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Going patially active with R105 MK1

 
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BobL
Intermediate Contributor 25+


Joined: 27 Jan 2004
Posts: 38
Location: West Midlands, UK

PostPosted: Sun Aug 07, 2011 4:03 pm    Post subject: Going patially active with R105 MK1 Reply with quote

It has been many years sinse I last had a crack at an active 105 using the diagram KEF supplied, I was never too impressed with the results and abandoned the project.

Active xover at <www.mightyoak.org.uk/kef/Active105.jpg>
Original passive at <www.mightyoak.org.uk/kef/R105-1.jpg>

I'm thinking of having a go at getting just the bass section active and leaving the mid/hf as they are for now.

I could do with some technical advice and would appreciate it if any of my questions below could be answered.

As I understand it, the crossover in the R105 is stated as 4th order and yet in the passive diagram it is clearly only 3rd order. I assume that the fourth part of the filter is to do with the natural respose of the B300 and the box that it is fitted in. Is that likely to be correct?

In addition, I would need to move the xover frequency from 150 to 400 Hz. I think I've already worked out the value changes for that. But the active xover is 4th order not 3rd and that complicates things a bit.

To just activate just the bass should I use a 3rd order crossover to match the passive? As otherwise the natural roll off would make it 5th order at this frequency though not at 150Hz.

If I do this, will I need to reverse the connections on the bass driver as I've seen odd order xovers with that before now?

The active xover has two 2nd order filters cascaded, is there an easy way to make one of then 1st order without changing its turnover frequecy?

Thanks in advance for any answers or help.

Cheers,

Bob.
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ColinR
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Joined: 31 Jul 2004
Posts: 1175
Location: Staffordshire

PostPosted: Tue Aug 09, 2011 10:58 am    Post subject: Reply with quote

You have another PM Very Happy .
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audiolabtower
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Joined: 06 Jan 2009
Posts: 544

PostPosted: Wed Aug 17, 2011 4:12 pm    Post subject: Reply with quote

Yes, the total response from the driver+crossover is 4th order.

Actually I think that 150Hz is possibly a better crossover point, which Kef did not do at the time passively, but did later with coupled cavity designs when adding many more crossover components for conjugate load matching. I don't think it would degrade the power handling by any significant amount running the B110 down to 200Hz, since the B300 still does the actual excursion work.

You could make the second opamp 1st order by just taking out a resistor and cap, and adjusting the values for 400Hz. But without some voltage graphs of the passive to match and maybe measuring pink noise or warble tone through the region difficult to assess how flat (ie as good as the original) the result would be.
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BobL
Intermediate Contributor 25+


Joined: 27 Jan 2004
Posts: 38
Location: West Midlands, UK

PostPosted: Thu Aug 18, 2011 7:58 pm    Post subject: Reply with quote

Hi guys,

Sorry I've not been back since asking my questions but there has been DNS issues with this web site and two ISPs (Plusnet & Virgin) were unable to resolve it for over a week.

So, If I make the active xover 400Hz 3rd order, the total will be 4th order and I needn't reverse the bass driver. Is that correct?

If so, I need to read up about active filters a little.

I would eventually like to go back to 150Hz xover but thta means using active for mid/hf in addition to the bass. I would like to see if a simple system would work well with only the bass active.

Thanks for your help.


Cheers,

Bob.
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audiolabtower
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Joined: 06 Jan 2009
Posts: 544

PostPosted: Fri Aug 19, 2011 5:59 pm    Post subject: Reply with quote

BobL wrote:
So, If I make the active xover 400Hz 3rd order, the total will be 4th order and I needn't reverse the bass driver. Is that correct?


As a first approximation yes, but the series large capacitor may have an effect if not there. One of the experts here would have to comment if better left in or out with an active drive.

I think the reversal of drive units with 3rd order crossovers is more pertinent at the 3kHz crossover, where the distance between drivers and the depth distance between driver centres at frequency overlap are factors that designers took into account as to whether they reversed or not.

However the 105 was "linear phase shift design" much more than most speakers so again without detailed measurement through the lower crossover region it's difficult to say which way round would be best.
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