(click on picture to zoom)<\/p>\n <\/p>\n The cross-over is very simple and uses a LR4<\/strong> filter topology (acoustical) between the tweeter and mid and a LR2<\/strong> topology (acoustical) between the mid and woofer. The cross-over consist of only 12 parts and all sections are second-order electrically, besides the tweeter section which is third-order electrically in order to achieve an acoustical LR4<\/strong> frequency roll-off.<\/p>\n Note!<\/strong> The woofer\u2019s are connected with reverse polarity.<\/p>\n The tweeter level can be adjusted by changing the value of the single resistor R1<\/strong>. R2<\/strong> and R3<\/strong> can be replaced with one resistor if it can handle >20W.<\/p>\n I don\u2019t reveal the actual component values until I\u2019ve evaluated the cross-over prototype. The cross-over schematic is only used here to inform how the cross-over looks like. Depending on the listening tests and further measurements it can be changed little or completely.<\/p>\n <\/p>\n Frequency response simulation:<\/strong><\/p>\n As in most of my designs the cross-over is optimized from the 15deg off-axis measurements and thus, the loudspeaker is intended to be used with none or little toe-in when setup.<\/p>\n Using the 15deg off-axis measurement generally means that you have a less pronounced baffle diffraction influence in the frequency response when optimizing the cross-over. Generally I\u2019m optimizing the speaker cross-over to have equally good phase behavior between 15-30deg off-axis frequency response, rather than having a good phase response at the on-axis measurement and a deteriorating phase behavior as you go off-axis.<\/p>\n When optimizing around the 15deg off-axis measurements, the on-axis frequency response tends to be a little \u201chot\u201d in the mid and upper treble. By using the appropriate amount of toe-in, I think this slightly added frequency response gives a more presence and airiness to the sound and are usually beneficial and desired.<\/p>\n However, each design and the drivers used in it are unique and sometimes it might not work out properly. The cross-over simulations and measurements alone doesn\u2019t tell if it\u2019s good or not and I consider this and the tweeter level adjustments to be the voicing and fine tuning part of the loudspeaker design.<\/p>\n <\/p>\n (click on picture to zoom)<\/p>\n Left:<\/strong> 15deg off-xis frequency response (no smoothing applied).<\/p>\n Right:<\/strong> 15deg off-xis frequency response with mid-range reverse polarity.<\/p>\n The mid-range driver has a built in \u201cBBC\u201d response dip between 2-3.5kHz. The tweeter has a very extended and smooth frequency response, especially considering that this is a co-axial driver unit. The loudspeaker has a system sensitivity of about 88dB @ 2.83v\/1m.<\/p>\n When reversing the mid-range polarity we can see very deep notches around 590Hz and 2.8kHz at the cross-over frequencies. The deep notches indicate a very good phase response tracking and summation between the driver units around the cross-over frequency.<\/p>\n <\/p>\n (click on picture to zoom)<\/p>\n Left:<\/strong> Frequency response 0-60deg (no smoothing applied).<\/p>\n Right:<\/strong> The frequency response curve is an average of four measurements (blue<\/strong>): on-axis, 15, 22.5 & 30deg and six measurements (red<\/strong>): on-axis, 15, 22.5, 30, 45 & 60deg.<\/p>\n The loudspeaker has a smooth power response with a slightly downwards tilting response at higher frequencies. The tweeter has an extended frequency response with very wide off-axis dispersion.<\/p>\n <\/p>\n (click on picture to zoom)<\/p>\n Left:<\/strong> Four different tweeter levels.<\/p>\n Right:<\/strong> System impedance simulation<\/p>\n The resistor R1<\/strong> can be changed to fine tune the tweeter level. The level 2 is the “reference” level used in the above frequency and cross-over simulations. The minimum impedance occurs @ 120Hz and is slightly below 3 Ohms. The system sensitivity of 88dB @ 2.83v\/1m is mainly achieved by the low impedance driver units used in the loudspeaker. A 4 Ohm stable amplifier is recommended.<\/p>\n <\/p>\n Summation:<\/strong><\/p>\n As I said before, I don’t intend to make a clone of any of KEF’s loudspeakers and I don\u2019t have a clue how the cross-over for KEF’s loudspeakers looks like, even though my chosen cross-over frequencies seems rather similar to those used in the R500 loudspeaker. It just happens to be very suitable frequencies to cross-over at. I would say that a suitable cross-over window is about +\/- 100Hz from that I have chosen, at least the drivers seems to be engineered in that way.<\/p>\n With that said, the KEF driver is really great and I\u2019m eagerly waiting for the cross-over components I\u2019ve ordered for the first prototype and I can\u2019t wait to have a listen to the Concentric Three \u2013 M5<\/span><\/strong> loudspeaker! \ud83d\ude42<\/p>\n For further details of the driver unit measurements see:<\/p>\n “KEF SP1632”<\/a><\/p>\n “SB Acoustics SB15NRXC30-8”<\/a><\/p>\n The KEF R500 loudspeaker using the same mid\/tweeter and similar sized woofer’s:<\/p>\n “KEF R500”<\/a><\/p>\n Independent KEF R500 loudspeaker measurements:<\/p>\n![\"\"](\"https:\/\/www.audioexcite.com\/wp-content\/uploads\/2014\/04\/CTM5-Cross-overx.jpg\" "\\"CTM5-")
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