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IMF Professional Monitor IV

As DSJR has said, big quoted damping factor figures for amps back in 'those days' - made all the difference in lots of cases - with the speakers they were coupled with. Yet an amp's 'damping factor' is not a popular stated thing, currently.

If only really full facts and detailed figures, plus genuine test graphs of performing equipment were readily published -as we used to know them - less purchasing mistakes would be made, today.
 
HiFi World quotes a damping factor of "40" as good these days. My Crowns were "several hundred" as were the AVI S2000's from ten - fifteen years ago (high damping means [I think] a very low output impedance from the amp, which many transformer and cap coupled amps don't have).

I suspect the amp and speakers should be chosen together, as a loose amp wouldn't work with many ported speakers of the "BBC" variety, whereas a crown style amp may sound too lean with tight-a*sed FE speakers without much bass...
 
opinions sought after...

How does the TDL Reference Monitor M compare to the IMF TLS 80 Mark 2 and the RSPM 1V? Considering the TDL to create a basement system but the price is high compared to the IMF's. Room is about 24' x 19'.
THX
 
Thanks for the response. What about the bottom end and scale? My reference points are the Tannoy Westminster Royals for sheer scale, warmth and drama and the IMF 1V for detail and the classic TL bottom end. Only criticism of the IMF was an almost studio monitor effect that seemed to narrow the field of impact. Sounded 'smaller' in other words. 'Dryer' perhaps. Maybe even not quite as 'musical' as the Tannoy IMHO. The Tannoy of course cannot compete in the bass. Hoping to hear that the big TDL bridges the gap between the Tannoy and the big IMF bringing the best of both into one.

THX

OD
 
Since my last post in Oct 2008 I've been busy playing! I had decided not to touch my IMFs, but to build something intended to be as good, from scratch. I thought of a novel physical external layout that would provide great flexibility and WAF, and thought I'd try first with total volume a bit smaller than the IMFs. I used Martin King's software to evaluate suitable line length, taper and starting point for damping, and got to work...

The result? Awful! Bass sounded so closed in and quite un-TL like, and changing damping did nothing to improve (testing included several positions in room, and even in garden).

Taking heed of adverse comments some had levelled at my choice of driver (the Peerless), and only having taken only one out of its box fand that only for a few tens of minutes of use I was given a fair exchange deal for a pair of Scanspeak 26W/8867 which seemed to model well with MK's software. But scarcely any improvement: it was indeed the box that was the problem! Back to drawing board. same external design concept to start, but now same volume as IMF RSPM, including same starting cross section (previous had been smaller, possibly responsible for that closed-in sound), remodelled from that starting point again using MK's algorithms. Vast improvement! However, no matter what I did with the stuffing in terms of density, profiling, materials, it never sounded as good, nor as effortless as the IMFs, and so failed to satisfy.

I gave up - at least in terms of one of original criterion, WAF. Now it was serious: I copied the IMFs. Blow computer modelling, blow even the difference in driver characteristics, yes this was simply a dimensional copy as exactly as I could measure by reaching and probing as deep as I could. The only change I made was to reduce height slightly to keep internal volume and cross sectional area the same with no midrange enclosure. Damping started off copying what I could of IMF's (but limited by available materials and the fact that I couldn't tell what was in the middle section), then tweaked. Ahhhh... the TL sound was back! As good as or better than the IMFs? I can't tell, but bass now was enjoyable, clean and extended, though certainly not identical. Until I make the second one and try some extensive listening tests I'll reserve judgement. One advantage of the new is much greater power handling capability - my IMFs use the older 'racetrack' SP6171 version of the B139, which from memory had only been rated at 15W RMS, and which is all too easy to overload if playing at elevated volumes.

Mid & HF are in small satellite enclosure on top, SACD-style, though freestanding so they can be angled independently of the bass enclosure. Drivers are the ATC SM75-150 and Scanspeak D3004-66000. Triamped using Behringer DCX2496 active XO, with Musical Fidelity P270/P170/P170 amps. The DCX2496 allows tweaking of the response to compensate for driver response - or indeed for room response. For comparison I set for a near-level response measured at listening position but boosted towards bass end in line with general shape of the in-room response of my IMFs. Overall, with the whole system one noticeable difference was a feeling that singers were in the room, which was startlingly good, though at the same time there seemed to be a harshness when the midrange got busy - both to do with the ATC mid I suspect, and will leave me wondering which of the two I prefer - again, until the new pair are done and comparable as a complete system it is hard to be definitive.

One thing that the testing has done is to reinforce how un-true much of popular 'Hi-Fi' hype really is. For example, simply moving the listening position few inches either way can produce dramatic peaks and troughs of 10dB and more in the response curve, and even having the door ajar, or leaving a cushion on the floor between the listener and the speaker can have similar though generally somewhat smaller effects: it is clear that nothing can approach the original sound unless tailored to the listener's room at the time of listening! The beauty is that with the DCX2496 I can now do that, and reset everything for any given room layout. To be fair, of course, I must adjust the IMF's response in the same way when I get down to serious comparisons. I yearn for the sound that my IMFs had the one time that I played them in the garden a few years ago - maybe not surprising as they were designed in 'free air', and maybe that is when they get closest to the mythical 'flat' resoponse (depending, of course, on where the listener is positioned...)

