Double electromechanical amplifier ("carbon amplifier")
In a mahogany box with a lid. Connections for input, output and battery voltage are provided at the rear and right. On the inside of the lid is a diagram with short instructions for use. The dual system amplifies the signal from a crystal receiver, so that a horn loudspeaker can be usd. The power gain is about 1000 times. Maximum output power is around 0.3 Watt. The frequency range is up to approximately 1800 Hz.
Under the wooden lid are two levers, AB and CD. The pre-amplifier is set with the left lever, the power amplifier with the right lever. Both amplifiers are set as a balance amplifier, to prevent saturation of the iron cores of the coils. The input signal is fed through two connections to a coil, causing a metal rod ("reed") to vibrate in rhythm with the sound signal. The rod is connected to a carbon microphone that is connected to a 6 volt battery. The current is amplified by this and then fed back into a second system, whereby the current is additionally amplified and fed to a connected horn loudspeaker via a matching transformer.
The carbon amplifier was invented in 1904 by Bell engineer Herbert E. Shreeve. He used the system (photo left) to amplify telephone signals between New York and Chicago.
The sound that would be obtained with this type of amplifier is advertised as "pure and undistorted". The truth is, the system was pretty scrambled, but it was cheaper than a tube amp.
In 1923 Brown introduced the Microphone Amplifier for single amplification and a speaker where the system was incorporated, the "Crystavox" (photo right).
Sidney George Brown (1873-1948) was a prolific inventor of electromechanical devices, including the well-known “A” headphones, the “Microphone Amplifier” and a gyrocompass. The company made loudspeakers and for a short time also radios. In 1906 he founded Telegraph Condenser Co and in 1910 he started a company under his own name, S.G. Brown Ltd., in Watford (north of London). Later he also had a business in North Acton, London W3. S.G. Brown Ltd. later became part of Racal, which again became part of Thales-Racal Acoustics.
Serieal number: None
Dimensions: 33.5 x 12.5 x 12.5 cm
Made in: 1926
Purchased in: 2011
DC resistance: 2000 ohms
Weight: 1.9 kg
Original price: £ 1.4.0, later 44 Shilling
In the same year, 1926, New Wilson Electrical Manufacturing Co., Ltd., 18 Fitzroy Street, Euston Road, London W1, also introduced a carbon amplifier, the Microphone Bar Amplifier for single amplification (photo opposite).

Front view and detail of the system
Left and center the two amplifiers, right the matching transformer and a loudspeaker capacitor.
Advertisement in Popular Wireless, January 15, 1927

This page was last edited on 22.01.2021