What are DMS & PMS Isobariks?

DMS = Domestic Monitor System (passive design, later ones convertable to aktiv)
PMS = Professional Monitor System (aktiv design, can be used with a passive crossover)

The original DMS has an internal crossover and one Canon/XLR socket. It's only possible to drive them in passive mode. Later these were changed by making the crossover accessible from underneath the 'speaker and the XLR socket was dropped in favour of banana sockets. They have 2 banana sockets for passive use and another 6 for conversion to aktiv. Conversion is done by disconnecting the 12 wires from the crossover (2 per driver) and soldering them directly to the 6 banana sockets. Converting them from passive to aktiv and back again is fiddly, and the wires get shorter every time you do it if you cut them! Linn do not recommend cutting them, nor do they recommend lengthening them with extra wire. The DMS was discontinued in 1988 when Linn released a 4 ohm external crossover for the PMS.

Some believe the late 1970's DMS third party crossover network is not very good. Others believe the older DMS XLR Isobariks have an "undeniable warmth" but they "tend to lose the plot with wooly bass" unless powerful amps and custom supports are used to "control" the 'speaker.

The PMS was designed to be used solely with an aktiv crossover, either from Linn or Naim. It was fitted with 3 XLR sockets but these were also changed later in favour of 6 banana sockets. It wasn't until Linn introduced the 4 ohm crossover mentioned above that they could be driven passively, use of non proprietary crossovers notwithstanding. The Linn passive crossover gives the facility for single, bi and tri-wiring/amping, depending on which crossover links are removed or left in place. The crossover sits in the stand, on the floor, with 'speaker cables plugged into it. Three cables run from the crossover to plug into the sockets on the Isobarik, terminated to suit the age of the 'speaker. Aktiv operation is still an option by reverting to an aktiv crossover and removing the passive crossover from the signal path.

Sources:
Linn DMS Isobariks - External Crossover questions
The Linn Isobarik DIY-Mods Bible
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