Yamaha CS Trigger Input Modification

Problem: Yamaha CS not triggering from an external Gate
Solution: small converter circuit

I had a Yamaha CS5 for some time, a neat little monophonic synth with one oscillator, one envelope, switchable HP/BP/LP filter, a simple LFO, white noise, and a single VCA. It has Control Voltage and Trigger input jacks round the back for interfacing with other devices.

The CS series uses a Hz/V (Hertz per Volt) CV, and the better modern MIDI-CV interfaces can handle this with no problem. The Trigger levels are comparatively awkward though, with ‘off’ being nominally +3 to +15V, and ‘on’ being nominally 0 to -10V. I say ‘nominally’, because the outputs of these CS synths are stated as +3V for off, -7V for on.

Why is this awkward? Well, there are two other common systems – Positive Gate (aka V-Trig), and Short to Ground (aka S-Trig), which I shall not discuss here – and whereas the other systems have been employed by several manufacturers, Yamaha was, and is, on its own with theirs. Though many CV interfaces are stated as being compatible with Yamaha CS synths, I have found this not to be reliably the case.

The problem comes when a Short to Ground signal will not trigger a Yamaha Gate. For whatever reason, some units just don’t provide a good enough trigger output to correctly pull down the inputs of some Yamaha CS triggers. I suspect a number of things, but won’t speculate here as I found an easy and practical solution.

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I owned both a CS5 and CS15, which use very similar, but not identical, trigger input circuits. My Kenton Pro-2 MIDI-CV interface would trigger the 15, but not the 5.

The Pro-2 is an older model, and has been long superceded by better units, but at the time I wanted to get the Kenton and the CS5 working correctly. My solution was to build a small buffer board and install it in the Kenton, adding a separate Trigger Out jack on the Kenton specifically designed for Yamaha’s system.

It works very simply. The Kenton provides a +15 Positive Gate by default. Its own subsequent conversion to S-Trig being insufficient, I added to the V-Trig output a single op-amp with a few resistors to provide both offset and scaling of the signal, transforming it into the ‘correct’ +3/-7V, and routed the new Trigger output to its own ‘CS-Trig’ jack socket. The schematic can be found below in both JPEG and PDF formats.

The circuit can be built onto a small piece of stripboard; I used a TL072 as it’s what I had to hand, but almost any op-amp will do. Mine was powered from the dual +15/-15 supply rails in the Kenton, but you could equally well install it within your CS synth if desired – just pay attention to where in the circuit you install it. Perhaps add a second jack for this input if you wish to leave the original in place (for example, if you wish to run your badly-triggering CS from another CS). Another option would be to install a switch to select the type of Gate input being used. That’s up to you; I present only the basic circuit that converts one gate to another.

NB: actual output values are 3.74V for ‘off’ and -6.45V for ‘on’, but they are within tolerance and much closer to Yamaha spec than the regular S-Trig.

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Schematic for a V-Trig to Yamaha CS-Trig converter

Schematic for a V-Trig to Yamaha CS-Trig converter

PDF version: CS Trig schematic

Here are a couple of photographs of the extra board in situ in the Kenton Pro-2. Note the angled PCB at the bottom left is Kenton’s own optional Hz/V CV board (from the factory the Pro-2 only provided V/Oct CV). My extra circuit is mounted on the small piece of stripboard at top left. It takes power from the Kenton’s 15V rails, and takes its trigger input from the Kenton’s V-Trig +15V Gate, and it outputs a near-Yamaha-spec +3/-7V off/on gate signal to a dedicated jack socket which I added myself. The unused half of the dual op-amp is not connected to anything other than 0V and itself, as indicated on the schematic. If you use a single or even quad op-amp in this circuit, re-arranging the pin-out is up to you.

V-Trig to CS-Trig convertor installed in Kenton Pro-2

V-Trig to CS-Trig convertor installed in Kenton Pro-2

V-Trig to CS-Trig convertor installed in Kenton Pro-2, detail

V-Trig to CS-Trig convertor installed in Kenton Pro-2, detail

 

Korg Lambda repair and modification

The Korg Lambdas I have encountered have each exhibited problems:

Fault: A very thin, almost silent, Chorus preset; all other sounds were fine.
Solution: A loose capacitor in the filter bank for that preset, which was a simple solder job.

