Electro-Voice EV-7600
A 52-pound rack-mount power amp with a damping factor of 250 and a service manual that assumes you own an oscilloscope.
Overview
The EV-7600 is a two-channel, rack-mount power amplifier from Electro-Voice's commercial sound line. The first thing a buyer notices is the documentation. The manual doesn't treat the owner as a hobbyist. It walks through restrapping the power transformer's dual 120-volt primaries for 220/240-volt operation. It warns that a bridged output is true balanced-to-ground and that test equipment without floating grounds must not be connected. Then it hands over a full trimpot alignment procedure that needs a Tektronix 2445-class scope, a Sound Technology 1700B distortion analyzer, a 15-amp AC ammeter, and dummy loads rated at 500 to 1,000 watts. That's not a consumer amp's service section. That's a page out of a touring-rig binder.
Electro-Voice built the 7600 to drive real loads hard and stay cool doing it. The amplifier will operate from line voltages as low as 90 volts AC at reduced output power, which helps in venues and installs where the AC sags when the house lights and the backline all come up at once. Two amplifier models exist. One has dual 120-volt primary windings that can be strapped in parallel for 120-volt operation or in series for 220/240-volt operation. The other is a separate export model for 100-volt, 50/60 Hz regions. The 120-volt version ships from the factory strapped for 120 volts. Changing it to 220/240 means pulling eleven screws from the top cover, reconnecting the barrier strip leads on the transformer, and installing an 8-amp Littelfuse 3AB slo-blo fuse. That's a ten-minute job for anyone who reads the instructions first.
The amplifier is a genuine 19-inch rack unit. It takes up 5¼ inches of vertical rack space and secures with the four rack-mount screws and cup washers included in the hardware kit. The chassis is 16-gauge steel with a 3/16-inch 5052 aluminum alloy front panel, finished black. Net weight is 52 pounds; shipping weight is 62. That's a two-person lift into a rack, and the weight tells you where the money went: the heatsinks. Each channel gets its own thermally equalized 3/16-inch aluminum black anodized heatsink. The output stage is a true complementary symmetry design with grounded collectors - no mica insulators, which means better heat transfer from the output devices to the metal. Thirty-two output devices are used across the amplifier, rated at 250 watts dissipation, 250 volts Vceo, 16 amps Ic, and a 200°C maximum junction temperature. A thermostatically controlled dual-speed ball-bearing fan moves roughly 55 CFM at low speed and 105 CFM at high speed, with a minimum life rating of 50,000 hours at 25°C ambient. Electro-Voice rates the amplifier for operation up to 60°C (140°F) ambient.
The spec that tells you what this amp is for is the damping factor. It's rated at greater than 250 from 20 Hz to 1 kHz (and still above 75 at 20 kHz) in dual mode into 8 ohms, which works out to an output impedance of about 0.032 ohms. Electro-Voice's own manual explains the reason. When a woofer's cone overshoots its resting point after the signal stops, the voice coil generates a back-EMF that travels back down the speaker cable. The lower the amplifier's output impedance, the faster that unwanted motion dies out. A damping factor of 250 is the kind of figure that keeps large cone drivers under control on kick drum and bass guitar. It also means the amp is unusually sensitive to what's between it and the speaker. The manual devotes several pages to cable loss calculations for exactly this reason: 160 feet of 18-gauge wire on a 4-ohm load will consume more than half the amplifier's rated 4-ohm power output. The 7600 will deliver the current, but it can't fix your cable.
Distortion figures are conservative, the way pro amplifier specs usually are. Total harmonic distortion is specified at less than 0.1% with a typical figure under 0.01%, measured across a 30 kHz bandwidth. SMPTE 4:1 intermodulation distortion is under 0.05%, typically under 0.01%. TIM (DIM 100) is under 0.05%. Rise time is under 6 microseconds, and slew rate is greater than 30 V/µs in dual mode or greater than 60 V/µs in bridge mode. Crosstalk is better than 80 dB below rated output at 1 kHz. Noise sits more than 100 dB below rated output with A-weighting, single channel operating into 8 ohms. Phase response stays within +25° at 20 Hz and −15° at 20 kHz in any mode. Voltage gain is 36 dB in dual mode into 4 or 8 ohms and 42 dB in bridge mode into 8 or 16 ohms. Input impedance is 15 kΩ balanced or unbalanced, and maximum input level is +20 dBu (7.75 V rms). The input section accepts a positive-going signal on pin 2 of the XLR or the (+) barrier terminal and produces a positive-going signal at the (+) output terminal.
