mjr7 mk3 mosfet audio power amplifier 70w

Posted on Feb 4, 2014

The Electronics Schematic Diagram MJR7-Mk3 Mosfet MJR6 page includes distortion extracted using the nulling method with a speaker load and a music signal to demonstrate that these designs have no audible distortion in normal use. The MJR7 has even lower distortion, for example the 19kHz + 20kHz intermodulation product at 1kHz is about 19dB lower.

mjr7 mk3 mosfet audio power amplifier 70w
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I have a new improved board layout, and may make boards available soon. Now there is an additional 220uF capacitor plus two diodes and a resistor which slow down the switch-on to keep the output pulse low enough for use with most high sensitivity speakers. For use in a bi-amp or tri-amp system with active crossover filters the pulse also needs to be small so that direct driven high frequency drive units are not damaged. The circuit addition is a simplified version of an idea suggested to me a few years ago. With the modification added the switch-on thump becomes almost inaudible. With the original MJR6 the output pulse was 4V peak, this has now been reduced to 1V. There is no audible thump at switch-off. I would prefer to leave out the 12V gate protection zeners, but I doubt whether the mosfet internal zeners have adequate current rating. With a negative output into a low impedance or shorted output the zener current would only be limited by the 300R gate resistor, which is why the zeners are placed after the resistors, and relatively high 300R values are used. There are possible alternative, but so far untried, input stage transistors, the Toshiba 2SA970BL and 2SC2240BL, which appear to be good substitutes for the Hitachi 2SA1085E and 2SC2547E. The BL` suffix is the high gain group. Alternatives for the Sanyo 2SC2911 and 2SA1209 are the Toshiba 2SC3423 and 2SA1360, which again are untried. Based entirely on my own...

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