KEF Q300 Crossover Information

gtyler

Member
2010-03-07 2:04 pm
If you have a set of KEF Q300's I have a huge favour to ask. I have happened upon a set of drivers that are used in these speakers, and I would like to build a copy with my own enclosure. What I need are some measurements, and a description of the internals. In order to do a copy I need the X-over specs, the volume of the cabinet, and the length and diameter of the base port. If you are able to provide this info for me I will be very grateful. Any help is appreciated. Thanks!
 
Well, this falls into "Any help" I guess. The typical crossover looks like a (4R) Zobel on the bass unit and a 4 ohm crossover third order on the treble.

http://www.diyaudio.com/forums/multi-way/213216-x-over-kef-iq30-2.html

You want to measure the DC resistance of the drivers. 3.5R is in the 4 ohm category. 6R is in the 8 ohm nominal region.

All looks a bit 4 ohm to me on the iQ 30:
Impedance Equalization (L-Pad) Circuit Designer / Calculator

Get the cabinet about right from pictures, and the reflex tube on the long side, then adjust it down. You can get adjustable tubes. It only seriously affects high-power handling. You could even use closed box. This IS diy audio, after all. If it's a metal bass cone, I'd probably add a 8kHz tank across the bass coil to reduce the ringing. 1mH would get something like 0.5uF and 16 ohms wirewound.
 

gtyler

Member
2010-03-07 2:04 pm
Thanks System7 for the ideas. This will actually be my first speaker build, and although I have plans to do a full speaker design and build after this I was hoping for a more straightforward build for this one. I hope that someone out there has access to these speakers, or their specs.
 

fabel

Member
2009-10-26 6:02 am
Hey gtyler,

here a link to a German site about the Q 300:

KEF Q300

there you can see the crossover - it is a electrical 6 dB type. The inductor should be round about 0,6 mH.

The internal volume should be easy to calculate from the external data ;). The port might be some thing like L 15 / D 5 cm.

But ! : did some experiments with the speaker and I would recommend a 12 dB/8 network to get rid of the of the aluminium cone resonance at 7 to 10 kHz. Like this:

picture.php


And working on the internal stuffing should reduce the unevenness at 1 kHz.
( Sheep-wool, Basotect for example )

Greetings, Fabian
 
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gtyler

Member
2010-03-07 2:04 pm
Hey gtyler,

here a link to a German site about the Q 300:

KEF Q300

Awesome analysis! Thanks!

there you can see the crossover - it is a electrical 6 dB type. The inductor should be round about 0,6 mH.

Is there anyway to know this value for sure?

The internal volume should be easy to calculate from the external data ;). The port might be some thing like L 15 / D 5 cm.

I guess I could measure the T/S parameters of the drivers I have, then find a box volume and port length that will yield matching Fc and Qtc to what was found in this test (Fc = 61.47 Hz, Qtc = 0.601). Am I right? May be more accurate than estimating the volume of the cabinet material and the bracing and subtracting it from the gross volume.

But ! : did some experiments with the speaker and I would recommend a 12 dB/8 network to get rid of the of the aluminium cone resonance at 7 to 10 kHz. Like this:

picture.php

I like the looks of this! Do you have the schematic for the cross over you suggest? I'm not at the stage yet where I can go form a 12dB/8 network to the actual schematic. If you don't, not a problem, I need to learn how to do that anyway.

Thanks for all your help!
 

fabel

Member
2009-10-26 6:02 am
Hey,

the exact value is not sooo important. Get an awg16 0.56 or 0.62 mH at Solen and you will be fine. The difference might be about less than 0,5 dB - nothing to write home about.

The t/s parameters you see in the test are taken from the chassis still built in to the original box. Therefore they are not suitable for simulation. If you measure it yourself in free air qts and fs will drop significantly.

Just build a 15/16 L enclosure and be happy .... ;)

Important is to make the transition from the surround to the baffle as flat as the original! If this is ignored the high frequency output will suffer badly.

