Cerwin-Vega Vega A-400 (ca. 1979)
A rack-width, fan-cooled brute that Cerwin-Vega built to live in the same rack as the studio gear. The service manual reads like it expects you to fix it yourself.
Overview
The spec sheet doesn't tell you what the A-400 is. The service documentation does. Cerwin-Vega opens its own manual by warning that these are professional, high-power amplifiers that "absolutely require proper test equipment and qualified service personnel," then lists what you need to work on one: a digital multimeter or VTVM, a dual-trace oscilloscope, an AC current meter, a 20-amp variac, a distortion analyzer with residual distortion of .002% or better, and load resistors rated for a kilowatt at 2 and 4 ohms. That is not the tool list for a receiver. It's the list for something that can hurt you and your bench.
The manufacturer's specification table backs that up. The A-400 is rated at a minimum continuous 225 watts per channel into 8 ohms, 20 Hz - 20 kHz, both channels driven, and 350 watts per channel into 4 ohms under the same conditions. Note the word "minimum." Cerwin-Vega wasn't quoting a peak, a one-channel number, or a 1 kHz burst. This is the floor, full bandwidth, both channels loaded. Period coverage worked from a slightly different figure: Modern Recording (August 1979, p.73) described the A-400 as rated to deliver better than 200 watts per channel into 8-ohm loads, or over 350 watts per channel into 4-ohm loads. Either way, the amp is built on the assumption that the speakers hanging off it are inefficient and the room is large.
The distortion numbers surprise people who file Cerwin-Vega under "loud." Total harmonic distortion across 20 Hz - 20 kHz, from a quarter-watt up to full power, is specified at 0.03% into 8 ohms and 0.05% into 4 ohms. SMPTE intermodulation distortion over the same quarter-watt-to-full-power range at 8 ohms is also 0.03%. Signal-to-noise referenced to rated output with A-weighting is 113 dB, an extraordinary figure for the period. IHF noise, referenced to 0 dBW with A-weighting, is −90 dB.
Now the bandwidth. Power bandwidth is specified as 7 Hz to 100 kHz. Slew rate is 80 volts per microsecond. Damping factor, referenced to 50 Hz into 8 ohms, is 250, high enough that on most speakers the amplifier isn't the limiting factor in the bass. Input impedance is 10 kΩ, input sensitivity for full output is 1.4 V, and IHF sensitivity referenced to 0 dBW is 0.094 V. In plain terms, it takes a normal line-level signal and turns it into a great deal of power without asking much of the source. Inputs are phono (RCA) or ¼″ phone jacks. The phone jack is the tell that this amp was expected to sit in a studio rack as readily as a hi-fi cabinet. Outputs are dual five-way binding posts.
The physical package is rack-oriented. Mix Magazine (May 1979, p.45) lists dimensions of 19 × 5.25 × 13 inches and a weight of 31 lbs, with the same RCA-or-¼″-phone input arrangement. Nineteen inches wide and 5.25 inches tall is a standard 3U rack panel, and the front panel carries handles. Modern Recording (August 1979, p.73) noted the handles and the power switch as the panel's contents, which is about as minimal as a power amplifier front end gets. The unit is fan-cooled, because 225 watts per channel of class-AB dissipation has to go somewhere.
Inside, the service manual lays out a fairly conventional but carefully executed topology. The drive circuitry uses a dual differential first stage, Q202/Q203 and Q204/Q206, running from a ±24 V supply set by zener references, providing the initial voltage gain. Q201 and Q205 act as level shifters driving a class-A stage, with drivers Q211 and Q212 providing further current gain for the output stage. Electronic current limiting is handled by Q213 and Q214. When the voltage across any output-stage emitter resistor climbs too far, one of them turns on and clamps the driver's base to the output rail, cutting off further current drive. The output stage itself is a bank of paralleled devices, Q601 through Q608, with Q301 and Q302 forming a bias compensation network. Q301 is mounted behind the circuit board directly against the output heat sink, sensing temperature and adjusting the idling current back in the class-A stage to keep thermal drift in check.
A lot of the manual is devoted to fixing the thing. There's a protection-circuit troubleshooting flowchart that walks you through disconnecting the three-pin connector from channel one's output board, waiting about fifteen seconds, and seeing whether protection resets, then repeating for channel two. There's a procedure for bypassing the output protection relay by moving the two outside yellow wires onto the inside terminals, with the manual shouting in capitals to remember to reconnect them afterward. There's a bias and DC offset adjustment procedure: connect a DC voltmeter between the output and the VI sense buss on the yellow and red wires, and adjust R246 to land between 22 mV and 28 mV. For DC offset, measure from circuit ground to the yellow wire on top of the output board and adjust R203 to get within about 10 mV of zero. The manual also warns that the fifteen-pin connector has high DC voltages on exposed pins, and that the finned heat sinks on adjacent driver transistors sit at high potential relative to each other. It suggests using an insulated-shaft screwdriver through the rear adjustment slot rather than reaching in.
The manual also documents its own revisions, which shows what went wrong in the field. R426, a relay resistor, was changed from 2.7 kΩ at half a watt to 2.7 kΩ at 1 W on units with serial numbers below 790700, to keep the resistor running cooler. R419 was changed from 15 kΩ to 8.2 kΩ on units below 790500 to reduce sensitivity to DC output voltage, specifically, the manual notes, when the amp is used with bass synthesizer boxes producing asymmetrical waveforms. The service addenda state that the listed modifications should be made to any unit that doesn't already have them, and that one or more could apply to serial numbers as high as 791000. If you're shopping for one, check that serial range.
Contemporary reception was strong. Modern Recording (August 1979, p.73) found the combination of very ample power output with ruggedness and reliability striking. The same issue's individual comment (p.75) observed that the dividing line between "professional" and "home audiophile" amplifiers was growing fuzzy, placing the A-400's performance in the top class of either category, taking together IM and harmonic distortion, TIM, slew rate, and transient response.
The record doesn't give up the retail price. Mix Magazine (May 1979, p.45) listed one, but the text is cut off after "Sug…" in the surviving source, so the figure is lost. Production years beyond the 1979 review window aren't documented either. The A-400 turns up in at least one period auction listing alongside a smaller A200, which suggests the two shared a lineup. Beyond that, the paper trail is short.
The amplifier is well documented technically and thinly documented historically. What survives is a service manual detailed enough to rebuild one from, a spec sheet written by engineers rather than marketers, and a pair of 1979 reviews that put it at the top of its class.
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