MOTM-480 Expansion Panel

The MOTM-480 has become one of my favorite filters. Internally it is two state-variable filters, hardwired in a configuration to reproduce the bandpass filter found in the Yamaha CS-80. It’s a great-sounding, versatile configuration, but it still seems a shame that the module doesn’t allow you to use all of the available filter modes present in its circuitry. Inspired by Richard Brewster’s extensive modification project, I decided to build an expansion panel for my MOTM-480.

Richard’s project provides complete, independent access and control to the two filters in the MOTM-480. I decided that I wasn’t quite up for something quite so elaborate and that I didn’t need all of the features his modification offers. Mostly I just wanted to be able to run stereo signals from my DAW through the filters, and have access to all three outputs (low-, high- and band-pass) present on each filter. In the end, the main functional difference between Richard’s project and mine is that his provides separate CV inputs for each filter’s frequency and resonance and mine retains the ganged inputs provided by the original module. Since the orginal provides separate frequency and resonance knobs for the filters I decided that I could live with this limitation. Living with this limitation means a substantially simpler and less-expensive project. My expansion project is also fully reversible in that it doesn’t require cutting any traces on the original PC board. (I have nothing against cutting traces on PC boards but it makes some folks squeamish, I suppose.)

I decided to retain the original MOTM-480 panel and add the new jacks and controls to a new 1U panel. Since a picture is worth 1K words, here’s a photo of the new panel:

I added two input jacks for the second filter, with level controls, and output jacks for each of the three outputs present on the two filters. The output signals are tapped from the MOTM-480’s PCB exactly as Richard describes. I removed the 150K resistor at the second filter’s input from the PCB altogether since this provided convenient connections to both the first filter’s output and the second filter’s input. I attached 150K resistors directly to the lugs of the input-level potentiometers to provide the proper gain scaling for the second filter’s input.

Following Richard’s lead, I copied the output buffers from the MOTM-480’s schematic to drive the new outputs. I built the buffers on Veroboard. This was the first project I’ve done on Veroboard that didn’t end up as mess of tangled wires. I decided that the secret is to very carefully plan the placement of all components, wires, and trace-cuts prior to assembling the circuit. I used a drawing program for the planning. I created separate layers in the drawing for the copper traces and components so that I could move things around and try different arrangements. When I was finished with the drawing, I printed it at 1:1 scale, taped it to the Veroboard, and pushed the component leads through the paper. This seemed a lot more convenient than counting rows of holes or using a 0.1″ scale to place the parts correctly.

Another technique possibly worth mentioning is how I mounted the Veroboard to the panel. Rather than gluing stand-offs to the panel as I’ve done in the past, I cut L-shaped brackets from aluminum angle stock. I drilled holes that fit around the largest part of the shaft of the panel jacks, and sandwiched the brackets between the jacks and the panel:

Here’s another view of the completed module:

Yes, those are cheap carbon Alpha pots you see. Good 100K audio-taper pots are becoming increasingly difficult to find so I gave up and used Alphas. So far they’re fine but I won’t be surprised if I have to clean them in the future.

Acknowledgement

Many thanks to Richard Brewster for providing all of the information about his MOTM-480 modification. Without his detailed description my project would have been considerably more difficult to complete.