Electro-Voice CPS-1
A Contractor Precision Series power amp rated at 230 watts per channel into 8 ohms and 650 into 2, built for the install rack that never gets a day off.
Overview
The CPS-1 is the smallest of four amplifiers in Electro-Voice's Contractor Precision Series. It is not a hi-fi amp and was never marketed as one. It is a fixed-installation power amplifier, the box that lives in a rack behind a bar, in a house of worship, in a conference center. The rated power figures show the priorities: 230 watts per channel into 8 ohms, 350 into 4, and 450 into 2, with maximum output at 1 kHz and 1% THD quoted at 280, 450, and 650 watts respectively across those same loads. That last number is the interesting one. An amplifier that more than doubles its 8-ohm output into a 2-ohm load is built to drive a lot of speakers off a single chassis - distributed systems, paralleled cabinets, long speaker runs.
The rated-power spec is quoted at less than 0.2% THD across 20 Hz to 20,000 Hz with both channels driven. The headline distortion figure is under 0.05%, with intermodulation distortion under 0.08%. Those are honest numbers rather than spectacular ones. The amp isn't chasing the last thousandth of a percent, it's chasing reliability at volume over a long day. Slew rate is quoted at 25 V/µs, the lowest of the four CPS models. The CPS-2 steps to 30, the CPS-3 to 35, the CPS-4 to 40, so the series was clearly engineered as a ladder rather than four unrelated boxes. Crosstalk at 1 kHz is better than 80 dB, and the balanced input impedance is 20 k ohms, so it will sit on the end of a long console snake without loading anything down.
What the CPS-1 hangs its reputation on is the protection scheme and the headroom. Electro-Voice built what the manual calls a 5-way protection system. Over-temperature protection keeps the amplifier from reaching a damaging operating temperature. DC protection shuts things down in the event of a direct short. An excessive back-EMF monitor guards against reverse current flowing back into the amplifier when a speaker fails, a failure mode most amps simply don't address. A low AC line voltage monitor prevents performance problems and potential speaker damage during brownouts. The cooling is a three-stage front-to-rear fan system, with the CPS-3 and CPS-4 stepping up to four stages. The fans are described as extremely quiet, which helps when the rack is in the same room as the audience.
The power supply was specifically designed to deliver high peak signals, and Electro-Voice documents a burst signal output capability in excess of 130% over the amplifier's average continuous rating. Live music and speech are mostly transients riding on a much lower average level, so an amp with 130% burst headroom sounds louder and cleaner than its continuous rating suggests. The mechanism is a circuit Electro-Voice calls the Nonlinear Signal Monitor, a dynamic limiter that holds amplifier distortion to under 1%, specifically to eliminate the hard-edged clipping that destroys speaker systems.
Around the back, the CPS-1 is set up the way install contractors expect. Inputs are electronically balanced on XLR-type and PHOENIX connectors, and those same connectors are used to loop the signal through to the next amplifier in the rack. Input routing switches select DUAL (stereo) or PARALLEL (monaural) operation, and monobridged operation is also possible. In bridged mode the CPS-1 is quoted at 900 watts into 8 ohms and 1300 watts into 4 ohms at 1 kHz and 1% THD, useful for subwoofer duty or for driving a single high-power cabinet where stereo separation is irrelevant. In normal operation the amp will drive loads down to 2 ohms. In bridged mode the minimum load is 4 ohms. Level controls are dB-scaled and mounted on the rear panel rather than the front, which suits an amp that gets set once and locked in a rack. The front panel LED display reports, per channel, whether the channel is operational, whether signal is present at the output, whether the limiters have engaged, and whether any protection circuit has tripped. Outputs are 4-pole barrier strip connectors on channels A and B. There's a ground-lift switch to break enclosure-to-ground potential and kill ground loops, and the mono bridged mode switch lives back there too. Power requirements are 120 V, 50-60 Hz, factory configured.
The documentation for the CPS-1 is unusually good for this era of pro gear. An owner's manual covering the CPS-1 and CPS-2 exists, complete with block diagram, dimensions, spec section, and warranty information, and the service documentation survives as well. The CPS series was sold with a three-year warranty. An amp with a documented protection topology and a surviving schematic is an amp you can keep running. The manual is also direct about its own advice: study it carefully, keep it, and the amplifier will provide a lot of fun and satisfaction in your work.
The CPS-1 is a 2-ohm-stable, bridgeable, fan-cooled, rack-mount professional amplifier from a major American pro-audio house. It was built for a market segment, fixed install, that doesn't generate the collector chatter of a studio monitor or a tube compressor. None of the surviving documentation gives production years, original price, dimensions, or weight, and the fact sheet carries no independent period review of the CPS-1 specifically. The CPS series as a whole is documented across the owner's manual and spec sheets, but the paper trail on the individual model is thin. A CPS-1 today is bought on function rather than on reputation: a quiet, 2-ohm-capable install amp with transient headroom and a protection system designed around real-world failure modes.
Similar items on eBay
No exact Electro-Voice CPS-1 listings are live right now — these searches show current Electro-Voice and similar vintage audio gear on eBay:
As an eBay Partner, we earn from qualifying purchases. This helps support our independent vintage technology research.