The Xaudia Blog

26-02-2015

Tannoy Ribbon Microphones Part 1

Introduction

This is my attempt to round up and discuss the various ribbon microphones that were made by Tannoy. I have split this survey across two posts, with part 1 describing Tannoy’s figure-8 microphones and part 2 focusing on their attempts at cardioid ribbons. Finding firm dates and model numbers is a little tricky as they are rarely marked on the microphones themselves, and so there is a certain amount of educated guesswork involved. If you know more, please get in touch!

Update 8th March 2015
Several kind and knowledgable readers have sent in links and articles with dates and model numbers. I have added these where possible. Thanks to all!

Figure-8 ribbon microphones
Over the years Tannoy produced three main models of bidirectional ribbon microphone, with some variations around those types. I have called these types 1, 2 and 3 in order of production.

Type 1
The BBC technical team reviewed this mic in 1947 and applied their usual exacting standards, rejecting the mic for broadcast use. But as usual the benchmark mic for comparison was the BBC Marconi AX model, which cost many times more, and in fact these mics can sound warm and rich, with an overtly vintage tone.

These mics are heavy with a cast body and large cylindrical magnet. I have come across mics with different output impedances, usually 50 ohms, 600 ohms or high impedance.

Type 1 variant
In response to the BBC comments, extensive changes were made to the motor design, although the body remained the same. The magnet structure is completely changed, with what looks to be a single cast magnet. I have never actually seen one of these on the bench, but Seth at the 2 Track Mind blog has written a nice post about his microphone here.
Type 2 – MR425
The Type 1 mic was succeeded by the type 2 or “Pitchfork” model, which is much smaller and has a completely new motor design. This model seems to date from the early 1950s and was available in different colours with different mounting options.

Most of these models have a 600 ohm output, and give a full well balanced sound. This is my favourite figure-8 Tannoy, partly because this was the first Tannoy mic that I owned, but also because it has a good response and a very usable signal / noise output.

The type 2 motor is simple but nicely made, with two tapered magnets either side of the ribbon, and the adjustable pole pieces allow one to set the ribbon tension.


Update – this model has been identified as the MR425, and it was briefly mentioned in Wireless World in May 1952:



Type 3 – Slendalyne
These mics are smaller again and use a lot of plastic in the body, which can be prone to cracking. The grills are stamped metal, and the connector is a simple DIN plug without a locking ring. They are a lot less common than the types 1 and 2, although perhaps not as rare as has been suggested by some sellers.

The magnets in these mics are made from a ceramic material which allows a smaller, lighter microphone than its predecessors.

Update: This microphone has been identified as the Tannoy Slendalyne model which appeared in Wireless world in June 1962:

In my next post I will describe Tannoy’s cardioid ribbon mics.
Stay tuned!

08-02-2015

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.
30-01-2015

Reslo Beebs in action… again

Thanks to producer and long-time Xaudia customer Ian Caple for sending these shots of his Reslo Beeb microphones in use:

Here are the Beebs in action as drum overheads, and for recording a string section…

He was using them to record the French band Watine at a country house in Sussex, working out of this amazing airstream mobile studio supplied by Yellow Fish.


We will link to the music as soon as it is published.

20-01-2015

Mic of the Month – Shure 508 Stratoliner

Here is our first Microphone of the Month for 2015. This Shure Stratoliner looks like a Zeppelin airship or perhaps Flash Gordon’s space ship. But is it a crystal, ribbon or dynamic microphone?

I had always thought that the Shure Stratoliner series were crystal microphones, and indeed the 708 model does have a crystal element. But although this 508C uses the same body, it is quite a different microphone inside.

This one says ‘dynamic’ on the label but it is really a rather strange ribbon microphone. Here is the business end of the mic with some kind of flat ribbon in place, possibly made from kitchen foil.

Once we remove the ribbon and clean the motor, we can see a series of slots right behind the ribbon. These make an acoustic cavity and help to control the directional pattern.

The motor design is quite basic, with two cylinder magnets behind the ribbon, covered in felt. The transformer is mounted behind the magnets.

This construction and slots behind the ribbon also do odd things to the frequency response. Here is the published response chart from the Shure 508a user manual:

Even the published chart is pretty bumpy, and it seems that Shure were willing to sacrifice fidelity for directionality. And the mic does sounds peaky – definitely one for the vintage / special effects shelf in your microphone locker.