Incidentally, my construction has been with 18mm MDF sandwiched with 2mm lead sheet and 3mm MDF (6mm for front baffle), with 0.5-1mm of silicone-type sealant material as a deadening adhesive. It gives a pretty 'dead' panel, but actually not quite as dead as those of my IMFs - yet the latter appears to be constructed simply of 18mm chipboard with Formica laminated to the outside (somewhere I read about stressed panels etc, but there's nothing on the inside other than the fibreglass wadding lining). Maybe chipboard is simply deader than MDF- yet I remember articles on speaker construction in the 70s that suggested that the then new MDF was ideally suited...

I'll be back with a further update after finishing the second speaker and doing some more exhaustive listening comparisons.
 
Hello again,

Looking to purchase some vintage IMF ASL40's for the bed room. Anyone have experience with this model? Also, I have pictures of the said pair up for sale. Two of the drivers appear to have different surrounds than those that appear in the pics in the IMF web site. Is this a factory variation or a recone job? Don't know how to include pics in this message. Could send them directly to a resident expert for comment.

Thanks in advance.
 
Hello again,

Looking to purchase some vintage IMF ASL40's for the bed room. Anyone have experience with this model? Also, I have pictures of the said pair up for sale. Two of the drivers appear to have different surrounds than those that appear in the pics in the IMF web site. Is this a factory variation or a recone job? Don't know how to include pics in this message. Could send them directly to a resident expert for comment.

Thanks in advance.

Put your pics into something like photobucketand then include a link with your post.

Just bought a pair of Pro monitor IIIs. They're in Plymouth at the moment at my son's house. I'll be going in a few weeks to collect them :) Be interesting to compare them with my TDLs.
 
Taking heed of adverse comments some had levelled at my choice of driver (the Peerless), and only having taken only one out of its box fand that only for a few tens of minutes of use I was given a fair exchange deal for a pair of Scanspeak 26W/8867 which seemed to model well with MK's software. But scarcely any improvement: it was indeed the box that was the problem! Back to drawing board. same external design concept to start, but now same volume as IMF RSPM, including same starting cross section (previous had been smaller, possibly responsible for that closed-in sound), remodelled from that starting point again using MK's algorithms. Vast improvement! However, no matter what I did with the stuffing in terms of density, profiling, materials, it never sounded as good, nor as effortless as the IMFs, and so failed to satisfy.

I gave up - at least in terms of one of original criterion, WAF. Now it was serious: I copied the IMFs. Blow computer modelling, blow even the difference in driver characteristics, yes this was simply a dimensional copy as exactly as I could measure by reaching and probing as deep as I could. The only change I made was to reduce height slightly to keep internal volume and cross sectional area the same with no midrange enclosure. Damping started off copying what I could of IMF's (but limited by available materials and the fact that I couldn't tell what was in the middle section), then tweaked. Ahhhh... the TL sound was back! As good as or better than the IMFs? I can't tell, but bass now was enjoyable, clean and extended, though certainly not identical. Until I make the second one and try some extensive listening tests I'll reserve judgement. One advantage of the new is much greater power handling capability - my IMFs use the older 'racetrack' SP6171 version of the B139, which from memory had only been rated at 15W RMS, and which is all too easy to overload if playing at elevated volumes.

Mid & HF are in small satellite enclosure on top, SACD-style, though freestanding so they can be angled independently of the bass enclosure. Drivers are the ATC SM75-150 and Scanspeak D3004-66000. Triamped using Behringer DCX2496 active XO, with Musical Fidelity P270/P170/P170 amps. The DCX2496 allows tweaking of the response to compensate for driver response - or indeed for room response. For comparison I set for a near-level response measured at listening position but boosted towards bass end in line with general shape of the in-room response of my IMFs. Overall, with the whole system one noticeable difference was a feeling that singers were in the room, which was startlingly good, though at the same time there seemed to be a harshness when the midrange got busy - both to do with the ATC mid I suspect, and will leave me wondering which of the two I prefer - again, until the new pair are done and comparable as a complete system it is hard to be definitive.

One thing that the testing has done is to reinforce how un-true much of popular 'Hi-Fi' hype really is. For example, simply moving the listening position few inches either way can produce dramatic peaks and troughs of 10dB and more in the response curve,

I yearn for the sound that my IMFs had the one time that I played them in the garden a few years ago - maybe not surprising as they were designed in 'free air', and maybe that is when they get closest to the mythical 'flat' resoponse (depending, of course, on where the listener is positioned...)

