The Marconi Company

PUBLIC SPEECH AND MUSIC
AMPLIFYING SYSTEM

1922 - 1928

MARCONI'S WIRELESS
TELEGRAPH COMPANY LTD
MARCONI HOUSE, STRAND, LONDON, WC2
Telephone No. CITY 8710
Telegraphic Address: "EXPANSE, ESTRAND, LONDON."
Codes:
Marconi, Western union, Bentley's, A.B.C. 4th & 5th editions

Marconi
No. I "Gigantophpone" Equipment

A. Carbon Microphone. B. Gramophone fitted with Adaptor. C. Microphone Accessory Unit. D. Microphone Control Power Amplifier. E. Sound Projector (small size).

This is a small power equipment especially designed for public address purposes. By taking advantage of the fact that the range of relative intensities embraced by articulate speech is considerably narrower for music, it has been possible to produce a substantial and less expensive equipment by using a high quality carbon microphone which will give the necessary volume of sound without in any way interfering with the intelligibility of the speech.

The volume of sound produced for speaking purposes is sufficient to reach an audience of about 5.000 people or to fill a medium sized dance hall.

No. 2 Marconi "Gigantophone"

This is a small power high quality equipment. The microphone system is the same as that used with the No. 3 Equipment, while the power amplifier and projector system is the same as that employed with the No. I equipment. The quality of reproduction is identical with that of No. 3 equipment.

The gramophone adaptor is particularly useful in enabling dances to be held without the expense of an orchestra. It is also recommended for permanent equipment in cathedrals, on board ship, for public address purposes, and for naval and military use.

No. 3 Marconi "Gigantophone

A Magneto Microphone. B Gramophone Adaptor. C Magneto Microphone Amplifier. D Control Amplifier. E Power Amplifier. F Sound Projector (large size).

This is a powerful, high quality equipment, designed especially for the repetition of music or speech on a large scale. The sound volume produced by the minimum equipment is sufficient for very large open-air audiences of up to 20,000 people, and by the addition of further power amplifiers and projectors can be extended indefinitely. Thus audiences of hundreds of thousands can be catered for by the provision of an appropriate number and disposition of additional projectors.

The microphone system with the first two amplifier units is identical with that used at the principal stations of the British Broadcasting Company.
Particularly recommended for dockyards, battleships, during military operations, and in political campaigns, or installed on board large ships for repeating the music of the orchestra in the several saloons.

The principal components comprise eight valves connected in parallel, input and output transformers, a filament voltmeter and a feed milliammeter. The components are mounted on a teak panel. A 130 volt grid biasing battery is provided as a separate unit.

A Marconiphone V2 receiver, amplifier and
Loudspeaker mounted on the running board of a car, 1922

Marconi Magneto Microphone Amplifier

Magneto Microphone Amplifier, Type G.A.1.

The microphone amplifier illustrated above amplifies the minute currents induced by the magneto microphone to a strength approximating that of the ordinary carbon microphone. The input circuits are designed for accepting ultra feeble audio-frequency currents.
The principal components comprise five resistance-capacity coupled valves connected in cascade; input and output transformers; anode resistances and condensers; a potentiometer; a feed milliammeter;a filament voltmeter; and two 9-volt grid biasing batteries, mounted on a vertical panel,in a teak case.

Marconi Microphone Control Amplifier

Microphone Control Amplifier, Type G.K.1.

The microphone control amplifier is used for amplifying the strength of the output from the "microphone amplifier" and is provided with adjustments which allow of great flexibility of control.

The input circuits are capable of accepting normal microphonic currents of average landline strength. The output circuits are capable of operating directly the modulating system of a broadcast transmitter or power amplifiers of a sound projector system.

The principal items of the amplifier comprise seven valves, input and output transformers, an inter-valve transformer, a total feed milliammeter, and a filament voltmeter. These are mounted in a strong teak travelling box fitted with loop handles.

The first three valves are resistance-capacity coupled in cascade. The third valve is coupled to the power amplifying valves by a I:5 ratio transformer, the four power valves being connected in parallel. A potentiometer in the grid circuit of second valve provides a means of controlling the amplification.

When used with No I "Gigantophone" Equipment

H.T. Batteries. One 240-volt 3 ampere- hour battery in 5 units is provided for supplying current to the anode circuits of the valves.
L.T. Batteries. The following accumulator batteries are provided for supplying current to the valve filaments and projectors. One 6-volt 80 ampere-hour for amplifier. One 6-volt 40 ampere-hour for projector field coils.
Grid Battery two 42-volt dry cells for bias adjusting.

