McIntosh Model MC240 Technical Description


The following description is from the McIntosh MC240 Owner Manual:

The patented McIntosh Unity Coupled circuit and output transformer have established McIntosh amplifiers as the unchallenged leaders in the audio field.

Before 1947, low distortion at high power and high efficiency was impossible. A completely new engineering approach resulted in an amplifier that for the first time permitted high power with distortion below 1%. That new engineering produced the McIntosh Unity Coupled circuit and the McIntosh bifilar wound output transformer. With the introduction of the McIntosh amplifier, new standards for distortion-free performance were established.

The McIntosh output transformer is unique. It has two primary windings which are wound bifilarly. In the bifilar technique, both primary wires are wound side by side. Each turn of primary number one is next to the same turn of primary number two. There is almost complete magnetic coupling between the two wires. The magnetic coupling is reinforced by the capacitance between the two wires.

In the McIntosh Unity Coupled circuit, one of the bifilar primary windings is connected through the power supply to the plate and cathode of one of the output tubes. The other bifilar primary winding is similarly connected to the other tube.

All low distortion high power amplifiers use push-pull output circuits known as Class AB1, AB2, or B. Two tubes are arranged in a balanced circuit. This permits each tube to operate alternately, somewhat over half the time. Compared to full time operation of the tubes, the push-pull method reduces heating and permits more power from a given type of tube. Despite this advantage of the conventional push-pull circuit, one problem in particular remained to be solved. When the current in each tube is cut off to begin the idle period, distortion is produced at the instant of cutoff and again at the instant when current flows. This form of distortion is known as Notch Distortion and was well illustrated by Mr. Pen Tung Sah in the "Proceedings of the I.R.E." Volume 24, pp 1522-1541 in 1936.

Imperfect coupling between the primary windings found in all conventional output transformers produces the condition which permits notch distortion. Trying to improve coupling in a conventional transformer decreases the power response at both low and high frequencies, heating the output tubes and lowering the available power output.

The McIntosh Unity Coupled output circuit and bifilar transformer is the first commercial breakthrough that eliminates notch distortion by coupling both output tubes almost to perfection. In the McIntosh transformer, the extremely close coupling of the bifilar windings removes the condition which permits notch distortion. Furthermore, the two output tubes are arranged as partial cathode followers. Half of the output circuit is in the cathode and half in the plate of each tube. The output tubes now are operating in a local feedback loop which reduces their distortion, reduces their internal generator resistance, and reduces their balance requirements. The McIntosh circuit in reality perfects push-pull high efficiency output circuits.

Leaking inductance (lack of coupling) between the primary and secondary windings of the output transformer limits the high frequency response of an amplifier. The primary and secondary windings of the McIntosh output transformer are interleaved five times to improve coupling. The interleaving is accomplished by winding groups of primary layers, then secondary layers alternately, until the total transformer is wound. Interleaving helps to extend the McIntosh power band width to over 100,000 cycles. Since 1947, this new technology has been built into every McIntosh power amplifier.

Good voltage regulation in the power supply permits overloads without overshoot or blocking, good transient resonse, and complete stability. To improve regulation, a silicon rectifier power supply is used in the MC240. In addition to better voltage regulation, the silicon rectifier allows even higher operating efficienty, cooler operation, and longer amplifier life.

To greatly extend tube and component life, a thermistor in the MC240 limits current surges produced when the equipment is turned on. The thermistor is a special resistor. Its resistance depends on its temperature. When the amplifier is off, the thermistor has a high resistance value (about 79 ohms). Just after the amplifier is turned on, the current which flows through the thermistor heats it and causes its resistance decrease to a low value (less than .1 ohms). Current is thus limited when the MC240 is first turned on, but is not limited as the unit warms.

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