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Active crossover question
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audiolabtower
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PostPosted: Fri Nov 26, 2010 3:33 pm    Post subject: Reply with quote

the Mk2 does not have a crossover at 150Hz, both circuit diagrams here show a crossover above ~350Hz if you work out the component values.
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BobL
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PostPosted: Fri Nov 26, 2010 7:57 pm    Post subject: Reply with quote

audiolabtower wrote:
the Mk2 does not have a crossover at 150Hz, both circuit diagrams here show a crossover above ~350Hz if you work out the component values.


OK, the maths of that is well beyond me. However, I still think the Mk1 has too small a mid range box for a 150Hz xover. i think the Mk2 is 8.5 litres and I think the Mk1 is only 7 Litres.

Bob.
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audiolabtower
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PostPosted: Fri Nov 26, 2010 9:27 pm    Post subject: Reply with quote

SpeakerGuru, the Kef designer here has said there was no design intent to increase the B110 volume in the Mk2, it was just a by-product of the different construction of the injection moulding head compared to the wooden box.

150 Hz is no problem in 7 litres, as confirned by Colin in the previous thread. After all the LS3/5a does a full range 70Hz in under 5 litres. So the cone excursion at 150Hz of the higher power version of the 110 will be enough to maintain the max output capability of the speaker at that crossover.
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BobL
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PostPosted: Fri Nov 26, 2010 11:20 pm    Post subject: Reply with quote

audiolabtower wrote:


150 Hz is no problem in 7 litres, as confirned by Colin in the previous thread. After all the LS3/5a does a full range 70Hz in under 5 litres. So the cone excursion at 150Hz of the higher power version of the 110 will be enough to maintain the max output capability of the speaker at that crossover.


Clearly it is possible to get lower than 150 Hz from a B110 in even less than 7 Litres. However, it will be at lower efficiency at the low end unless of course the box is bigger than it need be. Assuming the box is no bigger than it needs to be it will effect the correction done by the crossover in order to maintain a flat response. I think it will need more lift at the bottom of the b110 xover output.

Its the three point system again - size, LF response and efficiency. You can have any two at the expense of the 3rd.

Anyway that's my opinion, please do as you like.

Bob.
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audiolabtower
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PostPosted: Sat Nov 27, 2010 2:54 pm    Post subject: Reply with quote

BobL wrote:
I think it will need more lift at the bottom of the b110 xover output.


exactly, and that is precisely why the active is 5dB higher at 200Hz compared to 2kHz compared to the passive 400Hz design, it's needed for both in the active and is not there for both in the passive.

The passive curve for the 105.2 mid range is clearly shown exactly at ~400Hz in Keftopics Vol 4 No 2 in Kef's own graphs and matches exactly the curve for the Mk1. The difference in volume is going to make no difference at 150Hz, both practical experience and calculation show that.

You're welcome to your opinion but the facts show otherwise.
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BobL
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PostPosted: Sat Nov 27, 2010 4:37 pm    Post subject: Reply with quote

audiolabtower wrote:


exactly, and that is precisely why the active is 5dB higher at 200Hz compared to 2kHz compared to the passive 400Hz design, it's needed for both in the active and is not there for both in the passive.


I fully accept that but that in no way discounts or is in any way even relevant to the point I'm making.

audiolabtower wrote:


The passive curve for the 105.2 mid range is clearly shown exactly at ~400Hz in Keftopics Vol 4 No 2 in Kef's own graphs and matches exactly the curve for the Mk1.


You have the advantage as I do not have any keftopics that late. Mine are all earlier.

audiolabtower wrote:


The difference in volume is going to make no difference at 150Hz, both practical experience and calculation show that.



That may be true. However I've seen no evidence of that and certainly you have presented none so far. I would need to see the frequency response of the b110 in both enclosures down to the point of inaudability (compared to the B300) when operating with a 150Hz xover 4th order. I would guess that means down to perhaps 100Hz or may be lower.

audiolabtower wrote:


You're welcome to your opinion but the facts show otherwise.


That is just the point I've not seen any evidence what so ever that even suggests that the B110 has identical output at say 100Hz in the two enclosures.

You may be correct, I've seen not a shred of evidence.

All I can say is that I spent years working on the active version of R105 with a Mk1 box and Mk2 xover. Overall it never sounded as good as the passive version. My Mk1 speakers are passive to this day 30 years after trying the active xover which I got checked and calibrated at work to be spot on.

Bpb.
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audiolabtower
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PostPosted: Sat Nov 27, 2010 7:09 pm    Post subject: Reply with quote

fair enough, it's not worth arguing about. If you care to look up the Keftopics you will find you were wrong all along about the 150Hz passive in the Mk2.
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BobL
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PostPosted: Mon Nov 29, 2010 10:08 am    Post subject: Reply with quote

audiolabtower wrote:
If you care to look up the Keftopics you will find you were wrong all along about the 150Hz passive in the Mk2.


I can see it's important to you so yes, I did incorrectly remember the xover on the passive Mk2 but it has been 30 years sice I worked on my active project. In addition, my central point about us not having any data regarding the roll off point of the b110 in the two enclosures remains.

I would enjoy looking at the keftopics but I don't have a source for them.

Cheers,

bob.
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Lee in Montreal
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PostPosted: Tue Nov 30, 2010 3:00 pm    Post subject: Reply with quote

Just jumping in the conversation. Didn't Mr Cook himself design an in-house (for internal use) active crossover with 125Hz splitting frequency with a bump at 250Hz?