Fault: A loss of Percussion presets and a thinning of the Ensemble presets in Normal Octave mode; in Up Octave mode everything was fine.
Solution: A faulty CMOS switch (IC1 on KLM-184, a 4066) which failed to pass the divided-by-two clock to the first TOG, thereby effectively muting the first oscillator in that mode.

One case proved more problematic, however, and I have documented the process for reference.

Two main jobs were carried out on this machine:

  1. Tidying up a previous bad repair job
  2. Troubleshooting a bad oscillator

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Tidying up a previous bad repair job

The main PCB in the base of the Lambda is home to the TOGs, divider/keyer circuits, and associated per-key envelope generators. The lines of diodes/resistors/caps that form the envelopes can be seen quite clearly stretching along the board, and the nine divider/keyer ICs can be seen poking up, mounted vertically on daughter-boards. Each of these smaller boards holds a single divider/keyer IC and some zero-Ohm jumpers.

Korg Lambda KLM-185 main board

Korg Lambda KLM-185 main board

In this example, two of these daughter-boards had been damaged – someone had gouged away the tracks on the PCBs and bridged the gaps with wire, but solder was everywhere and the PCBs were in less than ideal condition. Rather than simply tidy up the existing mess, I chose to replace one of those boards with a fresh PCB. The manufacture was carried out by Jim Harris, who kindly documented the process in the video below.

The results are very satisfactory. I always install replacement ICs in sockets for ease of future repair. Turned-pin sockets are preferable as I find they assist in seating the IC firmly, but here a flat-pin type is used as it was the only one to hand.

Korg Lambda KLM-182 front

Korg Lambda KLM-182 front

Korg Lambda KLM-182 rear

Korg Lambda KLM-182 rear

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Troubleshooting a Bad Oscillator

When I initially tested this particular Lambda, it was exhibiting thinner sounds than expected. It became quickly clear that one of the oscillators was not sounding. The beat-frequency indicator LED B was locked in one state, revealing the culprit to be the third oscillator (Miii, as per the schematics).

There are multiple possible causes for a dead oscillator in the Lambda: faulty clock (including its output buffer logic), faulty octave switching, faulty wiring and dry/cracked solder joints/tracks, faulty TOG, badly seated ICs, corroded sockets, dirty switches, or even failed capacitors pulling down the power rails. There are probably more, but these are some options that occurred to me during testing.

In fact, everything seemed OK except the TOG itself, so I ordered a replacement vintage part and installed it. Despite initial success (all three oscillators worked, hurrah!) the new-old TOG failed after a few minutes. A pattern emerged of it working for the first few minutes after switch-on from cold, but then failing – and the machine had to cool down again for the TOG to function once more. It occurred to me there could be another faulty component bringing this TOG down, but isolation of the TOG from the other parts and testing it again showed the TOG itself to be at fault.

These ICs have been obsolete for many years, and are expensive to replace, if they are to be found at all. Stocking up with a batch of S50241s, of unknown provenance and with unknown remaining lifespans, is not a viable option.

How not to build a Top Octave Generator

I initially resolved to build a replacement using CMOS logic to fulfill the same function as the TOG – effectively it is a logic device, taking a clock signal and dividing it by fixed amounts to give outputs such that the pulse trains on them represent the frequencies of an octave scale.

Although the frequencies outlined on the S50240 datasheet are specified to give a certain pitch for a certain frequency clock, the Lambda’s keyboard begins at F, not C. To minimise the circuitry used, the TOGs are clocked so the outputs are pitched to match the octaves on the keyboard; thus, the nominal C output becomes an actual F, etc. This requires a higher speed clock than the recommended 2.00024 MHz, somewhere closer to 2.5 MHz to raise the overall pitch by a few tones.

