(1986 - 1988 - 1997)
to the C-90, C-90a
and C-91 preamplifiers
but with much less visible differences between them.
However, the original M-90 and the subsequent M-90a/M-91 are built
quite differently, although they look... identical.
M-90 is a 20kg beast with two
220VA EI transformers, 4 parallel push-pull pairs of 130W Pc transistors
and 48,000µF of smoothing caps - the japanese M-90a seems to
differ on this with only 40,000µF. Caps are aligned just behind
the front plate ; the two trafos make a straight line toward the
back plate with the heatsinks and power boards flanking the latter
on the sides ; total current capability is of 47A.
M-90 in repair, part
1 and part
M-90a has the same basic structure
but swaps the EI transformers with the cast-iron ones you can see
below and the thin fin heatsinks with beefier honeycombed ones ;
caps are maintained at 48,000µF. Also a thick two-part top
plate is added, 1,6mm honeycomb bottom chassis, resin-cast capacitors,
five cell-polymer-filled insulators - and more to mainly strengthen
mechanical stability (going from 20kg to 28kg !) and heat dissipation.
The EI transformers were however seemingly kept for the non-JP and non-US versions ; and the added top plate seemingly was for the units with potted transformers only... European models weigh 23kg and therefore only saw half of the upgrades between the original M-90 and the M-90a.
are otherwise identical and common features are Non-Switching structure,
70µ PCB tracks, input/output comparators, very large and very
flashy red meters and three inputs available : CD Direct, Line Direct
and Control amp. The first two bypass the input stages and buffers
but go through the front volume pot ; said volume and switches in
fact use a long shaft and relays, respectively, to keep signal paths
as short as possible. The small front flap hides speaker pair selectors
(A/B), a welcome display on/off switch and a headphones plug.
A two-position switch at the back allows to select speaker's impedance,
the 4Ohm...6Ohm position being restricted to using only one pair
of loudspeakers - the M-90/90a is a high quality powerhouse but
it isn't an MX-10000
the C-91 was replaced
in 1993 by more obviously home-theater oriented receivers,
the M-91 was kept available in the
USA until 1997, along the M-72 and