View Full Version : IMF ALS30
Based on some chats I had with Lawrence who recently got some IMF speakers I went and got a nice pair of IMF ALS30 (http://www.imf-electronics.com/Home/imf/speaker-range/studios/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?
https://imgur.com/7o7xrVV
https://i.imgur.com/7o7xrVV.jpg
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
07-09-2018, 19:36
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
07-09-2018, 20:26
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.
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.
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
07-09-2018, 21:48
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.
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 (http://www.imf-electronics.com/Home/imf/speaker-range/studios/als40)). 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
07-09-2018, 22:22
Earlier on I was trying to get my head around acoustic lines and active transmission lines (partly driven by this write up (http://www.imf-electronics.com/Home/imf/speaker-range/studios/als40)). 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.
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.
walpurgis
07-09-2018, 22:44
The length of the line is the main consideration. Like reflex cabinets, you can 'cheat' with the bass response by having a port resonance slightly below the driver bass resonance in system. Meaning that the port is excited by output from the driver during its roll-off. Say for instance, the bass driver resonates at 40 Hz and rolls of below that, tuning the port/line a little way into this roll-off, say 35Hz, allows the two to sum and extend the bass output level to remain nearer flat to lower frequencies.
There are loads of books and websites that would explain better than me, take a look.
Lawrence001
08-09-2018, 08:14
The length of the line is the main consideration. Like reflex cabinets, you can 'cheat' with the bass response by having a port resonance slightly below the driver bass resonance in system. Meaning that the port is excited by output from the driver during its roll-off. Say for instance, the bass driver resonates at 40 Hz and rolls of below that, tuning the port/line a little way into this roll-off, say 35Hz, allows the two to sum and extend the bass output level to remain nearer flat to lower frequencies.
There are loads of books and websites that would explain better than me, take a look.Is it really cheating, I thought that was the primary aim of the reflex design. Just semantics of course. The thing I'm really interested in with TLs is whether the line is designed to get rid of unwanted back waves from the woofer, or enhance the bass output, I've read conflicting views. I guess it can do both, plenty of long haired wool to dissipate higher frequency energy but leave the intended LF energy to output from the ports. The other thing I'm interested in is the phase effect of the quarter wave line. Ie if the back wave (180deg out of phase) travels down the line at 330m/s it will emerge 90 or 270deg out of phase at the frequency the 1/4 wave line was defined (presumably the resonant frequency of the bass driver, ignoring the impact of any stuffing) which is surely bad?
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In theory a transmission line should absorb all the rear output of the drivers, in practice this never happens and the low frequencies are used to augment bass response. BuT AFAIK they are in phase with the driver output, at least on the TLS50.
walpurgis
08-09-2018, 10:18
Is it really cheating, I thought that was the primary aim of the reflex design. Just semantics of course. The thing I'm really interested in with TLs is whether the line is designed to get rid of unwanted back waves from the woofer, or enhance the bass output, I've read conflicting views. I guess it can do both, plenty of long haired wool to dissipate higher frequency energy but leave the intended LF energy to output from the ports. The other thing I'm interested in is the phase effect of the quarter wave line. Ie if the back wave (180deg out of phase) travels down the line at 330m/s it will emerge 90 or 270deg out of phase at the frequency the 1/4 wave line was defined (presumably the resonant frequency of the bass driver, ignoring the impact of any stuffing) which is surely bad?
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Firstly, a tapered transmission line can be closed at the end away from the drivers and be treated as (hopefully) a total absorbtion system. However using the output from an open line is a neat trick to reinforce and extend bass, so why not use it.
Secondly, only part of the port bass output will be in phase, but the wavelengths at low frequency are so big that the output that is moving out of phase either side of the chosen resonant frequency will not be out by much, so won't affect the bass delivery negatively by causing cancellation.
Compared to a quarter wave line, the proportion of port output that is in phase will be halved in an eighth wave length design or doubled in a half wavelength type. A half wavelength line will totally invert, so the whole wave exiting the port will be in phase with the output from the front of the main driver at the designed frequency.
Most transmission line speakers are quarter wave types like IMF, but the B&W DM2 is an eighth wavelength design.
