Power Amplifier Design Philosophy

Technical Description

magnetic field. The force of the field causes the leads to bend 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 by logic circuits in the MA6900. The logic is changed by front panel push-buttons or by a microprocessor IR decoder. This microprocessor IR decoder is programmed with ex- clusive McIntosh software. It receives data from the front panel 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 a circuit configuration that is the equivalent of a series turned circuit. 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 overall gain of the stage is 0dB when the potentiometer is at its center detent position. In this position the tone control ele- ments are completely removed from the signal path.

The 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 or current amplification must be as linear as possible prior to the use of negative feedback. McIntosh engineers know how to properly design negative feedback circuits so they contribute to the extremely low distortion performance ex- pected from a McIntosh amplifier. The typical McIntosh owner would never accept the approximately 100 times higher 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 active region 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 in the 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-

Block Diagram of the Amplifer and Meter Circuitry

Figure 11