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) 

Very early tube ribbon microphone inspired by the RCA PB17

Ev of Vashion Island sent in these pictures of his wonderful tube ribbon microphone and has kindly agreed to share the photographs and description on this blog. The mic is a little bit of a mystery as it appears to be similar to an early Marconi design and the RCA PB17, yet has no maker’s mark and is different in many details.

The mic is clearly influenced by Harry Olsen’s design as described in his 1932 patent and the magnetic field for the ribbon is provided by an electromagnet, which is very rare and only usually found in the very earliest ribbon mics; this approach became redundant very quickly as strong permanent magnets became available.

As I have not inspected this microphone myself I will use Ev’s description and photographs – Ev’s comments are in italics:

“The outside diameter of the cylinder is 4.75 inches or 210mm. The  cylinder is aluminum. the top end cap and plate are machined aluminum. The hemispherical bottom cap is also machined aluminum.

The yoke mount is steel flatbar (I believe the PB17 yoke mount is made of cast metal).

There are three transformers including the one for the electromagnet.

Instead of having three UX864 tubes it uses two unknown tubes, one has 5 pins with a wire attached to the top, and the other is 6 pins. 

The resistors are made by Morrill, Germany. The transformers and capacitors have no makers marks that I can see. Whoever made this microphone obviously knew what they were doing. I wonder if this was a prototype made by RCA, or perhaps it is European (because of the German resistors)?

The bell…. is definitely cast aluminum. The inside plate at the connector end of the mic is also cast. The acorn nuts at the connector end fit a 7/16″ SAE wrench perfectly and the bolts with the wing nuts are US threads. 

Note the tiny piece of threaded stainless steel pipe bolted to the plate (to the left of the connector in the picture). I thought it might be a jack, but I think it is only a pipe. There is what appears to be a ground wire soldered to it inside the mic.

The number 13 etched beneath the bottom right connector blade corresponds with the number imprinted on the connector itself inside the mic.”

If any of our readers recognise this microphone or have any more information, we would love to hear from you.

New RCA thread adapters

We have some shiny new thread adapters for RCA microphones.  These thread adapters fit most of the ‘big’ vintage RCA microphones, including the 44, 74b, and 77 ranges, and also are perfect for Sony professional mics, including the C38b and C48 microphones.

We have been selling thread adapters for a while but the latest batch are a step up in quality. They are machined from solid brass and have a standard 3/8″ female thread that will fit most modern mic stands. They can also be supplied with 5/8″ on request.

It fits this RCA77B perfectly.

These fit the following microphones….
Most RCA 44, PB140, PB144, 74b, 77B, 77DX, MI-4010-A, BK-11.
Some (but not all) models of the RCA Varacoustic.
Sony C38, C48, FV300, Some C37s.
Many Aiwa, Toshiba and Matsushita ribbon mics
Syncron / Fairchild AU7a
Some older Amperite mics

Please ask about your microphone.

RCA 74A and 74B ribbon microphones

Happy 2014!

Here is a lovely pair of RCA junior velocity ribbon microphones awaiting service, a 74A and a 74B. We see a lot of ‘B’ models, but the earlier 74A (left) is quite rare by comparison.

RCA 74A (left) and 74B

Externally, the mics are similar in size and shape, but the 74A has a yoke mount and looks more like a baby 44BX, whereas the 74B has a fixed or swivel mount at the base of the mic. The ‘A’ model also has larger grill holes.

RCA 74A (left) and 74B ribbon mics inside

Inside, the ribbon motor assembly is the same although the mounting arrangements differ. The 74A motor (left) is screwed directly to the frame, whereas the ‘B’ is suspended by four small rubber mounts. The yoke would absorb some vibrations transmitted through the mic stand, and so the rubber parts were possibly introduced to compensate for the loss of the yoke. The earlier transformer has two outputs at 50 and 250 ohms, whereas the 74B transformer also has a high impedance output for public address use.

One can only guess about the reasons for the evolution from ‘A’ to ‘B’ models, but these things usually comes down to performance and economics, with fashion and marketing also playing a part. Losing the yoke might reduce costs, but adding the suspension and the swivel joint would probably balance this, so it is hard to tell.

Both mics need new ribbons!

The amazing Coutant website has a scan of a data sheet for the 74A.

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 Mystery 74B clone

It is said that imitation is the sincerest form of flattery. If that’s true then the RCA 74b junior ribbon was one of the most flattered microphones ever made!

