B&O BM5 stereo collet repair

This is a really common problem with B&O stereo microphones. They snap in half! At the time of writing I have over ten broken BM5s in my inventory and some repairs for customers, and so clearly something needs to be done.

The rotating mechanisms were (usually*) made from plastic. With time and use, the plastic parts become brittle and the teeth break away. First the mic becomes wobbly and then can break away completely. These parts are also make the top part of the microphone difficult to service because it can be impossible to remove the collet without causing further damage.

And so I commissioned some replacement collets and can now offer repairs for this problem.

Brass collet for BM5

The replacement part is machined from brass and won’t break easily. There is some re-wiring to be done. The BM5 used a 9 pin socket which was not easy to replicate. And so we simply run the wires through a hole in the collet and solder together. The socket can get oxidised and noisy anyway, and there is no real need to remove the top part in general use. 

Using a brass collet has the additional benefit of making a good electrical contact between the top and bottom mic, which means better grounding.**

Here is the repaired rotating mechanism which should be good for another few decades of use.

These parts are made specifically for Xaudia in the North of England.


* Some later BM5s used a different design with steel parts. B&O clearly realised that they had a problem. 

** I have addressed the grounding issue in an earlier post.

Andrew Cadie – Down Where the Drunkards Roll

Andrew Cadie recorded this great version of Richard Thompson’s song, ‘Down where the drunkards roll’, using just a pair of B&O BM6x microphones to pick up his voice and acoustic guitar.

You can hear more of Andrew as he will play some recent compositions and a handful of cover versions, on Thursday 3rd December 2020 at 20:00 (Ger) / 7pm (UK) – all from his cozy studio couch. You can tune in at his facebook page or  youtube channel. 

Andrew’s microphones were upgraded here at Xaudia, and fitted with new ribbons and magnets, and an upgraded transformer. These changes raise the output level and reduce the noise substantially, turning them into very fine ribbon microphones.

Stereophonic Star Girl!: Katy Bødtger with her BM3.

Here is an advertisement for B&O equipment, including the BM3 ribbon microphone, which appeared in the Danish publication Se og Hør (See and Hear), 11 December 1959.

Photo captions
(Main photo) 
Katy Bødtger with her B&O microphone which is engraved with her name.
(Top photo)
Using a B&O stereo gramophone with the new stereo pickup and stereo speakers, Katy Bødtger -the first national stereophonic singing star- listens to the results (of her recording session) from London.
(Right Photo)
Even when Katy Bødtger’s busy life calls for a visit to the hairdresser to style her frisky locks before the evening’s performance, her indispensable B&O Beolit all-transistor reciever is always nearby.
The sweet and fresh Katy Bødtger, who provided the vocals for the new B&O stereophonic test record, has sung her way to success in only 2 years
Many thanks to Ben Cahill for sending this in and for the translation from Danish.

B&O BM5 and BM6 magnet upgrades

A while ago, I blogged about some magnet upgrades for the B&O BM3 and BM4 ribbon mics that greatly improve the signal-to-noise performance of the mics. We have similar magnet upgrades for the later BM5 and BM6 models.

Dirk at V76 Studios in Hamburg was kind enough to share some sound files of his BM5 stereo set, recorded before and after we fitted new magnets and ribbons. Here they are…

‘Blackbird’ recorded with his stock BM5, before the modifications

Download file

‘Blackbird’ recorded with the upgraded BM5, with new magnets and ribbons

These are Dirk’s impressions of his recordings…

“The difference is amazing. The mic sounds awesome now. It’s about 10-12 db louder (needing only 52 db for the front for finger picking now) and it has got much more treble and presence. Still it sounds smooth and wonderful rich, without losing the transients. This mic is killer for acoustic guitar now, and also as a room mic. I also compared it to my modified RB500 Mics (with cinemag 9888 Transformers) and the BM5 now has 6 db more output and is sounding so much better. Also the Beyer M130 doesn’t match it by far, IMHO.

“Both were recorded with the BM5 (60 degree angle pointing towards neck and bridge) into a TRP Ribbon Mic Pre direct into RME Converter into my DAW. No lowcut. The older recording would have been 10-12 db lower with the same TRP-Gain. For a better comparison, I choose to normalize both files. Still the new one sounds louder, brighter and more detailed. The noise floor of the older recording is clearly audible, and unnoticable with the new one – due to the 12 db more output of the mic.”

Update 21 Feb 2015.
Here are some comments from Andy at Superfly Studios in Nottinghamshire, UK, about his B&O BM5 with magnet upgrades and new ribbons.

