Imf 50tls mk2

Hi all,i have just picked a pair of IMF50 tls mk2's bugger me they sound good. The midrange has perished slightly so getting new ones ordered and renew the electrolytic  caps .Anyway should the tweeters be on inside or outside of listing position?  


  • Like these...? (Just an image I've nabbed from the internet)


    I hadn't seen a pair before. Looks great. I can imagine they sound great too. :-)
    My instinct would be tweeters on the outside (as per the pic above) to sspprreeaadd that stereo image, but I have no real idea.
  • Thanks Doc,yeah i have put tweeters on outside.
  • I notice that there's 2 tweetery looking units in the top corners. Can someone talk about that please? Are these 4 way designs?
    Really like them from their appearance btw.
  • I love em,but my missus said they are FOUL looking lol
  • Docfoster said:
    I notice that there's 2 tweetery looking units in the top corners. Can someone talk about that please? Are these 4 way designs? Really like them from their appearance btw.
    All i know is they image like crazy and the bass is real not like over emphasized over blown crap bass form newer speakers.  Probly be changing top tweeters for coles 4001G's at some point. These are keepers get your self a pair if you can Doc. No doubt Paul will pop up with some info on these :)
  • The Coles is a supertweeter...?
    I'm obviously a big fan of speakers form this era. My 1973 Goodmans, once they'd been revamped by RFC Paul, I would now not be without.
    Though in my experience modern replacement tweeters often improve things further. (Not sure whether tweeters deteriorate over time, or whether modern designs are simply better.) So I'll be interested to read of how the Coles alter the presentation of these bad boys.
  • farout said:

    I love em,but my missus said they are FOUL looking lol

    Well, each to their own on the aesthetic front. But I'm enjoying their look. And would be keen to hear a pair from your description. Maybe at a future bake-off ??
  • That would be great Suzy, some of the IMF 50's came with the coles as standard so i shall be fitting those at some point. The super tweeter at the top i don't know what make it is but i shall find out. They really do punch a nice tight bass that goes very low .
    The helicopter in Pink Floyds The Wall is real chest pounding :) fantastic speakers.
  • edited January 2016
    God they are old there is a pair at Southend on Sea run with a 12W SECA I made in 197x old age is shit.

    You had a great pair of Spendor and sold them how long is this bargain going to last????
  • Hopefully i be keeping them a long time Col,if the missus comes can get her head around tge aesthetics but even she admitted they sound bloody good :) infact she sat down for a couple of hours just listening that says a lot to me .
  • The tweetery things are coles super tweeters which you may still be able to get from Jerry at Falcon if needed.  They oughtn't to be used with perished surrounds for fear of damaging voice coils and due to the fact they wont be performing as designed, so getting those sorted is important.

    The design is sound.  It's called a Transmission Line design but like all TL descriptions, IMF pinched the term from electrical signal theory of transmission lines, and in reality, TL speakers are usually 1/4 wave bass loaded designed. 1/4 wave uses a tuned horn which is 1/4 wavelength of desired tuning frequency (set by the fB of the woofer in the cabinet) and uses the Eigenmode principle rather than Helmholtz loading of bass reflex.  It results in bass which is in phase with the front firing woofer and more importantly, it flattens the impedance loading of the main woofer and only has a shallow roll -off of 6dB/Octave until the output of the tapered line and the driver resonant frequency coincide after which the bass rolls off at 24dB/octave (as with ported enclosures) the main difference being that the point at which coincidence occurs can be managed by tapering the line and altering the internal stuffing to result in a very shallow initial roll off.  This means that small drive units can be used to generate useful bass.

    IMF were founded and speakers designed by the American, Irving M. Fried, also known as Bud who studied at Harvard. It was an Anglo American company although Bud started it in the States but it wan't until IMF started up in the UK that the first TL hifi speaker designs were produced.  I think they became TDL in the 1980s after IMF were no more but the original and designs were to my mind superior.

    Hard to get right but when they are done well, they deliver great bass which due to the size of the opening and the eigermode principle acts as a velocity source and not a pressure source so less prone to causing room boom.  A guy called martin King is probably the expert in all things TL related and has written some great research and modelling software for TL design.  Worth reading some of his research for those interested ;-)
  • Cheers Paul,yes they certainly do bass very well without boom at all :) . I have ordered new midrange units from Jerry at Falcon and new electrolytic caps should be here tomorrow. Im not using them now until new parts come.
  • Always good to get your encylcopaedic historical knowledge on these things Paul. :-)
Sign In or Register to comment.