Module A-101-3 is a 12 stage phase shifter with vactrols as phase shifting elements. Vactrols are known for their smooth sound behaviour. For more general details about vactrols please look at the Vactrol Basics page.
In contrast to other phaser designs the A-101-3 is much more flexible and offering a lot of new features not available from other phasers on the market (as far as we know, please tell us if we are wrong). The main difference is that our design offers access to each of the 12 input and output stages leading to a lot of new filters that cannot be obtained in other ways. Especially the free patchable feedback loops (yes, not only one feedback loop is possible) between each of the 12 stages, the separate phase shift control for the stages 1-6 and 7-12, and the 2 polarizers intended to control the feedback loops lead to completely new filter types (a polarizer is a circuit that is able to generate positive and negative amplifications in the range -1…0…+1 with -1 = inversion, 0 = full attenuation, +1 = unchanged signal, for details concerning the polarizer function please look at the A-133 VC Polarizer or A-138c Polarizing Mixer module).
The module sketch and the frequency response curves below will help to explain the outstanding functions of the module:
Internally the module is made of 2 independent 6 stage phase shifters (1-6 reps. 7-12) with separate audio inputs (with attenuators), audio outputs (with mix control), and phase shift control units. The phase shift control units feature both manual and voltage controlled phase shifting (e.g. from a LFO, ADSR, Random Voltage, Theremin CV, Foot Controller CV …). For each sub-module a phase shift display (LED) is available. The LED shows the illumination state of the 6 vactrols of the sub-module in question as it is connected in series with the internal vactrol LEDs.
Each of the 12 phase shift states is equipped with an audio output socket and feedback input socket to obtain full flexibility to create a multitude of different filters. The audio input signal and the output signals of stage 6 resp. stage 12 are mixed with 2 manual controls to obtain effects at two audio outputs (for normal phase shifting effect this is 50% input signal and 50% phase shifted signal). The two submodules are internally connected via normalized sockets so that two 6 stage phase shifters can be obtained without external patches. Audio output of stage 6 is normalized to audio input of stage 7 and CV input 1-6 is normalized to CV input 7-12. But due to the open structure of the module even other stages than stage 6 and stage 12 can be used as outputs to generate different sounds (simply patch the desired stage output to the normalized mix input socket).
For a better understanding of the outstanding features a table with frequency response graphs is added at the end of this document.
The first 12 frequency response curves show the behaviour of the module when stages 1…12 are used as outputs for the final mixer (no feedback, no additional patching). This is the standard phaser application with a different number of phase shift stages. The frequency response curves of the higher stages show the typical comb filters of a phaser. The notches move through the audio spectrum as the manual phase shift control is operated or a control voltage is applied (for a standard phase shifter this is normally the triangle or sine wave from a LFO). The number of notches increases with the number of stages: number of notches = integer of number of stages/2. Odd stage numbers lead to different behaviours in the higher and lower frequencies (low end: high pass behaviour, high end: passage). Even stage numbers show the same response in the higher and lower frequencies (passage for both). Stage 1 is nothing but a high pass filter, stage 2 is the standard notch filter.
The second 12 frequency response curves show the behaviour when an inverter is inserted between the stage output in question and the final mixer (one of the polarizers can be used for this job). The result is the inverse frequency response compared to the output without inverter: e.g. low pass for stage 1, band pass for stage 2. The resulting frequency response curve is simply obtained by vertical mirroring the first 12 curves.
Additional feedback colors the sound. The third 12 frequency response curves show the behaviour of the filters with one feedback loop. Feedback comes from the stage used as output back to stage 1 (e.g. if stage 11 is used, feedback from stage 11 to stage 1).
But it is not imperative to use the same stage for feedback and audio output. Groups 4, 5 and 6 of frequency response curves show the behaviour with different feedback loops. In group 4 the same output 12 is used for all graphs but but the feedback goes from stage 12, 11, 10, 9 … and so on back to stage 1. Group 5 is nearly the same but output 6 is used for all graphs. In group 6 the output stage is the varying parameter and the feedback goes from stage 8 to 1 for all filters.
This is the result from all the response curves:
The number of notches is defined by the number of stages used as output (number of notches ~ stage number/2)
The number of resonance peaks is defined by the number of stages used for feedback (number of peaks ~ number of feedback stages used/2)
The height of the peaks is determined by the amount of resonance
Different numbers of notches and peaks are possible by using the corresponding patch for output in use and feedback loop !
Last but not least the open structure of the module allows multiple feedback loops (e.g. stage 8 to 3 and stage 6 to 1 simultaneously) and even “forward” loops (e.g. from stage 5 to stage 9). In combination with polarizers additionally the feedback or the output polarity can be normal or inverted. This leads to a multitude of possible filter types. Some examples for multiple and forward loops are shown in the last section of the response curves. Pay attention that for some examples e.g. varying the feedback leads to “moving” peaks. By means of VCAs (A-130, A-131, A-132) or the voltage controlled polarizer A-133 the feedbacks can be voltage controlled.
