|Linn Isobarik In-Stand Crossover|
The in-stand crossover as attached here is nominally a 4 ohm unit designed to be used with Linn Isobarik loudspeakers that do not have an inbuilt crossover, eg. the PMS and versions of DMS that have been converted to aktiv operation. The crossover consists of 3 parallel filter networks, one each for bass, mid and treble. Each Isobarik uses one of these crossovers and the in-stand name came from the fact that they sit on the floor within the base of the Linn Isobarik stand. One of the design features is that with the crossover sitting in the stands, the bass and treble cables to the loudspeaker have to be crossed over behind the loudspeaker to fit into the correct sockets on the rear of the PMS Isobarik.
The parallel filters are linked, close to the 4mm banana sockets, with three pieces of fairly stout wire. I have not been able to ascertain why three are used. With the links removed, as shown in the photograph, you can see three printed sets of lines where the links were (at the red arrow heads). With the links in place, the Isobarik can be driven by a single amplifier. Removing one set of links, say between the mid and treble section, the crossover is prepared for bi-amping and by removing both sets of links, tri-amping.
To bi-amp, the choice of whether to keep bass/mid linked or mid/treble linked is a topic of great discussion. I would opt for the former on the basis that most people would recommend the treble units are fed by the best amplifier in an aktiv configuration so it follows that the treble be fed by a separate amplifier in an bi-amped configuration.
Picture of Isobarik In-Stand Crossover (179Kb)
To remove the links, either cut them at each end or desolder them, keeping them for refitting should you wish to. If desoldering, you will need a very hot iron to melt the solder and will have to be quite quick to prevent the heat lifting the PCB tracks.
The method of wiring from preamp to loudspeaker depends on the preamp and amplifiers used, and whether single amping, bi-amping or tri-amping. The Linn Kairn preamp has three pairs of line output RCA sockets so if bi-amping or tri-amping, the extra pairs can be used to take the signal to the extra amplifiers. The Linn LK140 amplifiers have a line input pair and line output pair of RCA sockets so you could, instead, take a single signal lead pair from the preamp to one amplifier then link the output from that amplifier to the next in the chain and so on. If you only have one line output on your preamp or no line outputs on your amplifiers, you will need an RCA Y pair to split the signal for bi-amping and another to further split for tri-amping.
To wire between amplifiers and crossovers, treat each set of crossover inputs (linked bass/mid and unlinked treble if bi-amping or separate unlinked bass, mid and treble if tri-amping) as you would normal loudspeaker connections. WARNING: do not connect more than one amplifier to each pair of crossover sockets if they are still linked by the link wires, eg. do not connect an amplifier to the bass input and another to the mid input if those filter paths are still linked; it may damage both amplifiers.
When I ran my PMS pair triamped by 3 LK140s, I came to the conclusion that one LK140 driving the bass section of each loudspeaker, one driving the mid and one driving the treble did not give enough grunt at the bottom end. What I ended up doing, although being branded a heretic at the time, was to take one channel of an LK140 to the left bass, the other channel to the right treble and on a second LK140, one channel to the right bass and the other channel to the left treble. The third LK140 drove left and right mids. I believe this gave a deeper and clearer presentation, possibly because the power supplies in the bass driving amps where not being pushed too hard feeding through the bass crossovers high component count.