Decca Ribbon Tweeters part 2

Last week I posted about repairing some vintage Decca London ribbon tweeters. I was confident that I had done a good job but was unable to do listening tests in context as I only had the tweeters to play with. These are now back with Andy, their owner, who kindly sent in his impressions and some photos. Overall I think we have done a good job and it’s a nice feeling to have helped get these back up and running.

Decca Volt speaker with ribbon tweeter

“The [new] ribbon sounds smooth, crisp and clear with no tizz or distortion. I ran it for a while at different volumes and with different music and it sounds excellent with everything I gave it. It is very close in sound to the original in the other speaker with the soundstage being in the middle and correct. I had to try hard to notice a difference, Bob Marley’s Is This Love has some high frequency cow bell drum noises and you could hear they were slightly crisper and louder from the new ribbon. The crossover point in the Volt’s is about 1200hz so any imbalance between the two tweeters is easily heard as the singer tends to move off centre.

New ribbon installation at Xaudia

“I think the differences could be classed as being between an old and new ribbon the sound from them is so close. I assume as the new ribbon breaks in it will soften in sound a bit. I am tempted to switch the other new ribbon in as well.

Andy’s impressive hi-fi setup.

“Considering the lack of info on the ribbons I thought getting a close match sonically was not good but it’s turned out to be very close indeed. It is a huge relief to know that if a ribbon gets damaged again I can get help.

“I am not sure but you might get a lot of interest, when I was trying to find someone to repair my ribbons I came across a huge amount of forum posts on the same issue dating back about 5 years. Nobody had managed to find anyone so in theory there are a lot of ribbons to repair out there.”

Thanks to Andy Mcgregor.

STC 4136 revisited

STC 4136 microphone

A long time ago I posted about the STC 4136 condenser microphone and how to modify it to work on standard 48V phantom power, using a small circuit built on perfboard.

STC 4136 microphone, in pieces.

I had another opportunity to work on one of these microphones. This one was a challenge as it had no circuit inside at all, just the capsule and bodywork, although that also gave free scope to start from scratch.

STC 4136 microphone capsule.

Space is tight in this microphone, but with a careful layout and small components everything will fit neatly. My solution for this one was to fit a small KM84 circuit and Neutrik NTE10/3  transformer. This transformer performs well, does not break the bank, and can be squeezed into tiny spaces where nothing else will go. 

New circuit board and transformer

This time I commissioned a printed circuit board rather than working on perfboard, which does save time and look more professional. The board will be useful for other projects too – I have a handful of other small microphones which would benefit from updated, lower noise circuits.

Made in England

Thanks to Robert at Russell Technologies for the board layout and advice.

Decca Ribbon Tweeters part 1

Decca London ribbon tweeters with huge transformers.
I have not repaired speakers in the past, but I recently had my arm twisted to look at some vintage Decca London ribbon tweeters. In theory they work in a similar manner to a ribbon microphone and should not present too much of a challenge…. right???? Anyway, it is nice to see something different on the bench and maybe I will learn something in the process.
The horn of a Decca ribbon tweeter
The first challenge was to take the speakers apart and work out what is going on inside. They have a large horn with two vents, which is is held in place with six screws. With these out of the way it is possible to remove the large, powerful magnet assembly and ribbon frame.
Brass mess behind the horn,
I received three of these ribbon tweeters but they are not all the same. Two have cast alloy horns and the third had a plastic horn. And the alloy horns have a brass grill that sits in front of the ribbon (shown above), whereas the plastic one has a nylon mesh, which would better protect against incoming metal particles.
These have large, powerful magnets
The ribbon is mounted in a square frame made from two slices of stiff material. All three frames were different and used combinations of cardboard, Tuffnol and what looks to be copper-less circuit board material. Some have silver plated ribbon mounts and others are bare copper.
Old ribbon (left) and new one waiting to be soldered.

The ribbon material in these speakers is stiffer and thicker than one would use in a microphone. The thickest foil I had in stock is 6.5 micrometer thick, supplied by Advent Research Materials, so that is what is going into these. That is about three to four times thicker than would be used in a microphone. The gap is 10mm and I cut the ribbons at 8.6mm, based on measuring a ‘good’ ribbon. Corrugating is routine, but the challenge here is making the electrical connection from the ribbon to the mounts. There are no ribbon clamps but I can see traces of old solder. 

New ribbon in position.
Soldering thin aluminium needs a special solder and I used Alusol, which worked well after a little practice. All the contacts need to be cleaned before soldering. The solder needs quite a high temperature and you need to work quickly. To make this easier, I lightly glued the ends of the ribbon in place with nail varnish, which helped keep everything aligned. The top layer of the frame are then replaced – I used double sided tape to hold them together, and then the speaker can be re-assembled. 
New ribbon as seen through the horn.

I was only had the tweeters, not the cabs or woofers, and so could not test with full range music. Instead I ran a sign wave through the speakers. They sound clear and without distortion down to about 500 Hz which is a good indicator that I have done a good job. In use they should only be working above about 7 kHz. Now they will go back to the owner for proper testing.
Thanks to Andrew Mcgregor.

Update 18/11/22

Further reading. 

There is an excellent article about Decca ribbon tweeters by Rudolf Bruil at Sound Foundation.