The Korg Lambda in Detail: Outputs, Expression, and more Envelopes
As previously described, once at the Lambda’s output section, its two sonic ‘halves’, Percussive and Ensemble, are separately sent to two Chorus Phase lines and returned to the output mixer. The schematic below shows this section, which is on the KLM-184 PCB.
It is here we find the Tone controls for each section, formed of a simple potentiometer/capacitor network. The Chorus Phase sends are post-Tone control.
The four signal lines — Percussive, chorused Percussive, Ensemble, and chorused Ensemble — are routed through four VCAs (voltage controlled amplifiers) which here are the four halves of two AN829 attenuator ICs. The control for these VCAs comes via IC14, a dual op-amp, one half of which governs the volume of the two Percussive lines, the other the two Ensemble lines. IC14’s Ensemble control output is also routed to the Brass VCF cutoff as described earlier, mixed with the Brass fC control potentiometer. The input to IC14 is simply a potential divider and a switched input jack: on the rear of the Lambda the Expression Input takes a 0-5V signal that can be routed via its accompanying switch to govern either Ensemble, Percussive, or both.
The outputs of these VCAs are routed through two dual volume potentiometers and thence two mixer/buffers dedicated to opposite sides of the stereo field, such that the chorused signals appear in the opposite side to the dry signal. That is, half of IC17 (the output mixer dual op-amp) takes the dry Ensemble signal and the chorused Percussive, while the other half of IC17 takes the dry Percussive and chorused Ensemble. This means that when Chorus Phase is engaged for either of the sections, that section will be spread across the stereo field, but when used without Chorus Phase, it will be present on only one side.
Stereo Output 1 and Stereo Output 2 provide the two final audio outputs, Mix Out provides a simple passively-summed blend of the two, and the Headphones output is a low-impedance output taken from the same IC17, left and right fed as per the two Stereo Outputs.
This page of the schematic also covers some control functions not previously explained, basically consisting of the panel switches for selecting the presets and related user-variable parameters such as envelope controls. These are located on the KLM-186 PCB.
The preset selection switches are 1-pole, 2-way. When engaged, a positive voltage is applied to the Mi Off, Vibrato Off and Per. Envelope lines. The operation of the Mi oscillator has been discussed previously, in that engaging a Percussive preset disengages the Mi oscillator from the Ensemble sounds to free it up for Percussive use. The Percussive Envelope is engaged with its variable Decay setting, and Vibrato is removed from Mi.
The Sustain input jack and panel switch, when activated, turn off Q13. Q12 is turned on by the presence of an active Percussive preset switch, switching in turn Q11, and the resulting Sustain on/off control voltage (which, as described earlier, changes the release of the Percussive sound between abrupt and the manually-adjustable Decay setting) is applied to the Percussive Envelope circuit on KLM-185.
Lastly in this small section, Octave mode is selected, which switch selects the straight or divided clock signal for the TOGs.
Aside from switching in the presets, the selection switches function to govern the attack and release of the Ensemble envelope, as well as Ensemble Vibrato function.
Firstly, Vibrato: when either Strings preset or Chorus is switched in, the control signal is sent to the oscillator board, KLM-184, which switches the appropriate FETs in or out to add or remove vibrato to all three oscillators (recall also the LFO feeds are out of phase). Note that using Brass and/or Organ alone will have no effect on the Vibrato control signal – that is, Vibrato is only applied when switched on in conjunction with Chorus and/or Strings I/II.
The Ensemble amplitude envelope can be preset, or the user may independently vary its Attack and Release times. The switches select either potentiometers or potential-divers which provide voltages accordingly. The Attack Control (which goes to the Mi envelope control section of KLM-186 as described here) and Release (which goes to the divider/keyer envelopes for Mii and Miii) are directly connected to their destinations; Attack is buffered and level shifted before being applied to the divider/keyer envelopes for Mii and Miii.
Here we can see some operational quirks. Brass has its own envelope as standard, as described here; Organ employs the fixed quick-on, quick-off envelope too, by default; note that by default the base of Q27 is held high, applying a positive voltage to the envelope control lines via D19 and D20. However, when Chorus and/or Strings I/II are switched in, Q27 is switched off by the activation of Q26, hence removing that voltage from the envelope control lines – which are now governed solely by the Attack/Release preset/manual settings. The upshot of this is that a high control voltage gives a shorter time, and thus when only Brass/Organ are selected in the Ensemble section, the preset envelope is applied to all oscillators. However, when either Brass/Organ and Chorus/Strings are selected, it is possible to apply longer attack/decay times but they only apply to Mii and Miii. When no Brass/Organ is present, all oscillators are given full articulation.
To further complicate matters, as soon as any Percussive presets are selected, articulation of Mi is governed solely by their envelope. It is a cleverly designed system and makes sense when followed through, but rather convoluted. I hope my explanation makes sense!
And the rest…
The remainder of the schematic shows a basic power supply taking a mains AC input, diode rectifying it into two rails plus ground, and using fixed regulators to give +/- 15V, with a further +10V output obtained from the positive regulated rail. A power-indication LED and some supply smoothing capacitors completes the description.
I hope this detail of the technlogy behind the Korg Lambda is both complete enough to be useful for those who wish to repair one themselves, and to be comprehensible to those who wish to learn about the kind of electronics that went into these old synthesizers. The Lambda is a fine example, both physically and sonically, of its time and type; this kind of unit went out of fashion shortly afterwards, and manufacturing costs mean nothing like this will be mass-produced again – one of the attractions of vintage synthesizers, as many enthusiasts may attest.
Thank you for reading, and please let me know if I have made any errors so I may correct them.
For some added clarity, here is a block diagram of the Lambda’s structure: