A little best-seller of sorts the M-4 !
Perhaps more than the M-2, the M-4 circuit is placed within a truly excellent design - stark, bold but not loud.
Splendid Yamaha looks and an excellent amp, too, even if the basics of this design were first attempted at by Hitachi with the HMA-3790 and HMA-4590.
The japanese version of the M-4, named B-4 (to keep the "B" prefix inaugurated with the B-1 in 1975), is fairly different : two more PS caps, two transformers instead of one, LED meters replaced by an output impedance slider (as seen in a few other contemporary Yam' units) and, mostly, the possibility to switch the B-4 in Class A operation !
Thanks to Roger Bownds, here are some excerpts from the M-4's owner's manual :
"The voltage amplifying stage employs a cascode-connected differential two-stage circuit throughout.
The use of a Dual FET input and a new cascode bootstrap circuit in the first stage and a cascode-connection, current-mirror and push-pull output in the pre-driver stage ensures a high degree of stability, low distortion and improved high-frequency characteristics.
In the driver and the power amplifier, a three-stage emitter follower is paralleled with pure complementary OCL SEPP circuitry using high-fT transistors. This accounts for wide bandwidth, high output and low distortion.
Two extremely large 15,000µF capacitors, two separate constant-voltage power supplies for the voltage amplifying stage, and the use of a pure copper plate earth line for ultra-low impedance guarantee greatly reduced intermodulation distortion, improved separation and absolutely stable operation.
Except for the chemical capacitors in the power supply section, mylar and styrol capacitors are employed throughout ; layout of parts, wiring, earth line as well as many other aspects have been given due consideration."
I believe the transistors for the M-4 to be four pairs of 2SC1850 and 2SA889 ; those of the original B-4 are 2SA1050B and 2SC2460B.
Like the M-2 / B-5, the M-4 / B-4 are, specs-wise, also quite different.
Japanes B-4s at the indispensable amp8.com : 1 and 2.
Thanks to Hernan, B-4 owner, a comparison of the two versions can be seen here.