Yamaha M-4

Yamaha B-4

1 9 7 8 july 1978
1 9 8 2 1982

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.

Yamaha M-4, image 1 Yamaha M-4, image 2
Yamaha M-4 specifications
Title Value
8 Ohm power : 2x 100W (8 Ohm, 20Hz...30Khz, 0,005% THD)
4 Ohm power : 2x 170W (4 Ohm, 20Hz...30Khz, 0,005% THD)
IMD : less than 0,002% (half rated power)
Frequency response : 10...100Khz (8 Ohm, 50W, 0,02% THD)
Bandwidth : DC...100Khz (-0,7 / +0,5dB)
Damping factor : 200 (8 Ohm, 1Khz)
100 (4 Ohm, 1Khz)
S/N ratio :
Stereo separation : 118dB (IHF-A)
90dB (1Khz)
70dB (20Khz)
Input : 1V / 25kOhm (for 100W / 8 Ohm output, 470pF)
PC : 900W max.
Dimensions : 43,5 x 14,5 x 37,4cm
Weight : 18,5kg.
List price : 250,000¥ (1979)
Features : gold-plated RCA inputs
gold-plated speaker outputs
2,5mm pure copper bussing
phase-corrected polystyrene caps
ground termination
AC+DC inputs
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page online since : 2005
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