- Manual Cutoff, Drive, Resonance, Chirp, Envelope Mod & Accent Controls
- CV Inputs: Gate, Accent and Cutoff
- Audio Inputs: Signal In
- Audio Outputs: Filter Out
The TB Filter Envelope module is inspired by the filter, envelope and accent circuits of the Roland TB 303.
It copies the same 3 and a bit pole low pass filter and most importantly the resonance feedback loop.
Like the original it has pots 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 303 sound.
The 303 uses the same envelope to control its internal VCA, with the above setup you can have different envelopes and create a whole new set of acid-like textures.
R97, normally a 10k resistor in the resonance feedback circuit controls the amount of “chirp” to the characteristic 303 squeal. The TB-EF let’s you tweak this live by replacing with a 4.7k resistor and 5k pot. Anything lower than 4.7k may have you attracting all the local canines, but of course you are welcome to experiment! If you don’t have a 5k pot, you can use a 10k but remember the original is 10k, so you effectively reduce the resonance effect when you go above 10k (combined). Of course you could even reduce the 4.7k resistor if you want, but things get a little distorted if you go too low.
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 303’s. This can provide an inbuilt distortion effect at extremes, and again 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 I found via experimentation to be the best and most usable.
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.