The 12 dB/8 crossover locks like such in schematics ( the 13,6 capacitor before the HT is rubbish , should be 5.3 µF sry ) :

12-db-q300_296065.png


GF
 

gtyler

Member
2010-03-07 2:04 pm
Thanks for the schematic!

The t/s parameters you see in the test are taken from the chassis still built in to the original box. Therefore they are not suitable for simulation. If you measure it yourself in free air qts and fs will drop significantly.

Just build a 15/16 L enclosure and be happy .... ;)
I may be too picky for that,:eek: or maybe I just don't know enough to know how little that affects things yet.
I have a few other drivers that I have to measure. I plan to use them for my second project once I get off the ground with this one. Because of this I will be building an infinite baffle (not actually infinite of course, but large enough for proper measurements). What I meant when I said measure the parameters was to build this and use it to get proper driver Qts ,and Fs, then plug these into a box design software to get the Fc and Qtc. I could then iteratively change cabinet parameters until my Fc and Qtc match the ones that were measured in the analysis. Would't this give me the closest to the correct box volume, or am I totally out to lunch?


Important is to make the transition from the surround to the baffle as flat as the original! If this is ignored the high frequency output will suffer badly.
Thanks, I'll be sure to do this.
 
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fabel

Member
2009-10-26 6:02 am
Hey,

what you worry about would make only very minor differences. Especially in the bass region it is the room that dominates what you hear. The +/- of a few Hz or a fraction of dB is really not this relevant.

The suggestion by system7 of using a variable br-port is much more helpful then worry about half a litre more or less ... . In my calculation the q300 has about 15,5 L internal Volume; minus port volume and the space of the magnet plus the change because of a bit of stuffing material ... 15 L is a pretty valid guess.:eek:

BTW T/S parameter should be measured in free air - without any baffle. A baffle like a IEC Din-Baffle is to be used for frequency measurement etc. .

GF
 

gtyler

Member
2010-03-07 2:04 pm
Hey,

what you worry about would make only very minor differences. Especially in the bass region it is the room that dominates what you hear. The +/- of a few Hz or a fraction of dB is really not this relevant.

The suggestion by system7 of using a variable br-port is much more helpful then worry about half a litre more or less ... . In my calculation the q300 has about 15,5 L internal Volume; minus port volume and the space of the magnet plus the change because of a bit of stuffing material ... 15 L is a pretty valid guess.:eek:

BTW T/S parameter should be measured in free air - without any baffle. A baffle like a IEC Din-Baffle is to be used for frequency measurement etc. .

GF

Ok sounds good. I guess I better get building then. Actually planning first, then building.

One more question about T/S parameters though: when you say "in free air" what exactly do you mean. I was under the impression that an infinite baffle was free air because the driver is not enclosed, and completely open to air on the front and back. I assume it has to be mounted somehow, so why not on an infinite baffle?

I also wondered if you or anyone has any ideas of simple things that could improve this speaker. I was thinking of rounding the corners so that the baffle step would be smoothed slightly. I was also going to make the cabinets out of some hefty 1" MDF. Any Others?

Thanks again for all the great help!
 

gtyler

Member
2010-03-07 2:04 pm
Hi, I have measured my pair of KEF Q300 speakers and they measure 385mm high, 310mm deep and 210mm wide with a port diameter of 60mm and port length of 160mm, hope this helps.

Wonderful! That does help. I have slowly but surely been doing the T/S measurements so that I could do my own box calculations, but I was really hoping to have the true measurements so that I could compair and see how I was doing. I do have one questions for you though. I noticed on another similar KEF design that the inside end of the port also has a bell mouth on it. Could you tell if your Q300s has a bell mouth on the inside, and does your length include the front and rear (if it has one) bell mouth?
 
KEF Q300 crossover

Also one more question: can you tell if there is internal bracing in the cabinet? Thanks for the help.

