The Xaudia Blog

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

04-01-2014

RCA 74A and 74B ribbon microphones

Happy 2014!

Here is a lovely pair of RCA junior velocity ribbon microphones awaiting service, a 74A and a 74B. We see a lot of ‘B’ models, but the earlier 74A (left) is quite rare by comparison.

RCA 74A (left) and 74B

Externally, the mics are similar in size and shape, but the 74A has a yoke mount and looks more like a baby 44BX, whereas the 74B has a fixed or swivel mount at the base of the mic. The ‘A’ model also has larger grill holes.

RCA 74A (left) and 74B ribbon mics inside

Inside, the ribbon motor assembly is the same although the mounting arrangements differ. The 74A motor (left) is screwed directly to the frame, whereas the ‘B’ is suspended by four small rubber mounts. The yoke would absorb some vibrations transmitted through the mic stand, and so the rubber parts were possibly introduced to compensate for the loss of the yoke. The earlier transformer has two outputs at 50 and 250 ohms, whereas the 74B transformer also has a high impedance output for public address use.

One can only guess about the reasons for the evolution from ‘A’ to ‘B’ models, but these things usually comes down to performance and economics, with fashion and marketing also playing a part. Losing the yoke might reduce costs, but adding the suspension and the swivel joint would probably balance this, so it is hard to tell.

Both mics need new ribbons!

The amazing Coutant website has a scan of a data sheet for the 74A.

24-12-2013

Ribbon mics in action

I am lucky to have many great customers, and some are kind enough to take the time to share their music and other projects. It is always gratifying to see how these mics are used once they go back out into the ‘wild’.

Hewdie is a long-standing customer who was kind enough to send in a copy of his excellent new album.  I have serviced many of his ribbon mics over the years and these feature extensively on the recordings – including a B&O BM3, RCA 74B, Lustraphone VR53 and a couple of Beeb Reslos. The album is well worth a listen.

Andy Wright used his Beeb Reslos to record The Kimpton Players in their radio play production of Noel Coward’s “Brief Encounter”. Here is a photo of the Beebs in action…

And Jose Estragos was kind enough to send in these photos of his band Los Brandys, in action with their fine Reslo CR microphones. I will let you guess their influences!

Producer Tony Fitz used a pair of our ‘Beeb’ modified Reslo RBs for the drum overheads in this track..

And here’s how the drums sounded in the final track Stay by Free Manhattan

Have a great Christmas & New year – and good luck with all of your holiday projects.
Stewart
Xaudia

23-12-2013

‘The Beeb’ Reslos – also in black!

It is time to down tools for the holidays – this year we are part way through preparing a new batch of modified ‘The Beeb’ Reslo ribbon mics. The Beebs get a new ribbon, new transformer and XLR upgrade, along with a few other small tweaks. These will be available in January.

This batch now have their own metal badge, which looks great. In addition to the usual chrome and hammertone mics, we also have some rather smart looking black microphones, which have a tough powder-coating finish.

The sound is still the same 🙂

19-12-2013

I thought this was a switch… but it isn’t!

I had expected the red thing on the rear of this Amperite RB ribbon mic to be a simple on-off switch:

Amperite RB – switch on rear
In fact the knob is connected to a cloth-covered metal plate which slides up and down, blocking the rear of the mic. This is a mechanical method of altering both the pickup pattern and the frequency response.
Inside the Amperite – sliding plate, blurry photo 🙁

When open, the mic will be a normal figure-8 pattern. Closing the vent will reduce pickup at the rear, and also make the mic sound brighter by reducing the proximity effect.

Open and….
… closed

The mechanism is not as sophisticated as, for example, an RCA 77DX, but at least they made an early effort to offer engineers a new flavour.

Well done Amperite!

11-12-2013

MOTM – Melodium RM6

As promised (or threatened), MOTM is back, and December’s microphone of the month is the Melodium RM6.

Although much much smaller in size than the Melodium 42B, the RM6 takes some of its design philosophy from its predecessor. The motor assembly has horseshoe magnets above and below the ribbon (like a tiny RCA 44BX), and an inductive filter with multiple cut-off frequencies. It also has an obsolete and hard to find proprietary connector, albeit a different one from the 42B.
The body is made from cast metal which has a tendency to crack, and the first challenge is to get inside the mic without damaging it. There are three tiny metal pins pushed through the mic that fix the grill to the lower body of the mic. These need to be pushed all the way through the mic so that the grill can be removed, and usually these can be found stuck to the magnet, although one will inevitably go missing!

To remove the mic completely, the connector must be unsoldered and the switch tip removed. Once inside, we see something that looks like the Easter Island statues!

The ribbon is hiding behind the baffles, and the motor requires quite a lot of disassembly before the ribbon can be accessed. The ribbon itself is about 1.8 mm wide, so a bit fiddly to fit. Like most mics of a certain age, half the problem is that the ribbon has become oxidised and stiff, and the other half is small particles of wild iron that that have become stuck between the pole pieces, preventing the ribbon from moving freely.

Removing the strong magnets made cleaning and re-ribboning a lot easier! The transformer and filter inductor are housed in a mumetal can, screwed beneath the motor assembly and above the filter switch.

As usual, the mic sounds best without the high pass filter engaged, although I can imagine it being useful to compensate for proximity effect when close micing some instruments.

There doesn’t seem to be much information about this mic available on-line, but I have scanned an old Melodium catalog featuring the RM6. There is also a French language review of the RM6 over at Audio Fanzine. They seemed to like it, and gave the mic 9 out of 10!

07-12-2013

The Dead Zone….

Here is some amusing copy from the box of a Grundig GRM1, a.k.a.  Lustraphone VR53. Above the nice diagram of the the figure 8 pattern, it proudly states “There is virtually no sensitivity at right angles to the microphone front, and this we call the “Dead Zone”. Now we know what to call it!

The Xaudia blog has also been a bit of a dead zone in recent weeks, due to lots of repair work coming in. Things are getting back under control, so more microphone related posts and info will be coming soon!

14-11-2013

Lustraphone desk mic or speaker?

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!