Connections are aimed at installed and touring work, not a living room. Inputs include two ¼-inch phone jacks, two female XLRs, and two octal accessory sockets powered with ±15 volts DC at 25 mA. Those sockets are the expansion bus for optional plug-in modules, and the manual cautions that any accessory module must not draw more than 25 mA. Outputs are a four-terminal barrier strip plus three Neutrik Speakon connectors. The rear panel carries the mode switch and two input level controls. The front panel has the power switch, a power LED, two clip LEDs, and two protect LEDs. Protection circuitry covers shorted output terminals, overtemperature, and RF interference on the amplifier side, and startup/shutdown transients, DC faults, and subsonic signals on the load side. The power cord is a 4-foot (1.22 m), 3-wire, 14-gauge cable with a NEMA 5-15 plug. Power consumption at rated output with both channels running a 1 kHz sine into 4-ohm loads is 2,520 watts, producing 4.488 kBTU/h of heat. At 1/3 max midband power it draws 1,800 watts (4.465 kBTU/h), at 1/8 max midband power 1,440 watts (4.276 kBTU/h), and at idle 100 watts (0.340 kBTU/h). The unit's overall power requirement is listed as 120 V ac, 50/60 Hz, 1,800 watts, configurable to 220/240 V ac.
Electro-Voice offered three optional plug-in accessories for the octal sockets: the APX 24 dB/octave Linkwitz-Riley crossover, the APX-2 24 dB/octave Linkwitz-Riley crossover with external high-frequency output, and the APX-200 equalizer module for S200 and FR200 speakers. The manual also recommends a capacitor between the amplifier and any compression driver to block low frequencies and DC — a chart of values is provided for 8- and 16-ohm drivers at crossover points from 500 Hz to 6,300 Hz, with Mylar capacitors of at least 100 volts AC rating specified. Electro-Voice's reasoning is that compression drivers are far more fragile at low frequencies than cone woofers, and a misadjusted electronic crossover or a cable problem between the crossover and the amp can send damaging low-frequency energy or hum straight into a driver. Owners running the 7600 with horn-loaded mid/high sections should take that advice seriously.
Service access is straightforward for anyone qualified to open the chassis. The top cover comes off with eleven screws: three per side, three along the top-rear edge, and two innermost screws inset into the top bumper strip. Two of the side/rear screws are longer than the rest and go back into the rearmost position on each side. Inside, bias is set by R26, positive current limit by R24, and negative current limit by R23 (these two also affect clipping symmetry). The LF cancel trimpot R39 minimizes distortion caused by power supply ripple. The manual's R39 procedure involves feeding a 60 Hz, 0 dBu sine into each channel with 8-ohm loads connected and adjusting for minimum ripple on a scope. Electro-Voice's own warning is blunt: no user-serviceable parts inside, extremely hazardous voltages and currents within the chassis, and servicing is for authorized warranty stations and qualified personnel only. Modifications are explicitly not recommended and void warranty coverage on any resulting damage.
The 7600 is a workhorse in the Electro-Voice line, not a collectible. It was built for installed sound, touring rigs, and any application where the amplifier has to survive a hot rack and a long cable run. The 5¼-inch rack height, the Speakon and barrier-strip outputs, the octal accessory bus, and the ±15-volt module power all point at a system designed to be configured once and left alone. The published specifications don't name a rated wattage per channel in the manual's spec section as reproduced here, so buyers should verify output power against the specific revision of the manual or the unit's rear-panel label. What the documentation does establish is a conservative distortion spec, a very high damping factor, a heavy-duty thermal design, and a protection scheme that guards both the amplifier and the loudspeakers connected to it.
The practical questions for a 52-pound pro amp of this vintage are the usual ones: whether the fan still spins freely at both speeds, whether the output devices are original, whether the trimpots have drifted, and whether the transformer is strapped for the line voltage in your region. The dual-primary design makes the 120-volt model usable in 220/240-volt territories with a fuse change and a re-strap. The octal sockets are only useful if the APX modules come with the amp, since those accessories are long out of production and rarely surface separately. The Speakon outputs are the modern standard and remain easy to cable. The barrier strip is still the simplest way to land heavy gauge wire. The manual's own cable-loss tables make a strong argument for running 10-gauge on long 4-ohm runs.
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