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Kef 105 crossover and peak-level protection
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audiolabtower
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PostPosted: Mon May 07, 2012 4:24 pm    Post subject: Reply with quote

First thing I did with mine was bypass the fuses. They've ran tri-wired (actually quad wired with the led) from three 100W+ power amps with the crossovers external at the amp terminals for years now.

Many years ago a different power amp output stage failed to 40V DC rail on one channel with no damage to drivers - the large series bass tuning cap saved the day but I wonder if without that the B300 might even have survived a short time?
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speakerguru
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PostPosted: Mon May 07, 2012 6:01 pm    Post subject: Reply with quote

Maybe 30 sec or so. Depends if the d.c. took the coil in or out of the gap.
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audiolabtower
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PostPosted: Mon May 07, 2012 6:33 pm    Post subject: Reply with quote

Thanks Speakerguru. Maybe that's one argument for not going active? I suppose you can't leave the series cap in on it's own without the other passive components for the same design effect?
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speakerguru
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PostPosted: Mon May 07, 2012 9:17 pm    Post subject: Reply with quote

You can. I used it in the KM1, but you have to eq to take account of the removal of the resistance of series inductors. The theoretical gains are slightly greater without series losses but you need a different eq.
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bawnfree
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PostPosted: Tue May 08, 2012 11:38 am    Post subject: Reply with quote

All wired back together. I now have a low "buzzing" with rotary at any position other than "Off" on one speaker. I have tapped, flexed and squeezed this board in case it is just a crappy contact but the buzz is always there. As I was always going to leave this off anyway it's not a problem.
However; I may as well fix it when I'm changing out the caps and I can't make out any of the diode or tranny values until the board is de-soldered from the switch, so would anyone have a schematic for this circuit , or know the componant values I need so I can order them now and do all the work at once.
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speakerguru
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PostPosted: Wed May 09, 2012 8:14 am    Post subject: Reply with quote

bawnfree wrote:
...... I can't make out any of the diode or tranny values until the board is de-soldered from the switch


All the semiconductors would have to be high voltage to allow for the use of 200W+ power amplifiers. Other than that I don't think they were critical. KEF used BAY72 diodes, BC499A and BC450A transistors. There was a custom wound inductor on a small toroidal core. I don't know what the inductance was.


Last edited by speakerguru on Wed May 09, 2012 10:09 pm; edited 1 time in total
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audiolabtower
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PostPosted: Wed May 09, 2012 5:39 pm    Post subject: Reply with quote

I did not photo the indicator circuit board last time I had mine open. I suppose there must be a cap in there to average voltages? If this was an elcap it should be replaced at this age I guess to get back to design operation, the active components should still be ok if working?
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