Tag Archive | CV/Gate

Korg MS-04 Modulation Pedal

The Korg MS-04 accessory was contemporary with the MS-series analogue monosynths in the late ’70s and early ’80s. It looks like a standard volume pedal, and weighs in around 1kg owing to its sturdy metal construction. Essentially it’s a bender pedal that provides a variable voltage to control your synthesizer. It also provides an LFO with triangle and positive-only square waveforms, and a random output that is sampled at the LFO rate. The range of the LFO goes from around 1s/cycle at the slow end to 70ms/cycle at the fast end (around 1Hz to 14Hz). It adds a glissando feature, which puts the bender pedal through the sample-and-hold instead. It has two outputs, which can be switched on and off, and which can output either the LFO alone, the pedal voltage alone, or a mix of both. One output runs at ±1.2V peak, the other at about ±5V. An LED indicates the LFO rate. It is powered by two 9V PP3-style batteries.

Inside, there is a small PCB with a few dozen components on, and a whole lot of wiring connecting the panel-mounted pots, switches, and jacks. Some components are soldered directly to the panel parts, and there are multiple wires of the same colour that don’t always run to the same points, so trouble-shooting is slightly messy. Luckily, it’s a simple enough circuit, and the spaghetti wiring is the only thing that need cause any headaches here.

Inside the Korg MS-04 modulation pedal. So many wires!

Inside the Korg MS-04 modulation pedal. So many wires!

The only schematic I could find online was blurry and hard to read, and lacks component numbering. I did my best to clear it up, and added the missing information. Unfortunately, half way through, my software crashed and my only saved file was of lower quality than I would have liked – hence the soft text over about half the image. I’d already put plenty of time in by this point, so it’ll have to do!

My additions are self-explanatory, I think. Parts that are not numbered are soldered directly onto panel hardware. The only missing values are the diodes – D2 to D5 are plain old signal diodes, D1 is a zener that I failed to make note of during repair. Sorry.

There seems to have been a change in some component values during production. These are labelled with ‘1’ and ‘2’ in black squares. The unit I took my details from had the lower value resistors and larger capacitor at points ‘2’.

There is no protection against only one battery being installed. The unit is switched on by the insertion of a cable into either output jack, so it is advised to remove the connections before fitting/replacing the batteries.

Korg MS-04 schematic, enhanced with component designations

Korg MS-04 schematic, enhanced with component designations (JPG)

Here’s the Korg MS-04 schematic as PDF download.

And here’s a quick video of it working:

Synth DIY: Envelope Generators

An envelope’s basic purpose is to generate a changing control signal that ramps up and down between predetermined levels. Controls typically determine the rate of change of parts of the slope and/or the level a slope will reach. Some envelopes include controls for hold times for a certain stage of the envelope, or delay times before activation.

A more thorough explanation can be found in Sound On Sound’s Synth Secrets series, a very useful reference for many aspects of synth programming.

Various synthesizer envelope controls

Various synthesizer envelope controls

 

Even a cursory search shows there are so many envelope generator circuits to be found online that it might seem wasteful to present a few more. However, I wanted to collect a few of my own designs here to illustrate different kinds of envelope circuit, and to offer different ways of achieving them.

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Envelopes by type

AR (attack-release):

I will add more items to this list over time.

Moog Werkstatt: adding a proper Filter CV input

Note: I make reference to the Moog Werkstatt schematics throughout. Copyright prevents me reposting them here; they can be found on Moog’s website.

After the VCO Frequency CV and Gate inputs, perhaps the next most useful control we can modify is the VCF cutoff frequency. The Werkstatt already has switches to select either LFO or EG filter modulation in positive or negative amounts. Many synthesizers also have a Keyboard Tracking control which routes the CV generated by the pitch control source to the filter cutoff, allowing the filter to open up as higher notes are played. The amount of this modulation is often governed by a pot — giving continuous variable control — but is also often implemented with a switch — giving either preset amounts of modulation, or at its most basic just on/off (that is, 100% or nothing). At 100% Keyboard Tracking, a self-resonant filter can be used as a sine wave oscillator, the pitch of which will follow the keyboard.

Various filter tracking controls

Various filter tracking controls

 

The Werkstatt’s filter has a CV input on the header, which is fine for simple self-patching, but two problems show themselves when you want to control this parameter from an external source: firstly, the necessity of hacking a cable together as described previously; secondly, the accuracy of tracking. The Werkstatt’s existing filter CV input point does not, in my experience, give accurate 100% tracking from an external V/oct CV, which spoils sounds that require the resonance to boost harmonics that are locked to note pitch.

