The Korg Lambda in detail: Oscillator function
As has previously been sketched out, the Lambda uses TOGs (Top Octave Generators) to provide its basic waveforms. There are three of these in the Lambda, providing its three simultaneous, independently pitchable oscillator voices. There is an overall Tune control on the front panel, which governs the pitch of all three, and there are two individual Tune A and Tune B controls that allow a small positive or negative offset to the second and third oscillators. The oscillators sound simultaneously upon a keypress, permitting anything from a tight, almost phase-locked tone, to a drunken sprawl of a sound. Only the Ensemble presets make use of the 2nd and 3rd oscs. The Percussive presets use only the first.
Each TOG is clocked individually. Each clock is fed through to its TOG either as-is for Octave Up, or divided by two via a 4013 flip-flop for Octave Normal mode. The basic clock speed is around 2.5MHz. This means the TOG outputs straddle Octaves 8 & 9 by default in Up mode. The TOGs are employed in a way that differs from that suggested by the datasheet (the Lambda’s TOGs are S50241), giving not C-to-C but F-to-E. The 48-key keyboard is arranged in 4 octaves of F-E, so this makes sense.
The TOG clocks are generated by voltage controlled oscillators (VCOs). Each has its own VCO. These are fed by three summing op-amps, which are fed from a combination of sources: a common control voltage provided by a trimmer and the Tune control; individual detune controls on oscs 2 & 3; a common Pitch Bend control (the joystick); and Vibrato, a 3-stage low frequency oscillator (LFO) that provides each oscillator with a constantly varying control voltage at the same speed as, but out of phase with, each other. Hence, the three oscillators, when Vibrato is applied, will undulate at the same rate but will not exhibit the same pitch as each other at any given moment. This is a neat trick that enriches the sound. The Vibrato can be switched on or off and is passed to the summing amps via switching transistors.
One more thing to note about the oscillator circuit is that oscs 2 & 3 have a visual indication of detune in the form of a pair of pulsing LEDs on the front panel. Each TOG “F”-note output is fed through a pair of flip-flops to decrease the rate of pulsing, and the outputs of the three F/Fs are XOR-ed together; the LEDs brighten and darken more slowly the more close in pitch the oscs are. While not strictly necessary, because effective tuning can be accomplished easily by ear, it is helpful to realise that if any of the oscs goes down, the appropriate LEDs will freeze in one state. This can be a simple yet useful aid in troubleshooting.
Another handy page on the Lambda can be found here, including links to the schematics and user manual. There are errors on the above schematic, which I will detail in a later post.