todo: add image
You can get the schematics PDF here.
In essence there are two independent lowpass filters in parallel, only sharing the resonance knob and their outputs mixed in ‘anti-phase’, or in other words their output signals are subtracted from each other, instead of added. In practice this means that if both filters get the same input signal the LP band set to the lowest cutoff is subtracted from the LP band with the highest cutoff and this creates a bandpass response that only passes what is ‘between the two cutoff frequency settings’.
The trick in the Twinpeak is how the input to both filters is mixed before they go into the two parallel filters. So, not only the outputs are mixed but there is mixing on the inputs going on as well. The A and B input connectors both have their separate input mixers. For input A its signal always goes into filter one and the curve A knob sets how much of the input signal goes into filter two. So, if the curve knob is closed the IN A signal only goes into filter one and in the final output you only hear the effect of filter one on the IN A input signal. And thus it has a lowpass response as filter two does not get the input signal. That means that there is also not the subtraction at work that create the bandpass, as filter two gives nothing to subtract. Then , by opening the IN A curve knob, the signal level of IN A into filter two is increased and now there is something coming out of filter two and the final output curve changes towards a bandpass curve. For IN B it works the same, but this input always goes into filter two and through the curve B knob to filter one.
Basically it means the following options:
- When both curve knobs are at ‘lowpass’ IN A goes into filter one only and IN B goes into filter two only. Now the Twinpeak works as two independent lowpass filters with their outputs mixed into one output connector. Because they are mixed in anti-phase the IN B signal part will be in antiphase on the output, while the IN A signal part is still in phase. Modulate each filter independently by the PEAK 1 MOD and the PEAK 2 MOD knobs only, as the CUTOFF MOD knob will modulate both filters in a linked way.
- When curve A is at lowpass and curve B is at bandpass you hear the lowpass part of IN A, set only by the cutoff peak 1 knob. And from the signal on IN B you will hear the band between the cutoff peak 1 and the cutoff peak 2 knobs .
- When curve A is at bandpass and curve B is at lowpass you hear the lowpass part of IN B, set only by the cutoff peak 1 knob. And from the signal on IN A you will hear the band between the cutoff peak 1 and the cutoff peak 2 knobs.
- When both curve knobs are at bandpass both IN A and IN B will be bandfiltered with the same band between the cutoff peak 1 and cutoff peak 2 knobs.
- And there are many settings in between, also when there are two totally different signals on IN A and IN B. It is best to judge the effects by ear.
Note that the 1V-OCT input will affect both paralllel filters equally, like the CUTOFF MOD knob does.
Note that the filter has a very good ‘ping’ characteristic at its maximum resonance. It lets itself also be modulated very well by audio rate signals. E.g. feeding a slow B PULSE from the TriLFO into e.g. IN A on the Twinpeak and feeding the A SINE from the TriLFO into the CUTOFF MOD on the Twinpeak will create a wide range of ‘metallic’ percussive sounds when the LF-VCO A RATE is tuned to audio rates.
And note that the PEAK 1 MOD and PEAK 2 MOD have there zero level at twelve o’clock and turning them to the left will ‘reverse’ the direction of e.g. an envelope sweep.