The Gallery of Electro-Mechanical Amplifiers.

A celebration of an almost forgotten technology.
The title of this page has been changed to "Electro-Mechanical amplifiers", to distinguish them from mostly mechanical ones, such as the Frenophone, which can be found in the Mechanical Amplifiers gallery.

Page updated: 27 May 2009
More on Tannoy power microphone

The year is 1900. Cost and attenuation are seriously limiting the growth of long-distance telephony, particularly in the USA.This needs a little explanation. You can always reduce the attenuation per mile of a telephone line by using thicker copper to reduce the resistance. Copper is however expensive, and there are limits to what is practical in this direction.

A Digression on Line Losses in Telephony.

To us, the obvious solution is to apply some amplification to boost signal strength; but it is long before the invention of the transistor, and even the valve (the vacuum tube to US readers) is still years away.

Mere lack of resource has never blocked human ingenuity, and there was a handy fact to exploit. Carbon microphones, as used universally in telephones until the mid-1960's, are not mere transducers that turn sound power into electrical power, but actually give a power gain of about a hundred times. The microphone is a variable resistance, made up of fine carbon granules, that controls the flow of current from a DC power supply; it does not merely convert acoustic energy into electricity. This power-amplification technology was the essential basis of the first practical telephone systems.

From this, it is but a short step to the concept of coupling a telephone receiver to a carbon microphone to make an amplifier, and several people took it. Within a few years of the original Bell invention, patents on this notion had been taken out by Edison, Houston, Lodge and Hughes, not to mention others less famous. By 1896, 27 patents for a mechanical-electrical amplifier- though it was then called a "repeater"- had been taken out.

Carbon microphones are not high-fidelity devices. Anyone who has used an old-fashioned telephone will recall that they would intermittently go low-gain or noisy, due to the granules packing, and a sharp rap of the handset against the wall was required to restore normal service. It was clear that the mechanical amplifier was far from perfect, but it was the only amplification option that looked practical.

In 1903 H E Shreeve was given the job of designing a practical mechanical amplifier. He discarded the microphone and earpiece diaphragms, and replaced them with a simple plunger that was driven by the receiving coil and pressed against the carbon granules of the transmitter.

One of the requirements of a practical amplifier was stability over time, which meant a solution to the problem of granule packing was needed. Shreeve found that this was due to thermal expansion caused by the heat liberated in the carbon chamber. This reduced the resistance, causing an increase in DC current but reduced audio output. This problem was controlled by designing the transmitter so that expansion of its parts did not increase the pressure on the carbon; and this led to the 1904 model below.

Left: The basic operation of a mechanical amplifier is very simple.

The electromagnet attracts the armature in response to the incoming signals. This varies the pressure on the carbon granules in the transmitting part, which varies their electrical resistance, and hence the current flowing in the output circuit.

Left: The first Shreeve mechanical amplifier. Date approx 1904.

This model saw commercial use on a New York - Chicago circuit between August 1904 and February 1905.

Note the cooling fins for the carbon microphone section at the right.

Left: The early Shreeve 1a amplifier.

The 1a worked reasonably well, and saw some use. However it was not possible to run more than two in series without the audio degrading too much, and three were needed for transcontinental USA operation.

The degradation was usually referred to as "distortion" but this seems to have meant a poor frequency response rather than non-linearity.

The cooling fins appear to be at the bottom.

The Shreeve 1a included a simple form of feedback control to reduce packing. The transmitter current heated a zinc strip which withdrew the rear electrode to act against increases in this current. This was twenty years before the formal invention of negative feedback, though the principle had been in use for many years in the form of Watt governors.
Left: A cartridge type amplifier, Model 3A. This became the standard type of amplifier until valves were introduced.

It was developed in 1912 and standardised in 1914. Parts likely to need replacement were packaged in an easily replaceable cartridge.

Performance was still limited by poor frequency response and a rapid falloff of gain at low levels; three in series was the limit.

Here fixed annular cooling fins surround the base of the cartridge. The purpose of the two round things on top is unknown.

Above: The internal construction of the Model 3A.
The basic principle appears to be minimising the mass of the vibrating electrode, pushing the fundamental resonance to as a high a frequency as possible. The function of the "compensating winding" is obscure; since there appear to be only 5 terminals, presumably the magnetising and compensating windings have a common terminal.