The need to divide by 3-digit numbers in the low hundreds, some of which have no common factors, suggested the 4040 CMOS counter. My initial sketch is presented here for reference only, and in no way represents a functional circuit:

Erroneous TOG replacement using 4040 CMOS

Erroneous TOG replacement using 4040 CMOS

The clock, buffered by a 40106, pushes the 4040 along on each positive edge. The 4040 is a 12-stage (divide-by-4096) ripple counter, which means it has enough stages to divide by several hundred, but suffers the disadvantage of long propagation delays – each stage only flip-flops after the previous one, so there are several consecutive toggles to go through before a stable and desired output is obtained. The principle of using a clock divider is that when the correct number of clock pulses has been received (as derived by AND-ing together the correct combination of output bits), the counter is reset and an output pulse sent. This triggers a flip-flop to give a 50% duty-cycle output at the correct frequency. In theory, the result is a pulse train with a frequency of clock/n where n is the divide-by for a given pitch.

Experiments were sadly not successful. To illustrate one example, the note A requires division of the clock by 358. The total propagation delay using the 4040 to get through enough cascaded toggles is 65+(30*7) = 275ns. As the clock period is approximately 200ns, it can be easily seen that this is too long. In practice, I found this particular example gave a note around E – the incoming clock pushes the 4040 along before that count’s toggles have settled on the correct combination, erroneously triggering the next division faster than the desired division can be completed; a partial set of toggles is carried out and spills over to the next count – the resulting output is therefore at a higher frequency than desired.

Although there are ways around this, pragmatism demanded a tighter solution.

How to replace an obsolete Top Octave Generator the easy way

Time constraints eventually led to the purchase of a custom-built TOG replacement from FlatKeys, which is a small SMD circuit housed in a compact enclosure, and connected to the original IC position by a ribbon cable. It came configured as a 50240, which is identical to the 50241 except for the output pulse width – not an issue here as the Lambda further divides the TOG outputs at the divider/keyers. The enclosure fits nicely down the side of the main PCB, and the ribbon plugs into the TOG’s socket. It works perfectly, and there is no obvious difference between the original and replacement tones. It responds to pitch bend and modulation as expected. Though it would have been nice to make an entirely DIY circuit myself, simplicity proved the greater benefit.

Korg Lambda FlatKeys TOG replacement

Korg Lambda FlatKeys TOG replacement

The Korg Lambda in Detail: Outputs, Expression, and more Envelopes

Output Section

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.

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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.

Percussive Control

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.

Ensemble Control

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.

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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.

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Lambda output schematic

KLM-183, 184, 186 – Lambda outputs, power supply and panel switches

For some added clarity, here is a block diagram of the Lambda’s structure:

Lambda block diagram

Lambda block diagram

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.

Photo of the Korg Lambda KLM-214 chorus phase PCB

Korg Lambda KLM-214 Chorus Phase PCB

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.

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The KLM-214 schematic:

Korg Lambda KLM-214 Chorus Phase schematic

Korg Lambda KLM-214 Chorus Phase schematic

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.

Korg Lambda Brass envelope single trigger

Korg Lambda Brass envelope single trigger

Korg Lambda Brass Envelope retrigger

Korg Lambda Brass Envelope retrigger

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:

Korg Lambda multiple keypress voltage

Korg Lambda multiple keypress voltage

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:

Signal at TP33 Trig Out on a single keypress

Signal at TP33 Trig Out on a single keypress

Korg Lambda TP33 Trig Out signal on multiple keypress

Korg Lambda TP33 Trig Out retrigger signal

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.

Korg Lambda Tremolo waveform at TP50

Korg Lambda Tremolo waveform at TP50

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.

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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.

Lambda divider-keyer schematic

Lambda schematic, divider-keyers

Lambda wave mixers, filter banks, tremolo and Ensemble envelopes

Lambda wave mixers, filter banks, tremolo and Ensemble envelopes

The Korg Lambda in detail: Layering and Filtering

In previous posts, I have introduced the Lambda, and described how it generates notes and articulates them across the keyboard. The next step is to explain how those notes get to sound like different instruments. This is done by layering and filtering.

I will split discussion of this page of the schematic into two halves. This half will deal with the basics of wave layering and the fixed filters; the second half will deal with the remainder of the sound shaping at this stage.

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LAYERING

So far in our study of the Lambda we have arrived at a point where each key gives a set of volume-articulated square waves at its respective 2′ to 16′ frequencies, and these notes are allocated to 12 signal lines: Oscs 1-3, each with 2′-16′ outputs.

