The TB-EFA 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. 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.
This is a full DIY kit – including all components required to build the EFA Eurorack module
TB-EFA – Build Guide & BOM
- Manual Cutoff, Drive, Resonance, Chirp, Width, Envelope Mod, Decay & Accent
- CV Inputs: Gate, Accent, Cutoff
- Audio Inputs: Signal In
- Audio Outputs: Filter Out
- Width: 12HP