The JBL Model 6215 power amplifier has been designed to meet the most critical professional sound requirements. It is rugged and road-worthy, conservatively rated, and can handle reactive loads with ease. The engineering design approach stresses the optimization of each stage, allowing high slew rate and relatively low loop gain. Overall feedback has been held to a minimum and is employed only to stabilize the gain and the operating point. This design approach results in amplifiers with excellent performance under the most demanding dynamic input and load conditions. As evidence of the stress on dynamic rather than static or steady-state distortion mechanisms, transient intermodulation distortion measures less than 0.03% by the DIM 100 test. (Leinonen, Otala, and Curl, "A Method for Measuring Transient Intermodulation Distortion (TIM)", Journal of the Audio Engineering Society. Vol. 25, No. 4, April, 1977, pp. 170-177.) Reliable operation of the amplifier is ensured through the following protection modes: Current is limited under improper load or drive conditions. An output relay, with front panel LED indication, protects the loudspeaker load under conditions of DC offset or large low-frequency transients. The relay also provides power-up, power-down, and "brown out" muting to protect loudspeakers from AC power transients generated anywhere in the signal path. LED's on the front panel indicate the onset of clipping and standby mode. The JBL Model 6215 amplifier may be operated in the normal stereophonic mode, dual monophonic mode, or bridged monophonic mode. Rear panel switching sets these modes, obviating the need for patch cords, level matching, etc. Active differential Input circuitry offers the benefits of balanced operation without the use of input transformers. Input connections may be made via 3-pin XL-type connector, three-conductor (TRS) 6.3 mm (1/4 in.) phone jack, or barrier strip. The barrier strip has separate terminals for audio ground and chassis ground. The five-way output binding posts are arranged on 19 mm (3/4 in.) center so that bridged as well as normal connections may be made with standard dual banana plugs, bare wire, or terminal lugs.