There were other attempts to achieve amplification before the valve. In 1912 Dr H D Arnold had developed an amplifier in which beams of mercury ions were deflected by electromagnetic coils. This was based on some earlier work by Peter Cooper Hewitt on mercury-vapour discharge tubes.
The device worked, and had a good frequency response, but had such severe starting and maintenance difficulties it was unsuitable for unattended operation. It was used experimentally on the transcontinental circuit, but never commercially.

Left: The Arnold mercury-arc amplifier.

Here is my take on how it works:

Normally an arc is running from anode A to mercury cathode C, powered from Batt 2 and current limited by resistor R. The function of inductor L is obscure, but it might be something to do with promoting arc stability.

The arc is deflected sideways when currents flow through the deflection coils (green) mounted on the D-shaped iron core (in pink). These currents come from the carbon microphone on the right, energised by Batt 1 and coupled through transformer T1.
The arc thus moves between two subsiduary electrodes, in a way that is not too clear from this drawing, and the audio output is taken from the balanced to unbalanced transformer T2 to the earphone at lower left.

K appears to be the starting key. When it is pressed, starting anode A' is connected to Batt 2 and presumably an initial arc is struck through the shorter path A'-C.

Left: The Arnold mercury-arc amplifier.

This model looks somewhat different to the patent drawing; there is only one reservoir of mercury at the bottom, so presumably the starting arrangements were different.

Another possible amplifier investigated was the von Lieben cathode-ray amplifier; this proved to be no more promising.
The 1905 von Lieben amplifier was a cathode-ray tube, its electron beam being steered towards or away from a target electrode by magnetic coils. A patent was granted in 1906, and based on this, the two German companies AEG and Siemens founded Telefunken AG to build electron tubes.

Dieckman & Clag (of Strasbourg) filed for a patent on a similar device in 1906. This used electrostatic rather than magnetic deflection of the beam.

Meanwhile, the vacuum tube was being created. Fleming produced a thermionic diode in 1904, and in 1907 a US Patent was granted to Lee de Forest for adding a control grid, to make the first triode. By October 1912 de Forest was demonstrating a valve audio amplifier to Bell officials. By October 1913 improved valves had been built and tested on commercial circuits between New York and Baltimore, and at the end of 1914 valve amplifiers were in use on the transcontinental circuit. It was clear that valves were the way ahead, and the mechanical amplifiers built for this service were placed on standby.

So, was that the end for this technology? Indeed not. Now read on...

S.G. Brown Ltd., of London, were well-known makers of headphones. They manufactured several models of electromechanical amplifier, working as above with the input coils moving an armature connected to a carbon microphone. The microphone worked in push-pull; in other words one carbon capsule was compressed while the other was released, and the anti-phase outputs were combined in a transformer. What they called the "Microphone Bar Amplifier" was first manufactured during WW1 (1914-18) and was used by the British Army; after the war a large number were sold off as government surplus. I think 'Bar' is probably an acronym for 'Brown Audio Relay' but this is unconfirmed at present.
By the early 1920s, S G Brown were advertising their 'Crystavox' loudspeaker, which was an electromechanical amplifier combined with a horn loudspeaker in a single unit.

Left: A Brown Type V microphone amplifier from about 1924. Four models were available and this is a Type V (for valve sets) with a 2000 ohms input impedance, and 2000 ohms output load.

Note the useful circuit on the lid. The carbon microphone is a push-pull type, hopefully reducing second-harmonic distortion, feeding a step-up output tranformer.

Left: A contemporary advertisement for Brown amplifiers, showing the Four models were available. The one shown above is a R53/3 Type V.

Interestingly the amplifier is claimed to be "without distortion", which is quite untrue; distortion was high. The prices shown were a great deal of money for the time.

Rather strangely, the instrument is said to be suitable for all classes of communication, despite having a 1 kHz resonant frequency clearly chosen with CW in mind.

These two pictures are reproduced by kind permission of Lorne Clark, whose very fine website can be seen at

Left: A 1927 advertisement from the New Wilson Company.

By 1927, Brown appears to have lost interest in this technology and passed the rights over to the New Wilson Company. The amplifiers also appear to have been marketed by the Empire Electric Company of London NW1. Interestingly, New Wilson claim to be the patentees of the idea; this may simply mean they bought the patent rights from Brown.

The only report I have unearthed states that the gain was reasonable but the audio quality poor, as would be expected. Once again a very dubious claim of "no distortion" is made.

