Electro-mechanical amplifier ("carbon amplifier")
In a mahogany cabinet with a lift-up lid. On the left and right of the lid there are terminals for input, output and a 6 volt battery. There is a schematic diagram on the underside of the lid. The type plate is mounted on the front of the cabinet; on the left hand side a BBC transfer with the text "Type approved by Post Master General". A knob with the text "Magnetic Adjustor" - a "Vernier magnetic tuning adjustment" - as it is called in an advertisement, is positioned on top of the lid. A small magnet is hidden inside this knob. It can be used to fine tune the amplification.
The input signal is applied to two terminals (LF) and fed to the "receiving coils", which causes an iron reed to vibrate in sympathy to the changing magnetic field in the coils. The vibrations are then applied to a small carbon microphone, connected to a 6 volt battery. The amplified current is fed to the output terminals via a transformer to a loudspeaker or a set of headphones. In 1926 Brown made a 2-stage version that amplified the sound twice using another coil set and carbon microphone.
This type of amplifier was invented in 1904 by Bell engineer H.E. Shreeve. Shreeve used the device (left) to amplify telephone signals between New York and Chicago. In 1923 Brown introduced a loudspeaker with an integrated carbon amplifier, the "Crystavox" (right). Type "V" indicates that the Brown amplifier can be used for operating a loudspeaker or headphones from a 1-valve receiver. Type "C" was used for a crystal set.
Compared to a 2 valve amplifier, that would cost about 10 pounds, this amplifier was cheap: about 5 pounds.
Four models were available with different input and output impedances. This model has an input and output impedance of 120 ohms, the type number is RR53/1. The original price was 5-5-0.
The amplifier, called ("Microphone-Relais") was also sold in The Netherlands. The price was f 75,-
On the right an advertisement in magazine Radio Expres, August 7, 1924.
Sidney George Brown (1873-1948), the company founder, was a prolific inventor of electromechanical devices, including the well known "A" headphones, the "Microphone Amplifier" and a gyro compass. The company also made loudspeakers and radios. In 1906 he founded Telegraph Condenser Co and in 1910 he set up a business under his own name: S.G. Brown Ltd., in Watford (North of London) and later had a works in North Acton, London W3. The company eventually became part of the Racal organisation which in turn became part of Thales-Racal Acoustics.
On the left: the Brown 2-Stage Crystal Amplifier made in 1926.


Serial number: 7112
Dimensions: 16,5 x 12 x 9,5 cm
Made in: 1923
Purchased in: 2011
Impedance: 120 ohms
Click on the circuit to enlarge
The sound was described as "pure and undistorted" in the advertisements. But in fact there was quite a lot of distortion.

By 1926, Brown appears to have lost interest in this technology and passed the rights over to the New Wilson Company in London. In that year New Wilson introduced the Microphone Bar Amplifier.


The New Wilson Microphone Bar Amplifier

Advertisement, showing the various models of the Microphone Amplifier

Top view

Side view and detail of the mechanism

This page was last edited on 29.04.2019