KEF-Q70 Redsign

Speaker Asylum

General speaker questions for audio and home theater.

A few weeks ago a buddy brought over his broken KEF-Q70 Uni-Q speakers. This was the first chance I have had to hear these speakers. My first impressions of them were not good. They had awful bass that seemed to be tuned at several different places and the mids and highs had a sound kind of like a megaphone. You could clearly hear the bass ports. They were not fun to listen to. The only good thing about them was that they covered the range top to bottom.
The Q70 is a tall tower with three chambers. Two are for the two woofers and one for the midrange driver. All three drivers are five inch with the mid driver having the Uni-Q tweeter at its center. This arrangement seems to give the tweeter that funny megaphone sound.
The first thing I wanted to do was to measure and plot each driver (measure impedance, inductance and DCR) and also to trace out the crossover design and draw a schematic of the whole system.
The plots for the woofers and midrange were very similar and looked like the typical woofer. A resonance peak and drop to nominal impedance (usually between 100 and 300Hz) followed by a rise to a higher impedance value. I only plotted the impedance response to 10Khz. That was enough to tell me everything I needed to know in order to run computer sims on this system.
The bass rolled off at 300Hz and the tweeter came on at 3Khz. The mids were out of this world. My sims showed a response peak that started at 2Khz and went to 3Khz! Where did 300 to 2Khz go?
Since one tweeter was broken I brought out a pair Vifa 85-D25â€™s that I been meaning to use in another system. I ran plots for the Vifa and made a zobel for it and then designed a 3Khz crossover. This I placed on top of the tower with a one inch set back for time aligning. With the Uni-Q tweeter no long working the Vifa sounded pretty good but the mids and bass were still bad.
I worked out crossover designs for both of these drivers and ordered parts (from PartsExpress). When the part arrived I measured each component and marked the measured value on them. Next, I put the real values into the computer sims to see how they worked. I learned here that you have to have the exact values to get what you really want. Even a micro-farad or two or tenthâ€™s of an ohm can throw the response off. It can peak or dip or roll off badly. Also I found that to get the right response I had to adjust the Zobel values a little bit as well. The bass crossover was easy and I installed the new values and zobel into the towers.
The bass is now amazing. Clean tight deep and without any hint of the ports. You would never know it was ported at all. Next, came the midrange crossover changes. I installed these components and zobel and then set back to listen. The speakers were gone, only the music remained. With the Vifa tweeters the Q70â€™s mids and bass were as good as it gets. The image was deep and wide and solid. You can focus on any part of the music and listen to it alone or widen you focus and hear it all. There is space for every instrument or vocal. No crowding or traffic jamming.
There are good reports coming back from my friend on the Q70's. He reports all the music is there, plus some. The "plus some" he suspects is his old JVC CD player, and inexpensive preamp. They are now crying out for equal time. My buddy and I laughed that the old adage remains true; the sound is only as good as the weakest link.
What this has shown me is that every speaker system needs to have a zobel on its drivers before the crossover is designed. This is because without it the crossover point will not be fixed where you think it is. For example, if the design is for a four-ohm crossover and the driver impedance goes lower than four ohms, the crossover frequency will go lower too. If the impedance goes higher then four ohms then the frequency will follow for a bit and then begin to peak!
The driver impedance needs to be nailed down before the crossover is designed and the real impedance then used for the design of the crossover. I say real because the impedance may be higher or lower than four or eight ohms. Only when it is done this way will the system give you all that it can. This is the forth system I have redone using this method. Each of them is a winner after the rework.

This post is made possible by the generous support of people like you and our sponsors:

VH Audio

Topic - KEF-Q70 Redsign - bobwire 10:01:02 02/10/04 (3)

If you'd get the model number of the speaker correct... - suits_me 18:55:51 02/10/04 (2)

Re: If you'd get the model number of the speaker correct... - glennw 22:09:07 02/10/04 (1)

thanks for correction and add'l info (nt) - suits_me 12:54:48 02/11/04 (0)

You can not post to an archived thread.