Square waves, though they are the simplest kind of divide-down wave to obtain, are not very exciting to listen to, and greater sonic variety is required if we want our presets to sound suitably different and interesting. The two basic waveforms found on most analogue synthesizers are square and sawtooth:

sawtooth and square waves

sawtooth and square waves

Without going into detail here about their harmonic content (which information can be seen clearly illustrated here, a square tends to sound woody and a sawtooth buzzy. They complement each other well. If one begins with a sawtooth (as happens most often in VCO-type analogue mono- and polyphonic synths) it is a simple matter, electronically, to generate a square wave from it. Not many synths begin with a square and thence derive other waveforms. The Lambda is one; several, but not all, other divide-down models use this approach.

We have seen in the overview how layering square waves can produce an approximation to a sawtooth wave. Below is an illustration of the result of this process. Electronically speaking, this is achieved by summing op-amps (ICs 2 and 4-7 on the schematic) which use proportional resistor values to sum the incoming squares at particular ratios. For example, Mi (Osc 1) 16′ is fed to IC4 via a 100k resistor, and its 8′ is fed to the same via a 200k resistor. Thus the 8′ output of Osc 1 is present at half the level of its 16′ output. Its 4′ output is fed through via a 390k resistor, which is near enough to giving a quarter of the 16′ level, and so forth across the oscillators and their respective 2′ to 16′ lines.

The first photo shows the signal at Test Point 23 on the schematic. This is the summed pseudo-sawtooth described above. Note I have stretched the waveform to more clearly show the steps. As it is derived from four proportionally-summed square waves, there are 16 steps in total forming the pseudo-sawtooth, though not all are visible in the image:

image of the pseudo-sawtooth at test point 23

pseudo-sawtooth at Test Point 23

The next photo shows the signal at Test Point 35, again a summed pseudo-sawtooth from Mi, this time using only three of the four octave footings. Note how the reduction from four to three summed squares produces a much rougher, more notably stepped waveform:

an image of the stepped waveform at test point 35

stepped waveform at test point 35

If we follow the paths of all the summed square waves at this point in the Lambda, we can see that the purpose is to provide differently shaped waveforms, built from a combination of squares at different frequencies, that are suitable for use as the basic raw tones of particular preset sounds. These paths are visible on the schematic, and though they appear convoluted, it is possible to separate out the waves from which each preset should be derived and, in the event of a fault such as a TOG failure, to use this information to isolate the defective oscillator/s.

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FILTERING

Once the waves are summed, they are passed to fixed filter banks. Each filter bank represents one preset sound, except in the case of the Organ preset, which uses three parallel filters, and the Brass, which has its own voltage-controlled filter (VCF). More on that shortly. First, the fixed filters.

I will not here go into the technical details of how a filter works. Illustration can be found elsewhere on the matter, so I’ll simply state that the purpose of filters in analogue synthesizers tends to be to remove harmonics – start with a harmonically rich sound, smooth out harmonics below/above/around a certain frequency (the cutoff), and by the use of feedback boost at that frequency in a controlled way (resonance or Q). Harmonic content is one of the major determining factors in how a sound actually sounds, and this is one way to govern it that is easy to achieve in analogue electronics.

Fixed filters exist for the Strings II, Strings I, Chorus, and Organ presets in the Ensemble section. In the Percussive section, fixed filters exist for the Electric Piano, Clavi, Piano, and Harmonics. There is also a fixed filter for the Key-Click, a sound that is not switchable independently, but which is mixed with its own volume control into the Electric Piano sound. It is generated by filtering the key-trigger circuit output that provides a short pulse whenever a new key is pressed. The trigger circuit is used for other things too, more on which later.

The filters are broadly very similar across the set: they are a mixture of 2nd-order (ie. 2-pole, or 12dB) low- and high-pass active filters using RC networks and op-amps. Filter design and topology will not be elaborated upon here, but suffice to say the filter banks provide adequate tone-shaping to individuate the various sounds, despite them sharing their originating material.

