Bruno Velotron

The following is taken from the Xaudia blog.

The Bruno Velotron requires a polarisation supply and a buffer amplifier to operate - I am not sure if these came with the mic, but the tube amps of the era could easily have been modified to provide this from the high voltage supply. The excellent Coutant website shows a wiring diagram for a suitable polarisation supply and buffer amplifier...Coutant.org

Based on this diagram, I wired a 7205 subminiature tube into one of our tube microphone power supplies, to act as a little amplifier circuit for the mic.

Miraculously, the microphone worked on the first attempt, even after 74 years! The sound is very warm and dark with an obvious roll off at the top end, and it was very exciting to hear one of these for the first time.

I wondered if some of the roll-off may be due to cable capacitance - if the element really does have an impedance of the order of 4.5 MΩ then even a few tens of pF will become significant. However, swapping cables and using a triaxial arrangement didn't make any different, so it may well just be the natural response of the beast, and certainly how it would have sounded at the time.

The mic is unfortunately prone to noise and pops. They are notorious for breakdown of the insulation on the backplate and that seems to be the case with this one.

From Bruno advertisements inc. one in the Radio Amateurs Handbook, 14th edition. 1936.

THE Bruno "VELOTRON": A High Fidelity Velocity Microphone

The "VELOTRON" is more than "just another mike" ... it embodies in a simple and rugged device all the qualities of the various types, eliminating most of their defects and thus becoming the ideal microphone for the amateur and P.A, fields. Based on a new construction principle, the "Velotron" retains the high fidelity of the Velocity Dynamic microphone, surpasses the output of former, the "Velotron" will not pick-up stray r.f. currents, thus obtaining hum-less operation. It is ideally suited for outdoor transmission.

The unique design of this ultra-modern microphone makes high-fidelity operation over the entire frequency range a possibility. Frequency characteristics can also be changed to meet varying operating conditions by altering the polarising voltage. The "Velotron" may be quickly connected to the amplifier by means of a simple adapter circuit consisting of two small resistors and condensers. A diagram illustrating the necessary circuit is furnished with each microphone.

SPECIFICATIONS

Output level: -50 db.

Size: 6 x 2.25 x 1.5 inches. Weight: 1 lb.

Output impedance: to grid

Frequency response: 30 to 14,000 Hz (adjustable)

Finish: Gunmetal

Polarisation voltage: 150 to 350 volts

LIST PRICE: $19.50

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MoMics note.

The Bruno Velotron (made by and sometimes badged as Webster) was an oddball microphone that took inspiration from both ribbon and condenser technology. Strips of aluminium foil were attached to an insulated metal backplate, and a voltage applied between the two. As the foil vibrates, the capacitance of the device varies, and a signal is generated. They are notoriously fragile, and there are very few around in working condition. One astonishing thing to note is that the earliest version of the Velotron required a polarisation voltage of between 150 and 350 volts, which would undoubtedly give the user a surprise if connected in the wrong way. However, by 1939, Webster was offering a version with 'permanent polarisation' which could be used with any high impedance microphone input.

The following is from an advertisement shown is from the Radio Masters' Handbook of 1939:

"WEBSTER-CHICAGO MODEL 1240-A is a bi directional high impedance capacity device and its actuation is similar to the typical velocity microphones. In construction a residual charge is built on the dielectric, thus eliminating the necessity for a polarising potential. The output level of this microphone is relatively high and it can be used on cable lengths up to 1000 ft. The frequency characteristic is remarkably flat thus insuring natural reproduction. MODEL 1240-A as described. .....Price $22.50"

These were sold in Europe under the Radio Marelli brand.

The Velotron microphone and how it works is discussed in detail in Electronics Magazine, September 1936. This includes internal photographs and a circuit for supplying the required polarisation voltage to the microphone.

A technical discussion and attempted repair of the Bruno Velotron at the Xaudia microphone blog.

About the Bruno & Webster Velotron mics at the Coutant Website.