IMF "The Compacts"

The IMF Compacts

This Thread is about the smaller models of IMF. Some years ago, I just wanted to test the "IMF house sound" - I planned to buy one of the big transmissionline models of this manufacturer but than a pair of the smaller 3 way Super Coompact II came my way and I decided to buy them for small money, because they had the same mids and tweeter as their big brother. Since then I´m listenig to them and some other compact IMF speakers.

I want to start with the 2-way Compact II:

The tweeter is the well known KEF T27 that is also used at the little LS 3/5a BBC monitor.
The mid/woofer ist an coated version of an Audax:


...and this is the crossover:

Weiche Compact.jpg

The original capacitors are a typiclal electrolytic "elcap" (C1) and a Philips "chicklet" Type 341... (C2). Together they work far better, than one might guess.

Because of the age, it´s better to replace C1 - otherwise you have to listen 20 to 30 minutes untill it wakes up and the stereo image might not be very symmetrical. The little Philips is usally not on the end of its lifetime. The crossover is glued to the damping foam, wich is a big mess, if you want to clean it for soldering. After the repair it´s better to fix it on the bottom of the speaker ( you will need longer wires then ).

If you aim to an "next to original restauration", the english company Falcon Acoustics sells Replacements, that seem to have a little lower ESR and you might want to use a 1 ohm resistor to balance that.

It may look like this then:


But it´s not recommended to use the original circuit bord, because it is very weak and the conductors loosen from the pertinax board. I rather use something solid like that:


Now I´m hungry ... the thread will continue very soon ;)
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That brings back memeories, Audax HD17 has at least a bit of a coating.

I just happen to have a single sitting in the original box, they are a bit fragile — they ar efor a set of wall mont Divas which math them with an Audax tweeter.

Later, in their RS models, Tangent use dthe KEF T27 with HD20 in a similar arrnagent.

You can se ethe damping foam in the vent — does it have any depth or just a hole in the baffle? — will push the box towards aperiodic.

Fried later used the same technique in the Fried R, and Q.

Now I would like to write something about the sound of the speaker - what it can and cannot do...

I'm a big fan of a good spatial representation and so we're already in the middle of the topic. This speaker creates a holographic plasticity that you would only expect from full-range or coax systems. Beyond this there is nothing like a very sharp sweet spot. The best place to listen is in the middle - as ist allways is, but you can listen to music very appropriate at any seat in the room. The quality at low volume is very good too.
The Speakers play much bigger, than they appear - but of course you cannot reach the deepest cellar. Nevertheless they have that kind of sound, that makes you never miss anything...

The tweeter is without a doubt one of the best you can get on the vintage market and the performance in this setting is excellent. You can play with the two capacitors and use fo example that one (of course with 5.6 µF):


...or for the bypass C2 you can test a Vishay KP 1836, that makes the sound crystal clear.

What you should not try is playing loud. You find many pictures of Compact II speakers with damaged T27. They have little dents arround the dome next to the voicecoil.

The second problem is: The T27 is a bit too loud. The speaker ist designed to be used with a front cover, but even when you put it on, it plays 2 to 3 dB too loud. If you listen to some audiophile electronic music like Kruder & Dorfmeister or some triphop stuff, the sound is perfect - but when you listen to a screaming guitar, it is too much and your ears are bleeding.

I don´t know, why it is that way. The original measurements did not show that:


Maybe they did a little cosmetic for the catalogue ;)
I made some own measurements, but the diagrams died with the harddisc of my last computer.

Now I´m searching for a way to lower the level of the T27 - but without loosing any of the excellent performance. A simple L-pad with the usual kind of non induktive resistors does not work. The response is flat, but all life is gone. Using an autoformer is too expensive and sucess is not guaranteed. i have no experience with that stuff. The best result i get at the moment is with some oldschool wirewound resistors. There is a bit smoother sound with theese - but it´s nice and "shiny". This is a test-crossover:


Does anyone have further ideas, what I could try... ?

That´s it so far - thank you for your interest.

