IMF ALS30

[Edward]

Based on some chats I had with Lawrence who recently got some IMF speakers I went and got a nice pair of IMF ALS30. Not sure why I'm getting yet more speakers!

Picked them up today. Very impressed with the weight of these speakers and the quality. Build quality up there with Harbeths.

Plugged them and lovely sound. But ...

on the RH speakers (there are marked L and R) the two bass drivers are not working. I suspect something simple. Unlikely that both drivers will stop working at same time.

Any ideas?

Not had time to do a strip down or measure the drivers, only partly removed them and connections, spiders, basket, connection to xover all look fine. Probably crossover?

[Macca]

You won't know until you get into them but agree the drivers will be fine. Could just be a connection has come loose.

[walpurgis]

Have you put a meter or small battery across the driver terminals to check coil continuity? (don't use a battery on the tweeter)

[Lawrence001]

Very annoying. A surge through the drivers could fry both, they are probably in series or parallel so I wouldn't discount the possibility of both going. Fingers crossed.

[Edward]

Thanks Martin and Geoff.

Need to carve out some time to open her up and check stuff. Hopefully just a loose connection from a part of the crossover I can't see readily, or a cold joint or something like that. Bit disappointing not being able to listen fully now before stripping it all down prior doing the wood and recap etc. When I get the drivers disconnected I will take readings and label each driver accordingly. Just did that on different speakers (Radford M70) which is also requiring a wood restore and recap.

On the IMF the whole baffle can be taken off - so super easy to work on. The baffle is double braced so a great original build. Very solid.

[Macca]

They were expensive speakers back then. I've got a pair of IMF TLS50 lying around here that I really need to do something with. Proper 'high end' in the day. All these old quality loudspeakers deserve restoring for another 40 or 50 years IMV.

[walpurgis]

Like the larger ALS40, they appear to use what is probably an eighth wavelength acoustic line with a driver at each end, the lower unit, presumably augmenting the partially in phase back waves from the upper driver, probably over a very small bandwidth.

[Edward]

Like the larger ALS40, they appear to use what is probably an eighth wavelength acoustic line with a driver at each end, the lower unit, presumably augmenting the partially in phase back waves from the upper driver, probably over a very small bandwidth.

Earlier on I was trying to get my head around acoustic lines and active transmission lines (partly driven by this write up). I failed miserably to understand what the tech is behind this. I understand transmission line (as say employed by PMC) but not what seems to be derivatives of TL.

[walpurgis]

Earlier on I was trying to get my head around acoustic lines and active transmission lines (partly driven by this write up). I failed miserably to understand what the tech is behind this. I understand transmission line (as say employed by PMC) but not what seems to be derivatives of TL.

A while back, I designed and built a distributed port transmission line speaker. I'm sure it's been done before, but I don't know of any. Basically it's a quarter wave type, but with several port openings of reducing size going back up the line instead of only one port at the end. This spreads the column bass resonance (and energy) over a wider range than a single end port would, reducing the well known response 'hump' effect that can afflict TL's.

[Edward]

A while back, I designed and built a distributed port transmission line speaker. I'm sure it's been done before, but I don't know of any. Basically it's a quarter wave type, but with several port openings of reducing size going back up the line instead of only one port at the end. This spreads the column bass resonance (and energy) over a wider range than a single end port would, reducing the well known response 'hump' effect that can afflict TL's.

Interesting. How does this achieve the extra bass extension that is mentioned somewhere? In the case of the ALS30 I have the spec says it goes down to 29hz which, if true, is pretty good considering the cabinet size. Is it the extra bass driver at the end of the line that brings the bass response down so far?

There must be some pretty complicated maths behind the tuning of all this to achieve a flat neutral response.