.
Lawrence001
08-09-2018, 12:56
You've hit my ultimate point there, why are quarter wave lines the norm when a half wave will give the bass output in phase. Obviously cabinet size is an issue, but why didn't eg. IMF make their biggest speakers half wave?
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walpurgis
08-09-2018, 13:14
why didn't eg. IMF make their biggest speakers half wave?
I've not built a half wave box, but I'd imagine the bass output may be rather prodigious and tricky to keep under control with no useful benefit in performance over a quater wave, the extra complexity and bulk would be unwelcome too. I'd bet money that IMF would have experimented with this though.
A good quarter wave speaker can sound amazingly good through the bass. I've heard loads and built a few.
Even small ones deliver excellent sound, here's some info on a development of the seventies Daline design: https://www.blackdahlia.com/html/tip_80.html
The original used a 5" KEF B110 and went down to 30Hz believe it or not.
The original used a 5" KEF B110 and went down to 30Hz believe it or not.
Ah but what are your distortion levels at 30Hz with a tiny driver like that? There's no free lunch.
walpurgis
08-09-2018, 14:02
Ah but what are your distortion levels at 30Hz with a tiny driver like that? There's no free lunch.
Can't say offhand, but it was a well regarded design. I may even have the original series of articles that covered the build somewhere.
Lawrence001
08-09-2018, 19:23
I just missed one on ebay that went for peanuts a few years ago I was very upset.
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walpurgis
08-09-2018, 19:31
Horns are good fun too. There are several types, but the main ones are, constant width backloaded folded horn, quarter wave horn and flare coupled reflex (which is a hybrid). I've also built horns in the past.
So I found some time to look at the IMF ALS30 speakers properly. Unfortunately the bass drivers are both not working so I have started the return/refund process with the ebay seller.
Oh well. Big disappointment as I was looking forward to restoring the speakers to 'as new' condition.
walpurgis
08-09-2018, 19:55
That's a shame. They could have been fun, but it's the right move. You'd never locate proper replacement drivers and experimenting with alternatives would likely have only a moderate chance of success.
WESTLOWER
08-09-2018, 19:59
So I found some time to look at the IMF ALS30 speakers properly. Unfortunately the bass drivers are both not working so I have started the return/refund process with the ebay seller.
Oh well. Big disappointment as I was looking forward to restoring the speakers to 'as new' condition.
That is a shame Edward. Still you can take comfort in the other 36 pairs of speakers at Edwardo Towers!!
Odd they would both have gone though?!
That is a shame Edward. Still you can take comfort in the other 36 pairs of speakers at Edwardo Towers!!
Odd they would both have gone though?!
Now now Adam lets not exaggerate - I make it 8 sets - approximately! :)
Each speaker has two bass drivers - as Lawrence mentioned earlier probably a surge of some sort.
That's a shame. They could have been fun, but it's the right move. You'd never locate proper replacement drivers and experimenting with alternatives would likely have only a moderate chance of success.
Yep. I did have a quick look in the usual places if anyone selling drivers but no luck. Also not clear from IMF original sources what bass drivers they used or the spec.
It is all a bit odd as it was immediately obvious that the bass was not working when I plugged them in but the seller marked the speakers as fully working.
My guess is amp blew up and put DC on its outputs. The crossover would have blocked the DC to the mid and tweeter but the bass units took the full force and the voice coils burned out.
Have you looked at the drivers and found they were open circuit?
Yep I think you probably right Andrew.
Yep open circuit.
Waiting for a response from the seller as a first step.
My guess is amp blew up and put DC on its outputs. The crossover would have blocked the DC to the mid and tweeter but the bass units took the full force and the voice coils burned out.
Have you looked at the drivers and found they were open circuit?
Just a follow Andrew if I may? Is it possible for an amp to go DC only on one channel? What circumstances arise for an amp to go DC anyway?
cheers
walpurgis
08-09-2018, 22:02
Just a follow Andrew if I may? Is it possible for an amp to go DC only on one channel? What circumstances arise for an amp to go DC anyway?
cheers
Dodgy cap?
Dodgy cap?
