Hi all! A few weeks ago I purchased a set of 2 IMF ALS 30 speakers, that needed some attention: tweeters were not working and both mids needed a refoam. Nothing special: I already worked on ALS 40s, Compact IIs and Super Compacts in the past so I am aware of their sound and the issues you may have with these IMF speakers after 40-50 years. I was able to repair one tweeter, but as I was not happy with the sound it produced, I decided to replace both HD100D25 tweeters with new, technically identical Audax TW0-25A0 (also sold by Falcon for this purpose). Caps in the filters were 25-30% off spec, so I swapped the Elcaps with Jantzen Elecaps to the nearest available value (6uF with 5.7 / 30uF with 27). One sub (lowest) driver had to be recentered. Mids were refoamed and seemed OK after this procedure: so far so good. Still, after I carefully reassembled the units (made photographs to make sure that I would not mix up the colour coding of the speaker wies) and did some critical listening, the result was very unexpected. There is an awfull overrepresentation of mid/high frequencies, that also seem to be somewhat distorted (particularly when listening to female voices). The sound was quite harsh and fatigueing. I did some basis frequency response tests in REW, which showed a kind of U-shaped curve (see screenshot). The distortion plot shows a peak around 2.6KHz (which is in the mid driver range), but I am not really sure how to interpret these graphs correctly. Anyway, it seems like this is not the original sound, so something is going on here.
I tried to find the schematics of the filter (it says on the PCB that the MK2 version of the ALS40 was used, but layout is totally different from the ALS40s that I serviced earlier), but no luck so far. One photo contains pictures of the physical layout of the filters and the components that were used (please note that they show the Elecaps, not the original Elcaps): I hope this may be of help to other AK members some day. All components were checked and seem OK (although I could only measure the value of the inductors). There doesn't seem to be any trace damage on the PCB either.
So what to do next? Not sure of REW measurement of individual drivers will give much extra information. I expected the sound of the ALS30 to be not far ook that from the ALS40, which I know well and am quite fond of. Perhaps the mids did not come out right from the refoam procedure? Any advice that would help shed some light on this issue would be much appreciated. Meanwhile I have hooked up my IMF Compact IIs again, which I so far still prefer over all larger models the IMF family (given my ears and listening space). Many thanks for your opinion, and happy listening!
BW, Martin
I tried to find the schematics of the filter (it says on the PCB that the MK2 version of the ALS40 was used, but layout is totally different from the ALS40s that I serviced earlier), but no luck so far. One photo contains pictures of the physical layout of the filters and the components that were used (please note that they show the Elecaps, not the original Elcaps): I hope this may be of help to other AK members some day. All components were checked and seem OK (although I could only measure the value of the inductors). There doesn't seem to be any trace damage on the PCB either.
So what to do next? Not sure of REW measurement of individual drivers will give much extra information. I expected the sound of the ALS30 to be not far ook that from the ALS40, which I know well and am quite fond of. Perhaps the mids did not come out right from the refoam procedure? Any advice that would help shed some light on this issue would be much appreciated. Meanwhile I have hooked up my IMF Compact IIs again, which I so far still prefer over all larger models the IMF family (given my ears and listening space). Many thanks for your opinion, and happy listening!
BW, Martin
) so rather than mess about selling them I just offered them free to a dedicated IMF fan - even though it was a five hundred mile round trip for him to come and collect them he was there within 15 hours of me offering them to him!