Top view – Manley Neo-Classic 300B RC Preamplifier 2008 - present User Manual

Page 7

TOP VIEW

OD3

5U4G

300B

6SL7GT

A) HEADPHONE IMPEDANCE SWITCH - You should set this switch to best match
your headphones. You may have to look at the specification sheet to get the value in
ohms. If using two sets of phones divide that number in 2. If you don’t have this info then
pick the setting that sounds best - no harm will be done. If the phones are lower than the
switch setting, slight loss of extreme highs may occur. Expect the 300 ohm - 4000 ohm
setting to be louder. Electronically, we are simply either using the two transformer output
windings in series or parallel.

B) TRANSFORMER SWITCH - Switchable between “Direct” and “Transformer”.
Make sure to switch to “Transformer” when using headphones.

C) OUTPUT SWITCH - Switchable between “Line Output” (out to your amplifier) and
“Headphone” (which mutes the Line Out, as you are presumably listening with head-
phones and don’t need your speakers).

TUBE LOCATIONS - See the diagram above for the proper tube locations. The 300B’s
are not installed during shipping to prevent damage to these tubes. Power should be
unplugged and the tubes cool before handling them. We label the tubes and sockets to
help and so that the calibration we do will be valid in your home. It is possible to put the
300B’s in wrong. Note the two large pins and the two small pins on the base of the tube.
The two large pins go into the two large holes in each tube socket.

CALIBRATION: Each channel is factory-set for 12 dB of voltage gain (1V in = 3.2V
out ) via a pair of trimpots located on the main printed circuit board near the headphone
jacks. These pots adjust the amount of negative feedback in order to exactly match the
gain between both channels. Only 8.5dB of Global Negative Feedback is used which can
be considered minimal. Unless you hear a channel-to-channel imbalance after say, chang-
ing tubes, you probably do not have to re-trim the levels. There is no other adjustment that
needs to be made. There is no bias to have to set.