Curtis CEM3328

Eight of these filter chips sat inside the Ensoniq Mirage - and the people who knew them best warned that a careless patch cable could vaporize the traces feeding them.

Overview

The CEM3328 is a voltage-controlled filter chip from Curtis Electromusic Specialties, the Los Gatos outfit whose "wonderchips" (the word Electronic Musician used in its pages) turned up in a generation of analog instruments. That pinout is the whole practical story of the part. The filter's output feeds back internally to form one of its own poles. That gives the chip its resonance character, and it also makes the chip fussy about what you hang off the output.

The chip's most documented home is the Ensoniq Mirage, where eight CEM3328s - one per voice - handled filtering for the sampler. Electronic Musician published a modification guide for tapping those eight filters individually, and the article is the best surviving look at how the part behaves in real hardware. The author, Bob Yannes, was a driving force behind the VIC-20 and Commodore 64 and designed the C-64's sound chip before helping found Ensoniq and develop the Q-chip at the heart of the Mirage. Tom Metcalf, who worked out the resistor values for the mod, developed much of Ensoniq's sound library. Both knew the 3328 from the inside.

The core caution: the filter has no special output protection circuitry. In the Mirage, the mixing resistors were the only thing standing between the chip's output and the outside world. The article's authors had seen circuit boards on which the output traces were vaporized by high voltage, the kind that shows up when a musician plugs a keyboard into an improperly grounded audio system. "Think what that voltage could do to the filters - all eight of them," the piece notes. Anyone pulling a 3328 out of a donor instrument and wiring it into a homebrew circuit should buffer the output. Buffering also suppresses the chip's tendency to oscillate under certain capacitive loads, which can produce birdies at the output, and it keeps outside signals from affecting the cutoff frequency. The filter output forms one of the filter poles via local feedback, so a load that talks back to pin 9 is talking directly to the cutoff.

The 3328 also went through at least one revision during its life. The Mirage mod guide gives two different resistor tables for the stereo-output modification, depending on the date code printed on top of each filter chip. Units coded "8619" (the 19th week of 1986) or higher take one table, and units coded "8618" or lower take the other. Use the wrong table and the stereo outputs come out distorted or weak. The eight filters in a given instrument may not all be the same revision, and the date code is the only reliable way to tell them apart.

For experimenters, the part is accommodating. The differential inputs on pins 5 and 6 will accept external audio, so a bank of 3328s can be driven as a MIDI-controlled filter processor. You can feed stereo audio split across four filters per channel, or blend external signals with the host instrument's own oscillators ahead of the filter. The control input on pin 2 accepts external control voltages, which allows eight independent filters under sequencer control. The Electronic Musician article recommends pulling the chips from their sockets and running them via 14-pin DIP jumpers to a separate board for this kind of work, rather than soldering directly to the ICs, a risky practice the author explicitly avoided. Curtis Electromusic Specialties, at 110 Highland Avenue in Los Gatos, supplied data sheets, applications notes, and a catalog on request.

On the available record, the 3328 is not a collector's object in its own right. It's a component, the kind that gets desoldered, socketed, and swapped, and its value is bound up in the instruments it lives inside. The Mirage is where most of them went and where most of them remain. Buyers hunting a spare should look at the date code first, because a replacement that doesn't match the rest of the voice board may need different surrounding resistor values to behave correctly. Anyone building around one should budget for an op-amp buffer on the output. The SSM2134 was the article's recommendation at the time, a low-noise improved replacement for the 5534. The chip will not protect itself.

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