New to Forum, hello. The Q300 driver is an amazing piece of engineering, backed up by a mediocre cabinet and horrendous crossover (parts).
Having played with literally dozens of caps, inductors and resistors for this speaker I can recommend the following:
Stock 19-20 ga. inductor says .65mh but actually measures .665. It is in series with + lead to the woofer Having tried values from .62 to .68 a value of .665 to .67 is ideal. Too low a number brings the mids to far forward IMO and to large a number makes the mids flat & hurts the soundstaging. Try an ERSE 16ga. .68mh and wind 10-11' off. Soak the inductors in clear varnish for a couple of hours and let them dry. This cuts down on ringing.
Capacitor is 4.3uf. This is in series with the + lead to the tweeter.I've tried the Sonicap and the Clarity Cap ESA series. I prefer the Sonicap because I like to play with and change the sound with various .01uf bypass caps. More on this in a minute. Pay the $ to have then matched.
Stock crossover has a 1 ohm resistor in series with the + tweeter lead after the capacitor. The stocker is truly a bad sounding ceramic! Have tried the following:
Dueland- Amazing, worth the money IMO- can be unforgiving with a bad frontend, BUT they run 3-4% high in value. This is audible. If you use a Dueland be sure to pay the extra $ to have then matched and have a value no higher than + 1-2%. Or do a parallel network (I use Mundorf's with the Dueland to get the value down to exactly 1.0-.01.
Mills- soft, boring IMO. You can parallel to get to .98 ohm rather than 1.0 ohm to open them up a little.
Mundorf- 1.0 ohm 5 watt are OK, 20 m-resist watt's are better if a little bright just above the crossover frequency.
Build a Zobel Network for the woofer. It removes the HF impedance peak and tames it 7-10K ringing; similar the going to a 12db crossover. I use a Mundorf m-resist resistor of 6.5 ohm and a 15uf Clarity Cap ESA of 15uf. This goes in parallel with the + and - leads of the woofer after the inductor.
Bypass cap for tweeter- You can completely change the sound & presentation of the speaker by bypassing the 4.3uf cap with a small bypass cap (it goes in parallel with the 4.3uf cap). I simply have a sort pair of wires coming off each side of the capacitor with small alligator clips so I can change the bypass at will. Here is what I use:
Jupiter .01uf- adds a little sparkle and improves soundstage. Very smooth but not real lively.
Mundorf .01 SIO- Not very "accurate" but very addictive midrange and adds a little sparkle at the extreme top, where the KEF tweeter is a little down.
VCAP tftf- VERY lively, very "lit up" as the reviews say. Exciting but can be too much.
Sonicap Platinum .01- nice, but not worth the $$ IMO.
Clarity Cap MR .01- Very neutral, very open, great soundstage, stays controlled at all times. One of my favorites.
AmpOhm PIO, alum. foil- Except for the tippy top of the treble (where it is to my ears just a little soft), this is my favorite bypass cap. Smooth, detailed, beautiful mids, nice open yet stable soundstage; can listen for hours at high volume without fatigue or strain.
Vishay .01- For $1 each they work well with the Sonicap and adds a little sparkle and opens the soundstage a little.
Caps do have directionality to them, and they do need a break-in/settle-in period. The VCaps being the worse, horrible & bity for the first couple hundred hours.
The port does not have an internal flange BTW. It does have only one brace running horizontal about mid cabinet.
If you're having a cabinet built from scratch, over-build the crap out of it! Make the front panel from 1" at least, brace it well, and line the inside with some form of Damping material. Sonicraft.com has some good stuff (black hole?). That's also where I got the Soniccaps & Mills resistors. Amp Ohm caps from thetubestore.com Erse inductors from erseaudio.com and most everything else from partsconnexion.com in Canada. BTW, I'm in the U.S.
Have fun.
Bruce in Tucson