Doepfer A-100 series modifications by Nick Keller.

List of Mods:
Oscillator Mixer - includes A-110 to A-115 (hardwired), 5 channel mixer (hardwired)
Gate bus - includes Gate In to system bus, A-141 and A-142 to system bus
A-156 Quantizer - jumper option to front panel switch
CV3 In / Out / Thru - includes simple stand-alone attenuator
Gain Stage - stand-alone A-119 gain stage (coming soon)
A-110 Fine Tune and Linear FM (coming soon)
VCF Resonance inserts - A-120 and A-124,
A-102 + A-103 (coming soon)
A-118 White/Colored switch to Random
A-120 Input Level
Manual Gate
A-142 VCADSR Inv. Out (coming soon)

Many of the following modifications are more like re-wired shortcuts for patches I find I use regularly. The mods will most likely void any warranty and, if not done carefully, can damage the circuit board, IC chips, and faceplates. Aside from a few scratches I have had no real problems. I used a 15W soldering iron, rosin-core solder, de-soldering bulb, 22 gauge stranded hook-up wire, SPDT/DPDT switches, linear and audio potentiometers, resistors, blank faceplates, Doepfer style sockets, a Dremel-type tool, momentary push-buttons, IC chip and socket, male/female and multi-pin connectors, blank circuit boards and etchant solution, drill press, and spare knobs......but ALL are not required for every mod. Read through each to make a list of what you will need. I highly recommend ordering in bulk (when applicable) from an on-line source as this will save you a bunch of money. Faceplates and small quantities of sockets and knobs came from my country's Doepfer rep. A larger number of sockets came from a UK source. Iron, solder, bulb, and wire came from Radioshack, but would be cheaper on-line. Other parts came from a US on-line source.
Removing Faceplates - Some knobs are more difficult to remove than others. I carefully pried them up with a very small flathead screwdriver. The faceplate WILL scratch so try to put pressure only in places that will be covered by the knobs when replaced. Carefully remove the nuts holding the potentiometers and sockets to the faceplate. Put them somewhere safe! I've been using angled pliers and have left several scratches so there must be a better tool. These following mods, except for the A-118 mod, require no drilling of the Doepfer module faceplates, only drilling of blank faceplates.
Using sockets to interrupt - Many of these mods take advantage of the switched contacts on the 3.5mm sockets. On most A-100 modules the pins are connected to one another. [conn-vs-cut.jpg] By breaking the connection between the pins the sockets can be used to hardwire certain patches without losing front panel function. Signal interruption is done by using the 'normal' pin as the input to the socket and the 'switched' pin as the output. [socket-pins.jpg] Plugging into the panel socket can interrupt the hardwired signal, as in the A-109. [a109.jpg] With a pair of needle nose pliers, I carefully bent the 'normal' tip pin out at a 90 degree angle. Holding the pin at a 45 degree angle seems to work well. [45-degrees.jpg].....the plastic casing of the socket is easily broken. Hold the jack firmly in one hand and bend the pin slowly trying not to put any pressure on the plastic. [gap.jpg]
Removing sockets from PCB - If possible, use a de-soldering bulb to remove the solder and pull the socket from the PCB. My de-soldering bulb has too fat a tip to get to the solder when sockets are connected on the underside of the circuit board as in most of the A-100 modules. I used (Mr. D cringes.....) a Dremel-type tool with a cutting wheel to cut off the socket. Be careful not to cut into the circuit board or into any other components. I did cut through some of surrounding pins at times, but only the excess lengths leaving plenty of the solder. Because I cut off the socket completely, it can no longer be used with some modules. If cut carefully, this socket can be salvaged for use in other mods that don't require connection to a circuit board. I made some multiples like A-180 with the sockets I've cut off. Obviously a spare socket is needed, available from the Doepfer distributor in your area or from some other sources if ordered in bulk. Once the socket was removed, I used the de-solder bulb to remove the left over solder and the cut socket pins from the printed circuit board (PCB).
Potentiometer connections
Simple Stereo - Simple Stereo - Really the same as interruption, when two of the same modules are set side by side the module on the right can have its input socket altered so that connecting a cable to the left module also sends the signal to the right module. [gap.jpg] That signal can be interrupted by plugging a cable into the right input socket. I have done this with two A-125 Phasers and two A-138b mixers.

