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

*

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)

*

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 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 header connections provide a signal path, but there’s no ground. The user manual suggests cobbling interconnects together from hacked cables, taking a ground from the cable to a screw on the case (or the ground on the audio 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.

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 the problem still remains of the lack of buffering – the inputs and outputs are taken straight to the signal point, which may cause issues when mixing signals and/or there is a resultant loading of a signal in either direction. The mods here avoid these troubles.

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.

*

Parts Used

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

On the ‘Tube

So, I decided to set up a YouTube channel for Synthnerd: Here it is!

It’s a bit sparse right now, but I’ll be uploading all video material of my own creation there, and linking to any other videos I blog that have been made by others.

Cheers!

Roland Juno 6: the DCO

VCO vs. DCO: a non-debate

Much discussion can be found online of the differences between what are simply often labelled VCOs or DCOs (Voltage/Digitally Controlled Oscillators) but as with many things, the truth is less simple. Often the argument boils down to things like phase and stability. I do not want here to get into any kind of debate or pointless pontification about whether one oscillator is better than another, or to try to delineate boundaries between oscillator types in such a black-and-white way.

The reason, in this instance at least, is partly that the Juno 6 oscillators contain features that straddle what some may see as stricly analogue and strictly digitally-controlled. I offer below an explanation, as best I can give it, of the functioning of the Juno 6 oscillators.

The job of key assignment and voice allocation is something I am not going to cover here. The Juno 6 has a CPU that scans the keyboard and generates data, and this process is beyond the scope of my analysis. I pick up the process at the point at which note data is output from the CPU.

 

How does the Juno’s oscillator work?

Many (if not most) analogue synthesizer VCOs employ a ramp generator core; that is, a voltage representing pitch is fed (via a convertor that scales the control signal correctly) to a circuit that charges a capacitor, the charge across it generating a ramp. When the ramp reaches a pre-defined level, a comparator in the circuit quickly triggers the discharge of the capacitor, at which point the ramp starts its cycle again. The result is what is often called a sawtooth waveform. In the analogue domain, the stability of this process is subject to factors like temperature fluctuations. Digital control helps combat instability.

Though the Juno 6’s oscillator core is based on this analogue capacitor-charging approach, the ramp reset pulse relies not on an analogue comparator but a digital counter. Effectively, the CPU is programmed with the relative frequencies of each note, and provides a counter (one for each voice) with a value representing how long it should wait before resetting the ramp for any given pitch. The CPU also sends note data (via a DAC) as an analogue voltage to the ramp generator control input. Perhaps counterintuitively, this is not to determine its pitch – after all, pitch is frequency, and the frequency is determined by the resetting of the ramp – instead, it is to maintain an even ramp waveform across all frequencies.

Imagine a low frequency. The clock waits longer to reset the ramp. If the voltage fed to the ramp generator were constant for all frequencies, it would ramp at the same rate no matter which note was played. At low frequencies the ramp would reach its limit and stay there before being reset, and at high frequencies, the counter would reset much sooner, cutting the ramp off before it had achieved optimum level. In this scenario, the low pitches would be severely distorted, and the high pitches would be extremely quiet. Thus, in accordance with the frequency data provided by the CPU, an analogue voltage is generated which will charge the capacitor in the analogue oscillator core at a rate optimised for that pitch, while the resetting process (and therefore the oscillator frequency) is controlled digitally. The result is more stable than a purely analogue circuit.

The matter is slightly more involved, however. Firstly, the counters that provide the reset pulse have to know how fast to count. They are clocked not by the CPU, but by an analogue voltage-controlled clock, which is governed in turn by a sum of the pitch bend, fine tune, and LFO voltages as collected on the bender board. This ensures there is no stepping in the pitch during modulation – as the master clock is in the analogue domain, it is thereby continuously variable. The sum of analogue modulation voltages is also mixed with the CPU-generated ramp feed voltage, so that there is no distortion of the ramp during modulation.

To summarise, the basic process is as follows: note data is sent as a binary number from the CPU to a high-speed counter. An analogue voltage is also generated by the CPU via a DAC, and fed to an analogue ramp generator. When the counter completes its cycle, it resets the ramp circuit capacitor, and charging begins again. Thus, an analogue ramp – the sawtooth wave – is frequency-controlled by a digital device.

 

Schematic

It may help to study the schematic for the oscillator core and its logic control. Below is the appropriate page of the service manual. The complete manual contains further technical information.

The counters, DAC, exponential converter, and ramp generator are highlighted in red.

Juno 6 main board schematic with highlights

Juno 6 main board schematic

PDF version: Juno 6 main board

 

Moog Rogue: adding a Filter CV input

The Rogue in its original form includes keyboard CV and Gate inputs and outputs. It’s a slightly odd arrangement of TRS (stereo) jacks – one for the CV, with the input on one terminal and the output on another, plus signal ground, while the Gate connector has an even more awkward arrangement of short-to-ground on one terminal and positive trigger on another, either of which can act as input or output. It’s not great. But it does work, providing you have the right cables.

Rather than split these into separate jacks, which would be a handy mod, I just use custom made cables for my interconnects between CV devices. However, the Rogue lacks a CV input for the filter, which I thought might make a useful addition. It’s a simple enough job.

