Yamaha AX-550RS Owner's Manual page 8

Select either MM or MC position depending on your
PHONO cartridge, Moving Magnet or Moving Coil type.
However, if you use a high output MC cartridge, select
the MM position.
Using the PURE DIRECT switch
You can enjoy purer source sound by setting this switch
so that the indicator illuminates. Because, by doing so,
the input signal is output directly, bypassing the MODE
switch (AX-550 only), BASS, TREBLE, BALANCE,
LOUDNESS controls and SUBSONIC FILTER switch,
therefore avoiding any "muddying" caused by circuit
routing.
Concerning AX-550 only, the PRE OUT/MAIN IN terminals
are also bypassed.
PURE
O DIRECT
Setting the SUBSONIC FILTER
if you set the switch to the ON position, ultra-low-
frequency signals caused by turntable rumble or warped
records can be cut out without losing sound quality.
SUBSONIC
FILTER
This switch can be used for switching between stereo and
monaural operation. Normally this switch should be set to
the STEREO position.
MODE
& STEREO
When you listen with headphones
Connect the headphones to the PHONES jack. When
listening with headphones privately, set both the
SPEAKERS A and B switches to the OFF position.
PHONES