Leak varislope 3 owners manual

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H. ]. LEAK s: CO. LTD.

VARISLOPE III PRE-AMPLIFIER

INSTALLATION, OPERATION and MAINTENANCE

NOTES ON ASSOCIATED APPARATUS

e a a

The Varislope III pre-amplifier is designed for use specifically with Leak power amplifiers TL/12 Plus, TL/25 Plus and
TL/SO Plus. It will also operate perfectly with any previous Leak power amplifier, though the facility of a socket for
picking up heater and anode supplies for a tuner will not be available unless an additional octal socket is fitted to the
power amplifier. Instructions for carrying out this modification will be sent on request.

CONNECTING THE VARISLOPE III PRE-AMPLIFIER

1.

This unit may be used free-standing on a table, or it may be mounted on a panel of any thickness, through a cut-out
of 10% x 3% (27 cms. x985 cms.) To mount on a panel: remove the rubber feet by pulling smartly out of their
retaining holes : pass the body of the pie-amplifier through the cut-out until the front plate butts against the panel.
then pass the U-shaped bracket over the back of the pre-arnplificr and fix it by passing the wing screw through the
hole in the bracket and into the threaded hank-bush in the centre of the rear panel on the pre-amplifier. Tighten
the wing screw just enough to prevent the metal backing on the front plate of the pre-amplifier from slipping on the

panel.

A multiple cable of 4 ft. (1-22 metres) is supplied for connecting the Varislope III to the Leak power amplifier.
The male plug on this cable fits the socket on the power amplifier marked PRE-AMP. The female plug on the
cable fits the male socket on the Varislope III marked FROM AMPLIFIER. Longer cables can be supplied
to special order, up to a maximum of 16 ft. (5 metres).

To enable you to control the power amplifier from the Varislope III a switch is incorporated in the VOLUME
control. To make use of this facility a Z-core flexible cable is supplied with the Varislope III; one end of the
cable is fitted with a plug which inserts into the socket marked SWITCH on the rear of the Varislope III ; the
other end of the cable must be passed through the rubber grommet marked SWITCH CABL on the associated
Leak power amplifier, knotted behind the grommet, and the two bared ends connected to the terminals marked
SWITCH " (situated underneath the mains transformer), after removing the wire link joining these terminals.

We strongly recommend that the power amplifier should be earthed (grounded). If excessive hum is experienced,
particularly with the power amplifier not earthed (grounded) this can be reduced by reversing the mains input
leads to the power amplifier. Reversing the SWITCH connections will not be effective.

THE CONNECTION OF PICKUPS

Two pickups can be connected to the co-axial sockets marked PICKUP 1 and PICKUP 2 at the rear of
the Varislope III. Below these sockets are the associated input volume controls. On the front panel is the 2-way
selector switch marked PICKUP.

The greatest care has been taken in the design of this pre-amplifier to ensure that any pickup generally available
in the world can be connected to give optimum results, i.e., the highest quality obtainable from the chosen make
of pickup. Our prime aim is for you to obtain the optimum results from the pickup of your choice. Please
follow our instructions very carefully, even if they appear to conflict with other advice.

We know from experience that the main troubles encountered by the music-lover at home are with the reproduction
of records. There are five major reasons for these troubles :-

(a) No record can possibly give perfect reproduction, and many records (perhaps the majority) contain noticeable
distortions due to imperfections in recording and/or processing. These imperfections may show up as rattle,
high surface noise, recorded hum and rumble, and recorded wow". Shrill treble may be due to a poor
record, and/or due to a pickup having its high-frequency resonance within the audible range.

(l2) No pickup is perfect and the majority have performances very much below those which are attainable.

(0) Hum. This often arises because insufficient attention is given during the design of a pickup to the commonly-
found circumstancesin which it will operate, i.e., near an electric motor and near a power amplifier. Hum
can also arise from incorrect connection of the pickup by the user.

(rl) Rumble. Vibration from the motor is transmitted to the pickup stylus and appears in the sound output
as a rumbling or humming noise. Rumble disappears when the pickup is lifted from the record.

