Leach amplifier

Posted on Sep 17, 2012

A hot topic of amplifier design in the 1970s was `transient intermodulation distortion` (TIM). Other names which were used for this phenomenon were `slewing induced distortion` (SID), and `dynamic intermodulation distortion` (DIM). TIM occurs when a transient input signal overloads the input stage of an amplifier, causing it to either cut off or to become nonlinear for a brief instant before the feedback signal arrives. If the input stage cuts off, the amplifier output slews and the amplifier produces hard TIM. If the input stage becomes nonlinear but does not cut off, the amplifier is said to exhibit soft TIM.

Leach amplifier
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The amplifier will drive a 4 ohm load to full power without current limiting. Depending on the power supply regulation, the output power with a 4 ohm load is as great as twice the power with an 8 ohm load. With loads lower than 2 ohms, the protection circuits limit the maximum output current, and thus the output power, to protect the output transistors. The amplifier is stable with capacitive loads and will drive electrostatic loudspeakers with no problems. The 2N3439/2N5415 pair is the one that I originally used for my prototype amplifiers. It is the complementary TO-5/TO-39 case pair recommended in the RCA Power Transistor Manual for audio amplifiers. For all practical purposes, the 2N3440 and 2N5416, respectively, are equivalent to the 2N3439 and the 2N5415 for the Low TIM amplifier. Q12 - Q15 can run a little warm. I have never had problems with these running too hot. With the Version 4.4/4.5 amp, I have reduced the bias currents in these transistors just a little to keep them cooler. Although I don't think they are needed, you can put TO-5 clip-on heat sinks on them. There is not much room on the circuit board for the heat sinks, so they must have small fins. Another type of heat sink which will fit is in the shape of a flag (about 1 inch high by 3/4 inch wide) with rounded clips on one end that clip around the transistor. If you can't find them, they are easy to make from sheet metal flashing. The clips should make...

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