“I’m so happy with these microphones. They have been absolutely fab to use over the past few days.
First session in was an acoustic folk duo, I used the B&O BM5 on one of the guitars and it just sat straight into the mix and sounded great. Picked up all the detail of his picking and sounded nice and rounded on the full-on sections. Just the right amount of highs for my taste as well, I normally use a 414 for acoustic and cut some of the top end, but I’ve got a new favourite now.

Also used the B&O for the drums today and it sounded great in the room, I love using room mics on drums and had the beeb reslo’s as a spaced pair about 8 ft away (10ft apart). Seems to always do the trick in the live room and the B&O just in front of the kit at waist hight. Sounded fantastic lovely sustain to the cymbals and picked up the natural ambience of the room.”

…I’m gonna take my Ribbon to the United Nations….

Back in 1954, Bang and Olufsen BM3 ribbon mics were installed in the United Nations building in New York . The mics were chosen for their lack of feedback, and are clearly visible in these video clips of  Lester Pearson speaking about the Suez crisis.

Lester Pearson speaks into two B&O mics at the UN

Pearson later won the Nobel Peace prize for his work, and became president of Canada. Perhaps those little Danish microphones played a small part!

B&O were justifiably proud of their mics being chosen for the UN – here is a document from the time…

And a translation from the Danish….

“As we previously mentioned to you, we have received a message from our US representative, Fenton in NY, that the UN uses our BM3 microphones throughout the UN building in NY. They chose the B&O mic and an American mic because only they, out of several hundred different manufacturer products that were tested, did not give rise to any acoustic feedback in the several hundred loudspeakers that are mounted in the ceiling of the UN general assembly building. Our mic also had an advantage over the US product as it didn’t hide the speaker’s face, which is important with regards to film and TV reporting. The microphones have been in use in the UN since 1st October this year. The newspaper INFORMATION recently had an article with a picture that depicted the Canadian foreign minister , Lester B. Pearson , at the podium in front of B&O microphones when he presented his suggestion for involvement in Egypt. Via our advertising bureau we managed to send out a press release under the title “Danish microphones at the UN podium”, and a whole selection of magazines included the picture and advertisement. We have attached the picture for your information – Next time you see the program AKTUELT in TV with recordings from the UN building, take note of our microphones, they are clearly visible.

“Our American representative has also undertaken an investigation that shows B&O micophones are used at, amongst others:
[Mercury, ABC and Decca]
“We have large orders from Fenton conmpany for microphones and pick-up units, and the advertising that Fenton performs in the American technical journals has the effect that we receive requests from other countries. This has resulted in that we have recently sent 10 samples to Australia and samples to Hong Kong, and other countries are also showing interest.
“Likewise we have significant orders for AC motors from Norway, where we compete with German and English manufacturers, and samples of this product has been sent to other countries.

30th Nov. 1956

Bang and Olufsen BM5 grounding issue

Some Bang and Oulfsen BM5 stereo ribbon microphones are prone to hum on one channel. This is because the top half of the mic rotates around a plastic collet ring, but is only grounded through contact to the lower half of the metal body. With time and use, the contact between the two halves becomes loose, and the upper body is no longer grounded properly.

B&O BM5 stereo mic

The problem only occurs with early versions of the mic. Later, B&O recognised the problem and re-designed the mic’s rotating ring giving it an additional metal lip, so that the upper mic rotates around metal, ensuring good grounding contact at all times.

If you have a an early mic, in most cases the hum issue can remedied by improving contact between the parts as follows. Here are some tips, but remember that there is a delicate ribbon inside, so go slowly and if in doubt send it to someone more experienced!

1. Carefully remove the top half of the mic, and loosen the two grub screws at the base with a screwdriver of the correct size. Don’t lose these little screws – if you do then they are M2 thread!

2. Slide off the ring and put the rest of the mic somewhere safe.

3. Flip the ring over and rub over some fine abrasive paper to remove any dirt and oxidation.

4. Gently run the fine abrasive paper around the bottom of the metal tube that will make contact with the lower ring.

5. Replace the lower ring and tighten the screws, making sure that the metal parts are properly aligned and make good contact. You want a close fit between the tube and ring.

6. If that doesn’t work, addition of a strip of copper tape to the plastic collet can improve the electrical contact between the two parts. The tape will eventually wear out if the mic is constantly rotated, but many users tend to keep the same angle for most applications.

SJT, Feb 2014

Upgrades for B&O microphones.

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.

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.

The Bang and Olufsen ribbon microphone family

Bang and Olufsen are a Danish company that make high end hi-fi equipment. Once upon a time they made some decent ribbon mics too. Here are a few…
B&O ribbon mics, from left, 2x BM2, 2x BM3, 2x BM6, 3x BM5 stereo

As far as I can tell, the first commercially available B&O ribbon mic was the BM2 – which begs the question “what happened to the BM1?” There seems to a bit of confusion about this, and possibly there never was a model called BM1. According to Beophile, the first B&O microphone was a dynamic mic called MD1. However, others have listed this as BM1. Numbers 2 to 7 were ribbons and carried the prefix BM, which may have stood for ‘baand mikrofon’ – Danish for ribbon microphone. Although it could also have stood for beomic. The MD8 was also a dynamic.

