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 – Toshiba Type K multi-pattern ribbon mic

Last month I posted about a Toshiba copy of the RCA BK5. Here we have Toshiba’s take on the RCA77 DX….
Toshiba Model K
The Toshiba model K was one of the later Japanese ribbon mics. By this point they had really mastered the technology and were producing high quality microphones to rival to the American mics. It is slightly shorter than an RCA 77DX, but is equally heavy and well built. I very much like its stubby looks and slotted grill holes.
Rear of the model K, with pattern control
The model K was designed to broadcast standards, and this specimen was obviously made for NHK (Nippon Hōsō Kyōkai, or Japan Broadcasting Corporation), which is Japan’s equivalent of the BBC. Like most Japanese ribbon mics, this has a 600 ohm impedance and gives a strong output.

Inside the mic, the motor is based around a single strong horseshoe magnet, with the ribbon held between two chunky pole pieces.

Like the RCA 77DX, there is an acoustic labyrinth made from a series of holes with connecting channels, which goes up and down the centre of the mic. Two thick wires take the signal from the ribbon, through the labyrinth, down to the transformer below.

Acoustic labyrinth in the middle of the mic

The pattern control uses a choice of baffles to partly or entirely redirect the rear of the ribbon into the acoustic labyrinth. This turns the mic into a pressure transducer when the rear baffle is closed, giving a more omnidirection pattern.

Pattern control on the Toshiba type G.

It differs from the RCA design: the 77DX has a a cam shaped copper plate that allows the rear vent to be opened by incremental amounts, whereas the Toshiba has three discrete positions, which are labelled…

  • N (fully closed – non-directional)
  • B (fully open – bidirectional or figure 8), and 
  • U (a small opening – unidirectional or cardioid)

And in an attempt to beat the Americans, on the bottom of the mic there is a switch for a 6 position variable frequency high pass filter – the RCA77DX only has three!

High pass filter switch

Update 27/5/13… this is how the filter affects the frequency response….

Toshiba Model H mic – RCA BK5 copy

Although now more associated with household TVs and video players, Toshiba once made some very nicely engineered studio ribbon microphones. Very often they were close copies of RCA microphones, but with a few added innovations and improvements.
Toshiba type H ribbon mic – looks like a BK5

The ‘Model H’ is no exception, being almost identical to the RCA BK5 cardioid ribbon in both looks and function. The small format ribbon sits between powerful magnets, in front of an acoustic chamber or labyrinth that takes up the middle segment of the microphone

Rotating the base of the mic implements a high pass inductive filter.

In fact it seems slightly better engineered than the BK5, with a 5 position high pass filter switch that is actuated by rotating the dial at the very bottom of the microphone.

Toshiba type H ribbon

There is some additional electrical screening around the ribbon, but less acoustic baffling in front of it, which may be to give a more ‘open’ sound. In practice the ribbon is more prone to being stretched by air blasts than the RCA model.

Overall the type H is a decent alternative to a ‘real’ BK5, and is a little less peaky than most of the BK5s that we have measured.

Thanks to Kore Studios.

MOTM – Toshiba ribbon mic

Small Toshiba Ribbon mic

Toshiba are better known these days as a giant manufacturer of consumer electronics goods, so it is perhaps surprising to find a ribbon microphone with their name on. In fact back in the 1960s Toshiba made some pretty decent models, some of which were good copies of RCA mics.

February’s microphone of the month is this little ribbon mic, which lacks a model number, but has been referred to as an RCA74b clone. However, although from the outside it resembles a smaller 74b, inside it is very different.

Beneath the outer grills lies a perforated metal baffle backed by a finer mesh screen, which protects the full length of the ribbon against air blasts and pops. The top and bottom of the shield have a tendency to go ‘ping’ – I could actually hear this ringing when speaking into the mic, so a little bit of sticky foam was used to damp this.

Rear of the Toshiba showing transformer and magnets

From the rear, we can see that the field is suppled by a pair of strong horseshoe magnets glued to the pole pieces, which give a measured field of about 3000 Gauss between the poles. The transformer is a twin core ‘humbucking’ type, in this case wound for high impedance.

Once the inner screen is removed, two features stand out as unusual. Firstly, there are no ribbon clamps! The ribbon is simply glued to the supports, and then soldered to the terminals. The arrangement works well enough, but you only get one go at fixing the ribbon.

Toshiba ribbon

The second interesting thing is the small cross bar that bridges the pole pieces. This is actually glued to both the pole pieces and the ribbon itself, dividing the 3.6 mm wide, 60 mm long ribbon into two sections in a 3:2 ratio. I can imagine two purposes for this – to stop the long thin ribbon from travelling to far, and to minimise overtones from harmonic motion.

I have also seen this ‘node’ on another Toshiba ribbon model, so it does seem like a little trick of theirs. The nearest thing I have seen in other microphones is in the Cadenza mics, where the ribbon is glued to a support half way along.

Microphone cleaning

This mic was rather dirty inside. Lots of little bits of iron were interfering with the ribbons movement, making it sound like it was scraping against the sides – which it was! These were easily cleaned with some sticky tape, but the ribbon had to be sacrificed first.

New ribbon in the Toshiba mic

A new ribbon was fitted – soldering 1.8 μm aluminium foil is a bit tricky, but I got the hang of it after a couple of tries. And gluing the cross bar to the ribbon also requires a steady hand!

Once re-ribboned and reassembled (and fitted with a low impedance transformer), the microphone sounds nice, with a relatively flat response up to around 6KHz, where it begins to roll away.

Frequency plot for Toshiba ribbon mic at 35 cm.