Leak tl 25 plus owners manual

Page 1

OPERATION

TLIas PLUS'
power: AMPLIFIER

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LEAK

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INSTALLATION ' OPERATION ' MAINTENANCE

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l. The TL/ZS PLUS may be fed from the LEAK VARISLOPE MONO' pro-amplifier or from any other
suitable source. A source impedance higher than 25,000 ohms will tend to raise the hum level above the
advertised figure of 83db below 25 watts. An input of lZSmV r.m.s. will give a power output of 25 walls.

INSTALLATION

2. 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.

3. The amplifier should stand on its base in a well-ventilated position. lf placed in a case or cabinet. ventilation
must be provided. Four separate fixing feet with screws are provided with the amplifier.

. 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 TOP CHASSIS drawing) should be set appropriately.
On the U.S. model the mains transformer is tapped For voltages of 1 l0. 117 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 plug portion of the mains connector marked A.C. POWER.

In order that the amplifier may be remotely controlled we have provided two terminals marked SWlTCH
underneath the mains transformer (see UNDER CHASSIS drawing). The amplifier will not work unless
these terminals are electricallyjoined, and when the amplifier leaves our factory a wire link joins the terminals.
A remote switch may be run from the SWlTCH terminals, after removing the link, the flex being passed
through the adjacent grommet marked SWITCH CABLE. The flex should be knotted behind the grommet
to avoid strain on the SWITCH terminals. Most users will wish to make use of the switch incorporated in
the volume control of an associated LEAK pre-amplifier, which is supplied with a plug and twin flex for this
purpose. Full details are given in 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, seltipowered radio tuners, etc. The power taken from this socket should be limited to 100 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.C. POWER connector. This terminal is the one furthest away from the guide key and its corres-
ponding terminal on the fixed portion of the connector is marked on the chassis by the symbol (5. 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. No other earth connections
should be made elsewhere, particularly when a pre-amplifier is also used, iffreedom from earth loops and
hum is to be obtained.

. The loudspeaker should be connected by a twisted pair of wires to the terminals marked LOUDSPEA KER.
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.C. resistance of the connecting
wires should be as low as possible. and not more than one-tenth the D.C. resistance of the loudspeaker. It is
bad practice to operate any power amplifier without a loudspeaker, and if it is desired to mute the loud
speaker by switching it out of circuit this should be accomplished by use of a change-over switch which
replaces it with a resistor of corresponding value and rating. The selector plug on top of the output trans-
former (see TOP CHASSIS drawing) should be adjusted for the nearest match to the advertised impedance
of the loudspeaker.

Page 2

GENERAL NOTES ON MAINTENANCE

The circuit diagram provides a qualified engineer with all information required for servicing. However, the
following points may be of interest :-

(a) The TL/25 PLUS does not depend upon the output valves (tubes) being a matched pair to give the stated
performance, and if one output valve fails it is not necessary to replace both.

([7) Should it ever be necessary to replace the reservoir capacitor C14 (which is in the same can as C13) notethat
C14 must be of a type capable of handling a heavy ripple current. The C14 fitted has a very high margin of
safety, being capable of handling 220mA.

NOTES ON LDUDSPEAKEH SYSTEM.

Space does not allow for detailed explanations on this vast subject, but the following points should be noted :~

(11) This amplifier will operate any high fidelity loudspeaker system, both moving-coil and electrostatic types,
including the small low sensitivity assemblies currently popular, particularly in the USA.

(b) You cannot get high fidelity results if you mount a loudspeaker in the same cabinet as the turntable and
pickup.

(c) The assembly commonly called a loudspeaker consists of a magnet, a moving-coil and a diaphragm (or
cone). This assembly is a motor. You do not listen to a motor; you listen to a loudspeaker system, which
consists of one or more motors mounted in a housing (bafiie, box, cabinet, or horn). The housing plays a
profound part in determining the quality of. reproduction. The effects of various housings are discussed by
L. L. Beranek in Acoustics, McGraw-Hill Publishing Co Ltd, First Edition.

(d) One good motor, properly housed, is capable of giving fairly good results, but two good motors, properly
housed, will give noticeably better results: in this case one motor is designed to reproduce bass, and the
other, treble. A filter (dividing network) must be used in conjunction with the two motors.

(e) Remember that even the best conventional loudspeaker systems are by far the weakest link (i.e. the greatest
source of distortion) in a high fidelity chain, and that such systems are relatively expensive. The fundamental
weaknesses of conventional loudspeakers are discussed succinctly in a paper by H. J. Leak, High Fidelily
Loudspeakers: The Performance of Moving-Coil and Electrostatic Transducers, Journal of the British
Institute of Radio Engineers, Vol. 16, No. 12, December 1956. Copies are available from the Institution,
9 Bedford Square, London, W.C.l, price 7/-. However, such systems have become out-moded by the
revolutionary Leak invention of the composite sandwich diaphragm. The basic principles are described by
D. A. Barlow, M.Sc. (Head of the Leak Transducer Research Department), in Wireless World, December
1958, A further paper describing the complete Leak Sandwich loudspeaker system was delivered to the
British Institution of Radio Engineers by D. A. Barlow and H. J. Leak on the 24th January, 1962, and
will be published in a forthcoming journal of the Institution.