The mod below overcomes these problems by giving the Werkstatt a separate, tunable Filter CV Input that can be trimmed to give suitably accurate pitch tracking.

How it Works

As with the Pitch CV Input mod, we’re going to simply duplicate the existing control input and make a slight alteration. The existing header input for cutoff control mixes its CV via a 47.5k resistor. In order to be able to give tunable tracking, this mod is going to use a 43k resistor and a 10k variable trimmer in series. 100% tracking should be somewhere towards one end of the trimmer’s range.

Werkstatt filter CV mod schematic

Werkstatt filter CV mod schematic

 

Solder the two extra components to the board, take the third leg of the trimmer to TP17 (purple wire in the photos below), and take the outer leg of the resistor to the input jack, which is mounted and grounded exactly as for the CV and Gate jacks.

 

Werkstatt filter CV input mod PCB top

Werkstatt filter CV input mod PCB top

 

Werkstatt filter CV input mod PCB rear

Werkstatt filter CV input mod PCB rear

 

Werkstatt filter CV input mod rear wiring

Werkstatt filter CV input mod rear wiring

 

Werkstatt filter CV input mod jacks

Werkstatt filter CV input mod jacks

 

Fine Tuning

Tuning the tracking is similar to tuning the pitch (a process described in the manual) — with the Werkstatt open, connect the external CV and play as normal, using full resonance on the filter, with the cutoff tuned so you can hear the pitch of its self-oscillation. Adjust the trimmer so the filter’s resonant frequency scales up the keyboard at the same rate as the note pitch — that is, two notes played an octave apart should give a resonant filter peak an octave apart.

Because the Werkstatt’s VCO cannot be silenced without modification, it might be easier to disconnect the pitch CV control while tuning the filter; alternatively, if you have a way of multing the pitch CV, connect it to both pitch and filter. You might try setting the filter to resonate at an obvious harmonic such as a 5th above the pitch, as any deviation in the tracking will result in some noticeable sonic artefacts.

More Ideas

Of course, you might not want a simple fixed 100% tracking filter. It would be possible to add a pot to allow the user to vary the tracking amount; you could install a switch to select between different resistors to give preset fixed trackings; you could route the pitch CV to a break-contact on the filter CV input jack so that it tracks by default unless a jack in insterted to over-ride it. My own mod is simple and quick and functional, and hopefully will provide a point of departure for your own experimentation.

 

Werkstatt filter CV input mod in use

Werkstatt filter CV input mod in use

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Parts used:

43k 1/4W 1% MF resistor
10k trimmer
1/8″ panel mount jack socket

Moog Werkstatt: adding a proper Gate input

Note: I make reference to the Moog Werkstatt schematics throughout. Copyright prevents me reposting them here; they can be found on Moog’s website.

In its original form, the Werkstatt’s own keyboard generates the Gate signal to trigger the envelope, and there is no obvious ‘Gate Input’ on the header. The existing Gate Out can be (ab?)used as a Gate In, but it’s not ideal, because as with most of these header points, anything coming in here isn’t buffered from the internal signal.

Adding a proper Gate In to the Werkstatt is straightforward enough, though a little more involved than the CV input; my approach doesn’t require the cutting of any traces, the only hack-work being the hole in the enclosure for a jack socket. It does require the end of one wire to be soldered to rather small SMT (surface-mount) components though, so you’ll need a suitably fine tip for your iron and a steady hand.

Werkstatt gate mod schematic

Werkstatt Gate Input mod schematic

 

How it Works

The Werkstatt’s keyboard scanner outputs a logic high at U19 pin 3 when it detects a key press. As well as stopping the scan and loading the current key value into a latch (which feeds the VCO CV), this signal is buffered to provide a Gate, and differentiated to provide a Trigger. The Key On signal is buffered inversely by the Schmitt trigger of U14-F before being flipped back positive by U14-D. In order to add our external gate without affecting any other part of the keyboard circuit, we only need to bring the input of U14-D low. In this way, we can use both the Werkstatt’s own keyboard and an external Gate without having to switch between control sources.