Oscillator Mixer
includes A-110 to A-115 (hardwired), 5 channel mixer (hardwired)
[VCO-mixer-front.jpg] One of my favorite synths, especially for FM, is the Octave Cat. This is because there are level controls for each oscillator shape giving an "additive FM" effect. I wanted to make a mixer and connect it directly to an oscillator so I didn't have cables and multiples going everywhere. A regular A-138 wouldn't do as I wanted at least 5 inputs. So using the mixer schematic available from the Doepfer DIY page, I made a simple mixer with some extra features. I left out the inverted output and changed the input routing. I added some kill switches to two inputs and took advantage of the sockets by allowing interruption of the hardwired signals. I included an extra output for simple signal splitting.
Schematic
Example PCBs - Some of the modules I've tried to build from schematics have not worked at all, which may be because of the circuit boards I built. I've often wished that people that publish plans for modules would include an example PCB, even though it may not match the size/shape of the components the builder eventually uses. I thought I should include such an example for my oscillator mixer to help someone get started, and I know it works. The graphics are not to scale, but you should get the idea. This example is how I built mine, but this example would work for someone with different pots, etc.
Building - I decided to arrange the front panel in such a way that no cables would cross over the controls. In order to achieve this, I now have many criss-crossing cables behind the face. This may lead to signal interference, hum, and general confusion when soldering. [VCO-mixer-back.jpg]

I used one 8HP and one 4HP because I already had them and I was unable to get a 12HP. I wanted to include a sub octave and a noise source, so I built this mixer next to an A-110 VCO and A-115 Audio Divider with an extra input for noise. All the individual outs are still available at the VCO and Au.Div. The switches allow me to turn the 'sub' and/or noise signals on and off, and one switch toggles between Tri and Sine waves, as I didn't expect to use both at once from the same VCO.

Option: include a simple white noise source on the mixer circuit board.

As in the other mods, signal interruption is done by using the 'normal' pin as the input to the socket and the 'switched' pin as the output.

Two simple PCBs were made for this mixer, one with the mixer controls and circuitry and one with the input/thru sockets. [VCO-pcbs.jpg] The input sockets have separate connections for the 'normal' and 'switched' pins, except for the noise socket which doesn't require this. 'Normal' pins get wired to the tip pins of the VCO and A-115 outs. The VCO output sockets sit very close to the rails on the G6 frame so wires should be made to lie flat, perpendicular to the faceplate. [VCO-outs.jpg]

The mixer needs power, so short wires were soldered from the mixer PCB to the power pins on the bottom of the A-115 PCB. [Power-from-A-115.jpg]
VCO to A-115 - This mod simply wires the square output from the A-110 to the normal pin on the A-115 input socket. This is one of the first mods I did so it was a bit trial and error in regards to removing the socket. My de-soldering bulb has too fat a tip to get to the solder when sockets are connected on the underside of the circuit board as in most of the A-100 modules. Begin by CAREFULLY removing the nuts holding the circuit board to the faceplate. Some knobs are more difficult to remove than others. I carefully pried them up with a very small flathead screwdriver. The faceplate WILL scratch so try to put pressure in places that will be covered by the knobs when replaced. I used (Mr. D cringes.....) a Dremel-type tool with a cutting wheel to cut off the A-115 input socket. Be careful not to cut into the circuit board or into any other components. I did cut through some of surrounding pins this time, but only the excess lengths leaving plenty of the solder. Because I cut off the socket completely, it can no longer be used with this module. If cut carefully, this socket can be salvaged for use in other mods that don't require connection to a circuit board. Obviously a spare socket is needed, available from the Doepfer distributor in your area or from some other sources if ordered in bulk. Once the socket was removed, I used the de-solder bulb to remove the left over solder and the cut socket pins from the A-115 printed circuit board (PCB). With a pair of needle nose pliers, I carefully bent the 'normal' tip pin out at a 90 degree angle. Holding the pin at a 45 degree angle seems to work well......the plastic casing of the socket is easily broken. Hold the jack firmly in one hand and bend the pin slowly trying not to put any pressure on the plastic. Now resolder the three straight pins back into place on the PCB. I cut a wire to reach from the tip pins of the VCO's square output to the bent pin of the A-115 input. One end obviously connects to the bent pin and the other can be soldered in place to the VCO. A bit of solder on each end of the wire should make it easy enough. Just be careful not to bridge the gap between the Saw and Square outs. Bending the pin away from the PCB enables you to still use the front panel socket to interrupt the hardwired signal from the A-110 VCO.