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Kef Calinda - since 1979
Kef Cantata - since 2009
Kef 105/2 - since 2009
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speakerguru
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PostPosted: Wed Dec 01, 2010 9:33 am    Post subject: Reply with quote

Lee in Montreal wrote:
Just jumping in the conversation. Didn't Mr Cook himself design an in-house (for internal use) active crossover

No. Mr Cooke was MD and did not do any design in the period Laurie Fincham was Technical Director. My hazy memory is that an enthusiast wrote in with the circuit to Mr. C and he passed it on to the lab. The frequency response curves have an EE number, so that tells me that it was built up by in the era where technician Phil Granger was the senior technician. I will ask Phil what he remembers about it. Phil or his colleague Peter Neate will have built it up and EC measured it. KEF officially never had any serious plans to build an active 105.

Re crossover frequencies; I've explained why you can't easily do a low crossover frequency with passive filters before, somewhere on this forum.
Lee in Montreal wrote:

with 125Hz splitting frequency with a bump at 250Hz?
I don't understand what this means. Is it a typo?
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BobL
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PostPosted: Wed Dec 01, 2010 10:12 am    Post subject: Reply with quote

speakerguru wrote:
My hazy memory is that an enthusiast wrote in with the circuit to Mr. C and he passed it on to the lab. The frequency response curves have an EE number, so that tells me that it was built up by in the era where technician Phil Granger was the senior technician. I will ask Phil what he remembers about it. Phil or his colleague Peter Neate will have built it up and EC measured it. KEF officially never had any serious plans to build an active 105.


It's interesting to learn that KEF never had plans to build an active 105 and it wasn't their design. Perhaps that is why I never got it to sound any good. It does look very complex for an amateur design though.

I've always wondered why they were reluctant to market an active xover. I would have thought this would increase sales of both the active xover and speakers as some people do like active speakers and others like upgrade paths.

My poor memory of the time was that I wrote to Mr. Cooke and asked about an active version and he sent me the diagram that I've put on this site, all be it after a re-draw to make it electronic and more readable on the screen. Mr. C said at the time that they were looking at active 105.

A few weeks later, I got a letter from Mr. Cooke's secretary asking for the diagram back as they didn't have a copy. I returned it with a note suggesting they tied the input op-amp's input to ground as the original had it floating. When I got the diagram back, it had a resistor to ground. My single contribution to Kef technology. <smile>

Bob.
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Lee in Montreal
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PostPosted: Wed Dec 01, 2010 11:48 pm    Post subject: Reply with quote

"with 125Hz splitting frequency with a bump at 250Hz"

Meaning a crossover frequency at 125Hz but an increase in volume around 250Hz to perhaps compensate and end-up with a flat response. Wink
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BobL
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PostPosted: Thu Dec 02, 2010 8:53 am    Post subject: Reply with quote

Lee in Montreal wrote:
"with 125Hz splitting frequency with a bump at 250Hz"

Meaning a crossover frequency at 125Hz but an increase in volume around 250Hz to perhaps compensate and end-up with a flat response. ;-)


I can see where you're getting the 125 hz from, it's the point that the lines cross on the above graph - yes? The thing is though, the point the acoustic output of the B110 and B300 are equal ie. the crosover point is likely to be higher in frequency than the crossing point in the graph due to the efficiencies of the drivers in their enclosures. Kef said the acoustic xover was at 150hz.

As you suggest, the bump at 250Hz is to compensate for the B110 rolling off in its enclsoure towards the bottom of its response. Because the B110 is used as a bass driver in other speakers and yet here it needs to be heavily compensated as high as 250hz *suggests* to me that something is causing the B110 to roll off quite early and my best guess is the size of the enclosure, after all, it was designed to be done with below 400 hz.

This is why the Mk1 B110 enclosure being even smaller than the Mk2 for which the active xover was designed worries me. I *suspect* if it was measured, the Mk1 would need more lift. YMMV.

Bob.
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audiolabtower
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PostPosted: Thu Dec 02, 2010 1:33 pm    Post subject: Reply with quote

Not to compensate rolling off in the bass, but to compensate a rising response over the increased frequency range. Many BBC type smaller bextrene units have a naturally rising frequency response, around 6dB up at 1kHz compared to 100Hz which passive crossovers equalise by attenuating mid frequencies compared to the bass - by varying the inductor and other values from theoretical. The Spendor BC1 for example has exactly the same 5-6dB cut between 200Hz and 2kHz and the LS3/5a the same shaping with the older B110 to maintain a flat response down to 70Hz in a less than 5 litre enclosure. Efficiency is not a consideration here since each amp can be gain tuned to a flat response. Max power handling is the limiting factor but with an acoustic crossover at 150Hz and the throw of the B110B that should not be compromised. It is important to use identical amps with identical phase specs I would say.
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BobL
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PostPosted: Thu Dec 02, 2010 3:10 pm    Post subject: Reply with quote

audiolabtower wrote:
Not to compensate rolling off in the bass, but to compensate a rising response over the increased frequency range.

We obviously see things differently but to me those two things are the same.
audiolabtower wrote:
Efficiency is not a consideration here since each amp can be gain tuned to a flat response.

Der? The efficiency is the amount of energy changed from electrical into sound. the whole point of this conversation is that the efficiency of the B110 changes with frequency and enclosure. That is why you need boost at lower frequencies because it is less efficient there.
audiolabtower wrote:
It is important to use identical amps with identical phase specs I would say.

I don't disagree but I would ask why you say that? Talking of class A and class B amps I didn't think there was any phase difference unless you drive them via the negative inputs and then it's 180 deg. if you know more please expand.

Bob.
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