Part of the myriad of huge amplifiers Yamaha produced during the 1980s : well-built, powerful enough to almost drive a pair of Magnepans, looking in sync with the times (flashing red and black) and nearly indestructible.

Auto-Class A was for the later M-80 revision but the ZDR system was applied here as well as a fully DC structure. ZDR (aka Zero Distortion Rule) is a way to re-inject the detected distortion signal but in phase-inverted mode.

The M-70 was a "B-70" in Japan - "best product" there just the same.

The smaller M-50 was introduced alongside ; later replaced by the M-80 / M-60 / M-40 in 1984, all three with the added "Auto Class A" feature, somewhat bigger looks plus even more available power.