A-120 Input Level mod

First, I would like to thank the guys on the Yahoo Doepfer forum for their patience in this matter, that's Florian Anwander and both Ingos. I really appreciate it. This mod alters the input stage of the filter giving a boosted sound. [waves.jpg] According to the forum, two things must be done for this mod: adding one resistor and adding one capacitor. The Doepfer DIY page suggests simply lowering the value of the input resistor, but it seems that when they do the mod themselves it has the added capacitor. R15 is a 22K resistor while C1 is a 10uF electrolytic capacitor. The mod requires a 10K resistor to be added in parallel to R15, and a 100uF electrolytic cap to be added in parallel to C1. [R15-C1.jpg] The mod can be done permanently or with a DPDT switch so the original circuit's sound can be preserved [with-switch.jpg]
To do the mod, follow the examples in the two images above. Make sure that the electrolytic capacitor is installed the correct way, look for the negative signs on one side of the cap. I used a 'micro-mini' toggle switch from Radioshack, but found that it makes a poor connection, so I will exchange it for one slightly bigger. [panel-switch.jpg]


A-156 Quantizer Option

The A-156 Dual Quantizer is a great module to have. I personally use it in its Major/Minor modes most often. I wanted to have both channels of my quantizer in this mode most of the time, without restricting myself to that setting. There is a removable jumper near the power connections for the A-156 that allows Quant.1 to follow the function of Quant.2. By removing the jumper and attaching a switch, I can now toggle the function of Quant.1 between the Semitone scale and that of Quant.2. This saves me having to remove the module when I want to link the two Quantizers. This mod may require drilling of the faceplate.
Instructions - Remove the jumper. Next I bent the pins of the jumper away from the faceplate (do not bend if you ever want to remove this mod). [A-156-pins.jpg] Then I removed the module from the faceplate. I used the holes for Trig In and Transpose to line up my drill for the new switch. Using a mini-toggle SPDT switch, there is plenty of room between the switch and the circuit board. [A-156 switch.jpg] Once the switch was mounted to the faceplate, I remounted the module to the faceplate. Three small wires ( #22, stranded) were soldered from the jumper pins to the switch lugs as shown. [A-156 wires.jpg] The front panel is not overly crowded, and my rack space didn't allow for a blank plate just for this function. [A-156 front.jpg]


VCF Resonance Inserts

PCB Example
A-120 Moog - The A-120 (v1.1995, at least) has a jumper which is marked VCA INSERT, so one needs only to remove that jumper and use those holes for your wires. [120-wide.jpg] BE CAREFUL that you do not damage the solder pads.....mine lifted up when I tried to push my wire through and then just broke off completely. [120-pads.jpg] I followed the traces away from the pads and connected my wires there. I used a simple circuit board to connect my sockets [120-pcb.jpg], [120-pcb-dots.jpg] and mounted the mods to a 4HP faceplate. [Plate.jpg] The mod board shown has sockets for the A-103 also, not completed. I used wires rather than male/female connectors but they may be replaced in future. If using M/F connectors just remember to use Q Out one way and Q In the other so that removing the insert mod doesn't disable resonance..... [120-MF.jpg]
A-124 Wasp - The insert loop, as described on the Doepfer DIY page, is achieved by using two switched sockets between two points on a circuit. The 'normal' pins of the two sockets are connected to each other. I traced the line on the PCB from the center pin of the Q pot to where it connects to the next component. Using a diamond tipped bit on a Dremel-type tool (you could also use a fine drill bit) [tools.jpg] I cut through that line on the PCB. [Q-insert-detail.jpg] Then a wire was connected from the pot pin to the 'switched' pin of one socket and another wire was connected from the 'switched' pin of the second socket to that next component. [RIGHT-Q-insert.jpg]
[wasp-mods.jpg]


CV3 In / Out / Thru
Includes Simple Stand-Alone Attenuator
Example - [ PCB ] (coming soon)
In - All the A-100 filters have a CV1 input, as well as either one or two inputs with attenuators. I often use both LFO and ADSR to modulate a filter, but almost never use them at full 0-5V range. So I wanted to give all my filters three CV ins for cutoff, two with attenuators. Starting with my Wasp VCF I simply copied the pot and resistor values from the CV2 controls (B50k pot, 47K Res.) and wired them to the filter PCB. Follow the trace from the CV1 socket through its resistor where it meets the CV2 resistor. [LEFT-CV3.jpg] The socket's tip pins are connected to the pot input, the pot output is connected to one end of the 47k resistor and the other end is soldered to where the other two meet. The other terminal on the pot gets connected to ground somewhere, I chose where the pot braces meet.
Thru - Another option is to include a socket connected directly to the CV input socket (like a multiple, as on A-119 and A-126). This makes for easy patching when using one modulation source for several modules (VCF, VCO, VCA, PWM).
Out - Yet another option would be to insert a socket between the CV3 pot and the CV3 resistor (R). The pot out would connect to the 'switched' contact and the 'normal' contact would connect to the CV3 R. [CV3-out.jpg] When a cable is patched in here it interrupts the attenuator output so no CV is sent to the filter CV sum and instead the circuit acts as a stand-alone attenuator as shown on the Doepfer DIY page.