The Marconi-Sykes Magneto-Microphone illustrated right, is a moving coil instrument in which the moving coil itself acts as the diaphragm

It consists essentially of a heavy cylindrical iron container (A) with a central iron core (B). The central core and the container form a magnetic system, the field winding (C) being mounted on the central core. The pole pieces are so designed that a very uniform field is produced in the gap between the core and the outer casing. When the field coil is connected to an 8-volt supply, allowing a current of 4 amperes in the circuit, a flux density of approximately 1,500 lines per square centimetre is produced.
The moving coil in the form of an extremely thin annular ring (D) is wound with fine aluminium wire supported on a paper backing. It is suspended in the magnetic field between the centre core and the outer casing. Suspension is by means of small wads of cotton wool fixed to the face of the magnetising coil and to the underside of the moving coil. The coil is therefore free to move in sympathy with the sound waves impinging its surface
When sound waves disturb the neutral position of the coil feeble alternating current is induced in the coil circuit, The physical characteristics of the sound wave are thus transformed into electrical equivalent extremely small, 3 stage amplification is required.

The Marconi carbon microphone illustrated has been especially designed for use in connection with a wireless broadcasting set or a public address equipment. It is aperiodic to all audio frequencies, and is particularly suitable for picking up speech, but may also be used with good results for musical items where the expense of a magneto microphone system is not justified by circumstances.

Where perfect music reproduction is required the Marconi-Sykes Magneto-Microphone, with its greater flexibility and unlimited amplitude range is recommended.
The amplitude of the varying current delivered at its terminals is many times greater than that obtained from the magneto microphone, and consequently less amplification is required. A 3-volt battery is connected in series with the carbon microphone. A special microphone transformer of the "Ideal" type is used with the instrument.

Apparatus Manufactured by The
Marconiphone Company Ltd 1922-1928
From the Marconi-Man's Diary 1934-5 page 15

Decibel

The two main ratings of audio amplifiers are gain and power output, usually stated in decibels or watts above a given reference point. Some manufacturers also furnish a curve showing the response at various frequencies.

The decibel is a logarithmic unit which is an expression of a ratio. The gain of an amplifier builds up a signal from an extremely low input to a usable output. An amplifier that has twice as much power as another is not twice as loud. The difference between power levels and sound intensity is logarithmic, and as the uniform scale by which sound intensities can be expressed is more desirable than the logarithmic scale of power ratios, sound engineers in 1928 adopted the unit called the decibel to express sound levels. This unit represents the amount of change in sound intensity just discernible to the average ear.

In order to give meaning to the decibel scale, a starting or reference point is needed, and it was decided to place 0 at the threshold of hearing ( at which the faintest sounds just become audible ). This 0 on the decibel scale represents 6 milliwatts (. 006 watts ) in electrical power, and with this convenient uniform scale any sound that can be heard, can be expressed in so many decibels, preceded by a plus sign. Any sound too faint to be heard, can also be expressed in decibels, preceded by a minus sign.

If the sound level at the output of an amplifier is x40 db, this means 40 decibels above 0; and if a microphone is rated at -60 db this means that its output in sound intensity is 60 decibels below 0. In order to bring this -60 db up to 0 db a gain of 60 db will be required and to raise this to the above amplifier rating of 40 db an additional gain of 40 decibels will be required. In other words, to bring minus 60 db level up to plus 40-db level, the amplifier will require an over-all gain of 100 db.

The power output rating of an amplifier means the electrical power delivered at the output terminals, and is measured in watts or db above a specified reference level. It is always calculated for a given amount of harmonic distortion (usually 5%). The power output of an audio amplifier determines the sound level that can be obtained from the speakers. Peak power output is the maximum output possible regardless of distortion. It is considerably higher than the undistorted output rating.

Amplifier power output can also be expressed in decibels. However since we are dealing with absolute values rather than ratios, the decibel rating must refer to a particular zero reference level. For example, if we rate the power of a 10-watt amplifier in decibels, we must say that it has a power rating of so many decibels above the given zero level. Unfortunately, several different zero levels are in common use. The older one, used by many PA manufacturers, is a zero level of 6 milliwatts.

