DHT Line preamplifier
DHT Line
Lyrita Audio
L - 458, Sarita Vihar
New Delhi 110 076
Tel : 011-4140 1143
Lyrita Audio
Tubes, line stage
Russian 4P1L (2 nos)
Tube, rectifier
Russian 5C4S (5Z4G) (1 nos)
This preamplifier is guaranteed against any defect in material
and workmanship for a period of two years from the date of
purchase. Tubes are guaranteed for thirty days.
Frequency Response
10 Hz to 100 kHz
Distortion (THD)
less than 1%
Within this period, Lyrita Audio will repair the amplifier without
charge, provided that failure was not due to misuse.
12 dB (voltage gain of 4)
Output impedance
1250 ohms
CD, Tuner, Tape
Main Out / Line Out
Mains Power
230 V, 50 Hz, 150 W (max)
Size W x D x H
360 x 400 x 130 mm
10 kg
A separate guarantee card is not issued with this unit. The
guarantee period begins on the day you take delivery.
Serial No. _________________
To power on:
Flip the Power Switch down.
LED glows “Green”.
Power to the circuit ramps up gently. In about a half minute,
the preamp will be operational.
When making or breaking any connections, always turn off
the power to the amplifier first, so that no hum or transients
are transferred to the speakers.
When sitting down to a listening session, it is a good idea to
let the system warm up for a half-hour. This stabilizes the
circuitry, and makes it sound better.
The DHT Line preamplifier is a purist design, and provides
basic control functions in an audio system.
This design uses directly heated tubes/triodes (DHT) as
single gain stages, with their characteristic rich textured
The signal path is kept as simple and as short as possible.
This preserves the integrity of the music signal; as any
additional circuitry only adds complexity and distortions of its
own. This, of course, eschews the use of unnecessary
switching and filter circuits, as well as tone controls.
The overall design is simple and direct. Line level inputs have
only one amplification stage through the Russian 4P1L DHT
tube (configured as triode). This provides a low level of gain.
All wiring is point-to-point. All gain stages operate in Class A
mode. No global feedback is used.
Good ventilation should be provided around the preamplifier.
The tubes generate a fair amount of heat, and natural
ventilation is necessary to dissipate this heat to the
surroundings. The preamplifier chassis will get warm, but
should still be comfortable to touch.
Always place the preamplifier on a firm, smooth surface.
The 4P1L tubes are microphonic—emit some bell ringing
sounds when new, even though they are mounted on a plate
on rubber grommets to prevent vibrations. However, after
some use, the ringing settles down.
The power supply uses a large, shielded power transformer.
Tube rectification is used. The high voltage supply is
independently filtered through chokes and RC filters. The
tube heaters use a choke filtered DC supply.
The DHT Line preamplifier is housed in a naturally finished
solid sheesham cabinet.
! Warning : In operation, this preamplifier has very high
voltages. Always switch off, and unplug the amplifier
from the mains supply before working on it. Wait for 5
minutes for the high voltages to die down, before
opening the amplifier!
Mains supply connection is through a detachable power cable
with an IEC-type connector. The mains power switch is
mounted on the front panel of the amplifier.
All signal connections are made to RCA sockets on the back
panel of the preamplifier. These are clearly marked.
CD / Tuner / Tape
These are all line level inputs, and can be used with the
sources indicated.
Main Out
This set of output connectors sends the output signal to the
power amplifiers.
Line Out
This set of output connectors can be used to send the signal
to a subwoofer, or other controller.
The preamplifier is protected against overload by a fuse at
one point only. This is the mains supply fuse, mounted on the
rear panel and is easily accessed from the outside.
Mains supply fuse
2 A, 250 V (RQB 2000)
On operation of the preamplifier, if the fuse does blow, check
all possible causes before replacing the fuse. If any doubt
remains, contact Lyrita Audio for servicing.
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