Leak Stereo 20 Service Manual

Page 1

'STEHED 20
PDWEFI AMPLIFIEFI

INSTALLATION ' DFEHATION ' MAINTENANCE

OPERATION

The STEREO 20 may be fed from the VARISLOPE. STEREO' preamplifier or from any other suitable
\nurce. The LEAK pro-amplifier is supplied with It grey or brown multiple cable terMI ng in an octal

plug wh h fits the socket on the STEREO 20' marked PRE-AMP and automatically makes the input
connections.

When the 'VARlSLOPE 2 STEREO ore-amplifier is not used thei put connections should be taken vtn
two scrccned co-axittl cables to the plugs fitting the sockets marked 'INPUT L' and INPUT R. Source
impedances higher than 15.000 ohms will lend to raise the hum level above the advertised ligure of ROdh
below It watts. An input of IZSmV r.m.s. will give a power output of It watts.

INSTALLATION

Check that all \ alves (tubes) are correctly scatcd' In their holders and that the markings on the valves corres-
pond with those on the chussts adjacent to the holders. The amplifier will work equally well with any of the
alternative valves

The-amplifier should stand on its base in It well-ventilated position. llplacc
must he provided. Four tiring feet are titted to the amplifier. and these rI
and allow air to circulate freely.

It ease orcIIbiI-Iet. ventilation
e the bottom ofthe amplifier

0n the British model the mains transformer is tapped for voltages of 205. 225 and 245 and the voltage
selector plug on top ofthe mains transformer (see TOP CHASSIS' drawing) should be set appropriately.
On the U .S. A. model the mains transformer Is tapped for voltages ol HO 7 and [24 and the voltage
selector plug should be set appropriately The AC. power supply should be connected to the two terminals
nearest the guide key on the removable piug portion ofthc ijtnr markedM POWER'. In
order that the amplifier may be remotely controlled we have ptovt e two terminals marked-'SWITLH'
underneath the mains transformer (sec UNDER CHASSIS drawing). The IImplitiers leave our factory
thh It 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 tic knotted
behind the grommet and the wire ends connected to the switch terminalsiufter removing the wire link.
Most users will wish to employ the switch incorporated in the volume control of the 'VARISLOPE Z
STEREO' preamplifier. Which is supplied with a plug and twin flex for this purpose. Full details are given
on the installatton sheets which accompany every pre-ampltticr.

A double socket marked A.C, OUTLETS is fitted as a convenient source of power supply for gramophone
motors. \clllpowered radio tuners etc. The power taken from thi socket should be limited to I00 walls or
lhcrctlbouls. 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. C. POWER' connector This terminal is the one furthest away from the guide key and its corres-
ponding terminal on the fixed portion ofthc connector is marked on the ch 'hy the symbol r-\- It Is very
bad practtce to omit this connoctton. which may be made to the water system or to the steel conduit
cncasing the house w I..prmiding that these systems themselves are properly grounded To ensure
freedom from hum (caused by earth loops) no other earth connection should be made

The loudspcasters should be connected by twisted pairs ofwircs to the terminals marked LOUDSPEAKER
l. and LOUDSPEAKER R lt hill be seen from the circuit drawing that one side of the loudspeaker
winding is connected to the ch .and no part otthc loudspeaker wiring should be earthcd elsewhere. The
DC, resistance of the connecting wires should be In low as possible. and not more than one- tenth the D. C.

resistance ofthe loudspeaker. It is It had practice to operate any power amplifier without a loudspeaker. and
ilit is desired to mate either loudspeaker by switching it out ofcircuit this hould be accomplished by use of
It changeover switch which replaces it with a resistor of corresponding value and rating. If for 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 tsee
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) It single channel LEAK preamplifier to the
STEREO 30, Under these circumstances. the standard black multiple cable lsupplicd with the pre-
amplifier) should be connected to the PRE-AMP socket. The left-hand channel will be driven by this
pre-nmplilier and the rtght and channel should haven resistor ofapproxinttttely I6 ohms H watt)connocted
across in loudspeaker ter nIIls.

Page 2

FHA-IMO

to. [t is vitally important that the diaphragms ofyour two loudspeakers more in phase. i,

in the same direction

at the same instant To make sure of this note very carefully the following:-

til

0'")

lfyeu are using identical loudspeakers (as you certainly should it. you want true stereo) the manufac-
turer will doubtless have kept to a convention when marking the Iemtinals. very ulten simply red and
black. In this case the loudspeakers will he in phase when connected as shown below.

"E LOUDsPEAKER
t:

mnsvemm
'a'

Ifyour loudspeaker terminals are unmarked or you are using dissimilar loudspeakers you can check the
phase as followszgplace 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 | Ft.)totl1c 'PlCKUP R socket. Turn BASS' control to maximum and then turn up 'VOLA
UME control until the hum picked tip 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.
ie the loudspeakers are in phase,

Most nntorttmatcly. some mat tacturcrs ot stereo records and tapes ocrc inconsistent met lillilalllgr
We have stereo records containing several bands where the signals change phase from band to hand!
The same is Clll true ortl Hi-Fi demonstration tape! If you hate these earlier records and tapes you
will nccd to fit an external switch to raters-e the phase of ne otlhc loudspeakers .I sit-l'l Mm -*
mt!| ! h when 'y'ni operate the phase-(hange swt ~h rapidly. that the correct po' tton ls obvlot on
most r (die though less obvious on others: this is due to a combination of mu ical content and
recording techniques. When musical signals are out of phase you will generally have a loss ttfxlcreo
etchtr a deficiency in bass reproduction and a 'Itole in the middle [the sound appears to come from
each loudspeaker with little sound apparently coming from the ill" | between them). With .t good
stereo system properl) phased. there is no "hole in the middle' cchct.

-o
LOUW(AKER YEIMINALS

MARKED a

on rowan Awurlstt ?

/
\ LDoDSPEAKElI

l
l
I

_(§ Batu-Pot; cnmce- avast svnrcbt

MAINTENANCE

l I. The Circuil diagram provides a qualified engineer with all information required for servicing However. the
followtng paints may be 0 interest:-

(:1)

(b)

The STEREO 20 does not depend upon the output valves (tubes) being matched pairs to give the
stated performance. and il one output valve fails it is not necessary to replace both valves.

Should it ever be necessary to replace the reservoir capacitor CIJ (which is in the same can asClZl note
that the C13 must be ofu type capable of handling a ripple current of l90mAr