Though the filters are fed from common sources in varying amounts, their outputs are switched on a per-preset basis and the combined switched-in signals are summed by another op-amp, one in the Ensemble section and one in the Percussive section. Thus, for example, the output of the Strings II filter bank has a simple open/closed switch to include or exclude it from the mix input of IC25, as do the Strings I and Chorus filter banks, while the Organ is derived via three parallel filter banks that are mixed passively using resistors before being collectively switched into the Ensemble mix. The same procedure is followed for the Percussive section. IC25 provides, on pin 1, the combined Percussive signal, and on pin 7, the combined Ensemble signal.

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ADDITIONAL INFORMATION:

WAVEFORMS BEFORE AND AFTER FILTERING

Considering the rather rough stepped waveforms illustrated above, one might think the Lambda’s raw waves would sound too brash; the sharp edges of the steps are manifest as high-frequency content, and especially in the lower registers can effect a kind of aliasing tone, where one hears a high-pitched background whine behind the desirable tonal content that is likely to be displeasing, even distracting. It should be noted, however, that this is not necessarily a problem, as the Lambda’s filters remove much of the undesirable content from these stepped waves.

Below is an illustration of  the signal at Test Point 26. I have isolated Miii so there is only one oscillator output present here. This is the input to the Strings II filter, played somewhere near the middle of the keyboard. Note the obvious stepping:

A photo of a scope reading of the signal at TP 26 showing stepped wave

Osc 3 signal going in to the Strings II filter at TP 26

The subsequent filter removes frequencies below and above a certain window; note in the next illustration how the steps have been smoothed as the high frequency content is reduced (to put it more simply, the sudden jumps are ‘slowed down’ to a more gentle slope):

Waveform at the output of Strings II filter

Waveform at the output of Strings II filter

Three points to note here: firstly, the smaller steps are smoothed to a kind of slightly wavy slope, so although the harshness is gone, there will still be a slightly discernible brightness to the audio signal present as a quiet multiple of the fundamental of the note played; secondly, the falling edge of the sawtooth is also slowed to a slight slope, which is here the leading edge, and which leads to point three; that the waveform is inverted from the original, flipped upside down by the activity of the op-amp buffer IC25. This reading was taken at TP 48. Inversion has no effect on the audible content.

Due to the fixed cutoff points of these filters, low notes will exhibit more artefacts than high ones. With the user-adjustable filters wide open and the lowest notes played, some of these, though reduced, are still present; whether this is a problem depends on how one likes to use the Lambda.

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Here is the page of the service notes that both this and the next post deal with:

Lambda wave mixers, filter banks, tremolo and Ensemble envelopes

Lambda wave mixers, filter banks, tremolo and extra Ensemble envelopes

In the next post I will discuss the remainder of the KLM-186 board as depicted on this page of the schematic.

The Korg Lambda in detail: Dividing and Keying

In my previous two posts I have given a brief overview of the Lambda’s architecture, and a more detailed explanation of its method of generating top octave pulse waves. What happens next?

Our top octave needs to be turned into a note for each key, and one that only sounds when that key is played. This is a multi-stage process consisting of dividing, keying and mixing.

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Dividing

The first step is to divide down those 12 frequencies. Put simply, the constantly-running top octave is transformed into a set of keyed (that is, keypress-activated) outputs, each output here containing all the played notes of one octave.

The top octave waves are fed into a set of Divider/Keyer ICs. The IC used is the S10430. This, in a nutshell, takes these top octave waves and divides them down to cover the full keyboard. It also takes an input from each key on the keyboard that governs the level of the output of its own note. It then mixes the signals together and has outputs for each of the 2′, 4′, 8′, and 16′ pitches.

Study of the S10430 datasheet shows that the Lambda uses most but not all of the S10430’s capability: it can handle six frequencies per IC, whereas the Lambda employs it to handle four. There are nine S10430s in the Lambda – three for each oscillator, each handling four notes of the 12 from that oscillator’s TOG.

As has been mentioned in my Lambda overview, divide-down technology is a basic way of obtaining a full keyboard of notes. Once a waveform has been generated for each of the 12 notes of a scale, it is a simple matter to electronically halve that frequency to generate the same note one octave below, and then to halve that, and then halve that, and so forth. Various methods can be employed (for example, flip-flops or counters), but the result remains the same: after dividing down, there exists a square wave of the appropriate frequency for each note on the keyboard. In the Lambda, this operation happens within the S10430 divider/keyer IC. The S10430’s N-inputs take the TOG outputs, four to each module.

