B&W DM7 Kenneth Grange classic

[VanDerGraaf]

On the DM7 front, I am about to let mine go, and as with the above DM5s, I think the ferrofluid needs replacing.
Sad to see them go, despite Charles' opinions on them I happen to think they have a lot going for them.

[system7]

Ye Gods, what is all this fretting about ferrofluid! And its viscosity.

When it dries out, your tweeter clogs up and stops working. It's happened to me. It's a mixture of iron oxide and oily kerosene.

http://www.bluearan.co.uk/index.php?...e=manufacturer

That's enough for about 5 tweeters. Clean out the old from the magnet gap and put in the fresh usually. Ocasionally you need to unsolder the voicecoil leads, but this is rare.

It has a 2dB damping effect at either extreme and partially controls the low frequency resonance and provides some cooling to the voicecoil.

[tinear]

Still retain DM7 mk.1 from 1978. Always driven by Meridian 105's.

This speaker caused endless grief.
The original Tweeter would 'blow' (wire-stress when threaded through the doped fabric diaphragm?) - and had a c.5kHz resonance (identified by Peter Tribe when visiting the factory).
The Bass cone assembly would slap into the chamber rear & concertina/unravel the former/coil. The roll surround also originally too shallow.
The ABR initially had a metal slug connector - spider to foam - it would detach/unglue itself - replaced by a flanged plastic version.

Later took the precaution of getting 10 treble diaphragms (mk.2 polyamide - still mostly unused)- & noted some variation in coil windings (B&W considered that had no effect..). A residual problem was the kapton former could 'bubble' due to heating and attaching to the pole-piece (treble disappearing). There was no ferrofluid in the Mk.1 tweeters; wasn't aware it was in the mk.2.

The Bass unit was rather a mass of coloration - later production had doping applied to the cone rear - though the source mostly resided behind that..
Applying a thin epoxy 'veneer' from v/c winding end to cone eliminated much, and improved 'detail' (also has prevented further v/c buckling); but the bass unit is inherently 'sluggish' due to v/c mass (+ plastic former under the dust cover).
By comparison a modern QUAD kevlar bass unit is a Revelation for detail/lack of coloration.

Splitting the x-over had some advantage - as did slightly adjusting the treble resistors for output (the mass of bell-wire leading to the upper rotary adjuster being disconnected).

After all that, it became relatively trouble-free; but driver replacements had easily added 50% to the original (20% discounted) price.

[Handiman]

I've been using Yamaha C2/B2 combo since the late '70's. Only amps I could find to do the job properly.

[Lawrence001]

Nice thread resurrection.

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