I decided to use a 3.5mm minijack for the sake of convenience. Pick a spot on the rear panel and drill a suitable hole:

photo showing a newly drilled hole for the Moog Rogue filter CV input modification

Freshly drilled hole for the Filter CV input jack

 

photo showing a newly drilled hole for the Moog Rogue filter CV input modification (inner)

Inner shot of the Filter CV input jack hole

I had a switched 3.5mm mono socket to hand, so used that:

photo showing the Moog Rogue Filter CV input jack (inner)

New Filter CV input jack (inner)

 

photo showing the Moog Rogue Filter CV input jack (outer)

The new Filter CV input jack from outside

Solder a ground wire from the appropriate jack terminal to the Rogue’s jack PCB. Its large solder plane is suitable and easy to work with:

photo showing the ground wire of the new Moog Rogue Filter CV input jack

Grounding the Filter CV input jack

The filter CV input requires a 45k3 resistor to give the same scaling as the keyboard. As presented here, there is no onboard scaling, so if you want to run the input as anything other than 100% follow, use an external amplifier/attentuator. It would also be possible to build such a circuit into the Rogue, but I chose not to for simplicity.

photo showing a resistor soldered to the Moog Rogue Filter CV input jack

Solder a 45k3 resistor to the Filter CV input jack

The other end of that blue wire goes to the filter CV node on the main PCB. Rather than outline it on the schematic, I’ll just show you where that is on the board. This is the point where various filter CV inputs are summed. It’s an easy job to solder a wire at the top of this area:

photo showing the Moog Rogue Filter CV summing node

Filter CV summing node

Here’s a wider shot. Note the longer black wire is a previous bodge and not related to these mods and repairs:

photo showing the Moog Rogue Filter CV summing node and input mod wiring

Filter CV summing node and input jack wiring

I used a Dymo labeller to add the finishing touch to the back panel:

photo showing Moog Rogue Filter CV input jack mod

Filter CV input jack mod, finished!

Hey presto! A filter CV input scaled at the same degree as the keyboard. I use a Kenton MIDI-CV interface, which has an auxiliary output for filter CVs, amongst other things, so this little mod should come in quite handy.

Moog Rogue: replacing the power supply

The Rogue’s original power supply is not ideal. It uses an external transformer in a box with one cable going to the mains, and another cable plugging into a 3.5mm headphone-style minijack on the synth. The minijack delivers a nominal 24V AC, which is then rectified into +12 and -12V DC internally. The power switch on the synth simply connects/disconnects the AC input at the jack.

When I received my Rogue, the transformer was damaged. Rather than buy a replacement, I decided to install a power supply inside the body of the Rogue. There is plenty of room to do this safely.

NB: I take no responsibility for anyone’s actions regarding mains electricity. If you are not confident working with it, don’t. Hand the job to someone qualified.

First job was to desolder the power inlet jack, and solder two wires to the place where the jack was on the main PCB. Unfortunately, the photo below is the best I have at hand. Note the two red wires supplying low-voltage AC:

Photo showing replacement power jack wiring

Power minijack replacement wiring

These two wires go to a connector that allows one to plug or unplug from the transformer for ease of maintenance. There are several kinds of connector that would work; I chose the type found in computer power supplies, as it’s what I had in the spares box:

Photo of replacement power supply connector in Moog Rogue

Replacement power supply connector

The other part of this connector goes to the low-voltage AC output of a transformer. The service notes state the Rogue requires 24V AC at 200mA. I leave selection of a suitable transformer up to you. Note that the Rogue’s rectification is provided by a 78M12 and a 79M12. A 12V AC, 12VA unit should suffice – the Rogue is rated at 6W. Again, I am not taking responsibility for the safety of others here, only providing an outline of my own process.

The transformer is bolted to the base plate of the Rogue. An earth lead is connected to one foot of the transformer:

Replacement mains transformer for Moog Rogue

Replacement mains transformer for Moog Rogue

From the transformer’s mains-level connections, wiring goes to a newly-added mains inlet. I chose the clip-in type with an internal switch and fuse. I cut a rectangular hole in the rear of the Rogue, set low down so as not to interfere with the graphics, which also meant trimming a little of the base plate’s rear lip.

New mains inlet for Moog Rogue

New mains inlet for Moog Rogue

Here is an overview of the result, with tape to secure the looser wires:

Overview of replacement power supply for Moog Rogue

Overview of replacement power supply for Moog Rogue

Here is the result from the outside:

Replacement Moog Rogue power inlet

Replacement Moog Rogue power inlet

 

External view of the screw mounting for the Rogue's replacement transformer

External view of the screw mounting for the Rogue’s replacement transformer

My Rogue also came with a plug to stop up the hole left by removing the power jack:

Photo of the plastic bung used to cover the Moog Rogue's old power inlet jack

Plastic bung to cover the old power inlet

I tend to use the mains switch on the rear, and leave the panel switch set to ON. It would be a simple job to remove and bypass this, but I consider it unnecessary.

*

MAINS ELECTRICITY CAN KILL OR SEVERELY INJUR. PLEASE NOTE THAT SAFETY MUST BE PARAMOUNT. IF YOU ARE NOT CONFIDENT WORKING WITH MAINS ELECTRICITY, GET SOMEONE QUALIFIED TO DO THE WORK FOR YOU.

Follow

Get every new post delivered to your Inbox.

Join 41 other followers

%d bloggers like this: