Here’s a funny little thing from Lustraphone, found on ebay…..
This was sold as a desk microphone, but it may actually be a small speaker. Or both! It looks very stylish, with tolex covering and brushed nickel finish.
The ‘device’ measures 600 ohms at 1KHz, and 50 ohms at 100 Hz. Inside, the dynamic element looks more like a small speaker than a mic element, although of course the two things are fundamentally the same technology. When wired as a mic it is pretty lo-fi with a narrow bandwidth. And it does transmit as a speaker too. My guess is that it was part of an intercom system and serves both purposes.
Whatever the intended purpose, one can easily imagine this on a gentleman’s desk, so that he could talk to his secretary whilst smoking a pipe and considering important, worldly matters… like this chap!
I have created a new blog called MicIRP, which is a contraction of The Microphone Impulse-Response Project.
MicIRP will run in parallel to the Xaudia microphone blog, and aims to share impulse-response (IR) data from the many interesting old mics from our own collection and that come in for repair
The IR files can be used with convolution reverb programmes or plug-ins such as TL Space or Waves IR1 to simulate the sonic characteristics of the microphones. So they can be considered as filters or tiny spaces to modify your sound.
I have uploaded files for about 35 microphone to get things started, and will be adding many more mics as and when the opportunity arises.
Here is a UK patent by Eric Thompson of Simon Equipment Ltd, from 1959, that relates to the Cadenza ribbon microphone: Cadenza Patent
Drawing from patent for Cadenza microphone
The novelty in this patent is that the ribbon is fixed in the middle and clamped at either end. The inventors claim that this allows the two halves of the ribbon to be tuned to slightly different frequencies to give a flatter overall frequency response. I had suspected this for some time, but had never seen it written down! It also has the advantage of supporting the ribbon and giving it some protection against stretching. And here is a photo of the design in real life, from an earlier post.
Regular readers will be aware that I am a big fan of B&O microphones, and have serviced quite a few of them over the years. Whilst these mics are stylish and look great, the output levels can often be disappointing, restricting use to sources like guitar amps and drums. The low sensitivity is usually due to weak magnets and the fact that the early microphones have an output impedance of 50 ohms – an older standard. With some work we can make these handsome microphones sound as good as they look.
B&O BM3 deconstructed
Firstly, a replacement transformer can be used to convert the microphone’s output impedance. The stock transformer in the early BM2 and BM3 microphones has a ratio of about 1:20, and with the thick wide ribbon gives an output impedance of 50 ohms. Switching to a 1:40 transformer will give an increase of 6dB and raise the impedance to about 200 ohms – Xaudia make such a replacement. With a thinner and lighter replacement ribbon, the output and impedance will both be a little higher.
New magnets!
We also commissioned a batch of custom magnets for BM2, BM3 and BM4 microphones* which greatly increase the magnetic field, and therefore the output level. The magnets in an old BM3 typically have a field strength of 1000 to 1600 Gauss, whereas the replacements give a field of around 6500 gauss – a three or even four fold increase in magnetic field strength, which translates, at least in theory, to a 9dB to 12dB increase in output.
Here are some frequency response plots from three B&O BM3 microphones.
B&O BM3 with new ribbon (blue), new transformer (green), and new magnets (red)
Red = new ribbon, upgrade transformer, new magnets. Green = new ribbon, upgrade transformer, stock magnets Blue = new ribbon, stock transformer, stock magnets.
With the full upgrade, the output level is raised by about +18dB, which transforms the mic into a much more versatile recording tool that can be used for acoustic instruments as well as the usual louder sources. If your B&O mic needs a new ribbon, then it is well worth considering a magnet and transformer upgrade at the same time.
*We have similar magnet upgrades for the BM5 and BM6 mics.
