FILTER ON/OFF

Assuming you’re familiar with the FB383/MB33 you’ll know there is an external input on the back to patch a signal through the filter section of the unit. Assuming your unit works you know that when you connect a signal the internal synth is disabled which makes racking the unit a major hassle. By adding a filter on/off switch you can keep your unit racked and patched to a patch bay and use the filter easily. If your FB383/MB33 doesn’t work, like mine, then it’s basically a signal on/off switch. I plan on making a cv input for the oscilator and when i do, this mod will still be neceassry if i want to rack the unit.

The photo above is a shot of the finished mod. You can see more details, photos, and descriptions at the link below.

The Full Flickr photo set here.

FILTER TRIGGER

As with most external filter units, you need to trigger the envelope section before you will hear any sound. So if you have an input signal patched and the envelope is not triggered you hear nothing. To do this you have to supply a voltage to the bias input of the Operational Transconductance Amplifier (OTA). Instead of applying a direct voltage I opted to use a pot to drop the voltage at the base of a transistor controlling flow to the bias input… drop the voltage and current flows to the bias input. With higher voltages you get some gritty distortion which isn’t so bad in some cases. I soldered one lead of the pot to ground and one to an easy to reach lead on the end of a 0 Ohm resistor. That’s what you see in the photo above.

As before, you can see the full set of photos by the link below.

The Full Flickr photo set here.

RESONANCE BOOST

This one is pretty self explanatory. There is a resistor in place that restricts the resonance a bit. By bridging this resistor you can get some nice thick resonance and even some self-oscillation. Instead of simply bridging it though I added a switch to allow for ‘normal’ and ‘boost’ modes for a variety of applications.

As with the previous mods… photos and details by the link below.

The Full Flickr photo set here.

When initially looking into information on the FB383 I immediately figured out that nobody has any. Nobody. There was virtually no information available on the net with the exception of a brief background on MatrixSynth and some information on possible mods (for a fee) at Circuitbenders.uk. Of course there were reviews at typical locations like hyperreal and harmony central but other than that it was limited. I managed to track down someone familiar with MAM products through the company that ate them, Musonik, and figuring that somewhere someone would have service manuals, design specs, part sources, etc. I was wrong. Nothing.

So given the good number of these machines out there and the likelihood that there were a fair number of folks in need of a signal flow diagram I made a high resolution circuit overlay of the Freebass FB383 (MAM MB33). It’s not a schematic but it’s about as close as you can get. I’ve found it immensely useful and have it available here for any and all to put to good use. Enjoy!

The full high resolution (7920×1587) image here.