Microphones of the Month: Toshiba type A vs RCA PB144 / 44A

Toshiba are well known for making mid-range consumer electronic equipment. Once upon a time they used to make some fine ribbon microphones too! 

Toshiba A (left) and RCA PB144 ribbon microphones

The microphone on the left is generally know in internet-land as the Toshiba 'Type A', although it is labelled SN-1631. It is a very close copy of the RCA 44A and its relatives. I have been informed by a Japanese expert that this was made under license by RCA, most likely in the post-WW2 era. Having looked carefully at the Type A and compared it to my own RCA PB144 (right), I have no reason to doubt this assertion.

The mics look similar outside - my PB 144 has a film set style hanger mounting, whist the Toshiba has a simple cast yoke, and the grill hole size differs.

Inside the microphones there are many similarities and few differences. Both use three large horseshoe magnets to provide the magnetic field, and and the ribbon dimensions are very close.

Perhaps the biggest difference is that the RCA uses cast pole pieces, whereas in the Japanese version they are milled. This probably reflects the tooling and machinery available at the time. Casting is an expensive process for low quantity products.

Beyond the cosmetics, the Toshiba has a 200 ohm output transformer whereas my PB 144 is a 50 ohm microphone, with these no doubt being in line with the broadcast standards of Japan and USA at that time.

(Thanks to Takayoshi Sumitomo for his expertise) 

MOTM Tannoy MD422 cardioid ribbon mic

Here is Tannoy's cardioid ribbon microphone - the MD422.

Tannoy MD422 ribbon mic, front

Firstly, Tannoy lose a point for the name. 'MD' should surely mean "microphone dynamic" in any sensible society! Perhaps the D stands for "directional"? Who knows, but it puts them at odds with other the Sennheiser MD421, and it is just plain confusing.

Tannoy MD422 ribbon mic, rear

Whatever the D stands for, the mic itself has an industrial look, and this one is finished in a bronze-ish coloured paint. To the best of my knowledge this is the only cardioid ribbon model that Tannoy ever produced, and it uses an acoustic labyrinth to provide the necessary back pressure to the rear of the ribbon. The chamber is the black cylinder in the photo below.

Tannoy MD422 ribbon mic, chamber

Some of the parts were made to a budget, or perhaps a short production run, with thin stamped metal for the base and top cap, along with two layers of off-the-shelf mesh to protect the mic from dirt and wind.

The ribbon assembly and magnets are the same as found in the type 2 Tannoy 'pitchfork' microphone, which would have saved costs by sharing components.

Tannoy MD422 inside, showing ribbon motor

The MD422 was subject of a BBC technical report, which concluded:"..this microphone has nothing to commend it for use in the Television Service". That assessment seems rather harsh, but at the time the Corporation's selection criteria were flat, uncoloured responses and good signal/noise performance.

In the report, the bi-directional BBC-Marconi AXBT was used as a comparison, which was much more expensive, a different pattern, and therefore a tough benchmark. Cardioid (and non-directional) ribbon mics generally use an acoustic chamber on one side of the ribbon to apply pressure, and this damps the ribbon motion, reducing the output compared with its natural bi-directional response.

Drawing of the Tannoy MD422 from BBC technical document.

Unlike my mic, the BBC's example had a yoke mount rather than a fixed base, although there are mounting screw holes in the side of mine.

Despite my irritation with the model number, and the BBCs damning assessment,  I like this microphone very much. It has a gentle warm tone. More importantly, it was an attempt at technical innovation, which is always to be celebrated.

MOTM: Reslo NC-RB Lip microphone

Our Microphone of the Month for April is this rare oddball from Reslosound: the NC-RB.

This was Reslo's attempt at a commentator's lip microphone, and appears to be cobbled together from spare RB parts. The upper body and head are from an RB, but the head is rotated through 90 degrees and screwed to another chopped-down head.  The lower body-stroke-handle is a straight aluminium tube with a switch which terminates in a normal Reslo output socket.

Inside the head is a rotated  RB-style motor.  The magnets face the speaker, to provide some pop protection for the ribbon at the rear.

Behind the ribbon I found this metal baffle, which should control the pickup pattern and tweak the frequency response. There is also fibre glass and felt wadding for more protection.

Overall it is a strange little mic. The ribbon runs horizontally, which is generally considered a bad plan as, if it sags, it will droop into the metal pole pieces. They must have been a way of producing a lip mic without the expense of re-tooling for a completely new design. I have only ever seen this one, although I know of a customer who owns another.

What does NC stand for? "Not a Coles", perhaps?

MOTM: Gaumont-Kalee Type 1492 ribbon mic

This month's MOTM is the Gaumont-Kalee model 1492,

….made by British Acoustic Films Ltd. 