The solution is to use a simple NPN in saturation to take U14 pin 9 to ground when its base is taken high. In other words, a positive external Gate will take the gate inverter input low, just as does the keyboard gate detector. Because there is a diode in the way (D14), our added transistor is isolated from the keyboard scanner clock and data-bus, so there won’t be any accidental mis-readings of the keyboard CV.

Another advantage of this solution is that the Werkstatt’s own envelope retains its Gate/Trigger operating modes, as our external Gate also gets differentiated; we are activating the Werkstatt’s envelope, not over-riding it.

The modification takes just four components and a socket, and fits easily on the PCB. The hardest part is soldering the wire from the collector of the transistor to the appropriate point on the Werkstatt’s circuit – I chose to solder it across the connection between R89 and C64, as the two solder points make a convenient place to lay a thin wire and give it a firmer purchase.

I presume you’ll be doing both CV and Gate input mods; the socket ground can be wired to the CV In socket ground, which I wired to a solder tag around the nearby PCB mounting screw (see also the CV Input page).

Werkstatt gate input mod smt solder point

Werkstatt Gate Input mod SMT solder point

 

Werkstatt gate mod extra components highlighted

Werkstatt Gate Input mod extra components highlighted (PCB top)

 

Werkstatt gate mod PCB rear highlighted

Werkstatt Gate Input mod extra components highlighted (PCB rear)

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Parts Used:

33k 1/4W 1% MF resistor
100k 1/4W 1% MF resistor
1N4148 signal diode
BC549C NPN transistor
1/8” panel mount socket
wire

These parts are what I had handy. Pretty much any NPN with reasonable gain can be used here, and the signal diode is a generic one.

Moog Werkstatt: adding a proper CV input

Note: I make reference to the Moog Werkstatt schematics throughout. Copyright prevents me reposting them here; they can be found on Moog’s website.

The existing header on the Werkstatt allows for a VCO pitch CV to be patched in. Although the pitch can already be modulated by either the LFO or the EG (selected using a panel switch), the patch header input means you can use both modulation sources simultaneously – or an external CV, if you can cable it up.

When you start wanting to connect control sources to the Werkstatt, one problem is pretty obvious: the patch pin header provides a signal path, but there’s no ground. The user manual suggests hacking cables together, taking a ground from the cable to a screw on the case (or the ground on the audio output jack), but this isn’t a very neat solution. Better to add a proper CV input jack so you can directly and simply hook up your external CV source using standard cables.

Moog themselves (at the time of writing) do sell an add-on jack board, which provides both a row of minijacks and a signal ground, but I decided against buying it for two reasons: 1) it still doesn’t offer a true Gate input, which I felt necessary; 2) the jack board replaces the patch pin header – adding mods like mine means you can use them and the patch pins simultaneously, giving more possibilities.

How it Works

The circuit is very simple. Looking at p.2 of the official schematic, we can see the existing header CV input is mixed in via a resistor R46 and trimmer VR5. This trimmer can be carefully adjusted to give a 1V/octave response for your external CV.

It would be super-easy to simply wire a jack to the CV point on the header, but this has the disadvantage that inputs are not isolated from each other. Better (and still easy) is to replicate the two passive components and route them to the same mix point.

Here are my additions to the circuit:

Werkstatt CV modification schematic

Werkstatt CV modification schematic

Here is the mod in situ:

Werkstatt CV input mod (top)

Werkstatt CV input mod (top)

Werkstatt CV input mod, rear

Werkstatt CV input mod (rear)

The handiest solder points for connecting the extra components to the existing circuit are TP14 and TP10. Either will do:

Werkstatt CV input mod routing

Werkstatt CV input mod routing

The jack is wired to be brought to the side panel beneath the header. In this photo the Gate mod jack is also in place. The jack grounds are wired together, and then to a solder tag that connects to the nearby screw post. The existing screw is long enough to accommodate a washer or two:

Werkstatt mod ground point

Werkstatt mod ground point

Drilling the hole in the case is simple and quick, and a label finishes the job:

Werkstatt with CV and Gate mods

Werkstatt with CV and Gate mods

The accompanying Gate Input mod is also detailed on this site.

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Parts Used

68k 1/4W 1% MF resistor
100k trimmer
1/8” panel mount socket
3mm solder tag
3mm washers (x2)
wire

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: 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.

Oscillator circuit from the Korg Lambda

Oscillator circuit from the Korg Lambda

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.

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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.

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