The JBL Model 6290 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 an amplifier 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)”, JournaI of the Audio Engineering Society, Vol. 25, No. 4, April, 1977, pp. 170-177.) This JBL amplifier uses multiple 200-watt output devices in complementary configuration for high reliability and low distortion. At rated power Into 8 ohms, these output devices are operated at less than 25% of their rated power dissipation. The benefit Is high reliability and long component life. Reliable operation Is ensured through the following protection modes: current Is limited under Improper load or drive conditions; output relays with front panel Indicators protect the loudspeaker under conditions of DC offset or excessive low-frequency transients. The relays also provide power-up, power—down, and "brown out’1 muting to protect loudspeakers from AC power transients. Clip Indicators allow for system optimization. The JBL amplifier may be operated In the normal stereo mode, dual mono mode, or bridged mono mode. These modes are switch selectable on the rear panel. Active differential Input circuitry offers the benefits of balanced operation without the use of Input transformers. Input connections may be made via 3-pln 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 rack ears and heatsink of the amplifier are made of high grade aluminum extrusions; the chassis Is fabricated of heavy gauge steel. All Internal components are easily accessible through removal of top and bottom panels. Front panel graphic details are Incorporated on the rear-side of a polycarbonate laminate which is virtually Indestructible.