Ge0sync Synth

---

The TB-EFA Rev2 is the second version of our best selling TB-EFA module. It shares the same ‘3 and a bit’ pole filter circuit as the Roland TB-303. It also provides a complete version of the envelope used by the 303 with decay control. Rounding off the module is the BA662 based VCA circuit which is also controlled by the same envelope as the filter.

Like the original it has pots to control the Cutoff Frequency, Resonance and Envelope Modulation and Decay. These are true to the original in their operation.

The envelope is controlled by a CV gate input, triggering the envelope and amplifier. If no gate is input you will not hear anything as the VCA will not be opened.

TB-EFA adds an additional “chirp” control that allows you to tweak the upper levels that the resonance feedback can achieve. R97A sets the highest level – by default a 10K resistor in the original 303. In the TB-EFA this is replaced with a 4.7K resistor and combines in series with the 5K “chirp” pot. (Paul over at DINsync.info refers to this as the “dice roll”). You could experiment with an even lower value, if you like shrill bird song for example!

The original TB-303 has an internal trimmer, TM3, that sets the filter frequency maximum. The TB-EFA externalises this with its own pot control (resonance width) so you can adjust to your taste on the fly.

A drive control is also added which provides an inbuilt overdrive effect at extreme, and again builders can try different levels. A 33k resistor at R62 combined with the A100K pot lets you drive it hard but not too much (original uses a fixed 220k). You can reduce the 33k down even as low as just a few K for extreme distortion. This drives the initial input audio signal to the filter – so pre filter overdrive. Note that beyond about 2 o’clock on the pot drives the signal hot and will lose some of the resonance high frequencies as a result. This is normal.

The TB-EFA includes the VCA from the original 303. This uses the Open Music Labs (OML) BA662 “clown” clone circuit to stay true to the original. This VCA is controlled by the same envelope as the filter and only the decay portion can be adjusted.

The final control is accent. Here, a trigger CV input is provided as well as a pot to set the accent level. Note that the accent controls the envelope shape and level, so it needs the envelope to be triggered. Thus, both gate and accent need to be “high” for it to affect the sound – or at least the envelope needs to be running for accent to increase its effect. The accent also adjusts the bias on the BA662 circuit and increases the loudness of the note, so without a trigger you can increase the general accent tone of all notes with just the pot.

New for Rev2

REV2 adds an extra pot control for the VCA decay itself. This means you can control the VCA decay independent of the Envelope Decay, allowing for control right down to a “click”, through “plucky” short tones through past the standard VCA length of the original circuits.

REV2 also adds a toggle switch to turn the “maffez” rubbery control on or off. This creates an almost portamento-esque type effect on the filter decay itself, giving a more rubbery sound to the filter envelope – have a listen and hear for yourself.

In addition to the extra audio controls, REV2 adds an extra 5 CV controls for some of the pots. Resonance, Chirp, Envelope Decay, Drive and VCA Decay, all can be controlled via “carefully tuned” CV inputs.

Now, we could have added 0-10v full scale controls for each of these, however this would result in a module closer to 20hp (already Rev2 has gone from 12hp to 14hp) and so simplistic CV controls were added to the existing pot control circuits. These blend in an additive form, the CV and Pot and so both can be used to sculpt the incoming voltage. However, this does mean that in most cases, some form of attenuation or even offset/inversion maybe required to fine tune the incoming CV to a usable range – better than nothing. We did want to explore further refinement, but this module update is already very late – and stocks of Rev1 have now been exhausted.

The TB-EFA Rev2 is available as a panel and pcb set, a panel and pcb set with partial kit and in limited numbers as a pre-built, tested and calibrated module.

---

Module Notes

Check out Maffez page for some thoughts and modding options. Some of these have been integrated into have Rev2.

In addition we have fixed the two most common complaints, the >5v gate signals are now capped to below 5v in an attempt to stop certain CD4013 variant from stalling (which resulted in the accent being on all the time and the env decay appearing to do nothing!)

We’ve also added an extra output op-amp to boost the signal to be more in line with euro-levels (rev1 was true to the 303 filter output level – but of course the 303 also had a headphone or general volume gain circuit after the filter. If you wish to play with levels to match your needs, the default setup provides ~4.3x gain with R302 (33K) and R303 (100K) ( Gain = 1 + R303/R302)

The build is very similar to the rev1, we will update the build document soon, the BOM is available. The main differences are the on-board discrete circuit for the BA662 clone, and the optional pre-filter drive behaviour, I prefer the default (fit the jumper wire and use an A100K port for drive) – Maffez describes his preferred option (don’t fit the jumper, and use a B250K pot) in the link above.

---

---