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.
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!
November’s microphone of the month is a British long-format ribbon that was sold under the names “Lustraphone” and ‘Grundig”.
A Lustraphone-badged ribbon microphone in glorious brushed stainless steel finish
This mic was available in at least three different impedances, and I have come across 30, 200, and high impedance models. Unfortunately the badge often falls off so you don’t always know what you are getting! Most of the models I have seen are finished in a bronze hammerite colour, although there is a deluxe low impedance model which has a gorgeous brushed stainless steel finish. Despite the different badges and finishes, the mics are exactly the same on the inside (transformer aside).
Lustraphone ribbon microphones on the bench
One of the ribbon clamps sits on a spring-tensioned screw thread, which allows fine adjustment of the ribbon tension. This makes tuning the ribbon very straight-forward, and allows the owner a little bit of grace if the ribbon becomes a little stretched over time. This feature should probably be mandatory on all ribbonmics!
Grundig badged lustraphone mic, opened up for service
The magnetic field is supplied by a pair of horseshoe magnets (which unfortunately sometimes age with time, losing their strength). Connection to the rest of the world is made by a balanced three-pin paxolin plug, which are hard to come by now. The middle pin is ground, with the audio on the outer two pins.
Rear connector and original plug.
Fortunately, a male XLR connector can be modified to fit by slicing off part of the barrel.
XLR connector modified to fit the mic.
The long ribbon and motor design gives these mics a full bottom end and a pronounced proximity effect. Here are the frequency plots for three of these mics that we have serviced recently:
Lustraphone ribbon mic frequency plots.
(Thanks to Mark Stevens for additional information).
Update 23/1/12. These microphones were also sold under the brand Pamphonic. One appeared recently on ebay:
Update 29/2/12. And here is one with a Mimco badge!