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Xaudia offer microphone re-ribboning and repair services.

2013/04/30

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....