what are these BBC boxes?

[chatterbox]

Having a mass clearout of gear, and have a few of these boxes bought a few years ago from the BBC.

 

Not sure what exactly they are, or what they can be used for - I think they may be some kind of safety transformer?

 

They have a 3 pin male XLR at one end and 2 jacks with a 20db attenuator switch on the other.

Pics of the block diagram which is on top of the box can be found here:

 

http://www.kodakgallery.com/ShareLanding.a...;localeid=en_US

[mutley]

It looks like a sort of DI box, and from what it says on it, I would assume it's for connecting to non-BBC equipment. Eg; connecting the PA from a sporting event directly to the OB sound truck. The switch is there to select between high level signals, and line level signals, in a potential (voltage) divider arrangement.

Since the OB truck would probably be using its own generator, noise from an earth loop is likely when connecting to equipment which is not connected to the same generated supply. This box would help eliminate such noise.

 

Basically, just use it as a DI that has a permanent ground lift.

[boatman]

Links to much of the BBC Designs Department stuff can be found here. Just scroll down until you find U4/8 and the data sheet is there.

[johnb]

Isolating transformer for connecting "non-BBC" audio kit to the world of BBC audio. That way is there is any fault in the piece of kit then it can't cause problems further down the line. The non-BBC kit could be anything that generates audio from a whole OB to a single instrument.

[boatman]

The U4/8 was specially designed to allow a balanced microphone level to be taken from a standard or 100V line speaker feed. It's minimum insertion loss is 26dB with an additional switchable 20dB pad. The U4/7 is closer to a standard DI box.

[Dave SA]

Have you a good use for those? I do a fair bit of work in broadcast and those would be super useful.

[djandydee]

The transformer used in the UN 4/7 UN4/8 has a 3000V breakdown voltage so the box is designed to protect against an amplifier's output going to mains potential and frigging the input of an OB truck or mixer.

 

In the early days the BBC set up a designs department which produced some incredible bits of kit some of which are still used by the BBC today. The designs department has now been dis-continued due to companies like Canford Audio who provide some eligent off the shelf solutions to many engineering problems met in OBs and studio work.

[Dave]

I'm familiar with these - they are one of the classic BBC items that older guys like me have fond feelings towards (ooh-err missus). We used a load of the UN4/7s on the last BBC OB I worked on - the BBC crew referred to them simply as "grey boxes" !!!

 

A direct link to the BBC spec sheet is here.

 

I have to respectfully differ with John/Boatman - the UN4/7 is nothing like a "standard DI box" - DI boxes always have a high impedance input, which the UN4/7 doesn't. It's basically an isolating transformer - not quite a "rep coil" as the resistors mean it's not 1:1 ratio, but very similar. It's used as a balanced mic or line splitter.

 

But the UN4/8, which you have, is indeed a passive DI box, providing signal attenuation, impedance conversion, and balancing.

 

I'd need to check, but I think the transformers in both boxes (BBC designs) probably have rather good audio performance and can handle high signal levels. Their 3.75kV isolation is definitely top-notch - designed to protect BBC equipment crews from potential faults on unknown equipment. By way of comparison, regular EMO DI boxes (E520) provide 1kv isolation - you need the high-isolation one (E540) to give you similar isolation (4kV in this case).

 

Hope this helps,

 

Dave

[Dan Gruner]

Links to much of the BBC Designs Department stuff can be found here. Just scroll down until you find U4/8 and the data sheet is there.

 

Wow, what a fascinating site! Lots of interesting reading (and viewing).

 

I have the highest of respect for the BBC engineering department. Would dearly loved to have worked for them, had that been my era!

 

Dan

[Killyp]

The BBC LS3/5a is still considered one of the best mini-monitors made. Absolutely stunning as hifi speakers.

[dan slv-tech]

The BBC LS3/5a is still considered one of the best mini-monitors made. Absolutely stunning as hifi speakers.

 

 

these were built by rogers who were based in mitcham who sadly went several years ago I had a pair of there Ls1- amazing monitors. anyway slightly OT....

[Killyp]

Multiple companies manufactured them actually. The Spendor and Harbeth versions are also very good.

[Jerome]

Being of the youth (and not even as old as most of the equipment) I didn't even realise that the BBC made there own equipment once upon a decade, some of it is mind boggling. Why did they stop? Was it because other manufacturers jumped on the band wagon or was its due to regulations in electronics or some other reason?

 

Anyone know of any decent articles about them etc I'de love to have a read.

 

Jerome

[Dave]

I would say it was primarily because the broadcast equipment industry (which didn't exist when the BBC began, forcing the DIY approach) did develop over the years making self-manufactured equipment uneconomical. Financial pressures were obviously a related factor.

 

I also think that the culture in not just the BBC, but the other "technical" bits of the civil service (eg GPO/BT and BR), was historically suspicious of existing commercial equipment and took the view that it wasn't suitable for various reasons (quality, features, etc). So they either built their own kit, or designed it and got others to make it (eg LS3/5a). A "not invented here" mentality was probably part of the equation to support british industry, as was the tendency of bureaucracies to build ever-expanding empires.

 

I don't know of any resources but the web site mentioned looks like a good start and I'm sure I've come across a few others over the years - google is your friend!

[KevinS]

Dave is right - remember that until the 1970's the BBC had a monopoly on radio broadcasting in the UK (though BFBS was also UK-based of course). This meant there was effectively no domestic market for broadcasting equipment as it would not make business sense for manufacturers to invest in R&D just for one potential customer.

Whereas in the US, right from the 1930's, practically anyone could apply to open a radio station - this created a whole industry. This is why the pirate radio ships of the 1960's set sail equipped with off-the-shelf US gear like Gates turntables and Spotmaster cart machines.

 

As has already been noted, the BBC also had a rather high-minded approach to commercial gear - feeling it was not quite up to required standards electronically or sufficiently physically robust. This persisted until quite recently and the sell-off or closure of various technical departments. Commercial gear - even domestic stuff like CD players that already met BS safety standards and were only going to be used for office listening - was ripped apart and examined in some detail.

The UM4/8 in the OP is an example of the BBC not trusting anyone else - note the "BBC" and "Non BBC" sides with a "firewall" between. And of course any non-BBC gear (eg. a guitar amp) could only be allowed to be plugged into a BBC mains socket via a BBC mains isolating transformer for similar reasons.

 

Goodmans also made the LS3/5a but I always felt they were not quite up to the standard of Rogers and some others.

 

K