Oakley Sound Systems Super Ladder Filter

The SuperLadder is a voltage controlled filter module designed with modular synthesisers in mind. Cut-off frequency, resonance and envelope sweep depth are all voltage controlled. There are no pots acting as variable resistors in this design. Thus the unit could form the basis of an output card in a polyphonic synthesiser. The basic topology is the old classic, the ladder filter. However, there are some interesting twists in this design.

Schematic:

super.gif - (121K)

Firstly, the VCF may be built as a moog ladder or a TB303 diode ladder. Thus with the same PCB, you could make either a classic warm filter or a rather crude squelchier affair. Simply by fitting different links and a few component changes.

Secondly, this filter has two low pass outputs; the traditional four pole output, and a new one pole output. This gives a sharper edge to your sounds.

Thirdly, this filter has a band pass output. State variables watch out! Band pass, with moog warmth.

Fourthly, all the outputs are Q compensated. This means that the output level will not drop when you turn up the resonance pot. The traditional Moog ladder suffers badly from this. This design uses two VCAs to achieve voltage controlled resonance without the hassle. A beautiful sound.

Audio inputs are fed into the filter via a simple voltage mixer based around U2. Three audio inputs are shown, but you could add as many as you want. Four CV inputs control the filter cut-off frequency via a exponential convertor, U8 and U9. U10 provides voltage controlled levelling and inversion controlled with another set of CV inputs, ENV-CV1 & 2. This can be used to control a ADSR envelope. The inverting properties of U10 will allow negative ADSR sweeps to be easily generated by simply altering the voltage of either ENV-CV1 or 2. U11/Q12 provides the necessary CV to current circuitry to drive the OTA U10. Thus one pot, fitted between 15V and ground, could control envelope depth and sweep direction. Set PR4 so that the mid-point of the pot will give no ADSR sweep.

Resonance is controlled by one or two voltages, Q-CV1 & 2. These positive only voltages increase resonance with higher values. The current produced by U11/Q11 actually drives two OTAs. One, as you would expect, increases feedback within the ladder filter. This is U6. The other, U7, in conjunction with the op-amp summer, U5, keeps the one pole output constant as resonance is varied. This is what I call Q-compensation. The idea of Q compensation isn't new, and this form is similar to that used on the TB303. In the TB the output of the resonance pot reinforces the output of the differential amplifier by simply summing the two signals together. In the SuperLadder, it is the output of the OTA U6 that is summed with the output of the differential amplifier. The quad op-amp U5 performs this task for both the one and four pole outputs.

The one pole output is obtained by looking at the differential voltage across the first filter capacitor in the ladder. So that both four and one pole outputs are available, two differential amplifiers are used. For the diff amp itself, I have used the TB303 ones, because, they are physically small and they are similar to those used by Moog in the Mini.

The band pass output is obtained by mixing equally the four pole output with the inverse of the one pole output. Rather, than build a subtractor, I inverted the one pole output by simply wiring the diff amp in an opposite fashion to the other one. The two output signals from each diff amp are then simply added together with an op-amp summer. PR3 is provided to compensate for any variations in the gain of each diff amp. The change in phase in the one pole output is corrected later, just before it leaves the module.

Note that some of the resistor values have to be different for the TB303 filter. This is because the signal level drops as the signal is fed up the ladder. Thus to provide equal volume from all three outputs, the gain of the differential amplifiers and some of the following circuits will need to be altered. The TB303 components are marked up with a * next to them. Notice, also, the nine links in the ladder itself. Three links will need to be fitted if a Moog response is wanted. Six links need to be fitted if the TB303 filter is required. The TB ones, are again, marked with a *.

I have designed this filter as part of an output card for my new WalshBank synthesiser. If there is enough interest, I may well produce a PCB of this filter design. E-mail for details.

Tony Allgood - Oakley Sound Systems

Please visit The Oakley Sound Systems web site for the latest information!

Email the author: Tony Allgood