Manufacturer Specifications and Interpretation

[PENG]

I see where you are coming from.

Getting a power factor of one would be next to impossible.

However Raymond Cooke founder and CEO of KEF before his much too early death, did a lot of work on trying to get speaker loads as close to resistive as possible. In his time with a lot of tube amps that made more sense, with solid state amps, the issue is mute. Trying to solve this makes for a much more complex crossover and really ups the part count and expense.

I did however want to give a nod in his direction with the design of my speakers downstairs.

The speakers use his KEF B109 bass drivers, among my favorite drivers of all time.

The look of the speakers was designed to evoke the memory of some of the styling of the Cooke era. In addition I used his precepts to get the load as resistive as possible.

Here is the result. Not perfect, but all but one of the tuning peaks is blunted and the crossover peaks at around 400 Hz and 4 KHz are significantly blunted.

The phase angles deviate only briefly from zero in three areas, but over a narrow frequency range.

The crossovers are on three boards, and the component cost was more than a lot of people would spend on speakers, although not of that quality.

My Raymond E. Cooke Memorial Speakers.

I have to say, that in addition to sounding very good, they are an easy load on the amp, which stays nice and cool pushing them hard.

Anyhow that was the best I could do to get the power factor as close to 1 as I could.

Nice looking speakers, what elliptical? Technical reasons?

 

[Swerd]

Both sections are electrically second order. There is a zobel on the tweeter, for reasons not clear to me. May be to make the graph look pretty? Unfortunately the phase response is not shown.

The impedance flattening circuit' is on the woofer, W10 in the schematic. The blue impedance curve shows the major effect below the crossover frequency.

The only reason stated was to make for a flatter impedance curve to benefit users of a tube amplifier. He says solid state amps don't need it. Other than that, why would a zobel be used for a woofer or a full range speaker?

 

[TLS Guy]

Thanks Swerd, that's impedance flattening, but Slipper asked about the Zoebel network used to kind of flatten the phase angle, actually more, not just flattening but to zero degree. As TLS said, that could be next to impossible, but of course "next" is the key word. Even it that can be done, surely they would be some trade-off, or "catch".

I forgot this design. I think because I want to forget it! This crossover took me 10 years to get right. I would like to acknowledge lots of advice from the engineers at Dynaudio. That is the catch, trouble and lots of R & D.

It is the design of the crossover for the top three drivers of the speakers that are now my rear backs and used to be my studio monitors. Bob Carver got the hear these and stayed all afternoon. It is an odd ball crossover and a series network, not the pretty much universal parallel networks. Crossover frequencies are 900 Hz and 5 KHz It is also phase coherent for what that is worth. The rise of impedance at 100 Hz is because this is a short highly damped TL for the lower mid driver. The line does not "speak" as the crossover from the bass line to mid line is fully active at 180 Hz, and so the impedance curve below 100 Hz is irrelevant as the bass amps are directly connected to the KEF 139s.

 

[Seth=L]

Do the racetrack woofers add anything other than just helping slim down the cabinet width?

 

[TLS Guy]

Nice looking speakers, what elliptical? Technical reasons?

The elliptical drivers are the renowned KEF B139

As I said previously, this is one of my absolute favorite of all drivers. They have not been made for decades now, but go for a small fortune on eBay and deservedly so. All the T/S parameters line up for perfect bass extension in a modest cabinet.

They are elliptical so you can get good bass in a narrow cabinet.

If any driver should be copied and updated, it is this one. The fact no one has made a drop in updated driver for the B 139 absolutely astounds me. It seems to me that driver designers are not very well educated actually.

Here is the impedance curve of those drivers in the TL in the rear of this studio.

As you can see from the curve the polystyrene cones resonate violently at 800 and 1200 Hz. So you can only drive them to 400 Hz and then they need a fourth order electrical crossover to put them 24 db down by 800 Hz. This is were the updating should come in to extend their pass band.

In the impedance curve the second peak of impedance is not quite suppressed by the line damping.

This is where art takes over from science. They sound better that way!

 

[PENG]

I forgot this design. I think because I want to forget it! This crossover took me 10 years to get right. I would like to acknowledge lots of advice from the engineers at Dynaudio. That is the catch, trouble and lots of R & D.

It is the design of the crossover for the top three drivers of the speakers that are now my rear backs and used to be my studio monitors. Bob Carver got the hear these and stayed all afternoon. It is an odd ball crossover and a series network, not the pretty much universal parallel networks. Crossover frequencies are 900 Hz and 5 KHz It is also phase coherent for what that is worth. The rise of impedance at 100 Hz is because this is a short highly damped TL for the lower mid driver. The line does not "speak" as the crossover from the bass line to mid line is fully active at 180 Hz, and so the impedance curve below 100 Hz is irrelevant as the bass amps are directly connected to the KEF 139s.

The graph linked looks flat enough, nice job for sure.