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| Bang and Olufsen BM3 ribbon mic |
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.
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| 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.
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| 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.
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| 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.