Well done - it sounds quite a challenge, and at least you are prepared to judge the outcome. Do you have any photos including the reject design? Which Peerless driver? I always think Scanspeak are overhyped. The XLS will go as deep as that hole in Guatemala, but not in a TL as the Q is very very low [needs a 75 litre 30Hz reflex box - flat from 30 - 90 Hz. None of this 'in room' tomfoolery]:

http://farm4.static.flickr.com/3349/4632028934_2a2cffb563.jpg
 
DSJR:

i've just noticed a slip of your pen, i think.

i seem to recall listening to big IMF's in the mid-60's, whereas you refer to mid-70's.

is it not mid-60's to early 80's for IMF big speakers?

skyebridge
 
HI all.
first post here,
i also had good time with the TLS80 but did not have the opportunity to have one no money
i have seen clone of these beauty,as there are many people here working for the late imf company may be somebody have the plan of these speakers.i would like to diy those
thanks
Blessing
Abes
 
I finally managed to get my Pro Monitor IIIs home from Plymouth today. I set them up and tbh they sound pretty dismal. The right hand unit is ok but there are problems with the left hand unit. I can hear distorted voice from the bass unit so I'm assuming the cross over will need attention. The caps are probably all buggered by now so I'll be removing them for investigation sometime this week.

 
Are they round the right way? I found my SFs (with offset tweeters) sound better with the tweeters further apart, and bass units closer.

Richard
 
I tried them the other way around first as I thought that's how they should be. They sounded terrible with no bass cohesion so I swapped them over and everything seemed to snap into focus. It was my daughter who suggested swapping them around :eek: I could then hear that something wasn't right with the left hand speaker. Maybe when I get them fettled they might benefit from changing places again, we'll have to see :)
 
I removed the B139s and took out the crossovers. Jeez what a ****ing mess. One crossover board is broken in half and has been soldered across the broken tracks whilst the other unit has been rewired with some ridiculous QED cable and done wrong at that. This is the one that doesn't work correctly though I'd have gone with the unit with the broken board. The B139 in the unit with the busted printed circuit board is heavily corroded too with lots of verdigree on the speaker terminals. These were advertised as excellent condition and perfect working order so the seller must have no idea what an IMF TL should sound like and I'm sure he posted on this thread too :eek: I'm not too bothered as they were cheap at 200 quid and I can fix them but that doesn't alter the fact that they should be as advertised. I'll post some pics shortly.
 
I haven't repaired them yet Richard but when I had them with the mid and treble units to the outside they sounded lifeless with no bass cohesion. Swapped around they were much better. As I said, that may change when I get them fixed.

This is the damaged board


Some of the crap wiring


I am wondering if these speakers are not an original pair though the stickers on the bass units are the same. Can't work out why only one speaker has had its wiring altered for instance.
 
I've repaired the broken pcbs properly but I've just found that both super tweeters are open circuit. I'm wondering if I could replace the tweeter and supertweeter with a single unit, maybe a T27? The T27 certainly goes high enough to do the job. Anyone any ideas or comments on this?
 
Well done - it sounds quite a challenge, and at least you are prepared to judge the outcome. Do you have any photos including the reject design? Which Peerless driver? I always think Scanspeak are overhyped. The XLS will go as deep as that hole in Guatemala, but not in a TL as the Q is very very low [needs a 75 litre 30Hz reflex box - flat from 30 - 90 Hz. None of this 'in room' tomfoolery]:

http://farm4.static.flickr.com/3349/4632028934_2a2cffb563.jpg

I have some pics taken during costruction (incl failed) - have to sort out.
Peerless driver was XXLS830843, which actually modelled best of at least a dozen potential drivers that I'd taken through Martin King's software while working up my original design (more info on page 7 of this thread).

As someone pointed out when I first started on this project, in response to someone else's dampening remarks cautioning against spending lots of money on drivers for an unproven design, if a design fails the only cost is actually only the construction materials - only a few 10s of £s, as you still have the drivers and can always try again, and again, and again... Each time there's the challenge of redesign and construction, and anticipation of that perfect sound (..and the disappointment when inevitably it sounds better in your head than through your ears!) In my case the challenge is to at least equal the idol of IMF prof monitors, and if I do that with drivers 40 years younger than my IMFs they should last a good while, freeing up the IMFs to go to another loving home!

Paul
 
I've repaired the broken pcbs properly but I've just found that both super tweeters are open circuit. I'm wondering if I could replace the tweeter and supertweeter with a single unit, maybe a T27? The T27 certainly goes high enough to do the job. Anyone any ideas or comments on this?

Some designs of that era with 4-way drivers used same Celestion HF1300 tweeter with a Coles 4001 supertweeter, while others used the HF1300 with KEF T27 as supertweeter - that tends to suggest the T27 to replace the supertweeter not both the tweeter & supertweeter, though whether the crossover will need tweaking will depend on the characteristics of the original supertweeter. Not sure what the supertweeter is on your version - although Coles is often mentioned, I read somewhere that some models used a modified dynamic microphone capsule as the supertweeter rather than a commercially available loudspeaker driver, and I have to say my own IMFs appear to fit more with that (I think mine are PM Mk1).

Hope this helps,

Paul
 


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