Grüssle Henner
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Her is some data and information from IMF:

IMF Compact II​

This is the smallest IMF speaker, 2 ways.
Drivers reference : Tweeter : unknown; Bass : unknown


The original IMF Compact Loudspeakers were a great success. Despite opinions to the contrary, the Compact demonstrated that there is a sophisticated strata of the hi-fi buying public who do not judge the merits of a loudspeaker system on a cost versus size basis. The advantages of applying the same attention to design detail as in our IMF Monitor loudspeakers to enclosures of modest proportions are, apparently, obvious even to the novitiate. This demands a loudspeaker that necessarily needs to be made at a price, rather than to a price.
The IMF Compact Il employs a bextrene unit of specified piston area and excursion such as to provide homogenous niid-range and low frequency performance. This is complemented by a dome tweeter and the whole unit integrated by low loss crossover and phase correction circuitry. Bass performance is maintained via a critically tuned and optimumly damped reflex enclosure (see illustration). Indeed, had dimensions of the enclosure or drivers in the Compact Il been greater, it would not have been possible to achieve such smooth and extended response without resort to an additional separate mid-range unit and associate complex electronics such as apply to the IMF Super Compact. In all, the Compact Il sounds larger than its size, providing wide dispersion and remarkable low frequency extension, and represents superb value for space.
With the Super- Compact, enclosure size is sufficient to accommodate the use of a bass unit with a free ai resonance around 25Hz. The drawing illustrates the resistive loading conditions which, whilst not impairing the exceptional bass response, damps the system against 'cone weave' from unwanted subsonic signals The mid-range is isolated in its own short transmission line and this unit along with the tweeter is similar to those we used in our Studio TLS 50 and ALS 4 loudspeakers.
The decision was also made to adopt the same complex and expensive crossover and mirror image configuration as in these larger models. This results in two important advantages. The in-line crossover network has been developed as a no compromise optimum filter for the drive units employed and as such, provides the minimum of colouration and thus the maximum sense of acoustic transparency. Secondly, it ensures that the phase and dispersion characteristics of the SuperCompact are substantially identical to those of the TLS 50 and ALS 40. Thus all three models are compatible for four-channel applications. Research demonstrates that dissimilar loudspeakers cannot be used for four channels any more than unmatched speakers are acceptable for stereo. Even choosing all loudspeakers from the same manufacturer is no guarantee of compatibility unless the speakers have the same phase relationship throughout the range and a substantially similar integrated performance. With an eye to the future, IMF in their Super-Compact have ensured this compatibility and anticipate that many of these speakers will compliment IMF Studio or ALS 40 loudspeakers for rear channel information. Meanwhile the prospective customer can purchase now without .fear of pending obsolescence.
We have said very little about the sound of Compacts, which in a way is a good thing for both have little of the obvious 'hi-fi' qualities about them. Low frequencies are remarkably extended for their enclosures sizes, but are smooth and free from the exaggerations of 'one note bass'. The middle and top has a sense of 'sheen' and continuity - rather than the sound of multiple speakers working in a box. Although of moderate efficiency, such is the freedom from distortion and colouration, that listening at low levels as well as high, gives a sense of balance that reveals all that is worthwhile over a wide variety of programmed sources. The speakers are worthy of the best amplifiers and indeed the finest ancilliary equipment. We invite you to audition the IMF Compact 11 and Super Compact loudspeakers against others beyond their size category.


5" x 9" x 9-1/2" wide; 38 cm x 23 cm x 24 cm
Drive Units
6.5" 16.5 cm bextrene mid-range bass unit with domed tweeter
Electrical two way at 4kHz
Frequency Range
35 Hz 20 kHz
Frequency- Response & Dîstortion Characteristics
See Graphs
See Polar Diagram
Matching Impedence
4 - 8 ohms (see graph)
Efficiency Measured via Pink Noise at 1 metre on axis for 40 watts
Driving Power Requirements
15 - 40 watts
Nett Weight (each)
6 kgs
Gross Weight Packed (pair)
15 kgs
Subject to alteration without notice.
IMF International, Westbourne Street, High Wycombe, Bucks. Telephone High Wycombe 35576
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Interesting points about the resistors in the crossover.

Where is this going,, is it a study of the sound quality of commercial L pads, or hi spec resistors?

As the units are basically end of life or no longer state of the art, it might be interesting to re spin this with a Purif/Sb acoustics/ scanspeak/wavecor/seas driver combo using serial X overs and small box size.

My questions are based on interest and cannot see anything wrong with your current approach.
The particular IMF is interesting and thanks for sharing the information, interesting to see another serial crossover and high frequency bypass capacitor from so long ago.

