Wednesday, April 8, 2015

Trigger in Mod for TR-9090

I've been building Trevor Page's TR-9090 Drum Synth, a MIDI-controlled clone of the voice circuits of the Roland TR-909.  Since finishing the main build and testing the voices, I started adding whatever mods I could find or think up.  

One feature I wanted to add is a row of analog inputs so that I can trigger the voices with external signals from my modular and sequencers.  I tried unsuccessfully to find a circuit that would have a low-parts count and be relatively simple, not require ICs or rare parts, etc.  

Eventually, I asked a manufacturer of Euro-rack modules if he would be kind enough to share his method for developing an appropriate trigger pulse from just about any external gate signal.  While nervously planning an appeal to SDIY, I got a reply.  He sent me an original design that has been used in his own products and gave me permission to share with the world.  (Thanks again! ;D)

Here is that circuit, redrawn for the TR-9090:

This is merely one possible topology for the circuit, but it is basically what i used to test functionality.  I made it narrow so that each sub-unit can be inline with the socket.  The TR-9090 will need ten of these circuits to control all the voices, so some additional design will be required to get ground and power to each sub-unit and all units on one PCB.

Note: this mod by itself does not include control over Accent level.  The trigger inputs alone will not produce any audio output from the TR-9090.  To test this circuit, I tied the Velocity In for the BD to the Aux Power connector's +5V.  (see below for more on Velocity patch points)

Below are the points on the TR-9090 circuit boards where this circuit could be inserted.  PCB 1 uses jumpers from PCB 2 to get the triggers from the MIDI Interface to the BD, SD, LT, and MT voices.  These jumpers can be used rather than cutting any traces or lifting components for these voices.  

For the other voices, either traces need to be cut or components must be lifted. By "lifted", I mean that the component leg circled in yellow should be desoldered and raised from the PCB.  The now-empty solder pad provides a tapping point for the trigger signal from the MIDI Interface, while the free leg is the trigger return from the added circuit.  

If you decide to cut traces instead of desoldering, the straight lines show the origin of the internal MIDI triggers for the voices of PCB 2.  

Note: the trigger signal for the HH voice is connected to two components, shown circled in yellow and joined with an orange line.  Both points need to be isolated but must remain connected together.  

I'm not sure what effect lifting or cutting traces will have on the signals feeding the Tom Noise circuit.  See the noise schematic below.

Here are the Velocity patch points.  For Board 1, I recommend using the jumpers as the send and return points as all four voices have two destination points (circled in yellow and joined with an orange line).  For HT, HC, and RS on Board 2, I recommend either cutting the traces from U32 or lifting the pins of the IC as described above.  Note: C107 and C125 are missing from the schematics.  For Cymbal voices, lifting the circled leg and inserting a socket as described in the Trigger In mod is possible.  I plan to use a 3-way switch for Velocity so I can toggle between +5V, MIDI control, and External control.