Below is a rough sketch of what a divided pulse wave looks like. Apologies for the roughness, I have no mastery of drawing software…

divided waves

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Keying

Left at only this, we would be faced with a constant blare of square waves at every pitch. What is needed is a way to pass those waves into the audio chain only when the appropriate keys on the keyboard are played. This is where the S10430’s keyer circuits come in.

The S10430 has a number of K-inputs. Each of these inputs takes a varying voltage from one key on the keyboard. The voltage present on that input governs the signal level of that frequency that is passed through to the outputs. Thus, if a hard on/off sound is required, the K-inputs should take a straightforward low/high voltage; if a steadily varying volume of sound is required (such as the slow fade-in and fade-out of a string ensemble) then the signal present on the K-input should be a slowly moving voltage. The S10430 requires a voltage that is high for ‘off’ and low for ‘on’, or anywhere between.

The Lambda sports several controls for its individual note articulation: the Percussive section has a fixed attack but variable decay, as well as a Sustain switch that when active employs the Decay control as a release also (by default the Percussive envelope has an abrupt release); the Ensemble section uses a preset attack and release as standard but also has a Variable switch that activates a pair of user-adjustable Attack and Release controls.

These envelopes are provided on a per-key basis by discrete circuitry. For every key on the keyboard, the Lambda contains a compact but effective set of diodes, resistors and capacitors that, when the associated key-switch is activated, charges and discharges a capacitor according to the attack/decay/release control voltages present. Each key’s envelope voltage is then fed to the K-input for that key to govern its note’s audio volume.

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Mixing

The S10430 has outputs for four octaves: 2′, 4′, 8′, and 16′. This is standard nomenclature. At each of these outputs is present the sum of all the played notes handled by that particular S10430 in that octave and for its designated oscillator. For example, Module 1 (as designated on the schematic) provides C#, D, D#, and E for Oscillator 1. These outputs, on a per-oscillator basis, are mixed with summing op-amps to give buffered 2′, 4′, 8′, and 16′ signals; thus, at this point in the circuit, the Lambda provides Oscillator 1 at 2′-16′, Osc 2 at 2′-16′ and Osc 3 at 2′-16′ on dedicated audio paths.

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It can be seen that in this way, the Lambda gives individual articulation to each note played, and that there is plenty of circuitry on the boards inside…

Lambda divider-keyer board photo

The Korg Lambda divider-keyer board

There are several points to note about the Lambda’s divder/keyer arrangement:

1) The three oscs each use three divider/keyer ICs. Each S10430 handles four TOG notes. Modules 1-3 provide the total output for Osc 1, Modules 4-6 give Osc 2, and Modules 7-9 Osc 3.

2) The envelopes work differently depending on the switching of preset sounds. If no Percussive sounds are selected, the Ensemble sounds utilise the full range of oscillators as governed by their keying and mixing. However, if both Ensemble and Percussive sounds are selected, the Ensemble sounds lose the first oscillator from their mix – the first oscillator is given over to the Percussive sounds. The exception to this is the Brass preset, in which Osc 1 is retained while a Percussive sound is selected, but using the Percussive envelope instead of the Ensemble envelope (the other Brass oscillator retains the Ensemble envelope). A good way to hear this is to wildly detune the oscillators and minimise the Percussive volume while playing simultaneous Percussive and Ensemble sounds. Note also that some of the circuitry for this is employed after the mixing stage and will therefore be covered in another blog post.

3) The outputs at this point are still only square waves. However…

4) … each keypress generates not just one square wave but four. If, say, C3 is played, the Keyer/Divider IC in each oscillator that deals with the C notes will output a square wave pitched to be 16′ at C3. This will appear on the appropriate output for each oscillator. But there will also be an 8′ C3, a 4′ C3, and a 2′ C3. Why? Because, as we shall see in the post dedicated to wave layering and filtering, one square wave is not enough to make a large array of varied sounds. These extra octaved squares provide the potential for extra harmonic content, much as the drawbars on old organs combine simple waves at different frequencies to give a broad range of tonal colour.

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The above detail refers to the following page of the Lambda schematics:

Lambda divider-keyer schematic

Lambda schematic, divider-keyers

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