Formula Filter Array 24

Update: be sure to check out the comments for more. This was actually made by Bob Moog.

Title link takes you to shots, including some of the inside, via this auction.

"This auction is for one vintage Formula Sound Multiple Resonance Filter Array. This is a USA-made fixed filterbank / string filter from the 1970s that was (according to the engraved faceplate) custom made for San Francisco musician and producer Patrick Gleeson. Gleeson made some excellent albums on his own, but was better known as a session musician and producer/engineer who worked with artists as diverse and influential as Herbie Hancock and Devo. Though this model was advertised by Formula Sound in some magazines back in the 70s (see picture), this is the only actual unit I have ever seen or heard of. Since the faceplate says it was custom made for Gleeson, I believe it is the only one in existence. In theory it is similar to Moog String Filter but with greatly expanded and more tweakable facilities, but generally it excels at allowing you to electronically simulate the natural resonances of an acoustic chamber (like a violin or other stringed instrument).
This is also presumably somewhat of an inspiration for the Moog MURF pedal, as they seem to have taken the name from this unit (MURF stands for MUltiple Resonance Filter). Unit runs on 110V voltage and comes with a standard IEC power cord.

This piece is generally in very good cosmetic and functional condition, but has a few warts that I will be more explicit about in the following paragraph. All of the functions should be fairly self-explanatory... a mono signal goes to the input of the unit and is fed through 24 steep bandpass filters at various fixed frequencies, each of which can be assigned to one or both of the output channels via bus switches on the front panel. When all the sliders are down the unit is fully attenuated (silent).... as various frequency sliders are raised, that frequency becomes audible. There is a resonance shift rotary switch that allows you to change the overall harmonic tone of the selected frequencies and a drive knob which seems to have a compressor circuit built-in and sounds more like a tone control than a harmonic clipper.
In addition, there is a rotary switch that selects the overall mode---bypass (dry signal only), direct (unit engaged, frequencies sent directly to selected output bus), x-fade and x-pan (both similar in theory but different sounding). There are also 1/4" jacks for CV (control voltage) access so that you can integrate it into your modular system... one is a VCA input jack and the other is a modulation output jack that allows you to send the internal modulation LFO out to external equipment for coordinating sweep times. This is a very full featured unit that allows you to radically alter a sound, though it sounds very different and in a lot of ways a lot less radical than a standard lowpass filter.

Everything generally works as expected with a few exceptions, so I'll be pretty explicit here about everything that isn't quite perfect.
---The Drive pot is very scratchy.
---The Resonance Shift rotary switch is very loud and emits a loud thump through the outputs when switching, though this seems to be more by design than by a defective switch.
---The output Bus Selector switch for the 372 Hz frequency is wobbly and doesn't click into place like the others but it selects the output bus just fine.
---In Direct mode, in which is acts as a standard fixed filter bank, the B output bus is noticably quieter (about 15 db or so) than the A output when all frequencies are assigned to both buses. The overall volume difference, however, seems to depend on the drive level, input level and resonance shift amount. At certain settings the output levels are much closer to each other, though. I overcame this by running the 2 outputs into different hard-panned mono channels on my mixer and using the input gain pot to make up the difference.
---In X-fade mode it is the same as Direct mode---one channel is louder than the other. This is very noticable when using the 2 hard-panned mixer channel trick that is described above.
---In X-pan mode the output channels are pretty much equal volume and this mode sounds very similar to the X-fade mode, so I just used this mode instead.
---The front panels is very clean but there are numerous scratches and wear on the top, bottom and sides of the unit. There are some small paint marks that someone added to the selector switches and on one of the frequency slider caps.

This thing is built like a tank. All of the frequency sliders have very attractive machined aluminum tips. It does have some wear, but looks very good both inside and out, especially for a 30-plus year old unit. I've owned it for close to 10 years and I just had it serviced by analog guru Phil Cirocco in fall 2006 to fix a dead output channel. Though I don't have any schematics for the unit, any competent tech should be able to service it as the filter cards are modular (12, with 2 frequencies per channel) and all of the other stuff (VCAs, etc) are on breadboards and the circuits are clear and easily traceable. As you can see by the high quality parts and tons of circuitry, building something like this at today's prices would cost many, many thousands of dollars."