|Eleven Melodium 42Bs in for service at Xaudia
|Replacement magnet assembly for Melodium 42B ribbon microphones
The horseshoe type magnets found in some of these mics tend to hold their field well and rarely fail. However, I have occasionally encountered examples where the magnets have cracked and lost some of their strength. This may happen if the mounting screws are overtightened or the microphone has been abused.
Around half of the 42Bs use block magnets with a carrier bar to complete the magnetic loop. These are more prone to losing their field especially if they are removed from the microphone for some reason. It is hard to predict how these will behave. Luckily we can measure them and see if they are up to scratch. A healthy microphone should show above 3000 Gauss in the ribbon gap to give good signal to noise performance.
In this example, the field is down to around 2400 Gauss, which is at about 80% of where it should be. That amounts to 2dB loss of output – not yet a disaster but may be heading that way. With the new assembly in place we measure around 6000 Gauss, which is up + 6dB on a ‘good’ stock Melodium 42B.
*Magnet failure is rare. A bad ribbon is the most common cause of a weak signal in these microphones, and would be worthwhile testing the ribbon before embarking on any kind of magnet service.
It has been a long time since my last blog post!
This is mainly because the launch of Extinct Audio took up much more of my time and energy than I had ever imagined. But I have missed this blog and will be posting a bit more often in the coming weeks and months. Thanks for your support.
|Lustraphone VR53 ribbon microphone
This week I have been working on a few Lustraphone VR53 ribbon microphones. I have never been very enthusiastic about these mics because generally they have a weak output and can sound dark and flabby. Replacing the ribbons and transformers will get the mic so far, but they never quite get up to the performance of other British ribbon mics like the Reslos and Grampians. Which is a shame because they have a unique vintage design and look as though they should be good!
Part of the problem is the magnetic circuit. The two horseshoe magnets provide the field which is conducted (rather inefficiently) around a steel structure to the ribbon. This gives a field around 1500 to 2000 gauss. (For comparison, the field in a healthy Reslo would typically be around 4000 gauss.)
|Milling the pole pieces to make room for new magnets.
The steel pole structure is the limiting factor here and so swapping the horseshoe magnets gives only marginal improvements. For this upgrade I made a decision to mill out the pole pieces and fit some strong magnets right next to the ribbon.
The difference is impressive, bringing the field across the ribbon to 6000 gauss, which should provide about +9 to +12dB increase in output voltage. With a new ribbon fitted. the output and signal to noise were improved overall by 20dB. Now it can give the other microphones a good run for their money.
|Before and after… new magnets and ribbons.
As an aside, the stock 200 ohm transformer in this example is actually pretty good and don’t need to be replaced. The 30 ohm and high impedance models are not very well suited to a modern studio and in those cases it is worth swapping them out for a more sensible tranny.
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
‘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.”
|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.
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.
|Failed magnets in a T-Bone microphone
The coating around the magnets had peeled away, allowing the rare earth magnets to oxidise and expand. This process of course destroys the ribbon too. The magnets can be replaced, but may not be worth the expense.
I have seen a few microphones that look like this, and the worrying aspect is that it seems to happen spontaneously, possibly because the coating on the magnets was not of high quality, or perhaps they were scratched or cracked on installation.
Nearly all manufacturers now use neodynium magnets for their microphones, and I fear a epidemic in the future.
This is the inside of a client’s Avantone dual ribbon microphone, which was in pretty bad shape.
The cause of the failure is still a mystery – perhaps it could be due to a process failure in a batch of magnets, or maybe the mic was exposed to an excessively humid environment.
I have seen other mics with signs of flaking on the magnets, but nothing this serious. Neodynes are still a relatively new magnet technology, and how they will stand up to years of studio use and abuse remains to be seen.
Happily, we were able to bring the mic back to life by replacing the magnets and of course re-ribboning the mic.
Here’s an early Electrovoice velocity ribbon mic, model V1. These are great looking microphones, but the early versions are rather crudely made and this one, like many others, suffered from low output due to weakened magnets.
The motor of this model is based on a single cylindrical permanent magnet, clamped to a pair of metal plates which make up the pole pieces of the assembly. Because of the positioning of the magnet, the magnetic field is uneven, with a significant difference in field between the top and bottom of the motor assembly. In our example we found that the field varied from around 700 gauss at the bottom to 1000 gauss at the strongest point. This is very low for a ribbon mic, and, along with the oxidised ribbon is responsible for a low, noisy output.
Fortunately, we have sourced some very powerful cylindrical N42 neodynium magnets of a suitable size and shape, which are a perfect replacement for the original weak magnet.
With the new magnet the field is increased by a factor of around four, to about 3000-3200 gauss, a much healthier figure which should lead to an increased output and much improved signal-to-noise performance.
Now it’s time to cut a new ribbon, reassemble the microphone, and do some listening tests. In the meantime, we made a rather attractive bracelet from some of the spare magnets.