B&O BM2 ribbon microphone

Regardless of the BM1 (or lack thereof), the BM2 is a good-looking microphone, with a very different look to the later mics. It has a cast metal body and folded, chromed brass grill. The mics were painted in a green-yellow textured paint, which looks better than that sounds! They are usually* 50Ω mics, with an a switch which connects an inductor into the circuit, for a high pass filter. The ribbon is held in a removable frame, which slides out for servicing. (*They can be re-wound for 300Ω, to great effect).

The BM3 and BM4 look very similar to one another, and used an evolution of the motor assembly in the BM2, this time in conjunction with a steel tube body. This design set the style for all their later ribbon mics, and also inspired Speiden and Royer microphones, and a bunch of clones such as this Stellar mic.

B&O BM3 microphones

In the case of the BM3, the ribbon motor frame is larger than the diameter of the tube and sticks out from the sides of the mic, giving it the look of a long face with ears, or perhaps Doctor Who’s Cybermen. It has a three way selector switch which provides M (music – full range), T (talk – HPF) and  0 (off) positions.

Motor frame from the BM3, with Xaudia transformer, awaiting a new ribbon

The BM4 looks the same as the the BM3, but with an additional switch at the rear for selecting 50, 250Ω, or high impedance output. The BM3s were fixed at 50Ω (and benefit from a matching transformer or upgrade). Occasionally you see these badged as “Fentone”, although, oddly enough, they kept the B&O name on the mic too.

Fen-tone add showing the BM3 – from Preservation Sound

The BM5, BM6 and BM7 came later and formed a family of mics. The BM5 is the stereo model, and when rotated to 90 degrees, it is perfect for Blumlein pair recordings. The bottom half of the BM5 was available separately as the mono BM6, and the top was called the BM7, although it could not be used by itself.

Standard BM5 stereo set with stand

The design was an evolution of the BM4, but by this point the ribbon frame and been replaced by plastic mounts, and the cyber-ears have gone. The magnets are also slimmed down, with a semicircular or triangular cut-out, presumably in an attempt to increase the high frequency response. In these mics the body of the mic is made from steel and also acts as the magnetic return path, which helps to increase the output.

Insides of a B&O BM6 ribbon mic. Note the pistonic ribbon

Like all of Bang and Olufsen ribbon mics, the BM5/6/7 family have a pistonic ribbon, which is gently curved in the middle and deeply corrugated at each end. The ribbons were made from Duralum alloy, which contains copper in addition to aluminium, to improve the strength and stiffness. However, the alloy is more prone to corrosion than pure aluminium, and it is quite rare to find ribbons that are in perfect condition.

Delux BM5 set in posh wooden box!

From a technician’s perspective, these later mics are less robust  than the BM3/4. To me they feel more of a hi-fi design than one for a busy working studio. Although the sound is excellent, they are delicate in places and some of the plastic parts deteriorate with age, most noticeably the rotating ring in the top part of the mic. The switch tips also have a tendency to come off.

Plug for a B&O BM3 microphone

All the B&O mics used bespoke connectors, which can be hard to find. the BM2, 3 and 4 have a 3 pin connector only found on B&O equipment, whereas the BM5 and 6 used 5-pin DIN connectors but with their own threaded locking ring. The latter can be replaced with a standard bayonet DIN if the cable is missing.

Here are some links to wiring diagrams for some of the mics.
BM3 data sheet
BM4 data sheet
BM6 data sheet
BM7 data sheet

Thanks to Tom Day and Robin Kiszka-Kanowitz for additional information.

Update 1/2/18

Our friends at Extinct Audio now manufacture the BM9 studio ribbon microphone, which shows the  influenced of the B&O designs.

Update 4/3/14

We now have upgrade magnets and transformers for all B&O microphones.

Update 22/4/13

Many thanks to Evan Lorden for sharing these photos of his B&O MD1.

Bang and Olufsen BM2 microphone

Last month we were lucky enough to see this rare and beautiful B&O BM2 microphone pass through the workshop.

According to the BeoPhile.com website, these were made from 1951, and was replaced by the very different looking BM3 in 1962. Although different in appearance, the motor assembly and ribbon are similar in both cases.

The BM2 also has a switchable high pass filter to select between the music and voice modes, and also shares a hard-to-find connector with the BM3 & BM4.

Beyond that, we don’t know a whole lot about it!