Altec Lansing 2280B Incremental Power System
Seventy pounds of black steel that lets you bolt in amplifiers one card at a time. Altec's modular answer to how much power a rack actually needs.
Overview
Most power amplifiers arrive with their output stages soldered in and their future decided. The 2280B does not. Altec Lansing built it as a frame - a 19-inch rack chassis, 7 inches tall and 17% inches deep, weighing 70 lb. with eight power amplifier modules aboard - and the user slides output modules in one at a time. Altec called it the Incremental Power System, and the name is literal. You buy the frame, fill it with as many 2271 output power modules as the job requires, and leave the rest of the slots for later or for never. Each 2271 module is rated at 78 watts. The manual's paralleling table lays out what happens as you add them: two modules in parallel give 156 watts into 32 ohms, four give 312 watts into 16 ohms, and a fully loaded eight-module frame delivers 624 watts into 8 ohms. Every one of those configurations holds the same 70.7-volt RMS sine-wave output. The 2280B is a direct-output 70.7V amplifier, the classic distributed-line voltage for commercial sound, and the paralleling scheme lets a single chassis scale from modest to serious without changing the output voltage the rest of the system expects.
The frame is black, fan-cooled by a 75 CFM "Whisper" fan module, and fed by a primary power assembly with a pair of 2500 µF / 150V capacitors and a 6A bridge rectifier. It runs on 100, 120, 200, 220, or 240 Vac at 50/60 Hz, drawing 1200 watts at rated output and about 100 watts at idle. That multi-voltage primary tells you this was gear intended for installers, not for a single market's living rooms.
The input side is what makes the 2280B unusual. The modules include the 15335A input transformer (2284A only), the 15594A low-pass crossover/EQ module, the 15595A high-pass crossover/EQ module, the 15015A input bridging transformer, and the 15515A bridging transformer with resistive pad. The crossover and EQ functions that would normally live in outboard boxes can be dropped straight into the amplifier's input stage. For a permanent installation that means fewer boxes, fewer interconnects, and fewer things to fail in a rack closet nobody visits.
The measured behavior is respectable for a modular commercial amp. Total harmonic distortion is under 0.1%, with typical figures under 0.05%, measured across a 30 kHz bandwidth. Intermodulation distortion carries the same figures. Rise time is under 6 µsec and slew rate is 15 V/µsec. Damping factor exceeds 35 at 100 Hz. Crosstalk is better than 50 dBr referenced to 1 kHz at rated output into 8 ohms with eight modules installed, and noise sits better than 95 dB below rated output, A-weighted, with all eight modules paralleled. Phase response stays within +30° at 20 Hz and beyond −45° at 20 kHz. These are not audiophile-exotic numbers, but they are honest ones for a device whose job is to drive a distributed line reliably for years.
Input flexibility is handled properly. The 2285A, fully attenuated, accepts up to +15 dBu; the 2284A tops out at +1 dBu. Input impedance is 15 kΩ balanced at 1 kHz on either module. Polarity is straightforward: a positive-going signal at the (+) input of the barrier strip produces a positive-going signal at the (+) output. Ground lift is provided via a jumper between signal and chassis ground, shipped connected. The manual notes that systems with ground-related problems can likely be improved by removing it.
The monitoring and fault provisions are more elaborate than most home gear would ever need. Each plug-in amplifier module carries a red and a green indicator. Green lights when signal is applied, red lights on module failure. A fault-indicating relay on the main frame presents a normally open condition at the FAULT terminals during normal operation and closes on failure of any installed module - rated 1 ampere at 12 Vdc or 125 Vac. There is a system-level fault alarm connection using a 9-pin D-sub shorting connector across channels 1 through 8, and a separate channel fault indication connector above the output terminal board so each channel can raise its own alarm.
Serviceability follows the modular logic. The 2271 output module has published adjustment procedures. DC offset at the preamp output must be under 10 millivolts, offset at the amplifier output no more than 5 millivolts via potentiometer R17, and bias is set for 9 mA of output-stage quiescent current, which corresponds to 14 millivolts across R36 or R37. The module runs from ±120 Vdc rails and uses ±10 Vdc on its control pins, with pins 15 and 17 connected to enable the amplifier. Altec's own documentation warns in capitalized terms that there are no user-serviceable parts inside and that hazardous voltages and currents are present. Given the rail voltages involved, that warning is fair. The company also explicitly does not recommend modifications, noting that any damage from them falls outside warranty.
Collectors and installers who know the 2280B tend to describe it in terms of what it is rather than what it sounds like: a frame, a way of buying amplifier power in increments. The 78-watt module rating is modest on its own, but the paralleling architecture means the same chassis can be a small two-module amp or a 624-watt eight-module workhorse without a different model number. Complete, fully populated examples with a full set of input and crossover modules are the ones worth seeking out. A bare frame is a project, and a loaded one is a working tool.
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