SpeakerTalk

[dsmith]

Here is a question for the speaker guru (or any other gurus).

What was that Acoustic Butterworth thing all about? What was the difference between a 104 and a 104ab? Not only the technical differences but how (and when) did it come about? Was it the first product of the new KEF computers?

History please.

D Smith

[ColinR]

[speakerguru]

As Colin says, it's very well written up in Keftopics http://www.kef.com/Resources/KEFTopics/KEFTOPICS_vol2no1_a%20target%20function%20approach.pdf

In brief, it's concerned with making a passive XO work despite the varying load and radiation around the resonance of the unit. If you want to produce a particular acoustic cut-off slope, say 18dB per octave below 3 kHz and the unit cuts off at 12 dB per octave below its 1.2kHz resonance then the XO will have to have a dual (electrical) slope; 18dB/oct above 1.2 kHz and 6dB/oct below. That's what an aB XO does.

It's nothing at all to do with "compensating for the rising treble response of rigid dome tweeters".

[audiolabtower]

I seem to remember that the treble of the original 104 was not as highly rated as the 103 and one theory was that the 1.2 kHz resonance of the T27 was closer to the crossover frequency than the lower frequency 1.5in T52 in the 103, and this was upsetting the slope and thus integration of polar responses of the drivers?

Certainly the 104 had a "sheen" in the lower treble which was successfully reduced when I upgraded to the 104aB crossovers, leading to definitely more depth perception and even lower colouration. Thus the aB network was particularly advantageous for T27s at 3kHz.

[dsmith]

I think it was the first published instance of a loudspeaker's electrical filter having exactly the right response to give a combination of filter and transducer that meet a precise target shape. Before this plenty of electrical filters hit Butterworth or M-derived targets. The loudspeaker as a load for the filter, and the particular transducer response, were always ignored.

So, Speakerguru, was this an ah-ha moment or just something they knew they could do when they started developing the HP software? Was it a Mike Berman invention or a Laurie Fincham invention?

The "bridged T" topology is unusual, to this day

Dave

[proffski]

As the KEF article states, when I upgraded to the aB C/O the first most obvious enhancement was that massed strings sounded a lot clearer, as did the lovely soprano Victoria de los Angeles and others.

The differences between MC & MM cartridges was also more pronounced and I gave up using MM completely after that...

Now Diabolical Audio Broadcasting makes it all sound like screeching chainsaws again!
_________________
I contend that for a nation to try to tax itself into prosperity is like a
man standing in a bucket and trying to lift himself up by the handle.
-Winston Churchill

[ColinR]

[proffski]

[speakerguru]

[proffski]

THere is a short but heart warming article on the KEF 104 / 104aB loudspeakers in this months (Feb) issue of Hi-Fi News.

Oh what I'd give for a pair of brand new as were KEF 104aBs right now... I so miss mine.
_________________
I contend that for a nation to try to tax itself into prosperity is like a
man standing in a bucket and trying to lift himself up by the handle.
-Winston Churchill

[Nimo_jon]

Hi Proffski

You mentioned about upgrading from 104 to 104aB. Hope you would help in clearing up some doubts:

1. For the 3.3uf, 10uf and 10uf, did you use Alcaps?
2. For the 0.6uf, did you use a PP type cap?
3. What was the date code on the T27A?
4. Did you prefer to bypass the second inductor with 10R on the low pass section?

Thanks a lot.


Jonathan


ps: I fully understand the advantages of the aB network. I have the 104 original and 104aB original side by side. I often listen to baroque, quartets and light jazz at low levels. (usually 9 o' clock position on the preamp) I do not play my music loud. In this case, the 104 appear to be more 'pleasant' due to low listening level. When playing orchestral work, I would use the KEF 105.2 (slightly modified).

As mentioned in another thread, I might install a multi pole selector switch to select between 104 or 104aB network for the 104.

I am doing a project to modify a pair of 104aB using the Hiquphon OW1 tweeter. In this case, the simple 3rd order high pass may be good enough. I have not finished the integration yet (OW1 has a 94mm dia flange, needs some wood working on the baffle as the 127 is 108mm). I know this may well change the sound of the 104aB, but its a worth while investigation. T27A is getting more and more difficult to replace. Have you tried replacing the T27A with other tweeters? Cheers.