The newer one, used in broadcasting and by an increasing number of manufacturers is 1 milliwatt (0.001 watt). For power levels in decibels to have any meaning, it is essential to know which zero level is being used. When using zero level of 1 milliwatt the term volume unit (abbreviated V U) is often used instead of db. Another term used is dbm (m for milliwatt). Thus, a 10-watt amplifier has a power level of x32.2 db referred to 6 milliwatts and x40 V U (or x40 dbm) referred to 1 milliwatt.

The world today owes a great deal to Ampere. For a start, he established the law of magnetic interaction between two electric circuits - the basis of all electric motors. 1975 Nicolas Skrotzki, of French radio and TV, profiles the man to mark his bicentenary this year.

There are moments in history when ideas, hunches and experiments react to produce something great. At such moments somebody, by adding a new ingredient, provides the catalyst to make the whole thing work.

One such person was Andre-Marie Ampere, just as, much later, Einstein synthesised the theories that now explain a series of apparently uncorrelated phenomena in the field of physics.

Born in Lyons on 22 June 1775 Andre-Marie Ampere's contribution to the electrical sciences can only be appreciated in the context of the progress of his time.

In the 18 th century, two Italian men of science-L Galvani, a doctor, and A Volta, a physicist-prompted a complete revolution in our knowledge. Electricity is now seen, for the first time as a 'current' to be generated at will, transmitted through conductive wire and used to produce a range of effects: thermal, chemical and magnetic.

Simultaneously, Arago invents the electromagnet. But is left to Ampere, next in line, to formulate the laws governing the phenomena of electric current and magnetic fields. Others such as Faraday and Maxwell, perfect the science and pave the way for Gramme's generating machines and the wireless 'machines' of Branly, Popov and Marconi.

But back to Andre-Marie Ampere. To start with, he is self-taught. His father-a merchant, then a magistrate in Lyons at the time of the French Revolution-educates him in the principles of Jean-Jacques Rousseau. Flourishing in the environment of a happy home, Andre-Marie reads every-thing he can lay hands on: books on science, philosophy, literature. The shock is all the more severe for this sensitive young man when, at the age of 18, he sees his father go to the scaffold. His father's last tribute: I expect great things of my son.

In 1796 aged 21, Andre-Marie meets Julie Carron. Three years later they are married. In 1802 he presents his 'Considerations sur la Theorie Mathematique du Jeu', which reveals his qualities as a mathematician, particularly on the theory of probability. Fascinated by religion, he examines a series of parallel themes. What is the purpose of life Has God revealed himself to man? With his friend Ballanche, Ampere joins the school of mystics at Lyons. A second stroke of fate throws a question over his faith when his young wife dies in 1803. After a spell as professor at Lyons University, he goes up to Paris to teach mathematical analysis and mechanics at the Ecole Polytechnique. Ampere is now 34 years old.

Only five years later he is made a member of the Paris Institute . Mathematics alone is no longer a sufficient challenge for this remarkable mind, so he becomes a professor in philosophy at the Faculte des Lettres.

He then extends his interests to chemistry and works in close partner-ship with Berthollet. One of his conclusions is that the chemical molecule is composed of a group of atoms-simpler elements, variable in number-which assembled in the shape of a polyhedron, affect the behaviour of the different chemical combinations.

In 1824 he is offered the chair in physics at the College de France. At about the same time he invents a form of electrical telegraphy. But it is in 1820, following Oersted's experiments and Arago's demonstrations at the Academie des Sciences, that Ampere developes the theory governing the generation of magnetic fields. At the same time he demonstrates other aspects of the behaviour of electric current.

Foreshadowing the electronic theory of matter, in 1826 he wrote his famous and far sighted paper 'On the Mathematical Theory of Electro-dynamic Phenomena, as deduced from Experimentation.' According to this, all forms of magnetism result from the movement of electric charges. He invents the word 'current' to replace the term 'conflict' used at the time. These 'currents' are composed of molecular particles, a brilliant interpretation which proved entirely compatible with the structure of the atom later propounded by Rutherford and Bohr.

In the last years of his life Ampere embarked on a major project, never to be completed: 'An Essay on the Philosophy of Science'.

Inconspicuously going about his work as a university inspector-typical of this discreet, modest, sensitive man who had a most original and wide-ranging mind-Andre-Marie Ampere died quietly in provincial Marseilles on 10 June, 1836, far from the noise and pretensions of the world whose secrets he had helped to reveal.