I guess so.
But this does bring up an important point. How can we protect our (for many expensive) speakers?
When I try out a new (old) amps or speakers I always use sacrificial speakers (or amp) until I know things are ok. So far so good - but one of these days something will not be good.
walpurgis
08-09-2018, 22:14
I've never had an amp blow speakers, but I have had a couple of amps blow.
I've never had an amp blow speakers, but I have had a couple of amps blow.
Yep likewise. Not worth worrying about. I always switch off before swapping stuff around though. That's a good way to blow a speaker, changing leads and that with it all powered up.
The seller has refunded in full for the blown speakers. Seems that in Jan he had an amp go bad on him which probably blew the bass drivers. The speakers were fine immediately afterwards so he sold them as in good condition. Maybe the coils took a while to go open? The seller even offered to arrange a refuse collection or suggested, if I wish, I can just keep the speakers and try sell the drivers.
Good to deal with someone who is this straightforward.
I may hang onto them and try to find a broken ALS30 I can cannibalise. Anyone got one? :)
But I may just remove all the drivers, measure them and flog them. I said to the seller I would give him half of any proceeds if I go down this route. Fairs fair. Based on his email I do think he sold the speakers in good faith.
walpurgis
09-09-2018, 11:35
Modify the lower opening by converting it into a port (of the right size). Replace the upper drivers with some highish flux units of the right size and you have an eighth wavelength transmission line speaker. You could even use full range units and do away with the tweeter.
Modify the lower opening by converting it into a port (of the right size). Replace the upper drivers with some highish flux units of the right size and you have an eighth wavelength transmission line speaker. You could even use full range units and do away with the tweeter.
Thanks Geoff
Sounds interesting - but way over my head. :rolleyes: :)
If I try squeeze my coral bass drivers in will that work?
Alternatively would you like to have them so you can play with them? No charge.
walpurgis
09-09-2018, 21:48
Alternatively would you like to have them so you can play with them? No charge.
Thanks for the offer Edward, but I'm not in a position to take on projects at the moment, much as I'd like to.
No problem Geoff. If you change your mind give me a yell.
This question probably reveals my naivety but is there a way of measuring the working bass drivers such that I go and find equivalents?
walpurgis
09-09-2018, 22:17
This question probably reveals my naivety but is there a way of measuring the working bass drivers such that I go and find equivalents?
Unfortunately, it's the ones that are not working that you'd need that info for. They are different to the working set of drivers. I'm assuming it's the lower pair that are duds.
Unfortunately, it's the ones that are not working that you'd need that info for. They are different to the working set of drivers. I'm assuming it's the lower pair that are duds.No the blown ones are on the right hand speaker. The bass drivers on the lhs are the same and are working just fine. So I just need to replicate the lhs ones.
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walpurgis
09-09-2018, 22:35
Keep an eye out for a tired set of ALS 30 speakers that have (some) usable drive units then. Could be worth it, they are reputed to be a fun speaker. The bigger ALS40 certainly was. For their size the bass was ridiculous.
I had a bit of dc going out to a speaker once on a valve amp. it was a leaky cap. enough to give a hum but not enough to cause damage. replaced it and all was well..
What I said above refers to transistor amps only. Most valve amps have an output transformer which will block all DC. Some valve amps have output coupling caps, but they are quite rare. I think Croft used to do one and I think Graaf amps are designed that way.
What was your amp?
was a wad.. yup if memory serves it was a coupling cap
Yes, it's perfectly possible for one channel to get fried. Either thrashing the amp so the power transistors overheat or running into a short circuit could do it. Leaving speaker cables unplugged from the speaker, dropping them on the floor so the red and black touch and still having the cables plugged into the amp (whilst switched on) could induce instant and terminal injury to the power transistors. They can die in a fraction of a second and put the total DC from the power supply across the output terminals. Then you plug the cables back into the speakers and ... oh dear :doh:
Just as an aside, power amps can employ quite sophisticated circuitry to prevent this from happening and make the amp "bomb-proof". However, does it have an adverse effect on the sound quality? Many people believe so, so there are amps which can be quite robust as long as they're never shorted-out - but in danger of blowing up if they are.