Image from The Radio Times for 23rd Sept 1927


Left: A policeman demonstrating the power microphone PA system in 1937.

A related technology was the Tannoy power microphone, which could drive a loudspeaker directly, without the need for an intervening amplifier of any kind. This consisted of eight push-pull carbon microphones mounted in a heat-dissipating aluminum casting; they drove a centre-tapped transformer which in turn drove a horn-loaded loudspeaker. It was therefore possible to make a battery-powered public-address system without the fragility and complexity of a valve amplifier. In this early model the horn loudspeaker is mounted on a tripod with the accumulators slung underneath to give stability.

This apparatus was produced in anticipation of war. In 1938 about 800 were made for the Civil Defence Corps, the Auxiiary Fire Service (my mother was in that) and the Police for crowd control during bombing raids. There appears to be no information on whether they were ever used.

Even the final production model was not very efficient, delivering about 8 Watts to the speaker while drawing about 50 Watts from the accumulators. This was not really a problem because horn speakers are efficient at turning electricity into acoustic output, and the volume produced was quite enough. The apparatus would only have been used intermittently; there was an on/off pushbutton built into the microphone handle.

The same technology was used for intercoms in harsh and noisy environments, such as tanks in WW2.

Info from The Tannoy Story by Julian Alderton, pub Gaskell 2004

As I said above, a carbon microphone in itself is an amplifier. Until miniature valves became available in the late 1930's, a valve hearing-aid was not very practical. Early valves were rather large (imagine a hearing-aid constructed with octal valves) and battery drain was high for the filaments. There were a few large hearing aids with valves, but the majority of the electrical hearing aids sold between 1900 and 1938 were electromagnetic/carbon.

Left: circuit of the Western Electric No 38A Audiphone hearing-aid.

The rheostat is presumably a gain control, and appears to alter both the microphone energising current and the amplifier energising current. This no doubt saves battery power when the full gain is not required.

The circuit shows three cells in the battery. If this may be taken literally the operating voltage was about 4.5V

Left: internals of the Western Electric No 66B hearing-aid. This appears to have the same circuit as above. The rheostat is on the left, and the microphone, earphone and battery are all external to this little box.

Mr Hugh Hetherington (who kindly supplied this image) tells me that the hearing-aid carbon microphone employed a carbon ball technique rather than the carbon dust used in telephone transmitters; these apparently gave more amplification than the carbon dust transmitters, but were never used in telephones.

This topic was originally an external link to a very informative webpage. Sadly, this link seems to have died, so I give a much condensed account below. I have been unable to locate the original site builders- if anyone feels I have purloined information, please let me know.

In early 1912 several businessmen from New York were traveling in Austria-Hungary and while in Budapest, they were surprised to learn that they could listen to concerts or lectures without leaving their rooms. Budapest had an early cable distribution system that drove headphones.
These gentleman saw business potential in this, and formed the New Jersey Telephone Herald Company. It was decided to install the system in Newark, New Jersey, with the idea that if it was successful there, it would be introduced in New York. Wires were leased from the telephone company, installation was started early in the Spring of 1912 ,and regular programs were being broadcast by July. The programs were produced in a suite of rooms very similiar to a present day studio for a radio broadcast station, with acoustic treatment on the walls and Erickson microphones.

For the first two or three months the subscription department was swamped with orders for installations, and within the first three months there were about 5,000 subscribers. The service cost $1.50 a month. However, as with everything else, people soon tired of their new toy, mainly because loudspeaker reception was not available, although the signals that were received were very clear and of excellent head-phone volume.

The management of the company realized where the difficulty lay and Mr. Rainbault and his chief engineer, Mr. J. L. Spence, worked on the perfection of a "mechanical amplifier". No details are currently known but this probably worked on the same electromagnetic/carbon scheme as the telephone repeaters above. However, the results obtained were far from satisfactory, so in December of the same year it was decided not to fight any longer against such odds. The New Jersey Telephone Herald Co. was closed and the headsets removed from the homes of Newark. There had been an outlay of over $200,000.

Thus electromechanical amplifiers once more proved unequal to the task of amplification. Radio broadcasting in the USA did not begin until 1916.

I would be very glad to hear from anyone who has any more information on these remarkable byways of technology.

Acknowledgement: This article draws heavily on the book "Engineering and Science in the Bell System". All speculation is mine.

Back to Home PageBack to The Museum EntranceTop of this page