A-118 White/Colored Switch To Random
OK, so in this mod, I DID drill the Doepfer faceplate. I didn't expect to install a blank plate in the rack that houses my A-118, so I went ahead and drilled. There is plenty of room on the faceplate to install a switch and it looks fine now that I'm finished. [118-switch.jpg] I got the clues for doing this mod from the Yahoo Doepfer forum Database, and I thank Dieter Doepfer and Keld Soerensen for sharing the information. The A-118 random generator takes the signal from the colored noise as its input source, so any adjusting of the RED and BLUE levels will effect the range of the Random Output. If one wants to use the colored output for one part of a patch and the random for another part of the patch, it may be good to have the random fed by the white noise. This mod simply installs a switch to select between the two noise sources. As explained in the database, the resistor R12 connects the colored noise to random and there are marks on the PCB indicating the placement of R12'. [R12.jpg]
Doing The Mod - I used my soldering iron and desoldering bulb to remove the resistor R12. Next, I removed the module from its faceplate, measured halfway between the horizontal lines, and used my drill press to drill the hole. I went to put the faceplate back on and decided to cut the extra lengths on any pins between the switch and the PCB. The resistor was soldered to the center lug of the switch. The top lug was soldered a wire soldered to the Colored Out point, and the bottom lug was soldered to a wire soldered to the White Out point. [118-wires.jpg]


Gate Bus
Includes Gate In To System Bus, A-141 and A-142 To System Bus
[gate-bus-front.jpg]
I wanted to have all my envelopes connected to the system bus, but so far only the A-140 has allowed for this (not sure about A-143 range). This system bus signal can be interrupted at the Gate In socket on each A-140. I have A-140s, 142s and a 141. The A-141 has a ten pin ribbon connector which doesn't allow for +5v, CV or Gate signals from the system. [ribbons.jpg], [pinout-bus.jpg] The A-142 however has the full 16 pin ribbon connector, but the gate signal is not fed to the trigger input. A few things to consider: male/female connectors for easy separation of modules after modifications [male-female.jpg] and maintaining front panel function of Gate In sockets.
Building - I started by getting a mutli-pin connector so that individual modules could be disconnected if needed. Separate male/female connectors may have been better but would take up more PCB space. [multipin.jpg] I made a simple PCB to hold the sockets and multi-pin. One socket connects to the multi-pin base and to two other sockets creating a mini-multiple. One wire from the multi-pin was soldered directly to the Gate pins on the Doepfer bus board. Now plugging a cable with gate signal into the first socket sends the gate to the system bus and to the other two sockets.
A-141 - To connect to the gate bus signal, I used a wire from the multi-pin connector. The A-141 is awkward to remove from the faceplate, so instead I used the diamond tipped Dremel tool to cut the line between the two tip pins. This is awkward as well. [A-141 cut.jpg] The multi-pin wire is then soldered to the 'normal' pin. Plugging a cable into the socket now interrupts the gate bus signal.
Option: Add another output by simply soldering a wire from ADSR Out to another spare socket.
A-142 - The Trig In socket must be removed which I did with the Dremel-type cutting wheel. I replaced the socket with a spare after bending out the 'normal' tip pin. Next, a wire was soldered from the bent pin to the Gate pins on the bottom of circuit board. [A-142-gate-bus.jpg]
Option: Add another output by simply soldering a wire from Env Out to another spare socket.


Manual Gate
Manual gates are great for doing things in realtime as they can be used to trigger not only envelopes but also anything else in the A-100 that has a gate/trig in (A-155, S+H, LFO reset, A-151, analog arpeggiators, etc). I figured I would make three manuals with the panel space allotted and decided to separate the buttons as much as possible to avoid accidental triggering. [Gate-bus-front.jpg] Again, this circuit does not regulate to +5V so be aware when interfacing with other manufacturers.
Building - Doepfer informed me that the A-100 modules do not require regulation of gate signals to +5V and that I only needed to deliver a power source between +3 and +12V to the momentary push-buttons (momentary means it is only closed as long as it is pushed, as opposed to push on-push off). I used a two wire multi-pin connector mounted on the Gate Bus PCB (see mod above). [multi-manual.jpg] One wire was soldered to ground and one wire soldered to +12V using the holes provided near the left edge of the Doepfer bus board. On my PCB, the +12V connects to to one lug on each of the push-buttons . [mom-pb.jpg] The other lug on each push-button connects to a 470 Ohm resistor each, which in turn connects to its own socket. I drew lines on the faceplate to keep track of the sockets. I later added the LED options as shown on the Doepfer DIY Page.

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