In the middle is a ‘real’ RCA 74b and on the right is a 74b made under license in Italy by Magneti Marelli. But what’s that on the left? It’s our microphone of the month, and I don’t know what it is.*
When I first saw this I thought it must be another rebadged or licensed 74b. The outer dimensions are almost identical to the real thing, and it even has a red ‘meatball’ badge, although this time with some unidentified Eastern-looking symbols – perhaps from somewhere in South East Asia.** It even has a good copy of the swivel stand mount.
Inside the 74b clone
But inside the mic there are some differences. The ribbon has similar dimensions, but the magnet assembly is quite different, with two large, strong horseshoe magnets providing the field through a pair of U-shaped pole pieces.

Rear view with horseshoe magnets
In a ‘real’ RCA 74b, the magnets are at the front and bevelled, with a rectangular frame at the rear.
‘real’ RCA 74b microphones inside
In the photos above the transformer is a replacement. The original transformer, below, was faulty, probably due to insulation breakdown. Again, the marking on the transformer looks very Eastern in style, and appeared to have been written with a brush, in eastern style.
Perhaps the oddest thing about this microphone is that it has been shot! There is a pellet embedded in the side of the mic. Perhaps it was used as target practice. Or just maybe it was in use at a radio station in the last days of Saigon….
*Thanks to John Gooding!
** I have tried to identify these but no luck yet!


Update, 13 August 2012

A couple of readers have sent in messages about the provenance of this microphone….

My friends here in Thailand says the symbol is almost definately Chinese. It is certainly not Thai, Vietnamese or Cambodian. By most stuff here has been made in China since China started making things, so your Saigon theory is still the best.” James Browne, Hungry Ghosts

Hi Stewart. Sent picture to my factory in H.K, the marking is definitely Chinese,” Andy Hearn, ECM

Update, 15 June 2022 (ten years later…)

Thanks to Adam Pan from Taiwan for providing this additional information and photographs.
“The first photo left one ribbon mic is make by Shanghai Microphone Factory in 1940’s. 
After a long time looking for some China made microphone. when I move to live in Shanghai China from 2001, I found two ribbon microphones in China, one is the same as yours, the logo is the word “Ding”(鼎) in chinese, that is “tripod” the meaning is “one word is as heavy as nine tripods”. Reminder to “keep your word”. And the other one’s logo is “He-Ping”(和平),That meaning is “Peace”. 
A few years ago, I helped Superlux develop some products, and I met an older engineer who is retired, He was involved in the manufacture of Type 74 microphones at the Shanghai Microphone Factory.”

MOTM: The Australian

This rather lovely RCA44 style ribbon is our microphone of the month for March. He was found on Australian ebay, and so will be known as ‘The Australian’ until his true identity is discovered!

‘The Australian’ Vintage RCA style ribbon mic

The mic has a chrome plated steel bottom, a steel yoke, and brass grills, painted black. Connection to the rear is via a pair of screw terminals for balanced output, but with no ground connection.

Although the body shape is very similar to an RCA44, the interior is more like the early Harry Olson prototypes, with a single large horseshoe magnet and broad flat pole pieces, giving a magnetic field strength across the poles of around 1000 Gauss.

The transformer is a dual bobbin type, with a primary inductance of 255 µH at 1 KHz, and 770 µH at 100 Hz. The thick primary wire gives a measured DC resistance of 27 mΩ, and the overall turns ratio is 1:50. With a 2.4 micrometer ribbon, the Australian has an output impedance of about 600 ohms.

There is no maker’s name plate, and no sign that there ever was once. However, next to the ribbon there is some writing in pencil. This is a little tricky to capture on film, but it reads “RIBBON 23825 B1154”.

Could this be a clue? Perhaps the 23/8/25 is a date from a previous ribbon installation? Sadly, I don’t think so. 1925 seems a bit too early – although ribbon mics were invented in the 1920s, the RCA PB31 (first commercial ribbon and forerunner to the RCA44A) wasn’t introduced until 1931. It seems more likely that this is a copy or prototype made by a small engineering firm, based around patent drawings.

Sound-wise, the mic has a rich warm tone with a decent output for its age, and the 600 ohm output makes it very usable with modern equipment. The lack of an earth connection makes hum an issue, and I am contemplating adding a third terminal to the rear, if I can find one that matches.

Perhaps it was made by an Australian manufacturer? AWA made copies of RCA microphones, and Zephyr were another Australian company that made some nice ribbon mics. For now it is a bit of a mystery, but we would love to hear from you if you know more about this.

Microphone of the Month – Framez ribbon microphone.