Doepfer A-101-6 Opto FET VCF/Phaser
A-101-6 is a new filter module that uses so-called opto FETs to control the filter frequency. Opto FETs are very similar to Vactrols but use light depending field effect transistors (FETs) instead of light depending resistors (LDRs). A opto FET is a combination of a light depending FET and a LED (light emitting diode) both put into a small light-proof case. The advantage compared to vactrols is the much faster response of opto FETs compared to LDRs. This allows much faster attack/decay times and even FM effects. The disadvantage compared to vactrols is that the FET behaves as a normal Ohm resistor only for small levels. With higher levels the FET begins to distort. Apart from this the notes mentioned on the Vactrol Basics page are valid.
Module A-101-6 is made of six serial 6dB filter stages. Each stage can work as lowpass, highpass or one of two allpass types. The following schematics show one of the six serial stages in the four possible modes:
The variable resistors shown in the schematics corresponds to the opto FET. The brightness of the Opto FET LEDs and consequently the filter frequency can be adjusted manually (Frequ. control) and controlled by means of an external control voltage (CV) with attenuator. The LED at the front panel reflects the LED brightness inside the opto FETs.
The type of filter is chosen by jumpers on the pc board (factory setting: low pass). The type of filter determined by the jumpers positions can be marked by means of a water-resistant felt pen at the front panel. The following document shows the jumper positions for the four different filter types: A-101_6_Jumper.pdf. The document describes also the adjustment of the trimming potentiometer for the max. resonance/feedback.
The resonance is controlled by the Feedback control up to self oscillation. By means of a trimming potentiometer the maximal feedback can be adjusted. High feedback values can be used mainly in the allpass mode to obtain very extreme self oscillation sounds. Even an external feedback signal can be used instead of the internal feedback connection (FB In socket). Whenever the filter type is changed by means of the jumpers the trimming potentiometer for the maximal feedback has to be re-adjusted (see A-101_6_Jumper.pdf last page for details)
The Mix control is used to pan between the original signal (CCW position) and the effect signal (CW position). In filter mode (LP/HP) this control is usually set fully CW. In the allpass modes one obtains phasing sounds at center position or “pure” allpass sound in fully CW position.
Doepfer A-106 X-Treme Filter
Module A-106-1 has it’s origin in our experiments to built a MS20 filter clone. The module is not exactly the same as the MS20 filter but similar. The famous original MS20 included two filters: a 12 dB lowpass and a 6dB high pass filter connected in series both with a very special design (the MS20 highpass if very often described as 12dB high pass, but this is not true). During our researches we found a way to use the same circuit simultaneously as lowpass and highpass for 2 different audio signals (a bit similar to the A-101-1 Steiner Vactrol filter that has even different audio inputs available, but with the special MS20 circuit). For this two separate audio inputs for lowpass (LP) and highpass (HP) with separate level controls are available. The sockets are normalled, i.e. the signal applied to the LP input is available for the HP input too provided that no plug is inserted into the HP input socket. The level control of the HP input is realized as a polarized input. This means that the signal can be added with the same polarity (+ range) or opposite polarity (- range) compared to the LP input. This feature enables notch (+) and bandpass (-) filter functions too. From our point of view this is the most flexible solution that enables e.g. these functions:
Lowpass: the audio signal is fed to the LP input, HP level control is set to zero, LP level control is set to the desired level
Highpass: the audio signal is fed to the LP or HP input, LP level control is set to zero, HP level control is set to the desired level (in this special case it does not matter if positive or negative amplification is chosen with the polarizer control)
Lowpass/highpass mix with one audio signal: the audio signal is fed to the LP input, LP and HP level controls are set to the desired levels.
special setting 1: if the level controls for LP and HP are set in a way that both levels are identical with the same polarity (i.e. + range of the HP level control) and no or little distortion only one obtains ~ a notch filter (the “~” indicates that the notch is far from beeing perfect, the attenuation in the passband is not as good as for other filters of the A-100 system, look at the frequency response curves at the bottom of this decoment for details)
special setting 2: if the level controls for LP and HP are set in a way that both levels are identical with the opposite polarity (i.e. – range of the HP level control) and no or little distortion only one obtains ~ a bandpass filter (the “~” indicates that even the bandpass is far from beeing perfect, there is a significant feedthrough of frequencies below and above the center frequency, look at the frequency response curves at the bottom of this decoment for details)
Remark for settings 1 and 2: The original MS20 circuit was not planned for notch or bandpass applications. The ~notch and ~bandpass filters should be treated as a free bonus and have the disadvantages mentioned above. The reason is that the lowpass has a 12dB/octave slope and the highpass 6dB/octave. This leads to phase relations that do not allow a “perfect” bandpass and notch simply by adding/subtracting signals as for other filter designs (for insiders: there remains always a 90 degree phase shift). For better notches and bandpasses other A-100 filters should be used – or two A-106-1 patched in series (bandpass) or parallel (notch) with suitable frequency settings.
Lowpass and highpass with two different audio signals: the two audio signals are fed to the LP input resp. HP input, the level controls for LP and HP are set to the desired levels. For the +/- control of the HP input it is essential in this case if the two input signals are phase correlated (e.g. two different outputs of the same VCO or VCO output and a frequency divided signal derived from this VCO) or there is no fixed phase correlation between the two signals (e.g. two different VCOs). In the first case the the – and + range of the HP control leads to different filter results. In the second case there is no difference if the + or – range of the HP control is used.
