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Actively driving a KEF R107

 
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SaSi
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PostPosted: Sat Feb 11, 2017 1:16 pm    Post subject: Actively driving a KEF R107 Reply with quote

I just added the KUBE to the amplification chain (removing the quick and dirty solution of the parametric equalizer I used) and it makes a huge difference. But it really strains the amplifier (2x110W/4Ohms) if I crank up the volume.

Of course I could look into finding a more powerful amplifier, but that's too simple.

I'm considering using multiple amplifiers to actively drive the drivers (4 per enclosure).

I had a look at the crossover schematic (to try and decipher how to configure filters) but I don't know how to handle the "conjugation" parts that make the loudspeaker a resistive 4Ohm load. Do I ignore these? Will it matter?

As I see it, apart from the slopes at the crossover frequencies, there's a notch filter at the ~180Hz peak generated by the 4th order bandpass enclosure configuration that obviously needs to be part of the active crossover.

Is that all there is to it?
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speakerguru
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PostPosted: Sun Feb 12, 2017 1:53 pm    Post subject: Re: Actively driving a KEF R107 Reply with quote

SaSi wrote:
...I don't know how to handle the "conjugation" parts that make the loudspeaker a resistive 4Ohm load. Do I ignore these? ...


Yes. Leave them alone. Your amplifiers will love you for leaving them resistive loads to drive. They will run cooler compared to when driving reactive loads.

Just split the input to the LF, MF and HF sections and feed the resulting three inputs with your separate amplifier outputs. That's all there is to it.

p.s. you can unsolder and lift the input end of the first component in each section and use those as your inputs, if you don't want to butcher the pcb tracks.
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SaSi
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PostPosted: Tue Feb 14, 2017 8:34 pm    Post subject: Reply with quote

So you are saying, tri-amp, in effect, by splitting the crossover and drive each section alone, and avoid the active crossover stage.

That's an idea, however, if IIRC, I've read that the R107 main crosover includes a small section that affects the midrange. I could be wrong, and remember wrong that the KUBE slopes extend to the midrange affecting the B110.

I have traced the main crossover and have the schematic somewhere. Will look it up and check it out.
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audiolabtower
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PostPosted: Tue Feb 14, 2017 9:20 pm    Post subject: Reply with quote

I think it depended on the exact series. Some Kubes equalised the 107 bass only, so could be left out of mid and treble sections if tri-amped, but others did the shaping through the mid and higher to get maximum efficiency and volume out of the system, so would need something in front of the mid and treble amp to give the same shelving effect, otherwise the output will not be flat (although some of the cheaper magazines said the sound was better without the kube, undoubtedly because the "reviewers" had no idea about natural sound and liked some kind of JBL peaky impact).
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SaSi
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PostPosted: Wed Feb 15, 2017 6:20 pm    Post subject: Reply with quote

If the KUBE is extending it's slopes to the midrange, it can be set before the bass and midrange power amps and take care of both ranges, leaving the treble alone to feed directly from the preamp to the power amp, and in effect (perhaps small) reduce IMD perhaps caused by the equalization in the bass affecting the tweeters.

The easy thing to try first is unhook the heads from the main crossover and drive them directly from a different amp pair and use my ears as a first test.

Something should be wrong there, as checking the main crossover schematic shows two 100uF in parallel running the signal to the mid/high plug with a 6,4mH inductor across. So there's a second order high pass filter in the main crossover tuned to 140Hz where there's more space for the big inductor and capacitors.
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speakerguru
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PostPosted: Wed Feb 15, 2017 10:58 pm    Post subject: Reply with quote

SaSi wrote:
So you are saying, tri-amp, in effect, by splitting the crossover and drive each section alone, and avoid the active crossover stage.

What do you mean by "active crossover stage"? The Kube? If so, then no, leave it in ahead of all the power amps.
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audiolabtower
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PostPosted: Thu Feb 16, 2017 12:58 pm    Post subject: Reply with quote

SaSi wrote:
two 100uF in parallel running the signal to the mid/high plug with a 6,4mH inductor across. So there's a second order high pass filter in the main crossover tuned to 140Hz where there's more space for the big inductor and capacitors.

From memory the midrange crossover has another couple of inductors and caps in series as the low pass part, followed by impedance compensation components - that 140Hz should not bother the tweeter. So taking into account the ouput impedance of the preamp and input impedance of the power amp it should be very simple to implement that high pass 140Hz with a couple of passive components at the power amp input?

I would try and get a frequency response at the kube outputs, if flat it should be relatively simple, if sloped to counteract all the drivers in enclosure it is more complicated.
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audiolabtower
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PostPosted: Thu Feb 16, 2017 1:11 pm    Post subject: Reply with quote

By the way, here is an example of the Kube shaping from Mk1
http://www.stereophile.com/content/kef-r107-loudspeaker-1987-measurements#rGwm0Ajm3vlQjdpc.97
as you can see without the Kube the high crossover will be peaky and treble too strong.

For the Mk2
http://www.stereophile.com/content/kef-r107-loudspeaker-1991-measurements#fwXDcvWPKLl97wkc.97

I would be inclined to stick with the Kube, and if you reckon you can hear hiss or something try more modern opamps, some of which have spectacular performance, just being careful to limit bandwidth on high spec ones to avoid radio freq oscillation.
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SaSi
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PostPosted: Thu Feb 16, 2017 6:28 pm    Post subject: Reply with quote

Thanks for the KUBE curves. I have mk1.

I had that issue with bright treble before adding the KUBE. The parametric equalizer had been set for -3dB beyond 5kHz and that tamed it enough. Thought it was something wrong with the tweeters, like dried out ferrofluid, but then I thought that should make them sound dull instead.

I don't intend to remove the KUBE from the equation. Since it extends its influence until the high end of the response, it obviously needs to drive all three amps.

One thing that needs to be done to the low pass crossover is to isolate the first part of the highpass filter and drive it separately. That leads to a succesful bi-amping (low and mid-high separated).
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