Purple Audio MC76: MONO PEAK LIMITING AMPLIFIERThe popularity of vintage audio gear has driven up prices and made desirable models hard to find. Some manufacturers are already emulating older equipment 3/01/1999 7:00 AM Eastern
The popularity of vintage audio gear has driven up prices and made desirable models hard to find. Some manufacturers are already emulating older equipment designs or duplicating the vintage products of now-defunct companies, and we can expect to see more retro packages in the future. Based on-but not identical to-the popular UREI 1176, the MC76 by Purple Audio Inc. is a monaural FET peak-limiting amplifier. Priced at $1,800, the MC76 features an improved circuit design that has eliminated the need to duplicate some original components.
THE BASICSThe MC76 front panel is simple and features the familiar illuminated VU meter flanked by eight push-button switches, four on each side. Four buttons are for compression ratio (4:1, 8:1, 12:1 and 20:1), three buttons are for meter functions, and one is for power. Two large knobs control input and output levels, and two small knobs modify attack and release times. Attack times range from 20 usec to 800 usec, while release times range from 1.1 seconds to 50 msec. Turning the attack knob fully counterclockwise turns the limiting action off. The knobs are all continuously adjustable, but the attack time knob has a different, almost "micro-stepped" feel.
Between the input and output controls a small hole allows for screwdriver calibration of the gain reduction "0." There are two XLR connectors-one for input, one for output-all pin 2 hot. Two 11/44-inch jacks provide for linking of two or more units. An IEC power inlet also contains a fuse holder and a line filter.
A look within reveals many of the favored components of a familiar vintage compressor, with a rearranged circuit board. The Roman numerals "MC" denote the number 1,100, but, according to John Klett of Purple Audio, the letters actually stand for "Mono Compressor." Klett points out that the input attenuator in the MC76 is the same Allen & Bradley component used by UREI in the original 1176. The UTC input transformer is also the same as the original, as are many other parts. Purple Audio has "improved upon the original circuit design by cleaning up the amplifier circuit, biasing things a little bit differently," according to Klett, and by choosing alternate parts based on greater availability and better consistency. At the output is a Class A amplifier and a custom-made transformer. The specifications claim a frequency response of 15 to 80k Hz, +/-1 dB. The manual is readily understandable and respects all levels of users, from its definition of compression to its detailed instructions for calibration andmaintenance.
LISTENING TESTSBefore taking the MC76s into the studio, I tried them out on some jazz CDs. Of course, these CDs had already had their dynamic range set at a comfortable level during mixing and mastering, so it follows that any additional limiting might sound unnatural. With that in mind, I put on an alternate take of Charles Mingus' "New Now Know How" and listened to one channel without the units in link mode. With the limiting function turned off, the gain alone added a beautiful warmth. It beefed up the track in a thickening, yet clean, sort of way, and what little addition of harmonic content it imparted seemed to be the perfect selection of harmonics. As with most good compressors, it's very hard to hear the compression with gain reduction under 3 dB. This was the case when I set the ratio at 8:1, the attack fast, and the release medium/fast. For most of the song, the gain reduction stayed around 3 dB, but when the trombone solo came in, the GR went up to 9 or 10 dB, and it didn't sound bad-slightly squashed, yes, but not stepped-on; strangely, the rest of the band didn't sound stepped-on either. At 4:1, the effect was very pleasing and smoothing, with the threshold set so limiting only occurred at loud kick drum hits, solos and the head. The trombone solo was reduced only by about 6 dB and sounded more even in the mix, with less effect on the other instruments.
I also tried that old 1176 trick of pushing several buttons at once, combining 8:1 with 12:1 on the Mingus tune. The meter behaved very erratically, sticking mostly to the far right (approximately +3 dB), whipping down 2 to 15 dB at some unpredictable moments and some very predictable moments, such as a -15dB jump at the trombone solo. The funny thing is, it didn't sound bad, just interesting. It may not have been the way Teo Macero wanted it to sound, but it gave a new bounce to the recording in a very interesting way.
Again, with this mastered CD higher ratios aren't appropriate, and the lead instrument often triggers the limiter to clamp down on the rest of the band. A more equitable test for the higher ratios is achieved with a single instrument.
The gain alone of the MC76 added a welcome warmth to an Aria Pro II bass, which was played direct into the console. Turning on the compression at an 8:1 ratio provided a lovely sustain with up to 4 dB of gain reduction. I also used the pair of MC76s on the room mics, going to tape. The drummer, who kept referring to the Purple Audio pair as "Barney," was about 15 to 20 feet from the room mics. I used two AKG C12s in omnidirectional mode, spaced about 15 feet apart and raised eight feet above the floor. These went through a Neve console with 1081 modules. The MC76s were inserted in the signal path pre-EQ, but I didn't end up adding EQ, as the limiting action was so transparent. Although the attack time is doubled (i.e., slower) when the units are linked, the Purple pair performed exceedingly well, with little or no color added to the signal by a 4:1 ratio.
Overall, the Purple MC76 is a very useful tool, both for subtle signal control and for wild production techniques. Most compressor/limiters tend to dull the high frequencies of the processed signal. Although the MC76 does this at very high levels of gain reduction, in general it is more efficient in this respect than its predecessors and most of its contemporary competition. In fact, the only modern mono limiter I like more costs twice as much and requires a year-long wait on delivery. The MC76 also seems to be a much more logical choice than searching for a similar vintage unit and paying to upgrade it.
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