April’s microphone of the month (MOTM) is this glamorous shiny blue ribbon microphone by Framez (not to be confused with Framus!)

This has no model or serial number, but is probably from later 1950s or early 60s. Framez were an Italian brand related to Meazzi – according to one source the name is a contraction of Fratelli Meazzi (trans. Meazzi Brothers), which does sound plausible. Framez / Meazzi also made some cool oddball guitars, and were associated with Wandré Pioli. Fetish guitars have much more information about these guitars.

Back to the microphone! This looks very much like a copy of the RCA 74b ‘junior’ microphone, but us physically somewhat smaller. And it is a pretty good microphone in its own right. The magnets have retained their strength over the years, measuring a healthy 4500 gauss between the pole pieces.  It has a hefty transformer with taps for both low and high impedance, making it suitable for both recording and PA use.

This one arrived with a thick flat ribbon – probably a DIY ‘kitchen foil’ repair. With a proper 2.5 micrometer ribbon installed, and the mic rewired for balanced, low impedance operation, the sound is clear with a strong output, just lacking a little of the low end proximity boost that you find with many ribbon microphones. The ‘low’ impedance tap is 1:45 ratio, which gives around a 450 ohm output with the 2.5 micron ribbon.

I’m actually very impressed with this little microphone, and it looks great too!

Update 23/4/11

We managed to get hold of a Meazzi ribbon microphone for comparison. It’s a little less glamorous, and has a similar ribbon dimensions, but a very different motor assembly.

Thread Adapters for RCA, Sony and other microphones

RCA to modern mic stand thread adapter

A few months ago I was trying to find some thread adapters for a customer with an old RCA junior ribbon microphone, who wanted to use it with a modern standard 5/8″ microphone stand. Most suppliers were charging around $25 dollars, plus post and the inevitable import and handling duties.

So we decided to make some.

These fit most of the ‘big’ vintage RCA microphones, including the 44, 74b and 77 ranges, and also are perfect for Sony professional mics, including the C38b, FV300 and C48 microphones. They also fit Syncron AU7A / Fairchild F22 microphones, and several other American and Japanese microphones with a large thread.

These are now for sale on the Xaudia website priced at £6.00.

If you want one, please get in touch.

Mic of the Month – RCA Junior ribbon mics

October was a bad month for blogging – I was busy with the haunted house sound installation, and this was compounded by a fault with my Macbook, which took the Apple repair centre three weeks to find and fix, a long time to track down a faulty cable. With a microphone, that would be the first thing to check! Amongst all the chaos I completely forgot to do the ‘mic of the month’ column.

Back in the real world, I have chosen the RCA Junior ribbon for November’s Mic of the Month. This is because they seem popular at the moment, and we’ve seen four at the workshop for service or repair. The fun thing about this family of microphones is that they vary somewhat in construction, so it is possible to compare and contrast versions from different eras. They tend to be a bit more affordable than the bigger RCA 44 and 77 mics, but still have a good tone that is very usable in a modern studio, especially if the ribbon is in good condition and the transformer is healthy and wired correctly.

The ‘Junior’ was created as a budget version of the RCA44, with a similar motor assembly but smaller magnets and housing. The most commonly seen models are the ‘black badge’ and ‘red badge’ versions, and these are actually quite different inside – the black badge model has a 3.0 mm x 55 mm ribbon, whilst the red badge version I examined has a wider, 4.5 mm ribbon and a stronger magnetic field.

The output transformers on these microphones can be set for 50 Ohm, 250 Ohm or 10KOhm output impedance, and it is worth checking that the mic is wired correctly to get the best performance with modern studio equipment. Normally that will be the 250 ohm setting.

The earliest and rarest version, the MI-4010-A, is shown on the right in the picture below. It is slightly larger than the later versions, with a different ribbon assembly which has horseshoe style magnets around the back of the ribbon. The magnetic field in this example is weaker, and the output lower than the more modern versions, although the tone with a new 1.8 micrometer ribbon is very pleasing. 

Finally, some RCA mics were actually made in Europe, and it would seem that some appear under different names. The microphone on the left is badged as ‘Magneti Marelli, Milano, Italy’ but is almost identical to the black badge RCA 74b. The only difference is that the Magneti has an alternative transformer, but still with high and low impedance options. The sound is every bit as good.

We’d love to hear from anyone who knows more about the Magneti Marelli microphones and their relationship with RCA.

(Thanks to Jules at DADA Studios in Belgium and Jørn Christensen at Rodeløkka Studio in Norway.)