ModWright Instruments LS 300 Owner's Manual

LS 300

Owner’s Manual

Manufactured by ModWright Instruments, Inc.

www.modwright.com

USA

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Introduction:

Thank you for purchasing a ModWright Instruments product. This unit is designed to
offer high performance and exceptional reliability.

The LS 300 preamp offers high bandwidth, low distortion, and exceptional musicality.
We have designed this unit to allow the reproduction of recordings that is accurate,
transparent and musical. Please note, absolute phase is variable and may be set via the
remote

NOTE: When the unit first powers up, it is DOES NOT invert absolute phase. Absolute
phase may be toggled IN or OUT via the remote control phase button. When the PHASE
led is illuminated, then absolute phase is inverted. Pushing the phase button on the
remote again will revert to absolute phase being non-inverted.

CAUTION: DO NOT use any other signal tubes other than 6DJ8, 6922, 7308 or you
WILL DAMAGE this unit. The rectifier tubes may be exchanged for other equivalent
type rectifier tubes such as 5AR4/GZ34, 5U4G, 5U4GB, 5V4G, 5V4GB, 274B and
equivalents.

CAUTION: Any source input connected to HT/BP inputs, will feed RCA and XLR
Main outputs with NO VOLUME ATTEUNATION. It is important to ONLY use this
input for signal from the front channels of a Home Theater Pre/Processor or other
source WITH built in volume control. When the LS 300 is powered down, the HT/BP
input defaults to HT/BP mode. When unit is powered up, any source connected to
HT/BP input may be operated in HT/BP mode. Importantly however, when the LS 300
is powered down, the HT/BP input will default to bypass mode. IF A SOURCE WITH
NO VOLUME CONTROL IS CONNECTED TO HT/BP INPUTS AND THE LS 300
IS TURNED OFF, FRONT CHANNEL AMPS WILL RECEIVE FULL OUTPUT
SIGNAL LEVEL FROM SAID SOURCE WITH NO ATTENUATION. THIS COULD
LEAD TO SYSTEM DAMAGE.

Caution: Do not operate or power up unit without tubes installed in tube sockets.
Failure to do so will result in damage to power supply.

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Setup:

It is always recommended to remove all tubes before shipping the unit. Be sure that the
LS 300 is NOT connected to AC power and that power plug is disconnected from the
external supply and wall outlet. Install (2) 6922 signal tubes in 9-pin ceramic sockets and
(2) rectifier tube in ceramic octal sockets in preamp prior to use. All tubes may only be
inserted one way.

After tubes are installed, connect both Left (L) and Right (R) umbilicals to the ‘male’ 8-
pin barrel sockets on the back of the preamp, labeled PWR(L) and PWR(R). The
umbilicals must be lined up with pins and then outer barrel securely tightened until
connection is secure. Connect the other end of the two umbilicals to the two ’female’ 8­pin barrel sockets on the external power supply labeled (L) and (R) and secure in the
same manner. Both umbilicals are identical, but they must be connected to the

corresponding (L) or (R) connector at both power supply and preamp end. If they are
not connected properly, the unit will not operate as intended. They will only fit one

way, via ‘male’ and ‘female’ pin connections at umbilical and chassis/jack connectors.
Only after this, should the power cord be connected to the external power supply.

The LS 300 has two sets of fully balanced inputs, located on the back of the unit, labeled

‘BAL IN 1’ and ‘BAL IN 2’. The next two inputs are single ended RCA connections,

labeled ‘III’ and ‘IV’. The fifth set of RCA inputs is the Home Theater Bypass inputs,

labelled ‘HTBP’. Inputs are manually selected only, via the left hand rotary selector

switch. There are five positions, starting with BAL IN 1, BAL IN 2, III, IV and HTBP.
HTBP (Home Theater Bypass operation may be selected at any time by depressing the
HTBP button. All source inputs are received the same by the unit, except for HT/BP
input. HT/BP input should ONLY be used as input from a Home Theater
Preamp/Processor or other source with built-in volume control. This input only, may be
configured to bypass all gain and volume control stages, directly feeding RCA Main
outputs. When HT/BP button is depressed and LED illuminated, any input connected to
HT/BP RCA jacks will be routed straight to RCA main outs only, bypassing all gain
stages and volume control. CAUTION: WHEN LS 300 IS POWERED DOWN,

HT/BP INPUT AUTOMATICALLY DEFAULTS TO HOME THEATER BYPASS
MODE.

The LS 300 has two sets of fully balanced main outputs, labelled ‘BAL OUT 1’ and

‘BAL OUT 2’. There are also two single ended RCA main outs, labeled ‘MAIN 1’ and

‘MAIN 2’.

NOTE: There is a ground toggle switch on the rear of the unit labeled ‘GND(1)’ and
‘GND(0)’. When this toggle is in the GND(0) position, then RCA and XLR outs may be

used at the same time, with XLR outs acting as SE outputs, by grounding pin 3 of the
XLR jacks. When operating in fully balanced XLR output mode, you must flip the toggle
to the GND(1) position. This then allows fully balanced output operation ONLY. Use of
RCA outs with ground switch in GND(1) position will cause a system hum.

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You may ONLY operate both XLR and RCA outputs at the same time with the Ground
toggle in the GND(0) position. You may only operate the XLR outputs in fully balanced
mode with the ground toggle in the GND(1) position. In this setting, the preamp will
operate with fully balanced XLR outs ONLY, but not RCA and XLR outs at the same
time.

The external power supply is fitted with an IEC input, for use with detachable power
cords. Unless otherwise specified, unit is designed for use with 120V-60Hz AC power.
Main fuse is accessible from the back of the supply, in a twist lock module. (ONLY
REMOVE FUSE WHEN UNIT IS POWERED OFF AND DISCONNECTED FROM
ALL SOURCES OF AC MAINS POWER). Only replace fuse with 3A (120VAC mains)
or 1.5A (240VAC mains) Slo-Blo type, 5mmx20mm.

There is a toggle switch located on the rear of the unit, adjacent to the power entry (IEC)
input. In the ‘DOWN’ position, signal and power supply grounds are connected to
chassis and earth ground. When toggle is in ‘UP’ position, chassis and earth grounds are
disconnected from signal and power grounds. It is ideal to leave this switch in the

‘DOWN’ position unless system experiences ground loop hum or buzz issues due to

system grounding configuration. You should ALWAYS use a grounded AC cord with
this product. The chassis is always connected to earth ground if unit is fitted with proper
grounded AC power cord.

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