The Korg Lambda in detail: Brass, Tremolo, and Articulation
LAYERING AND FILTERING: Part 2 – Brass, Tremolo, and articulation
There are a number of other sections of this page of the schematic that require our attention. These are the Brass sound shaping circuit, the Tremolo circuit, and the Ensemble Mi (the schematic designation for the summed output of oscillator 1) envelope circuit.
THE BRASS FILTER
The Brass filter is a little more complex than the others in that it is not tuned to a fixed frequency but is voltage controlled and has its own envelope generator: when a key is pressed, the envelope sweeps the (12dB resonant low-pass) filter cutoff frequency up and then back down in a predetermined way. This happens every time a new key is pressed across the sum of the audio signal for the Brass preset, the upshot of which is that if a number of keys are pressed at the same time, they will articulate simultaneously; if some keys are held and others added, the new notes will be articulated as expected, but the existing notes will be re-articulated also. This sounds more limiting in description than it is in practice. The envelope can be observed at Test Point 45.
The trigger for the filter is given at every new keypress. This is generated by a dual comparator formed around IC3 (see the schematic, below): when the input voltage (from the keyboard switches) changes, the comparator generates an output pulse. The keyboard trigger line provides a voltage that increases in (negative) magnitude according to the number of keys pressed. In the following image, you can see the result at Test Point 24 of holding a number of keys, releasing them one by one, then pressing more keys again one by one:
One half of the comparator is fed directly, the other through a low-pass filter formed of C3 and R23, which delays the duplicate signal to the other half; the resultant flip-flopping output is fed to logic gates and then to both the Brass filter envelope trigger, and the transistor buffered output of the Trigger Out jack. The signal at the jack can be observed to be normally positive (around 5V) but when a key is pressed the trigger goes low (around 0V) for the duration of its holding. If, during a held note, another key is pressed, the signal goes positive briefly (for around 10ms) before returning low. In this way every keypress, solo or in combination, results in movement of the trigger signal. Test Point 33 shows the Trig Out in action:
Within the Brass circuits, the trigger pulse is directed through a simple network of diodes, resistors and capacitors to produce a fixed-time attack and decay envelope. This passes through a transistor pair via a resistor to the current control inputs of IC13, an LM13600 dual OTA (operational transconductance amplifier). Without going into detail about what that is, its purpose here is to act as a kind of voltage-controlled resistor, in the place of a fixed resistor in a standard fixed-frequency low-pass filter. The two stages make this a 12dB LP. The filter is given its audio input at pin 13, consisting of two pseudo-sawtooth waves at 16′ from Mi and Mii. The cutoff is varied by the signal present on the current control pins, resonance is fixed by IC20 in the feedback path, and the audio output at pin 8 is then switched into the Ensemble mix as with the other presets of that section.
The cutoff frequency is also governed by the front panel control Brass fC which is simply a potentiometer providing a variable voltage, and by the signal present at the Expression input jack, which takes a variable input from 0-5V and can be directed to various control points. Here, the Expression voltage, if selected to control the Ensemble section, is mixed with the Brass fC voltage and the envelope before the transistor pair Q3, which in turn drives the current control inputs of IC13 via a resistor.
THE TREMOLO CIRCUIT
Tremolo, a slow undulation of volume, is an effect that is applied in the Lambda to the Percussive section mix. The output of the Percussive summing op-amp (IC25, pin 1) is directed to both the Tremolo switch and the input of the Tremolo circuit. The switch selects either the straight output or the Tremolo output, and routes it to the next section of the Lambda.
The Tremolo is accomplished by using an LFO (low frequency oscillator) to drive a VCA (voltage controlled amplifier), which passes the Percussive signal. The output is buffered and amplified by two transistors and associated components before feeding the switch. The LFO is comprised of IC11, Q17 and their associated passive components. The depth of the effect is fixed, but the speed may be varied by use of the Tremolo Speed potentiometer. The VCA is a two-transistor form of a type common in Korg synthesizers of the time. Either one or two transistors may be found in this design, with the control being sent to the base, and the audio given to the collector, with the output at the emitter. The input signal is kept low to avoid distortion, and there is appropriate post-VCA amplification/buffering. This type of VCA occurs in the MS series of monosynths, and the PS series of polysynths, among others. It also appears in Roland’s JX-3P, which is unusual as Roland tended to use OTAs in their VCA designs.
Mi ENVELOPE SHAPING FOR THE ENSEMBLE SECTION
As has been previously mentioned in the Dividing and Keying section of this series on the Lambda, the shared use of source signal for all the presets leads to a problem when different articulation is required: how are the resultant sounds to be differentiated, given that they are comprised of the same raw waves? This is only partly answered by filtering. The other part of the solution is to cleverly manipulate the volume envelopes.
If no Percussive preset is selected, the Ensemble presets (Brass excluded, as it uses its own envelope and filter system) use all three of the oscillators. We already know that the Percussive section uses only the first oscillator, Mi. When any of the Percussive presets are selected, Mi is given over entirely to the Percussive articulation and is removed from the Ensemble sounds. This allows clarity of articulation for the Percussive section, though the Ensemble sounds are thinned slightly as a result.
This is accomplished by the Mi Off signal line as seen on the schematic, activated by the Percussive switches. As seen on the divider-keyer schematic, the primary articulation envelopes for Mi are Percussive, those for Mii and Miii Ensemble; but switching off the Percussive presets brings into play the extra envelopes on the KLM-186 board shown below, effectively replacing the default envelopes. If only Ensemble sounds are used, sets of VCAs are activated by the Attack Control line (Q 7-10) and Trig line (Q19-24). The former pairs of transistors, coupled with their associated diode/resistor/capacitor arrangement, provide variable length attack and decay parameters as for the Mii and Miii envelopes on KLM-185; the latter transistor pairs, along with their passive components, provide a secondary trigger-pulse articulation to Mi during multiple keypresses. It can be seen that if Percussive switches are activated, the Mi Off line is high, Q4-6 are turned on, and the VCAs are held closed, thus allowing the default envelopes of KLM-185 to be used instead. In this way, either a full Ensemble tone can be heard in isolation, or a slightly thinner Ensemble tone accompanied by a clearly and separately articulated Percussive section can be heard.
We have seen these pages of the schematic before, but for convenience I have posted them here again, as they are referred to in the above text.