Leak tl 12 owners manual

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H. I. LEAK 8: CO. LTD.
April, I957

TL/ 12 PLUS POWER AMPLIFIER

INSTALLATION, OPERATION and MAINTENANCE

NOTES ON THE CHOICE AND PERFORMANCE OF LOUDSPEAKER SYSTEMS

The TL/12 Plus may be fed from Leak Varislope III and Point One Plus pre-amplifiers, 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 85db
below 12 watts. An input of l25mV r.m.s. will give a power output of 12 watts.

Ex)

Check that all valves (tubes) are correctly seated in their holders and that the markings on the valves correspond
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. If 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 fused voltage selector
plug on top of the mains transformer (see Top Chassis drawing) should be set appropriately. On the US. model
the mains transformer is tapped for voltages of 105, 110 and 117 and the fused voltage selector plug should be set
appropriately. The AC. power supply should be connected to the two terminals nearest the guide key on the remove-
able plug portion of the mains connector marked AC. Power.

In order that the amplifier may be remotely controlled we have provided two terminals marked Switch underneath
the mains transformer (see Under Chassis drawing). The amplifier will not work unless these terminals are elect-
rically joined, and when the amplifier leaves our factory a wire link joins the terminals. A remote switch may be
run from the Switch 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.
self-powered radio tuners, etc. The power taken from this socket should be limited to 100 watts or thereabouts.
This socket is not controlled by the amplifier switch or fuse.

Many British tuner units have no built-in power supplies and require a source for heater and anode currents. The
octal socket marked Spare Supplies on the back of the amplifier is provided for this purpose. The pins are
numbered as indicated on the circuit diagram, and connections should be made to the pins similarly numbered on the
removable plug which is supplied. The high tension supply is 350V and on some tuner units it may be necessary
to provide a dropping resistor of suitable value, and some units may also require a condenser for smoothing and/or
decoupling after the dropping resistor. An earth connection should not be made directly to the tuner unit, as this
is automatically effected by the above connections via the power amplifier. The maximum current available from the
6-3V terminals is 2-lA and the maximum high tension current is 40mA.

A connection to earth (ground) should be taken from the third terminal on the removeable plug portion of the AC.
Power connector. This terminal is the one furthest away from the guide key and its corresponding terminal on the
fixed portion of the connector is marked on the chassis by the symbol A. 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, if freedom from earth loops and hum is to be obtained.

The loudspeaker should be connected by a twisted pair of wires to the terminals marked Loudspeaker. 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 DC. resistance of the connecting wires should be as low
as possible, and not more than one-tenth the DC. resistance of the loudspeaker. It is bad practice to operate any
power amplifier without a loudspeaker, and if it is desired to mute the loudspeaker 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 transformer (see Top Chassis drawing) should be adjusted
for the nearest match to the advertised impedance of the loudspeaker.

Ref. 106

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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)

(b)

The TL/ 12 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.

Should it ever be necessary to replace the reservoir capacitor C14 (which is in the same can as C13) note that 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 380mA. The values of 60mfd and lOOmfd are made high for the sole purpose of minimising
the hum resulting from the unavoidable earth loop which must be occasioned by the connection of non-powered
tuner units.

NOTES ON LOUDSPEAKER SYSTEMS

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

(a)
(b)

(0)

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

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, (baffle, box, cabinet, or horn). The housing plays a profound part in deter-
mining the quality of reproduction. The effects of various housings are discussed by L. L. Beranek in Acoustics
McGraw-Hill Publishing Co., Ltd., lst Edition.

One good motor, *properlyheusedg-is-eapablewof-giving-good-resukszwogood 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.

The fundamental laws of acoustic propagation stipulate the impossibility of obtaining deep bass notes from small
structures, whether they are musical instruments, foghorns or loudspeakers. If you want very good bass you must
have a large loudspeaker system, i.e. 7 cubic feet or more.

Remember that even the best modern loudspeakers are by far the weakest link (i.e. the greatest source of distortion)
in a high-fidelity chain. The very best loudspeaker systems are relatively expensive and large. The fundamental
weaknesses of modern loudspeakers are discussed succinctly in a paper by H. J. Leak, High Fidelity Loudspeakers :
The Performance of Moving Coil and Electrostatic Transducers, Journal of the British Institution of Radio
Engineers, Vol. 16, No. 12, December, 1956. Copies are available from the Institution, 9 Bedford Square, London,
W.C.1., price 7/-.

The above paper points out that even when a loudspeaker manufacturer publishes an accurate steady-state frequency
response that this characteristic is of little value in assessing the performance of a loudspeaker when it is the
only measurement given.

The paper also states, When one speaks of a loudspeaker having a certain degree of naturalness of reproduction
one is assessing the subjective impressions derived from listening to it, and herein lies the difficulty of defining high
fidelity, for it is not often that any two people will agree entirely on the degree of naturalness or be capable of
describing it in words ..... These difliculties of agreement arise because all loudspeakers have known imperfections
or distortions, and because the psychological attributes of hearing vary greatly between individuals. The tolerance
or acceptance of distortions varies with individuals as regards the types, amounts and admixtures of the distortions.
It is, therefore, impossible for us to tell you which is the best loudspeaker in any price range. You will have to
listen to a selection and make your own judgment, which may not coincide with ours for the reasons given above.

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