15-01-2015

HMV 2350 ribbon microphone documents

These HMV and EMI ribbon microphones show up quite often for repair. The models are identical, just branded differently, and very often they come complete with wooden box, which survives decades in a damp shed or attic better than cardboard.

It is rare to find any paperwork with these but I was lucky enough to come across an HMV 2350 complete with brochure and manual

The manual confirms that they were designed to drive a 20 ohm line. I have found that most of these mics measure around 15 ohms and have really low output. A suitable matching transformer makes a big difference to the usability of these microphones.

HMV 2350 brochure

HMV 2350 manual

PS, some of you might recognise this mic from Marvel’s Captain America movie – it was used by Howard Stark in the scene where Steve gets his treatment!

14-01-2015

Review of the Reslo RB by F.C. Judd

Here is a scanned review of the Reslo RB miniature ribbon microphone,

The review is written by Fred C. Judd, who was an early champion of both electronic music and DIY audio electronics. You can hear and buy some of Judd’s creations at this bandcamp page.

This article arrived with a microphone but I am unsure which magazine it was originally published in. Perhaps “Amateur Tape Recording”, of which he was technical editor. If you know more, please let me know.

11-01-2015

Shure Unidyne 545 transformer bypass switch mod.

I have read a few posts on internet forums wherein the transformer is removed from Shure mics such as the SM57. This will inevitably reduce the output level and also reduce the impedance to that of the capsule alone, and will of course remove any chance of overloading the transformer! Which means that the result is a quiet mic with low output, perfect for putting close to a drum head.

I decided to try it out. I had a Shure Unidyne 545 with a faulty transformer, so I took that out and wired the capsule directly to the output socket. The result was absolutely ideal for close-micing snare drum.

One of my regular customers asked about making this into a switchable feature. Well why not? With a double-pole double-throw switch it should be possible. The only challenge was finding a switch small enough. In the end, after trying several brands, I found that a spare switch for a Fender Jaguar would fit… but only just!

Firstly the capsule needs to be removed, unsoldered and new wires attached to the capsule. I also ran new wires to the XLR socket to be safe. The mic is filled with quite a lot of sticky brown gunk so that slows down the work.

The trick is to fit the switch from the outside of the microphone, and so the wires need to be fed out through the switch slot. It looks a mess in this state, so they are trimmed back to roughly equal length and soldered to the switch, and it all goes back together. I marked the transformer position with an X. (‘Trans’ in Latin can mean ‘cross’ or ‘across’, and X is a cross!).

Here is my sketch of the circuit, with the capsule on the left and the XLR on the right. Note that the black and white transformer wires are a high impedance winding and are not used in this circuit. Transformer wire colours may vary for other models and examples, and need to be checked before starting work.

The final job is to check the polarity of the mic against a known reference. If it is out of phase then the wires need to be swapped either at the capsule or the XLR socket.

With the transformer bypassed, the mic measures around 15 ohms at 1 Khz. The transformer brings this up to 200 ohms, and increases the output level by 10 to 12 dB. but also introduces a little bass cut. The plot below shows the difference the transformer makes. I am uncertain if the bump at around 800 Hz is real.

With the transformer bypassed, the mic can be placed close to loud sources and there is also a bass proximity effect, so the mic sounds really nice and fat!

Thanks to Lee Mouatt.

30-10-2014

2-Track Mind’s Tannoy mic restoration

Here is a nice article by Seth at the 2-Track Mind blog, describing how he restored a very early Tannoy ribbon microphone. Seth went to great lengths, building his own corrugator and refinishing the mic, and the results look great. I played a tiny role here and was able to supply a little brass mount so that he could put it on a stand.

The mic itself is quite unusual, and the motor assembly (above) is different to most of the other Type 1 Tannoy ribbon mics that I have seen. Below is the more usual look of these mics inside, with a large cylindrical magnet and simple flat pole pieces:

It is becoming clear that Tannoy made many different variations on their ribbon mics. I feel a big Tannoy blog post coming on…
25-10-2014

Syncron AU7A schematic (again)

Many thanks to Dan Zellman, a former Syncron employee, who sent in this schematic for the AU7A condenser microphone. This was the ‘last issue’ diagram.

Syncron AU7A factory schematic
There are a few differences from my traced circuit that I posted in an earlier blog entry, most noticeably I had the 42V battery supply reversed.  Also, I have worked on several of these mics, but have never seen one with the 470pF capacitor present. 
SJT 25/10/14