Mystery massive hi-fi speakers ID required

[Ted Kendall]

We shall just have to agree to differ.

[mhennessy]

About what?

[Aub]

Wow, I've never seen a B139 before. Never seen a speaker with no apparent centre either. Didn't realise they are like that.

What a find.

Have fun with them.

Aub

[Radio Wrangler]

The B139 has an expanded polystyrene cone with glued-on aluminium foil on the front and rear surfaces. The basket is a hefty diecasting. The cone works in piston mode to a good frequency and typical crossover frequency is 400Hz. All told, an excellent driver.

The funny shape is reputed to be a response to a crazy way of assessing things for tax.

The B139 can be found in some very reputable speakers.

David

[vidjoman]

Originally Posted by Radio Wrangler

The funny shape is reputed to be a response to a crazy way of assessing things for tax.

I remember that now - it was something to do with size and I think 12" was the cut off point for domestic speakers and the point at which Purchase tax no longer applied as larger speakers were deemed to be professional. So a 13 x 9 speaker escaped that P/T which was quite a large amount in those days.

[unitaudio]

If memory serves, the Kef B139 was first produced in 1964. Amazingly advanced at the time along with the earlier (about 1962 I think) Wharfedale RS-12-PS.

Regards,
Paul

[mhennessy]

Originally Posted by Ted Kendall

We shall just have to agree to differ.

Originally Posted by mhennessy

About what?

Hi Ted,

I left it a few days before replying as I hoped you might come back to me on that.

When someone says they "agree to differ", that means they disagree with someone else's opinion. But my previous post was mostly factual (I think the only opinion I expressed was about PMC's marketing stance, but I gave the facts that informed that view). So some clarification about what you disagreed with would be appreciated.

Perhaps you disagree with my report about the frequency response of the TB2?

As it happens, we measure that speaker several times a year as part of a loudspeaker measurement practical - the figures were from memory, but actually they vary from sample to sample in my experience. I've attached a curve showing the midrange dip and the HF lift - this example is actually one of the better ones, as the dip isn't as pronounced as some.

I've also got measurements of the LB1 and DB1. They both show a similar trend, whereby the HF level is up by some 6-8dB, depending on where you choose to pick the starting point.

Just in case there's any doubt about the measurements, I've also included measurements from an ATC SCM7, a Harbeth M30.1 and a Genelec 1029A. As you can see, they are all broadly flat, with the Harbeth showing the most lift at the top, but perhaps only 3dB.

Yes, these on-axis plots are only part of the story, and of course measurements alone aren't enough to give the full picture, but measurements can often explain subjective impressions - as they definitely do in the case of the PMC models I've met. I've measured many monitor speakers, and heard many more in addition, and PMCs have always leapt out as being "outliers". Earlier you said "I don't know what is supposed to be so extreme about PMC loudspeakers" - well, now you do

Or perhaps it was my comments about Chartwell LS3/5As?

Again, it's obvious that I was reporting facts, but as I said, I can't share the data I was looking at. It occurred to me that I could take some new measurements, but as I only have access to a few pairs at the moment, you might (understandably) question the validity of that (but am happy to do so if you like). So perhaps a quote from Derek Hughes will help? From a recent thread on the LS3/5A mailing list on groups.io:

Just to say I have measured a number of 15ohm LS3/5a speakers of different manufacturers over the last 10 or so years.
All of them were well outside BBC specifications, and hence technically 'not BBC ls3/5as'.
The later 11ohm versions fare better.

Like I said, there's no doubt your Chartwells will have changed over the last 40+ years. You're welcome to bring them over in due course to have them measured

As you own both LB1s and Chartwell LS3/5As, those are the two main areas where I think you might be agreeing to differ, but perhaps it's something else? If so, let me know - I'd be happy to explore further.

Either way, I hope this is of some general interest.

All the best,

Mark



PS: Just a few words about the measurements: all taken under the same conditions at the same time - 1m from the tweeter, gated (which is why they start at 500Hz), signal level unchanged so you can see relative sensitivity (though the Genelec is active, so the level there is arbitrary). Measurement system is Omnimic, which isn't as comprehensive as Cleo or Praxis, but was created by the person behind Praxis. My example is one of a batch that has known issues with results above 15kHz, where it under-reads substantially because of a temporary problem with their calibration procedure - so the HF lift with these PMCs is actually worse in practice. I could send it back to be recalibrated, but that involves shipping it back to the US. I have a modified calibration file that I made in conjunction with a friend (who has a calibrated mic and Cleo), but while I use that "internally", I only use the supplied calibration file for measurements I publish, in the interests of openness and traceability.