An active box idea based on KEF Q1.

[Dxvideo]

Hi all,

I've a pair of KEF Q1 in my listening room for two years. I have tried them with various types of DIY amplifiers in this while and finally decided to use with my LM4702+Lateral MOSFET amplifier.
It's not a bad speaker but may be sounds a bit bright and cripsy and some distorted on high mids all I can say. Of course all of those most dependent on amplifier you use with it.
Anyway, last weekend I've decided to open it and see the crossover.... And disappointed a lot! Because they use standard bipolar type (8u, 10u and 14u) caps on it.. Then I thought that will be good to replace them with MKP type hi quality ones to make its sound more balanced. However the spaces were not suitable for that MKPs! So I've reassembled it with a big sadness..
However after some hours, a shiny idea has appeared in my mind! It would be great to replace that crossover with an active crossover + two channel diy amplifier. And the space was ok for this type of modding of course if the PSU would be outside..
So if you consider the dimensions a two channel LM1875 amplifier and 24dB/octave active filters (LPF+HPF) before these amps would be very suitable for my idea..
As KEFs manual says the crossover frequency is 2.8Khz in the native configuration. So I'd like to ask that;
- Has anybody tried something like that? And has good results? May he share with us the experiences on that matter..
- Should I use the same crossover frequency for my active ones? (Please consider that I dont like the distorsitons on hi-mids and its bright sound)
Thanks a lot in advance for the replies..

[planet10]

1st thing that will need doing is to trace the existing crossover circuit ... it will be invaluable info to have as you design an active XO.

There is no reason you can't rebuild the passive XO either, just toss the circuit board -- 2nd step in converting to active is to rework it so you have terminals direct to the midwoofer & tweeter on the back anyway.

dave

[cbdb]

There are advantages to active crossovers and a lot of high end studio monitors are going this way. One main advantage are the steeper roll offs (especially when the x-over is very near the most sensitve part of your hearing). Also less TIM in each amp (less freq for each amp to deal with).

A couple of things to keep in mind; the tweeter amp can be a lot smaller, typicaly for 100 watt speakers 20 watts, and 80 for the woofer. You will need to calibrate the amp levels so the balance between tweeeter and woofer are the same as they are now. If you can run a ( lets say 1 volt) test tone into the speaker and measure the voltage on the woofer at 500hz and then measure the voltage on the tweeter at 8khz (freqs far away from the x-over point) this will show you the relative levels you should be aiming for with the active system (you could try to sweep the entire 20 to 20khz range just to make sure the x-over isnt also an EQ to fix driver/cabinet problems) . Then adjust your amps so these levels are the same ( or put a HF level adjust on the outside of the speaker so you can adjust the tweeter amp to taste, if you want to make it less bright, assuming the amps are in the speakers.)