A ideal of killer small 2 way has always been appealing to me.
Where is this going,, is it a study of the sound quality of commercial L pads, or hi spec resistors?
Sometimes I find vintage speakers and ask myself: What is the progress in the past 40 - 50 years in developing speakers??? If this happens, I buy 3 pairs of them. The first pair stays original, the second ist restored to a most possible close to original performance and with the third one I´m looking for the best performance with modern parts in the crossover.
This is such a speaker model and btw the tweeter is build again by Falcon Acoustics and the woofer can be restored. If I succeed in teaching the speaker a more balanced tonality, then it will be one of the better ones I've heard in my life.

When I try to use modern parts in the x-over of vintage speakers, I often have the problem, that the original fine-tuning works no longer.
I have here about 10 pairs of speakers and some of them wait for a restoration while others are used at my listening room. Looking for the best parts for a perfekt and authentic sound is the biggest task, when I restore a vintage x-over...
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That one i have only seen on IMF drivers.

But i have tried many coatings. And seen at least a few others.

What i mostly use now is a PVA on paper cones, and i am experimenting with acryclic gloss on metal cones. PVA tends to wipe out the highs (very useful on the midbass i tried it on, but on the FRs i mostly play with now that is too much.

Lacquers like Damar and C37 can be useful in small amounts or in specific places on a cone, it stiffens them, use too much you kill the driver. They smrll nice (but it is probably not good for you).

The IMF coating is more like a silicon, same kind of messy and heavy. I have a pair of give-away 15s that are coated in silicon sealant.

Commercial products like BL100 et al are pricey and harder to get. Something like this is used on the KEF bextrene cones, i have used the same PVA (ModPodge) on a couple that somehow shed their original coating.

bextrene cones
Troels Gravesen writes that abeout the Spendor BC1 coating at :

May 2021:
Thanks to Hans-Martin, Germany, I now have this information on coating (and additional information):

Today I want to give you something in return:
@ printpage 117 (PDF pages 114-118) it reads:

"As already mentioned, the bass unit employed is the 305mm
plastic cone unit described last month. A chassis with a more
powerful magnet is now available and an increase in sensitivity
of about 2 dB is thus possible. Further experience with the
unit revealed a slight colouration in the 1.5 kHz region,
and this is accentuated with a later material manufactured as a
replacement for the type of Bextrene formerly used. It is
however completely removed by painting the cone with a
layer of polyvinyl acetate damping compound known as
Plastiflex type 1200 P, even though this treatment does not
cause any appreciable change in the frequency response."


" Listening tests, however, showed a
noticeable colouration in the 1.5 kHz region and chopped -
tone tests were therefore applied. In the region 1.2 kHz to
1.7 kHz these tests revealed three resonances with Q- factors
of the order of 500, some 40 dB below the steady -state con-
dition. If in phase with the steady -state condition, these
resonances represent irregularities of no more than 0.1 dB
on the axial curve and can only therefore be measured by
chopped -tone techniques. It was however shown that the
application of a layer of Plastiflex type 1200P damping corn-
pound to both sides of the cone reduced the resonances to a
marked extent; furthermore, the use of pink noise and the
recording technique mentioned for the bass unit demonstrated
a great improvement in the reproduction and the colouration
was reduced to a very low level. "

polyvinyl acetate
PVA aka wood glue....

The citated article actually refers to the making of BBC LS5/5
The series starts in
and ends :
(pages 92-95)
(pages 66-69)

Another article
" The cone was coated on both sides with Plastiflex 1200P to reduce slight colouration in the 2 kHz region and in this regard listening tests show that the reproduction from the coated unit is remarkably "clean"."

Another article
"There were, however, some signs of a lower-middle colouration below the frequency at which the 200 mm unit is used in the LS5/5. Treating the cone with a second layer of p.v.a. Plastiflex type 1200P removed the colouration without seriously affecting the response/frequency characteristic; "

Spencer Hughes and HD Harwood used PVA as coating at the time when BC1 was designed
Best regards
Chiming in with the LTSpice models of the crossover with 3 dB L-pad and autoformer. Might be worth measuring the response to see, if the hump at 5-6 kHz is necessary.

With the program installed you can open the asc file and play around with values and see what it does. Click on Simulate -> Run then you can probe around the circuit with the mouse and click on measuring points to see the voltage.


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Hi I wondered if you had any knowledge of these IMF bass/mid units - I am trying to find their specification. The cone is 18cm and the outside dimension 21cm. I am pretty sure they are 8 ohm. Have had them for some time and was looking to use them in a 2 way speaker build.


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