The Xaudia Blog

31-07-2013

Inside the Beyer M360 (again) …

Beyer M360 cardioid ribbon mic
Back in May I posted about the Beyer M320 and M360 directional ribbon mics and mentioned that they used acoustic labyrinths to provide damping to the ribbon, adding omnidirectional character to the ribbon’s native figure-8 pattern, to give, hopefully, an overall cardioid behaviour.
Since then I have had the opportunity to peek deeper inside the M360, and below is a picture showing the labyrinth. It is not a great photo as the shiny varnish on the top confused my autofocus, but we can see what is going on.
Beyer M360 labyrinth (left) and motor.

The magnet assembly is larger than the standard Beyer motor, in order to accommodate a smooth curved cavity at the rear of the ribbon. This tube curves through 90 degrees and out through a hole in the aluminium base plate. This connects to the inlet of the acoustic chamber, which consists of 25 interconnected tubes that go up and down through the block. There does not appear to be any kind of outlet at the far end, and the tubes are stuffed with string to damp internal reflections.

The mic is really directional, with relatively little pickup from the rear of the ribbon. The back pressure from the sealed rear chamber also has the consequence of making the output brighter and lower than it would otherwise be. These are really very bright sounding for ribbon mics.

SJT 31 July 2013
23-07-2013

Reslo MR1 ribbon mic (MOTM)

The MR1 was an attempt by Reslosound to re-invigorate the classic RB model for a 1970s market, whilst trimming down the cost of manufacturing parts and assembly.

Reslo MR1 and RB models

The MR1 (left) has the same format as the RB, with the ribbon element and magnets housed in a tilting head attached to a cylindrical body that contains the transformer. The grills are now flat, rather than domed, and the custom pivot screw is replaced with a standard hex socket bolt.

Reslo MR1 and RB

The MR1 still usesd Reslo’s custom 3-pin connector, and still makes ground contact through the body only. One assumes that this was to ensure backward compatibility with their older mics and cables, but it seems like madness  that they did not take the opportunity to switch to using XLR connectors, which were very well established by this time.

The classic stamped brass badge on the RB is replaced by a printed foil sticker that hides the join between the barrel of the mic and the cast base of the pivoting head. The sticker shouts ‘Reslo MR1’ – note that it no longer says ‘Reslosound’ – in a very trendy 1970’s style font, with fattened letters typical of the era. I am also 1970s vintage, and the label reminds me of watching programmes like Magpie on TV in my childhood!

Magpie – classic 70s font!

The ribbon and motor assembly (top in the picture below) is exactly the same as the RB, but the transformer is smaller. Over time, better understanding gave rise to alloys with higher magnetic permeability, enabling smaller transformers to be made with the same inductance properties.  The RB mics used a EE24/25 size core (pictured below the motor). The MR1 has a narrower body and a smaller transformer marked 4414.

Top to bottom… Reslo MR1 motor,  transformers from RB, MR1 and Xaudia

Xaudia, as you might expect, make a suitable replacement transformer (bottom) for upgrade and impedance matching duties. 😉

Compared to the RB, I see very few MR1s, and can only conclude that they were not a great success. By the 1970s, good dynamic mics were becoming the first choice for home recording, and professional recording studios would probably favour condenser microphones for many applications. Reslosound Ltd. disappeared sometime in the 1970s, and this must have been one of the last models that they produced.

17-07-2013

Lustraphone microphone catalogue

Here is a scanned product sheet from Lustraphone, which includes the VR53 and VR64 ribbon microphones, along with dynamic microphones and accessories. Like many other manufacturers, they supplied stands and impedance matching transformers to use with their mics.

With a typically British quirkiness, the price of the VR65 stereo ribbon is given as 30 Guineas rather than pounds, shillings and pence. A guinea was equivalent to one pound and one shilling, so at about 4 times the cost of the mono VR64, the stereo VR65 was a very expensive mic!

In other Lustraphone related news, I have commissioned a batch of replacement enamel badges for Lustraphone VR53 and other microphones. They will be available through the Xaudia website very soon.

16-07-2013

Electro-Voice Microphone Catalogue

Here is a scan of an old Electro-Voice catalogue, probably from the 1950s or early 60s, featuring a wide range of microphones. It includes the multi-pattern Cardak, which appears to have been the flagship model, and the V-series ribbon mics. There is no date on the document, but at this point in time the V1 would set you back US$27.50, and the multi-impedance V3 was $50. The Cardak II would set you back a whopping $75!

In the velocity microphone description I was amused to read that “the woven housing allows the sound to pass through without reflection”. This neglects to mention the whopping magnet located at the rear of the ribbon! All of the dynamic ribbon mics were available in 50, 200, 500 ohm and high impedance models, which reflects the plethora of input types around at that time.

The second page deals with carbon and crystal microphones and accessories. The bottom of the page was uppermost in the box, and consequently is somewhat grubby.
14-07-2013

Cadenza Crystal Mics – a mystery solved!

This Ebay auction has solved a little mystery for me!

Cadenza Crystal Microphone

Cadenza was a British brand remembered primarily for their ‘Rocket’ ribbon microphones. They are still quite common, and I have repaired many of them in the course of my work.

Cadenza microphone with ribbon element

Most of the Cadenza mics are grey or pale blue in colour, and have a removable stand. However, I have come across three or four of these mics with black bases and a fixed stand. These often have a crystal element rather than a ribbon motor, and I and speculated on their origins in a previous blog entry. When I first encountered one of these, I had thought that it was some kind of DIY ‘repair’ job, but having seen more than one, it seemed more likely that they came out of the factory this way.

