Saturday, October 26, 2013

Input to Comparator : a mod for the Doepfer A-119 and A-142

While working on what will hopefully be my next post, I was aware that I still do not have a dedicated comparator module in my system.  However, I do have several modules which contain internal comparators.  So, I decided to modify some of these to allow me access to the comparator function for use with other external signals.  

A comparator circuit is often used to derive a gate signal from a varying voltage source based on when the source exceeds a certain voltage (defined by the Threshold setting). The modules I have with internal comparators like these are the Doepfer A-119 Ext. Input/Env Follower, the A-142 VC Decay/Gate, and the A-143-1 Quad AD/LFO.  All these modules use a very similar circuit to Doepfer's A-167 Analog Comparator module.  The A-142 VC Decay/Gate (like the A-143-1), generates its Gate Out (Comparator) signal from its Env output while the A-119 generates its Gate Out from its Envelope Follower Output.

There are times when I want to use the VC Decay or Envelope Follower (or its pre-amp) but don't want to generate a Gate from their own outputs.  Instead, I'd like to use the internal comparator function with an external source.  By performing a simple modification to the A-119 and A-142, an input socket can be added to allow one to use the internal comparators with any signal (LFO, Sequencer, Min/Max, Random, etc) while still being able to use the VC Decay and Ext In/Env Follow functions separately.  Note: this mod is only described for the Doepfer A-119 and A-142, but it probably could or probably has been applied modules from other manufacturers. 

If you are already familiar with this type of socket, you can skip ahead to the videos or instructions below them.  

But, if this is your first time working with this type of socket, this might explain how it works (and other sockets with normalized connections:

You'll notice if you look at a socket on your module that the legs/pins on either side have a "normalized" connection, meaning the opposite pins are connected when NO plug is inserted into the socket.  This allows signals to be routed through the socket and interrupted with patch cables.  One leg/pin bends into an arm at 90 deg on the top of the socket and lies underneath the other side's longer arm.  I'll refer to the shorter arm as the normal arm and to the longer arm as the switched arm. Think of the socket as having a switchable input and an output for each terminal (Tip and RIng of a mono 3.5mm patch cable), and this is regardless of whether the job of the socket is a that of an Input or an Output.  Normally (when no plug is inserted into the socket), the signal at the NORMAL pin passes through to the OUTPUT, or switched pin.  But when a plug is inserted, the SWITCHED arm lifts off the normal arm, breaking the normalized connection, and instead the switched arm is only in contact with the plug's signals.

Watch these two short videos to see the Comparators reacting to external signals.  You will notice that the Env LEDs to not match the Threshold LEDs.  

A-119 "Input to Comparator" Mod from N K on Vimeo.

A-142 "Input to Comparator" Mod from N K on Vimeo.

Disclaimer:  Hang on.  Yes, this is a simple modification.  But be aware that any mods done other than by the factory will void your warranty.  Also, you should know and follow safety procedures and have well maintained equipment.  You should know how to solder.  Don't solder while in the bath.  Turn off power and remove from rack/enclosure before attempting to modify.  I cannot be held responsible for damage to your modules, burns on your fingers, spilled coffee, etc.  If you follow the instructions and look at the images, you should be fine.  I am neither an electrician nor an engineer, but I managed it.  If you're unsure, stop.  Ask a question in the comments.  If you do need to remove a PCB from the faceplate, be careful not to scratch it.  Do not touch your tools to the faceplate! It is very easy to scratch them. 

A-119 Mod:

On the A-119, the Envelope Follower signal passes through R24 (100K) to the comparator circuit.  We want to place the interrupt socket on the Env Follow side of R24, so the signal you patch in will pass through R24. 

Now rather than drill a hole in the faceplate or use a blank 4HP plate to mount my insert socket, I chose to re-purpose the 2nd Audio Out socket on the A-119 for use as my Comparator Input. You need to carefully de-solder and remove this socket from the board, or if you have extra replacement sockets, you can cut it from the board using a Dremel or wire cutters, then desolder what's left of the legs.

Next I bent the rear pins/legs of my socket up at a 90 degree angle and soldered two short pieces of wire to the pins. See Image.

Solder a 2" wire to the normal pin of the socket.  Solder another 2" wire to the switched pin of the socket.  Desolder and remove R24 from the circuit board.  Resolder the front two Ground legs of the socket to the PCB, or if you are building this mod on its own faceplate, you must connect the front two pins of the socket to Ground somewhere on the module, like the solder points for the pot brackets.  

Twist and loop together one end of R24 and the switched wire and solder together (1, 2).
I slid some shrink tube over R24 and its connection to the switched wire (3). Resolder the other end of R24 to the open through-hole on the PCB (4) and cover R24 with the shrink tube (5).  Solder the normal wire to the IC2/Pin1 junction point (6).  My wire was too heavy to get through the hole at R24, so I soldered it to some of the component legs at the same junction (6,7).  

Now the Envelope Follower signal will be sent through the socket to R24 and the comparator circuit.  Patching an external signal into the socket interrupts the internal Env Follow signal and replaces it with the external source.  

A-142 Mod:

Modifying the A-142 is a very similar process to the A-119 mod above, however the module doesn't have an "extra" socket which I can re-purpose.  Instead, I used a 4HP blank plate which I already had on hand and was already drilled to fit the Doepfer-style 3.5mm Cliff Sockets.  I threw some more sockets in the other holes and made some extra Ins and Outs.  Here's what it looks like now (left) and what it might look like if I rebuild it (right).

The resistor we want to desolder and remove is R21, located here near pin 1 of the TL084.  In this image, the signal path is travelling from right to left, so the right side of R21 is the junction of the VC Decay circuit and the comparator circuit.  We want our external signal to pass through R21. Since my 4HP expansion will be positioned on the left of the A-142 in my rack, I want to move R21 to the solder-side of the PCB, just like in the A-119 mod above.  

Basic mod:  prepare a spare socket by soldering a 2" long wire to the normal pin, another 2" long wire to the switched pin, and a 2.5" wire to the ground pins as shown here.  Twist together the end of the 
switched wire and one end of R21 and solder together.  Slide shrink tube over the switched wire.  Feed the other end of R21 through the hole in the PCB (shown left, "In to Comparator") and solder in place.  Move the shrink tube down to cover R21.  Solder the normal wire to the other solder pad (shown left, "Out from "Env Out signal") from R21's original position.  Solder the Ground wire to a GND pad on the module, pot bracket pads are good (shown left, "Ground"). Mount the socket on a blank plate.

Since I had extra holes in my plate and spare sockets, I added a few more Ins and Outs:
-  a socket is connected to the Trig In socket of the A-142 and simply acts as a multiple for the Trig In source.  
-  a socket is connected to the CV In socket and acts as a multiple for the CV In source.
-  a socket is the Comparator In socket from this mod
-  a socket is connected to the "switched" signal from the Comparator In socket.  If no patch cable is inserted into Comp In, this socket is a multiple of the A-142 Env Out.  If a patch cable is inserted, this socket is a multiple of the inserted signal.