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Converting Kef 104/2 to biwire/amp

 
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we5004
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Joined: 01 Mar 2014
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PostPosted: Fri Apr 11, 2014 7:56 pm    Post subject: Converting Kef 104/2 to biwire/amp Reply with quote

I have a pair of single wired Kef 104/2s and have added a second pair of SP2050 crossovers. I have used 5.6 ohm 50 watt wirewound resistors to provide dummy loads where appropriate on both crossovers. I am using two Arcam Alpha 10 amplifiers but am surprised to hear that, whilst a degree of presence and air have been achieved, the tweeters are too bright to be comfortable. Can anyone explain why this might be the case please, and the best way to evercome? Many thanks. Peter

Do I understand your post properly? You are trying to bi wire/bi amp a single pair of 104/2s by using 2 sets of crossovers on 2 stereo amps with the relevant non-used drive units replaced by resistors to load the crossovers?

It will not work because the drive units are a combination of inductance and resistance mainly and thus the frequency balance will be altered from original design by the removal of the drive unit inductance?

The way to do it is to separate the bass and treble circuits of a single crossover in each speaker by cutting tracks on the printed circuit board and running 4 cables from each speaker back to each stereo amp. I would suggest using each stereo amp for each channel in a pseudo monoblock arrangement - left channel stereo amp for left channel speaker (amp left for speaker treble, amp right for speaker bass). Ditto for other speaker and stereo amp.

Good luck! I have a similar arrangement on my 105s with 3 power amps and feel it is really worthwhile upgrade equivalent to a much more expensive hi-end single amp.

By the way this the music forum so maybe a mod can move the thread to the Speakers forum?

Anymore comments please?? Thanks to audiolabtower for this response.
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SaSi
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PostPosted: Sat Apr 12, 2014 10:30 am    Post subject: Reply with quote

Quote:
It will not work because the drive units are a combination of inductance and resistance mainly and thus the frequency balance will be altered from original design by the removal of the drive unit inductance?

I think this is the reason why it's not working for you.

I would not recommend removing the crossovers and going active with an active crossover and 3 stereo amps, mainly because of the complexity of the crossover and the difficulty in emulating it with a simple slope in an active crossover.

I also think the correct way would be to cut the tracks and separate the "ways" from each other. I understand this is a three way speaker, so you have the option of going bi-amp or tri-amp.

The fact that you have obtained a second set of crossovers means you can experiment without ruining a working set of crossovers you can keep for backup.
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we5004
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PostPosted: Sat Apr 12, 2014 12:30 pm    Post subject: Reply with quote

Thank you for your replies. Please be patient with me as I am not electronically qualified. If I isolate the crossover low frequencies and high frequencies and then run cables back to the amp, will I not be attempting to pass full range frequencies from the amp down the high frequency section of the crossover, resulting in damage? I am using two integrated amps, one has a switch inside that cuts out the preamp section and converts it into a power amp version of the integrated. Hopefully you understand my question!
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SaSi
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PostPosted: Sat Apr 12, 2014 10:00 pm    Post subject: Reply with quote

I understand the question (I think) but don't understand why you think it would occur.

Consider the crossover as 3 parallel filters that are wired at the input so they get power from the same amplifier. Each filter gives output to a speaker (low, mid, high).

If you "cut" the junction that shorts the input between low and mid, and then bypass the original input and wire the amplifier directly to the low pass filter, power will be fed only to the low frequency, low pass filter where the amp power would be filtered (low frequencies only would be allowed to the speaker) thus effectively blocking all output from the mid and hi frequency units.

Now, consider you use another amplifier to connect to the mid-hi section, bypassing the original input. That power would be fed to the mid (bandpass) and high (highpass) frequency filters only.

What the above achieves is that the outputs of the two amplifiers are not shorted together (very, very bad thing).

Final step: Find a way to run cables from the above points down to the input binding posts, adding a pair of binding posts, so that you can easily connect your two amplifiers, one to the low frequency filter and the other to the mid/high frequency filters and you're done.
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we5004
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PostPosted: Sun Apr 13, 2014 8:55 am    Post subject: Reply with quote

Thank you SaSi. I do understand now - I wasn't picturing it correctly in my head. Please may I bother you for one more comment. It is a lot of work to strip the 104/2s down to remove the crossover, and I am somewhat limited for time, plus I'm not too sure I have the expertise to undertake your remedy. I do have a pair of 8 ohm 100W L pads. I was thinking of firstly installing them between the amp driving the upper frequencies as all three drivers are a little bright but especially the tweeter. In addition, if I put an ohm meter accross the crossovers I get 3.9 ohms on the upper crossover and 4.8 ohms accross the bass crossover (I know the meter is DC not AC). If that didn't work well I was going to try the same exercise directly accross the tweeter - I know this would shift the crossover point a little being an 8n ohm attenuator. Please bear in mind I am not looking for audio perfection - I can hear the improvements in sound quality now against using one amp - I just need to tame it a little! I appreciate your help very much.
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SaSi
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PostPosted: Sun Apr 13, 2014 7:44 pm    Post subject: Reply with quote

I'm not sure how you expect to attenuate the high frequencies only if you go "before" the crossover.

It might help greatly if you could make a sketch of what wiring you have in mind. That way we may understand a different layout from what has been asserted so far.

I'm also trying to figure how can you find a KEF speaker too bright. They were mainly aiming for a balanced response overall and most comments I've heard from people regarding several different KEF models were too tame and too bass shy. True, I've never heard the 104/2.
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we5004
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PostPosted: Sun Apr 13, 2014 8:00 pm    Post subject: Reply with quote

This model has a tweeter and an upper and lower midrange speaker (as well as the two bass drivers of course). One of my amps is feeding these three drivers in each channel. Having listened to my speakers closely it appears that all three are somewhat louder (brighter) than when I ran them in a one amp/one crossover configuration. It just appears more noticeable on the tweeter. So I thought that perhaps using an L pad attenuator just across the 3 speaker input before the crossover and reducing their volume might balance the balance the overall sound better. I will provide a drawing but it is just a standard L pad configuration. If this doesn't work, I will try the same just across the tweeters, failing that, I will enlist some professional help to carry out your recommendation. If it is of any use to you, I have the 104/2 crossover schematic which I am happy to email to you if you wish. Thank you for your continued support!
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keffan
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Joined: 19 Feb 2011
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Location: USA

PostPosted: Mon Jun 13, 2016 7:53 pm    Post subject: Reply with quote

Hi we5004,

Would appreciate you could email me the 104/2 crossover schematic (kycheng@yahoo.com). I am considering to recap the crossovers. I will also replace the ferro fluid in the next couple of weeks.

Thanks

Kwok
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