Crystal element in a Cadenza body

Until now I had never seen an advertisement, manual or box that could confirm the existence of a Cadenza Crystal microphone. The photos in the auction show the original packaging, marked ‘Cadenza Crystal. So this was indeed a Cadenza product, using the same body as the ribbon microphones.

I like the triangular box!

Thanks to Benchmark Collectibles for permission to use this photo.

24-06-2013

MOTM: Melodium / Radiodiffusion model R

Here is a rare and magnificent microphone: the Melodium type R (number 12).

The microphone is dated 31.12.40, which makes it 72 years old at the time of writing! The mic was made by Melodium for the national broadcaster “Radiodiffusion Nationale”, which would have been the French equivalent of the BBC. This is particularly fascinating as it dates the manufacture to during the period of occupation in the Second World War. I was surprised that microphones were still  being manufactured during that period, but I suppose that the government needed to broadcast their propaganda, or perhaps this one was stolen and used by Le Resistance!
 

There is a bit more about the history of the French national broadcasters here, and it seems that Radiodiffusion Nationale began in 1939 but was subverted by the Vichy government in 1940. It was replaced or perhaps rebranded in 1945 after the end of the war. There is more history here for our Francophonic readers.

Inside, the mic is very similar to the 42B, with the same magnets and motor assembly, but the internal grills are different, using a wider mesh and cloth.

The high pass filter section of the mic is also different to its square cousin.  Instead of the three way rotary switch on the 42B, this mic uses a metal bar to bridge a terminal inside the base of the mic, and it just as ‘Music’ and ‘voice’ settings. A hole in the bottom plate lets you see whether the jumper bar is engaged or not. It is a more reliable arrangement, but you need a screwdriver and spanner to make the change. I guess the guys at the radio station did not want the filter to be engaged accidentally, causing them to wonder where the bass had gone!

Remarkably, the ribbon on this one was still intact, and after cleaning the motor the mic sounded very good indeed, with a big bottom end and nice proximity effect to make voices sound richer and fuller.

New parts for the old microphone.

The mic arrived without some parts, so we made replacement brass washers and thumb nuts on the lathe. They look really smart, and in a few months will be a lot less shiny!

Melodium 12 side by side with the 42B
In comparison to the 42B, the model R has a bit more bass, although this could simply be due to the differences in tension and mass of the ribbons. There are other small differences to the high frequency sonic signatures, primarily due to the differences in grill shape and mesh. 
Frequency sweeps for Melodium R (red) and 42B (Blue)

The mic is now back home at Kore studios, in good company!
15-06-2013

Oktava mod FET biasing

Here’s one way to bias the FET on an Oktava microphone….

Our good friends at Valley Wood Studios in Leeds had followed one of the popular projects for upgrading Oktava MK319 microphones, which involves changing a handful of resistors and caps, along with the field effect transistor (FET). After the mods one of the mics was about 3dB louder than the other.

This kind of thing is normally due to variations in the FET’s characteristics, and to optimise the modification, it is necessary to adjust one or both of the bias resistors (R7 and R8 in the PDF document). Between them these set the gain of the mic, and the bias point for the FET, which gives the best noise performance.

I often do this by hacking a decade box into the circuit, and in this case we put the mic in our test chamber so that we can tweak and scan as we go along. At the end of the tweaking the pair of mics were well matched, and noise free.

We celebrated with tea and biscuits!

13-06-2013

Cadenza Mic Transformers and Mounts

Xaudia now supply drop-in upgrade transformers for Cadenza ribbon microphones.

Xaudia transformer (left) and original Cadenza tranny
The original transformers had a tapped winding made to give an output impedance of either 30 ohms or high impedance, neither of which is ideal for interfacing with modern equipment. Our upgrade transformer gives an output of 300 ohms with a good original ribbon, which gives about +10 dB compared with the 30 ohm connection. It also has a lower DC resistance and noise floor.

The transformer is supplied in a mumetal shielding can of the same dimensions as the original, and slots into the base of the mic below the motor assembly, as shown.

Cadenza on a stand!

And here is a funky adapter that we made for a customer, to mount his Cadenza onto a normal mic stand. It recycles the plastic connector and locking ring from the original tripod mount.

Cadenza mic stand adapter

*Or other impedance values on request.

10-06-2013

Reslo RV and UR microphone variations

Reslo RV ribbon velocity microphone.

The RV was one of Reslo’s earliest ribbon microphones – possibly their first production model, although I am not sure if the UR series came before or after. They share parts so it is hard to pin down which one came first.

Inside Reslo RV mics

The two mics shown have the same frame for the ribbon, but the magnets are very different. I believe the mic on the right to be the earliest RV design, or at least the earliest I have come across, and it uses a single cast magnet with quite a complex shape.

Reslo RV magnets

The mic on the left uses two simple horseshoe magnets at the top and bottom, attached to a pair of metal pole pieces, and this really sets the style for all subsequent Reslo ribbons – later models have three magnets but use a similar layout.

The UR model has a similar construction but has a rear mounted transformer. This is effectively blocking the pickup from the rear of the mic, giving it a more cardioid pickup pattern. It seems likely that ‘UR’ stands for Unidirectional Ribbon, although I have not seen any documentation to support this.

Reslo UR magnets and rear-mounted transformer

But the story doesn’t end there. Many of the RVs that I have worked on have a later, smaller motor which is essentially the same as found in the RB model. Keeping common parts with the later models must have kept costs down.

Reslo RV mics with different motors.

Until very recently I had only ever seen 30 ohm versions of the RV and had assumed that they were all low impedance. However, the one on the left, which came in recently, has a high impedance transformer, and measures around 6.5K ohms with the original ribbon.

Reslo mics don’t have serial numbers or dates on them, so it is hard to make a definitive time line for the development, and always if you have any further information we would love to hear from you.