Month: June 2013

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.