LS3/5  -   LS3/5A 

FL6/16   FL6/23   FL6 /38

 

There seems to be a bit of confusion over the two (actually three) types of LS3/5s.  The very original one was LS3/5, FL6/16 from 1970 and used the B110 A6362, the original 4-8 ohm version, and T27 A6340 with the black plastic case. The 8 Ohm version of this model A6535 was never used in any LS3/5 system. There were only a few of these earlier units made.  However if anyone has these units and wants a crossover then we can supply a ferrite version, Falcon No.100T, with a tapped H.F. ferrite inductor and matching input capacitors.

The original network was a standard 4th order configuration for the L.F. section, but this was not right and the changes to the topology were made on 9th October 1970, where the circuit topology changed to that on the FL 6/23. The values were of course different as the impedance and unit parameters were different.

The first production crossover was LS3/5A, FL6/23 from 1974 using the B110 - SP1003 and T27 - SP1032; this went through various modifications over the years, due to variations in the KEF units.  These were caused by various things including the variation in Bextrene. This plastic was bought by KEF ½ a ton at a time, but when they protested about the changes - usually not notified - they were told they were a very small user compared to the car industry using tons per week.

The problem was not helped by the original selection of the reference unit.  About 24 units were tested and instead of an ‘average’ unit being selected, one of the flatter units was chosen as the reference - hence there was always a reject problem.  Subsequent changes in the network had to be made to correct for the changes in production materials etc.

The basic LF changes were:-

C5 (BBC circuit/PCB ref) or C3 (Falcon PCB7), from 6.2mFd to 8.3mFd to 10mFd.
R2 from 33 Ohm to 22 Ohm.  See Table 1 below:

Please take these changes into account if you have 1970's units. Please also note that as the B110 response changed during its production span there were changes in the crossover to compensate. These are tabled below. It may be preferable, if the date of unit manufacture is known, to substitute the original values. Obviously the crossover changes came in after the 'unit' changes. We can make up the crossovers to the required values if you let us know on ordering.

TABLE 1 ( LF SECTION)

 

How to work out KEF Date codes eg. D 1576:

D is the month - April.
15 is the 15th.
76 is the year.
Some just the month and year  A 80

The basic HF changes were:-

The tweeter input capacitors C2 were increased by about 10% to compensate for a tendency of the T27 to have a rising response. This was spotted in the early 80’s when Goodmans took on one of the licences from RAM, who had gone 'bust'. There were only 3 licencees at a time.). The capacitor increases increased the output in the 5-8kHz. range. This was notified in April 1984. See Table 2 below:

There was also a change to the tweeter crossover input capacitor R2a & R2b (R5a & R5b on PCB7). This was dated 4/84. Again the tweeter response change occurred before the circuit modification.

TABLE 2 ( HF SECTION)

 

Falcon Acoustics applied in January 1982 for a licence to manufacture the LS3/5A, (we had a subsidiary company, Nightingale Acoustics Ltd.) but we lost out to Goodmans.

A comment by the BBC Designs Department on validating the sample systems supplied by Goodmans using our manufactured crossovers, was

"All capacitors bridged and found to be very close tolerance. It was noted that these components were probably individually selected so that the crossover was therefore not typical."

We were a little upset, as this was a standard crossover. All we had done, as with all our polyester capacitors was to select them all out into 5% batches. Many of the capacitors are made up from 2 capacitors in parallel, so we put a 0-5% and a 0+5% in parallel to give a cancelling tolerance variation. Other values, such as C4, 5.03mFd. is specified as 4.7 + 0.33mFd. We used a 4.7mFd selected to +5 to +10% an overall variation of only .235mFd, this was an outside variation as due to production spreads, there were less values at the +10% than the +5% (Gaussian distribution).

GOODMANS & /RAM LS3/5A FL6/23 CROSSOVER AS SUPPLIED BY FALCON

This network was available as the full spec version above as supplied to Goodmans (FL6/23), with transformer type inductors and H.F. tapped transformer but is presently not available.  

 

Falcon Crossover 23 is a ferrite version with resistive H.F. attenuation.

Falcon Crossover 23T is the same but with a tapped H.F. inductor and matching input capacitors.

Falcon Crossover 23aB is a  Falcon modification using the acoustic Butterworth (aB) H.F circuit originally devised for the T27 in the 104aB system. It gives a true 18dB acoustic roll-off and suppresses the rise due to the tweeter resonance. It was used in most of the KEF designs. It has been said by many to be the best.

All 23 type networks are supplied on Falcon No.7 PCB.

Falcon Crossover 102/ID/T Component Kit is also available using Iron Dust Inductors and Polypropylene caps with an Iron Dust auto tapped HF inductor.

This gives network power handling well in excess of the units, helps to keep the distortion in transients as low as possible,  and the Polypropylene capacitors give very low losses at high frequencies.

Some years ago, we sold some of our 23's and old RAM cabinets to a customer who had been buying the full spec ones. He was a little disappointed that the middle was not as good. He experimented after we said there should be no difference and he found the difference to be the actual values of the caps/resistors ref. the manufacturing dates, hence our emphasise on the date /component value table. During the testing he swapped over ferrite to Radiometal and found no audible difference.

LATER LS3/5A VERSIONS

Later LS3/5A’s use different units and the crossovers (FL6/38)/units are not interchangeable. It has become common to use the 15 Ohm and 12 Ohm versions to describe the different types. This is not particularly helpful as the impedance curve below will show.  What impedance you classify this system is anyone guess! It is best to specify the units or the Spec. i.e. FL6/23 etc.

 

 

KEF LS3/5a F6/38 NETWORK

We can supply a variety of component sets for the new version of the crossover FL6/38, using the B110C - SP1228, with full size hard wire layout.

Falcon Crossover 101  is supplied with ferrite inductors and polypropylene capacitors.

Falcon Crossover 101/ID  is supplied with iron-dust core/ferrite inductors and polypropylene capacitors. These inductors are of higher power rating than the official KEF versions. This version has superior distortion/power handling and lower loss/dielectric distortion in the capacitors.

Falcon Crossover 101/ID/T   Similar to 101 but with a 3 tap HF inductor and matching caps.

All are ideal for use as an external bi-wired crossover.

KEF B110 comparison data dated June 1997.

The B110C is nearer the B110B both in looks and performance than the B110A, so it is easy to tell them apart.

 

July 2010 update ( orig. M Jones 2003)