A-170 Dual Slew Limiter

The A-170 Dual Slew Limiter smooths control voltages – abrupt changes are “smeared” in the process.

Switched before the frequency control of an oscillator, this function causes the well-known “portamento”, which transforms a jump on the keyboard into a slurred note.

The two sub-modules differ, the upper slew limiter only allows a simple adjustment of the rise / fall time and is suitable for controlling VCOs. The lower one offers more complex options, but is not suitable for precise control of VCOs (a slight voltage drop due to the special circuitry leads to somewhat inaccurate tones).

User interface







Possible uses

Amplitude modulation

Doepfer himself suggests a very interesting patch that uses a second VCO to amplitude modulate the first. When shifting the two VCO frequencies (only one VCO is controlled via the slew limiter) you basically get a waveshaper:

The slew limiter ensures that one of the two VCOs reacts to pitch modulation with a delay. During this time, the tonal result of the amplitude modulation changes!


Structured similarly, but with a completely different sound: hardsync between the two VCOs, again only one VCO is controlled by a slew limiter:

Here, too, the slew limiter delays the reaction of one of the two VCOs to the pitch modulation.

Several slew limiters, set differently

You get a “bombastic” sound if you control several VCOs via several A-170s with slightly different settings:

Two A-110-1 VCOs are controlled by two different A-170 slew limiters – with different settings of the slew limiters, the sound “drifts” apart, only to find a common tone again in the end.

AR generator

The A-170 Slew Limiter creates an AR envelope here (periodic because controlled by the LFO).

If you don’t have enough envelope generators, you can use the gate signal (e.g. from a keyboard, a sequencer or an LFO) and the lower sub-module to create a simple but usable attack-release envelope.

Buffer amplifier for older series

Older modules before May 2009 should also switch a buffer amplifier (A-180-3, A-180-4, A-185-1 or A-185-2) between the A-170 and VCO for the upper slew limiter. The newer series (printed “Version 2” on the circuit board) have an integrated buffer amplifier.

Sound examples

Amplitude modulation as described above, the modulating VCO gets its own VCA. Both VCAs are modulated by the same A-140 ADSR. This means that there are no continuous tones when the “CV2” control in the A-131 is opened wide. The sequence is controlled by an A-155, the intensity of the amplitude modulation is controlled manually (starts at around 20s), as is the delay time of the slew limiter a little later:

Hardsync, also almost as described above, I use an A-111-1 as hardsync slave, again an A-155 with an A-140 controls the sequence. The delay time of the A-170 is manually increased and decreased again:

Technical specifications

Width8 HP
Depth40 mm
Power requirements20 mA (+12V) / -20 mA (-12V)