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…