Both GB Kalee Ltd. and BAF Ltd. were subsidiaries of the parent Rank Organisation, which owned various other companies associated with the film industry, including Odeon Cinemas.

Back to the microphone - this specimen comes in a nice wooden box, and although is painted in a rather drab shade of mustard yellow, there is something pleasing about its proportions and utilitarian design. The grill has a coarse outer later and a very fine layer of woven wire inside.  It is designed to sit on a boom mount, like this (nicer looking) gold-painted example from Pinewood studios. 

Sadly we don't have the mount, but as this shares a connector with the Reslosound mics, we can use a Reslo stand mount for it. None of the other parts are common to the Reslos, so they must have just sourced the connector from them.

Inside, the mic is a fairly conventional figure-8 ribbon mic, but has added baffles both sides of the ribbon, and thick wires run from the top ribbon clamp, giving some blast protection along the way.

The ribbon motor uses tapered triangular pole pieces, which compensate for the decrease in magnetic field with increasing distance away from the two block magents, and ensure a more even magnetism along the length of the ribbon.

And of course there is a suitable transformer in the base to convert to a sensible impedance - in this case 50 ohms. The mic sounds rather nice, rich and dark close up - it may make a vocal session very soon! 

MOTM - Melodium RM6

As promised (or threatened), MOTM is back, and December's microphone of the month is the Melodium RM6.

Although much much smaller in size than the Melodium 42B, the RM6 takes some of its design philosophy from its predecessor. The motor assembly has horseshoe magnets above and below the ribbon (like a tiny RCA 44BX), and an inductive filter with multiple cut-off frequencies. It also has an obsolete and hard to find proprietary connector, albeit a different one from the 42B.

The body is made from cast metal which has a tendency to crack, and the first challenge is to get inside the mic without damaging it. There are three tiny metal pins pushed through the mic that fix the grill to the lower body of the mic. These need to be pushed all the way through the mic so that the grill can be removed, and usually these can be found stuck to the magnet, although one will inevitably go missing!

To remove the mic completely, the connector must be unsoldered and the switch tip removed. Once inside, we see something that looks like the Easter Island statues!

The ribbon is hiding behind the baffles, and the motor requires quite a lot of disassembly before the ribbon can be accessed. The ribbon itself is about 1.8 mm wide, so a bit fiddly to fit. Like most mics of a certain age, half the problem is that the ribbon has become oxidised and stiff, and the other half is small particles of wild iron that that have become stuck between the pole pieces, preventing the ribbon from moving freely.

Removing the strong magnets made cleaning and re-ribboning a lot easier! The transformer and filter inductor are housed in a mumetal can, screwed beneath the motor assembly and above the filter switch.

As usual, the mic sounds best without the high pass filter engaged, although I can imagine it being useful to compensate for proximity effect when close micing some instruments.

There doesn't seem to be much information about this mic available on-line, but I have scanned an old Melodium catalog featuring the RM6. There is also a French language review of the RM6 over at Audio Fanzine. They seemed to like it, and gave the mic 9 out of 10!

BBC-Marconi Ribbon Microphone Type B

Our microphone of the month for September is this a rare and lovely BBC-Marconi 'type B' ribbon microphone. The type B is closely related to the 'A' series of mics (AX, AXB and AXBT) which were made from around 1935 onwards. The model B was made from 1937 onwards and used the same  motor assembly, but with a smaller (yet still massive) magnet and without the ribbon tension adjustment facility.

BBC Marconi type B, side view

The smaller magnet format allowed the mic to be packaged into a smaller cylindrical body, and these were apparently were often used for outside broadcast, attached to the chest of the broadcaster! One would have to be pretty strong to carry that around all day.

Although this is widely know as the model 'B', the nameplate calls it 8559A.

BBC Marconi type B badge

As well as the outer grill, there are two extra windshields inside the mic, which would have protected the ribbon from wind blasts and dirt. These windshields did their job well - in this example, the ribbon is unbroken, but this has oxidised and stiffened over the years and will need to be replaced.

BBC Marconi type B ribbon motor

The magnet and motor assembly are held in place with foam rubber, which has mostly survived the years. The output transformer is hidden behind the magnet. This had a break in the secondary winding, but luckily I was able to unwind it by a single turn and bring the mic back to life.

BBC Marconi type B, transformer

At the rear of the mic, connection to the outside world is made by three screw terminals, and square bulge at the bottom shows the location of the transformer.

BBC-Marconi ribbon mic, rear

Overall, the mic sounds warm and rich, although the metal can gives it a certain 'boxy' sound that instantly defines it as a very old microphone!

There is some more info about the BBC-Marconi ribbon mics at the Coutant and Orbem websites.

Thanks to Tom Barwood

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.

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!