Leak stereo 20 owners manual

Page 1

v

STEnED 20'
power: AMPLIFIER

m""""""""" INSTALLATION ' OPERATION ° MAINTENANCE

OPERATION

The STEREO 20 may be fed from the 'VARISLOPE 2 STEREO preamplifier or from any other suitable
source. The LEAK pre-amplifier is supplied with a grey or brown multiple cable terminating in an octal
plug which fits the socket on the 'STEREO 20 marked PRE-AMP and automatically makes the input
connections.

When the 'VARISLOPE 2 STEREO pre-amplitier is not used the input connections should be taken via
two screened co-axial cables to the plugs titling the sockets marked "INPUT L and INPUT R'. Source
impedances higher than 25.000 ohms will tend to raise the hum level above the advertised figure of 80dh
below It watts. An input of l25mV r.m.s. will give a power output of II watts.

INSTALLATION

3.

*1.

Check that all valves (tubes) are correctly seated in their holders and that the markings on the valves corres-
pond with those on the chassis adjacent to the holders. The amplifier will work equally well with any of the
alternative valves.

The amplifier should stand on its base in a well-ventilated position. lfplaced in a case or cabinet, ventilation
must be provided. Four fixing feet are fitted to the amplifier. and these raise the bottom of the amplifier
and allow air to circulate freely.

On the British model the mains transformer is tapped for voltages of 205. 225 and 245 and the voltage
selector plug on top of the mains transformer (see TOI CHASSIS' drawing) should be set appropriately.
On the U.S.A. model the mains transformer is tapped for voltages of HO. 7 and l24 and the voltage
nearest the guide key on the removable plug portion of the rain connector marked A! POWER'. In
order that the amplifier may be remotely controlled we have provided two terminals marked-SWI'I'LH'
underneath the mains transformer (see UNDER CHASSIS drawing). The amplifiers leave our factory
with a wire link joining the terminals. A remote switch may be run from the switch terminals. the switch
flex being passed through the adjacent grommet marked SWlTCH CABLE'. This flex should be knotted
behind the grommet and the wire ends connected to the switch terminals-rafter removing the wire link.
Most users will wish to employ the switch incorporated in the volume control of the VARISLOPE 2
STEREO~ pre-amplifier. which is supplied with a plug and twin flex for this purpose. Full details are gi\ en
on the installation sheets which accompany every pre-amplifier.

A double socket marked A.C. OUTLETS is fitted as a convenient source of power supply for gramophone
motors. self-powered radio tuners etc. The power taken from this socket should be limited to too watts or
thereabouts. This socket is not fused but is controlled by the amplifier switch.

A connection to earth (ground) should be taken from the third terminal on the removable plug portion of
the 'A.(. POWER' connector. This terminal is the one furthest away from the guide key and its corres-
ponding terminal on the fixed portion ofthe connector is marked on the chassis by the symbol .1... It is very
bad practice to omit this connection. which may be made to the water system or to the steel conduit
encasing the house wiring. providing that these systems themselves are properly grounded. To ensure
freedom from hum (caused by earth loops") no other earth connection should be made

The loudspeakers should be connected by twisted pairs ofwires to the terminals marked 'LOUDSPEAKER
I. and LOUDSPEAKER R. It will be seen from the circuit drawing that one side of the loudspeaker
winding is connected to the chassis. and no part of the loudspeaker wiring should be earthed elsewhere. The
D.(. resistance ofthe connecting wires should be as low as possible. and not more than one-tenth the DC.
res .tance ofthe loudspeaker. It is a bad practice to operate any power amplifier v\ ithout a loudspeaker. and
i It is desired to mute either loudspeaker by switching it out ofcircuit this should be accomplished by use of
a changeover switch which replaces it with a resistor of corresponding value and rating. lffor any reason
only one loudspeaker is to be used then a similar resistor should be connected across the loudspeaker
terminals of the amplifier channel not in use. The selector plugs on top of the output transformers (see
'TOP CHASSIS drawing) should be adjusted for the nearest match to the advertised impedance of the
loudspeakers.

Some users may wish to connect (as a temporary measure) a single channel LEAK pre-amplifier to the
STEREO 20'. Under these circumstances. the standard black multiple cable (supplied with the pre-
amplifier) should be connected to the PRE-AMP socket. The left-hand channel will be driven by this
pre-amplifier and the right-hand channel should have a resistor olapproximately 16 ohms watt) connected
across its loudspeaker terminals.

Page 2

PHABING

10. It is vitally important that the diaphragms ofyour two loudspeakers move in phase. i.e. in the same direction
at the same instant. To make sure of this note very carefully the following:-

(i)

(ii)

If you are using identical loudspeakers (as you certainly should if you want true stereo)_the manufac-
turer will doubtless have kept to a convention when marking the terminals. very often Simply red and
black. in this case the loudspeakers will be in phase when connected as shown below.

RED WIRE

RACK WIRE

cx
LOUOSPEAKER r717] LDUDSPEAKER
.L. .R.

lfyour loudspeaker terminals are unmarked or you are using dissimilar loudspeakers you can check the
phase as followszflplace the two loudspeakers as close together as possible: set the FUNCTION
switch to INPUT R and the input selector to PlCKUP. Connect an unscreened length of wire
(about I ft.) to the 'PlCKUP R' socket. Turn 'BASS' control to maximum and then turn up VOL-
UME control until the hum picked up by the unscreened lead is fairly loud. Then reverse the leads to
one loudspeaker: the condition which gives you noticeably more bass hum is the correct connection.
i.e. the loudspeakers are in phase.

~

Most unfortunately. some manufacturers of stereo records and tapes were inconsistent over phasing.
We have stereo records containing several bands where the signals change phase from hand to band!
The same is even true ofa Hi-Fi demonstration tape! If you have these earlier records and tapes you
will need to fit an external switch to reverse the phase of one ofthgloiudsgmakers. as shown below. You
win find. Whefryrnt operate thephase-chfn'ge swutch rapidly. that the correct position is obvious on
most records. though less obvious on others: this is due to a combination of musical content and
recording techniques. When musical signals are out of phase you will generally have a loss of stereo
elTect. a deficiency in bass reproduction and a 'hole in the middle' (the sound appears to come from
each loudspeaker with little sound apparently coming from the area between them). With a good
stereo system. properly phased. there is no 'hole in the middle efi'ect.

LOUDSPEAKEH TERMtNALS
manta R

ON POWER AWUFIER ?

l < Louosenm
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DOM-PM CNANGE- OVER SWITCN

MAINTENANCE

ll. The circuit diagram provides a qualified engineer with all information required for servicing. However. the
lollowmg pomts may be of interest:-

(u) The STEREO 20' does not depend upon the output valves (tubes) being matched pairs to give the

stated performance. and if one output valve fails it is not necessary to replace both valves.

(b) Should it ever be necessary to replace the reservoir capacitor C13 (which is in the same can as CIZ) note

that the C13 must be ofa type capable of handling a ripple current of l90mA.