The Korg Lambda in detail: the Chorus Phase circuit
So far in the Lambda’s signal path we have looked at the way the waves are generated, combined, filtered, and articulated to obtain the preset sounds. This covers the bulk of the Lambda’s tone generation, and what remains is a small amount of control circuitry, effects, and overall output.
After the preset sounds have been switched in or out and mixed together, the resultant two separate signals (Ensemble and Percussive) are passed to the output section on the KLM-184 board. Looking at the schematic we can see there is a send from each of these to the Chorus Phase circuit, as well as two returns from that circuit to the output mixer. We shall look first at the effect section, which here is called Chorus Phase, and is present on the KLM-214 board at the rear of the machine.
Chorus Phase is the Lambda’s name for what is most commonly known as simply a Chorus effect. The basic idea is to enrich the sound by adding into the original a slightly delayed copy, the delay of which can be varied in a constantly shifting manner to give a sense not only of expansion, but of movement. They typical delay time for a chorus effect hovers around a few milliseconds; longer times sound more like slap-back echo, shorter times result in a more phase-like or flanging tone. Typically a chorus uses no feedback, so there is only one non-repeating delayed copy of the input signal.
The Chorus Phase circuit is housed on board KLM-214. The schematic is below. The Ensemble and Percussive signals go through low-pass filters before being fed into IC4, which is the MN3010 BBD (Bucket Brigade Device) comprised of two separate 512-stage BBD lines.
Without going into too much detail of how a BBD works, it basically is a clocked chain of transistors and capacitors that are opened and closed by the clock and thereby pass the input signal down the chain like people passing along water in a bucket. The time taken to pass through the BBD is the overall delay time; this is dependant on the clock speed and the number of stages. Because of the nature of the technology, there is an optimum range if reasonable fidelity is required for any particular delay time, but 512 stages is adequate to be clocked at a rate that gives a pleasing chorus without too many noisy side-effects. Noise is a part of life with BBDs, and vintage synths are full of noisy chorus circuits, but these days this is often seen as part of their charm and not something to be ‘corrected’.
The MN3010 takes its clock from the oscillator based around IC1, its rate governed by the joystick control, and its output driving a transistor bistable that in turn clocks the two BBD lines of the MN3010 with out-of-phase pulses. The delayed signal is again filtered to remove the artefacts that are an inevitable part of a raw BBD output, especially at slower clock rates. Aside from the practical element, part of the pleasing character of BBD signal processing lies in this low-pass filtering, as it gives the copied signal a muted feel that sits behind the original and complements it well.
The two signals are then passed through FET switches, which are controlled by IC7’s flip-flops, in turn driven by debounced panel switches, and which also drive panel LEDs to indicate the on/off status of the Chorus Phase effect.
From here the signal returns to the output section, which will be discussed in the next post.
The KLM-214 schematic: