Description

The TB-EF module is inspired by the filter, envelope and accent circuits of the classic Roland TB 303. TB-EF copies the same 3 (and a bit) pole low pass filter and most importantly the resonance feedback loop.

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

The envelope is controlled by a CV gate input that triggers the envelope and the filter modulation. If no gate is supplied the filter will open and close as you would expect but without any modulation. You can therefore connect to an external envelope and VCA but part of the character of the 303 is down to the way the cutoff and resonance behave when the internal envelope is triggered.

The sound is transformed when you apply a gate. The internal envelope then modulates the filter and tweaks the cutoff during the envelope cycle. With the envelope modulation pot control you can then create those classic 303 like tones, from chirpy squeals, to, for want of a better word, wumps… you know what I mean!

Ouput the filter to a VCA modulated by an external envelope with no attack, low decay, even lower sustain and almost no release and you will really get that distinctive 303 sound. The 303 uses the same envelope to control its internal VCA, and with the above setup you can have different envelopes and create a whole new set of acid-like textures.

The TM3, filter frequency trimmer is on the back of the module so you can trim the cutoff filter peak to your taste as per original. A drive control is also added, similar in design to the common overdrive mod applied to many original 303’s. This can provide an inbuilt distortion effect at extremes, and builders can try different levels. A 33k resistor combined with the 100k pot let’s 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 – be aware this really does over drive the signal into the filter and when reducing it back down it takes the circuit a little time to catch up. 33k has been found to be the best and most usable value here.

The TB-EF does not include envelope decay control. The BA662 is intrinsic to how the decay affects the VCA envelope in particular, and there just isn’t room in 10hp!. For that see the TB-EFA module which includes everything here and has the 303 VCA too.

The final control is accent, again a key part of the 303 sound. A trigger CV input is provided with an attenutation pot to set the accent level. Note that the accent needs the envelope to be triggered, so both gate and accent need to be “high” for it to affect the sound. That is, you need to sync up the accent trigger with the envelope gate trigger. The accent really just momentarily opens the cutoff a little more and changes the envelope shape to reduce the decay phase a little.

This Partial kit includes:

• 1 x 10HP TB-EF Black Panel
• 1 x TB-EF PCB Set
• 3 x 2SA733P Transistors
• 21 x 2SC945P Transistors
• 1 x 2SK30A-Y Transistor

TB-EF – Build Guide & BOM

Features

• Manual Cutoff, Drive, Resonance, Chirp, Envelope Mod & Accent Controls
• CV Inputs: Gate, Accent and Cutoff
• Audio Inputs: Signal In
• Audio Outputs: Filter Out
• Width: 10HP