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300B
300B
0D3
0D3
5AR4
5AR4
6SL7GT 6SL7GT
HEADPHONE
IMPEDANCE
HANDCRAFTED IN CHINO, CALIFORNIA
OUTPUT
MODE
LEFT RIGHT
PREAMP
LINE
DRIVER
RECTIFIERREGULATOR
HEADPHONES
CAUTION! TUBES RUN HOT - DO NOT TOUCH TUBES
DO NOT OPEN CHASSIS - HIGH VOLTAGES !
REFER SERVICING TO QUALIFIED TECHNICIANS
NOTE - PIN SIZE FOR
PROPER ORIENTATION
DESIGNED BY DAVID MANLEY
ALL TRIODE VACUUM TUBE AUDIO PATH
TUBE RECTIFICATION AND REGULATION
AUDIOPHILE 2dB STEPPED
ATTENUATORS
LINE OUTPUT
(TO AMPS & SPEAKERS)
(MUTES LINE OP)
THE 300B PREAMPLIFIER
POWER SEQUENCE : ALLOW SEVERAL MINUTES FOR WARM-UP
TURN ON SOURCES FIRST, THEN PREAMP, THEN AMPLIFIERS
TURN OFF AMPLIFIERS FIRST, THEN PREAMP, THEN SOURCES
300 OHM - 4000 OHM
30 OHM - 400 OHM
TOP VIEW
A
B
A - HEADPHONE IMPEDANCE - 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 - OUTPUT MODE - Determines whether the output signals go to the back panel
RCA output jacks or to the headphone jacks. In HEADPHONES mode the main
outputs are muted - In LINE OUTPUT mode the headphones are muted.
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 ? Internally there is a trimmer for each channel to set gain. We set
each channel for 18 dB of voltage gain (1V in = 8 Vout ). The trimmer adjusts
feedback from 5 dB to 10 dB and ensures consistancy between channels and unit to
unit. This amount of feedback can be considered minimal.