David Broad’s LEM ribbon microphone

I re-built this old LEM ribbon microphone for David, with improved magnets, new ribbon and a medium impedance transformer. You can hear the results on David’s videos.

Ervox R58 – French ribbon mic

Today on the bench we have an unusual microphone from France. I have never heard of this brand!

Ervox R58 ribbon mic

The Ervox R58 is a high impedance ribbon microphone designed for public address or home taping to a reel-to-reel recorder. It is made from a two part cast body, perhaps inspired by the RCA Varacoustic, and is nicely finished in hammertone paint. Paint still covers two of the screws, so I can see that I am the first person to look inside this one, which is always a nice feeling, like unwrapping a present!

Ervox R58, rear

Inside the mic everything looks nice and clean. The motor is built around a sturdy block of clear plastic with a large horseshoe magnet to the rear, and the ribbon itself is a piston-style design similar to those found in B&O and Altec microphones.

Inside the Ervox R58

Unfortunately the transformer is not working properly – the inductance measures only 14 microhenries, which for an (estimated) 0.2 ohm ribbon would make a HPF filter at around 2KHz… I’m sure it can’t have been designed like that, so one suspects either insulation breakdown and a short, or the laminations have deteriorated with age and lost some permeability.

Ervox ribbon motor assembly

I’m not sure if it is worth the effort of re-winding this one, particularly if the lams are below spec. Luckily we can make a new low impedance tranny for it and bring it back to life.

The previous owner kindly included this scan of a wiring diagram for the mic.

MOTM: very old LEM ribbon

April has been hectic! We have been repairing microphones, installing a new coil winder, and developing some new & exciting products. And suddenly it is already the 30th and just time for a very brief MOTM.

This month’s mic is a beautiful and very old French LEM ribbon mic, probably from the 1930s….

Very old LEM ribbon mic

Inside, it is very similar to the smaller Amperite ribbon mics, with a large fibreboard frame supporting the ribbon, and a pair of (rather weak) magnets behind.

Old LEM mic deconstructed

The mic has a classic design, with brass sides, a folded steel grill and a cast steel yoke. These solid-sided microphones look strange now, but I guess the thinking at the time was that figure-8 microphones didn’t pick up sound (or reflections) from the sides, which allowed for a very simple construction. The ribbon itself is positioned right at the front next to the grill, with a metal plate across the magnets at the rear, so there is some attempt at making the mic more directional.

The transformer had failed due to insulation breakdown, but with new magnets, a fresh ribbon and repaired transformer, the mic has a reasonable output and nice vintage tone. And it looks great!

MOTM – LEM 305 ribbon mics

December’s Microphones of the Month are these impressive art deco-style LEM ribbon mics. LEM are a French manufacturer who still make reporter-style dynamic microphones. They used to make really cool looking ribbons!

LEM 305 / 306 microphones
These are all high impedance mics, and although they are substantially similar, there are some subtle differences between the models.

Inside the LEMs
The microphones all use large, 6 mm wide ribbon which sit between two steel pole-pieces. One of the mics arrived with fully corrugated ribbons, whereas the others had thicker, half-corrgated ribbons.
LEM 306 ribbon motor with holes in the pole-pieces.

The magnets are different – in the one, a pair of block magnets are connected at the rear by a welded steel plate, whereas two have a pair of horsehoe magnets. The smaller, more powerful magnets in the later models allow holes to be made in the pole-pieces, which (in theory at least) should give better high frequency response.
LEM 305 magnets

The other obvious difference between the microphones is the transformer, with the older models having a larger, iron core, whereas the newer ones have smaller mu-metal laminations, and these transformers are housed in screening cans.

Sadly all three transformers were faulty – the old rubber insulation around them had become brittle with time and the wires were broken. We were able to re-wind the two, but the third had suffered from an earlier bodged repair attempt, and had to be replaced completely. In this case we took the opportunity to give it a 300 ohm output impedance.

LEM transformers in metal screening cans
LEM transformer with larger core

Once serviced, these mics work well, with fair output for old ribbon mics, and a good vintage tone.

Frequency plots for LEM ribbon microphones, after servicing.
Update: Here is  a product sheet for the LEM305, in French and English. Thanks to Microphone Online Museum for sending the link.

Update 5/1/15: Thanks to Philippe Le Gourdiol for sending in this picture of his LEM ribbon microphone, which is a low impedance model with a high-pass filter. The filter inductor is located below the motor assembly…