxury dilemma- wanting to have a good 303 sound without the sequencer in Eurorack… TD3 is plastic, RE303 is expensive, yadda yadda yadda… TB-EFA for the win!
Building the module
Building the module is easy, and though the part labelling seems illogical first, sticking to the 303 schematics here is a boon. What’s in the partial kit is transistors and an OML BA662 clown, and the rest of components needed is easily available. I built per BOM document rather than per building guide, but either way, none of this is too complicated. What’s easily apparent from the number of shootouts printed onto the PCBs and anecdotal explanations in the BOM and build document is that someone dedicated was at work here. For my taste, a separate document about changes/nerdy facts about the original might have provided some further clarity, but that’s maybe just me getting carried away by non-building-related pieces of information. Long story short: easy build.
Tweaks and modifications
The TB-EFA sports some classic 303 modifications, and while they’re not always to my taste, they totally cover what many users might want. There’s input drive, increased resonance, filter-trimmer-as-pot, and FFM per CV. To start with complaints on a high level: the only tweak needed in my opinion is a slight rewiring of the drive potentiometer.
Input drive tweak
The audio input signal as per 303 schem would pass a 220k resistor before entering the filter. Robin Whittle’s classic overdrive modification replaces this resistor with a potentiometer and resistor in series. My understanding of this mod is that you use the potentiometer as a variable resistor, not as a voltage divider, and that’s how I do it in my TD3 version. Now, on this module here, VR1 (100k) goes PIN1 to +5v, wiper = input, PIN3 = to 30k resistor, to filter. The +5v is for biassing (303 VCO is a bit different that way), but un practice Ido not find this super appealing. This is because mostly, the range between no signal and clean input is rather narrow, and, which is personal taste, I don’t like that I can accidentally dial my input fully out.
In the end, I put in a 250k lin pot with PIN1 unconnected. This way, my maximum resistance on the input side is 280k, and the minimum, like in the version intended by Geosync, 30k. This is well enough range to tame some louder input signals and to beef up some quieter ones.
Changing the existing setup is easy: use a 250k linear pot instead of the 100k mentioned in the build docs, and leave pin1 unconnected.
Rubberduck (filter slew) modification
One of my favourite modifications in my TD3 explorations is the rubberduck (not “rubber mod”) quack you get from switching a capacitor between the middle PIN of your Envelope Amount pot and ground. The capacitor works as an RC filter here that softens the transitions of the filter envelope. The beauty of this is that the more you turn Envelope Amount up, the less effect this modification has, so you can easily move between moderate envelope amount QUACK and full Env Amount stabs.
I wired the cathode of a 2.2uf cap to PIN1 of the Accent potentiometer, which is grounded, and the anode via a sub miniature switch to the wiper of the Envelope Amount potentiometer (middle pin).
As this Mod needs a simple on/off switch (you don’t want that permanently on), I reworked the “Resonance Width” mod on the TB-EFA panel by simply using a 500k multi-turn trimmer on the back of the PCB. For me that’s something you set and forget – similar to the maximum resonance amount.
Variable VCA Decay
Depending on whether I want my maximum resonance setting fixed, I’d like to repurpose that “Resonance Chirp” potentiometer for a variable VCA decay modification. The latter would not only ben somewhat logically placed – right above the normal decay potentiometer. It would also drastically increase the variations you can get from a simple melody line. (see TD3 modding page)
Let’s start with the target mod though – for the VCA decay side of things, you simply replace R123 (1.5M) with a 1M linear potentiometer, set up as a variable resistor (i.e. don’t wire anything to ground). Since R123 is on the back PCB, you need to lead some wire from the Pins of your 1M pot to the back there.
This way, you can dial the VCA decay of normal (i.e. unaccented) notes fully out and in – cool for making notes “disappear”. 1M is well enough for long notes, so don’t sweat finding a small enough 2M pot.
When using the place for the original Resonance Chirp knob for our VCA decay, we need to do some further things. First, just use whatever value you deem fit for R97A (here 10k would be conventional), and short PIN3 and WIPER for VR97 on the PCB. Your maximum resonance is now fixed.
You won’t be using VR97 itself but you will place your new pot for the VCA decay in its place, so just solder in the braces holding the pot in place, not the actual pins – these need to be bent up so that they don’t touch the PCB or any other components.