19Technical Descriptionmagnetic field. The force of the field causes the leads tobend and contact one another inside the sealed glass tube.The inert gas eliminates corrosion of the contacts and in-sures a low resistance, distortion free switch.All inputs, outputs, and data ports are controlled bylogic circuits in the MA6900. The logic is changed by frontpanel push-buttons or by a microprocessor IR decoder.This microprocessor IR decoder is programmed with ex-clusive McIntosh software. It receives data from the frontpanel or external sensors and provides the command sig-nals for input switching, data switching, and volume con-trol.The equalizer section uses several high technology op-erational amplifiers. The amplifier stage has been opti-mized for the best transient performance and minimum dis-tortion. Five other operational amplifiers are arranged in acircuit configuration that is the equivalent of a series turnedcircuit. A control potentiometer inserts this series tuned cir-cuit into either the feedback or input section of the equal-izer amplifier. This provides a 12dB boost or cut at the fre-quency of the tuned circuit. Refer to figure 10. The overallgain of the stage is 0dB when the potentiometer is at itscenter detent position. In this position the tone control ele-ments are completely removed from the signal path.Power Amplifier Design PhilosophyThe design philosophy incorporated in the MA6900 in-volved several different techniques, all based on sound sci-entific logic. Refer to figure 11. Every stage of voltage orcurrent amplification must be as linear as possible prior tothe use of negative feedback. McIntosh engineers knowhow to properly design negative feedback circuits so theycontribute to the extremely low distortion performance ex-pected from a McIntosh amplifier. The typical McIntoshowner would never accept the approximately 100 timeshigher distortion of many non-feedback designs.All transistors are selected to have nearly constant cur-rent gain over the entire current range they must cover.Output transistors in particular, have matched uniform cur-rent gain, high current-bandwidth product and large activeregion safe operating area. An automatic tracking bias sys-tem completely eliminates any trace of crossover distor-tion. Precision metal film resistors and low dielectric ab-sorption film capacitors are used in all critical circuit loca-tions.The output signal of the circuit is coupled together inthe unique McIntosh MA6900 Output Autoformer. It pro-vides low distortion power transfer at frequencies from be-low 20Hz to well beyond 20,000Hz with optimum imped-Figure 11Block Diagram of the Amplifer and Meter Circuitry