(e) Acoustic feedback. If a loudspeaker is placed in the same cabinet as a pickup, then vibration from the move»
merit of the loudspeaker can be transmitted to the stylus of the pickup. As the volume is increased a stage is

reached where a sustained roaring noise is set up. At volume levels constderably below this point distortion
is noticeable. Acoustic feedback disappears when the pickup is lifted from the record.

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(f) Connecting pickups having a single wire within a screen.

PICKUP

q ______________________ 1/- THE INNER

. \ 3 WIRETAKEN

\\t---___---" \ THROUGH
\ HOLOWCENTRE

L

SHORTEST POSSIBLE SCREEN , PIN AND
LENGTH OF SOLDERED to 5 AMP PLUG SOLDERED AT

UNSCREENED WIRE, OUTER SHELL. THE TIP.

lf hum is to be kept to a minimum the outer screening should either have an insulated covering, or it should
be prevented from touching any metal on the motor, motor-board or anywhere else. The outer screening must
not be used for earthing any part of the motor and turntable assembly, which should be earthed by a separate
wire taken to the a terminal on the Leak power amplifier. If the tone-arm is of metal and the outer
screening is connected to it, then the arm must not make metallic contact through its bearings With the metal
turntable and motor assembly.

Unfortunately, some record-players and record-changers are wired as shown above, the screen then being
connected to the body of the motor mounting-plate. This is bad practice on the part of the makers, and is
very likely to cause hum, particularly when using a low output pickup. If you have this type of wiring, you
are most strongly urged to replace it with the system (g) below.

(g) Connecting pickups having two wires within a screen.

T - K, . --j ONE INNER
,..'_ _______________________ . J WIRE TAKEN
'\ \ THROUGH

PICKUP

HOLLOW C ENTRE

, sc REEN AND ONE PRE-AMR PLUG.

SHOEgEZTr:§S§iBLE INNER me softlERErboAT
SOLDERED TO THE TIP.

UNSCREENED WIRES. OUTER SHELL.

Again, as in (f) above, the outer screen should either have an insulated covering, or it should be prevented
from touching the motor. motor-board or anywhere else. If, however, the screening does touch, then hum is
less likely to be caused than by using the single-wire system of (f).

(h) Connecting low impedance pickups using a transformer.

(- _______
PRIMARY I/l SECONDARY
\l\ I

PICKUP L _ _

NOTE WELL.

(1')

The transformer must be enclosed in a screening can of highpermeability metal, i.e., mu-metal or permendur.
The primary winding must be balanced, the centre-tap being taken to the chassis.

The primary terminals are to be as small as possible and to be as close together as is practicable, in order
to obviate a loop in the wiring. The primary leads to be tightly twisted for the same reason. The live"
secondary terminal to be sereened and to be as small as possible Ideally all terminals should be inside the
can. The above precautions are not yet universally followed, though they have been standard practice on
Leak pickups for ten years. These precautions are essential if the lowest hum level is desired,

Hum.

The Varislope Ill pie-amplifier has an extraordinarily low hum level, which can be checked by removing
the pickup plug and turning up the VOLUME control. The connection of any input device to the
pickup socket will lower the input impedance and should, therefore. reduce the hum level. If the hum level
increases on connecting the pickup, the cause of the hum must lie outside the preamplifier, and our instructions
on the choice and connection of pickups should be carefully read in an attempt to locate the cause of the hum.

Hiss.

With the input control switched to a pickup position and the VOLUME " control at maximum, a certain
amount of hiss will be heard This hiss is as low as is possible to obtain at the present day, and it is inherent
in high-gain vacuum tube amplifiers.

It is a fact that with the highest quality pickups (moving-coil types used with a transformer) the VOLUME
control will not need to be anywhere near the maximum position, and therefore the hiss will be unobtrusive ;
but with competitively-priced variable-reluctance (moving-iron) pickups the output will be so low, for reasons
explained later (see 60 (2), 135), that hiss will be more noticeable. This is the price that the user has to
pay in return for a modest outlay with reasonably good results i

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