More likely power transistor failure.
If using an unknown amp, switch it on first without speakers connected. Use a multimeter to measure the DC at the output terminals. Up to about 100mV is ok. Over a 100mV, looks dodgy. 40 volts? Switch it off!!!
You could put a capacitor in series between the amp and speaker as a safety measure, but it would have to be such a huge capacitor that it isn't really practical.Thanks Andrew
This actually I think completes the picture of the blown bass drivers of the IMF speakers.
The seller mention that when he plugged in an amp it went bang/pop etc.
The speakers came with some very dodgy cable which I threw out. Just retrieved them to take a pic and as can be seen the covering was brittle exposing the wires. Likely that when he plugged the amp in he disturbed the wire, creating a short, blowing the rh amp channel and then the bass drivers.
He had the speakers for 18 years and one lapsed moment destroyed his pride and joy.
Thanks for the tip on measuring a solid state amp output.
On RCA outputs, to test say unknown items, what would I be looking for?
Edward
https://uploads.tapatalk-cdn.com/20180910/df87b1cb99a71331c6d3946ebca94ea2.jpg
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that speaker cable is one hell of a lapse
spendorman
10-09-2018, 10:43
I've not built a half wave box, but I'd imagine the bass output may be rather prodigious and tricky to keep under control with no useful benefit in performance over a quater wave, the extra complexity and bulk would be unwelcome too. I'd bet money that IMF would have experimented with this though.
A good quarter wave speaker can sound amazingly good through the bass. I've heard loads and built a few.
Even small ones deliver excellent sound, here's some info on a development of the seventies Daline design: https://www.blackdahlia.com/html/tip_80.html
The original used a 5" KEF B110 and went down to 30Hz believe it or not.
I have original Dalines 3 unit version, KEF B110, KEF T15, Coles 4001G, and yes, even with the small B110 they go down really low, I would say lower than 30HZ. However, one can't have disco levels!
The two unit version was B110 and T27.
24152
that speaker cable is one hell of a lapseQED Qudos shite.
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Yes, those speaker cables were in a very poor state and look like an accident waiting to happen - which it did :doh:
RCA outputs? As in pre-amps etc? They're often DC blocked anyway but it wouldn't do any harm to measure the output for any DC. Any more than a few millivolts (ten?) would give me concern. Even so, it probably wouldn't cause catastrophic failure because whatever it's connected to will probably have DC blocking at its inputs.I was just thinking of how to check for correct operation and possibly have a source playing through it. So, for example, an old cassette player or CD player.
As mentioned earlier I normally use a sacrificial amp or speaker to test things but easier just to use a multimeter.
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Lawrence001
10-09-2018, 16:18
QED Qudos shite.
§I got some with an amp I bought, it was also cracking in places, I thought it must have been placed behind a radiator or something but may have been a design flaw in the choice of plastic.
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walpurgis
11-09-2018, 19:35
As there were questions about amplifier failure, I thought this might clear things up: https://www.analogbros.com/tech/articles/goodgobad.html
So after a bit of a lul I've managed to source some correct drivers for this project from the USA (thanks Grant) and today plugged them in. Mind you a bit of a faff bringing stuff in from abroad what with import duties and Parcelfarce seemingly having a monopoly on imports. But no damage thankfully.
Immediately obvious that these speakers are something special. Full coherent sound across the range with great definition and punch. Played my usual array of test tracks and the IMF comes up trumps. No wilting around the edges and running away.
As the speakers have the active lines to boost the bass I played tones through it. And yes they do go low. Good tight sound at 30hz and improves from there. Obviously not bass that rocks the walls (and anyway only two 6 inch drivers) but nice bass. On the other extreme I could get to circa 15khz but nothing beyond that. But I assume that is more a factor of my hearing and/or the aged caps.
So now will ponder if I do a recap and wood restore. I think a recap will enhance the definition and open the sound up in a worthwhile way.
Although the baffles have screws it looks like they are solidly glued down as well so may be an issue of getting to the crossovers easily. The crossovers can be seen from the bottom bass driver but can't be retrieved without removing the baffle.
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