ACME Opticom XLA-3

Leveling Amplifier

ACME Opticom XLA-3 Leveling Amplifier

Review by George Shilling

The Opticom is a 2U mono opto limiter which combines high speed photocells with all-valve circuitry. Designed and built in the USA, it provides three different Response modes which each employ different optical circuits, and with these it aims to produce a range of tones from ‘clean’ to ‘dirty’.

ACME’s founder Al Sutton is best known as an engineer and producer, most extensively collaborating with Kid Rock, but also with credits including Sheryl Crow, The Detroit Cobras and R. Kelly. He also runs his own finely equipped Rustbelt Studios in Michigan. Dan Russell, the designer of the Naylor guitar amplifier came to him with the concept of taking the optical compressor to the next level, and the Opticom is the result. The ACME name was chosen to reflect the glory of the industrial revolution – Sutton no doubt inspired by his Detroit surroundings.

ACME Opticom XLA-3 Leveling Amplifier

The Opticom certainly has an old-fashioned industrial appearance. The front panel and indeed the entire case is finished in glossy military green paint. On the front, two retro-style backlit kidney-shaped VU meters dominate, accompanied by some unusual switchgear. On the rear are mounted five valves (four are covered), all protruding outside the case by quite a margin. Neutrik and Cliff connectors are employed, and everything is apparently built to military specifications. Connector sockets are hidden amongst the protruding components: XLRs are provided for audio Input and Output, and there is a (surprisingly plasticky) jack socket for stereo linking two units.

Mains is on a flying lead. The review model supplied was a 115V unit, requiring a converting transformer; a proper 220V export version is planned. Internally there is no circuit board; components are bolted to the bottom of the case. It is all hand wired point-to-point, with mil-spec potentiometers and ceramic tube sockets. But it’s not all old-tech - LEDs are used to activate the photocells. A front panel toggle fires the Opticom up, illuminating a lovely Pultec-style jewel lamp and the meters’ backlights. A slightly larger toggle labelled Limit enables the circuitry or bypasses the limiting. The Threshold meter continues to operate in Bypass, and gain controls are also bypassed, rather than it just defeating the opto circuit. However, the Output meter changes in bypass to indicate the actual output level and, remarkably, this continues to operate even with the power lead pulled out! The deep red coloured Bakelite chicken-head Input gain knob is used to drive the limiter circuitry, much like using an 1176, although this sounds rather different.

ACME Opticom XLA-3 Leveling Amplifier

Hot signals can be attenuated with this knob. But the Threshold meter is somewhat unconventional, in that it operates almost like a normal gain reduction meter in reverse. With no limiting, the needle rests to the left. As the gain reduction threshold is crossed, the needle starts to move to the right. There are two selectable ranges, labelled x1 and x2, toggled with a black sliding switch (which appears to be borrowed from a Fender Jaguar guitar). This metering is not calibrated or accurately measurable, but in x1 mode, zero represents about 9dB of limiting, whilst in x2 mode this is (predictably) roughly 18dB. This unconventional metering works extremely well in practice, as it is easier to see the point where the threshold is crossed; I soon got used to interpreting the needle movements. The Output meter is conventionally calibrated to +4dBm, and adjustment of the final signal level after limiting is achieved with another dark red chicken head Output Gain knob, with up to +10dBm on tap. The only other control is a three-position lever switch (apparently pinched from a Fender Telecaster guitar) labelled Response. This selects which opto circuit is employed. The simple legending of Slow, Normal and Fast settings belie the power here…

Slow provides the most transparent signal flow. Here the signal remains pretty clean, although there is a lovely warmth to the sound, and fairly heavy limiting is still possible – the limiting takes no prisoners and the compression ratio is very high – waveforms in the DAW are visually very ‘contained’, and when recording a very dynamic vocalist, he found the limiting a little too containing when performing, which meant I had to ride the level to their cans in order to raise the level enough in the choruses. Although this is the ‘clean’ setting, there is a wonderful chocolaty warmth to the overall sound, even with minimal limiting. The high frequencies have a serene smoothness, with a hint of glue to the mids and lows making everything gel nicely. There is a little initial overshoot on the attack, with a small emphasis to transients that varies depending on the amount of limiting.

Medium Response mode ups the ante; a little more harmonic richness becomes part of the character, along with a noticeably faster recovery. Things start to pump a bit here, and I found this great for smoothing out and adding character to acoustic guitar in particular. Transients are noticeably enhanced with a bit of attack. It also worked really well with a hyped-up bright and aggressive ELO-style cello part, bringing a richness and glow, and taming some harshness brought on with the massive EQ boosts.

Fast setting is the truly the acme of this ACME, with an incredible fast recovery, and noticeable juicy distortion becoming apparent as gain reduction is cranked up. This seems to affect the middle and lower frequencies more, so it is not too harsh. According to the manual, even-order overtones are the primary constituent of this distortion. For drum ambience crunching this is truly super, although Medium retains a little more attack for drum sounds if that is needed. With a moderate amount of limiting, one is often tempted to use the faster modes as they seem to provide a higher output level as the mode lever is moved up. But all three modes’ appeal is much to do with the varying release which is a property of all opto limiters, with transients recovering more quickly than longer periods of gain reduction, and the mathematically complex yet audibly appealing rate change that occurs during release.

The difference from other opto limiters that the Opticom boasts is the employment of cadmium-selenide photocells, which are faster than more commonly employed cadmium-sulphide photocells. And a lot of the sonic weight is accredited to the generously specified power supply. Whatever the theory, this is a breathtakingly great sounding limiter. The only improvement I could imagine would perhaps be a lower ratio compressor mode to complement the hard limiting.

Determining the Response mode when tracking is sometimes a matter of engineering bravado – it is certainly tempting to overcook things, but the sound is so luscious that nothing I recorded was ever regretted the next day – the warm characterful sounds always seem to engulf you in a warm fuzzy feeling and bring a smile to the face. I’m not one to boast, but after extensively using the Opticom on a fun one-day session, a successful 21-years established band declared the track to be “the best recording we’ve ever done”! And I humbly suspect most of that was down to the Opticom.

Pros: Wonderfully characterful fast opto valve limiting; Great build; Simple to use - amazing results achieved quickly

Cons: Presently only 115V operation

Reproduced with kind permission from Copyright ©

Visit George's website to find more great reviews.