A type of voltage controlled filter, originally developed by EMS in order to work around a Moog patent. The diode ladder works on a similar principle to the Moog transistor ladder; the diode, under the influence of an applied control voltage, permits or limits current flow to a filter capacitor, such that as the control voltage is varied, the filter's cutoff frequency changes. A main difference between the transistor ladder and diode ladder is that in the transistor ladder circuit, each pole of the filter is electrically isolated or "buffered" from the others. In the diode ladder, this is not the case; all the poles interact electrically with each other. This makes the diode ladder much more difficult to analyze mathematically, and its distortion behavior as resonance is increased is more non-linear and harder to predict. Performers often say that the diode ladder filter has a "dirtier" sound, which is probably the effect of the less predictable distortion behavior.
Besides the EMS synth models, the diode ladder was used mainly by Roland in certain models. The famous TB-303 filter is a diode ladder.