Hi all,
There is much of interest on this site, including discussions of TL loading - but although it is steering me in a particular direction, I've found nothing to answer my query - which may be of interest to others as the drivers used in my speakers are common to many TL designs from the 70s & 80s.
I have an ancient pair of IMF Professional Monitors (=RSPM Mk I). For more information they are visually identical to the model featured in these two reviews: imf-electronics.com/Promon/ontest/index and marklev.com/IMF/Promon/promonmkiii Maybe not as good as later versions such as the RSPM IV, but still pretty satisfying! The best recorded sound I ever heard was once when I had a party in the garden – nothing like free air loading and good volume to make them sing, and the bass was incredible – which I can never achieve indoors (despite 22’ x 14’ living room).
However when a pre-amp fault blew one of the B139s recently (managed to find a replacement on e-bay), that started me thinking about the longer term if things go wrong, and also about whether modern drivers might be better, especially in the upper registers. And the thought of the cost of that made me wonder if there was any way of achieving as good a sound in a more ‘spouse friendly’ package, such as a pair of smaller standmounts supplemented by a sub. The more I think and read about it the more I become convinced that even throwing large amounts of money at the latter, the bass end will never be as good as from big transmission lines (someone correct me if I’m wrong!)
So to upgrading the IMFS, the main question is what bass driver to use. It would be possible to fit up to 280mm diameter (dependant of course on the size of the mid unit) Any thoughts regarding a suitable unit for this transmission line? The existing driver is a KEF B139 SP6171, which is the older version, KEF’s declared Fs was 20Hz with freq range 20-1000. Crossover would depend on the mid unit, but probably ~400Hz. Wilmslow Audio has suggested the Volt 250.8 – is this reasonable? What other drivers may be appropriate? Does anyone have Thiel-Small data for the SP6171 version of the B139? (The TS website only gives the rather different later version, the SP1044.)
As for mid/HF, Wilmslow Audio have suggested the ATC SM75-150, and some choices of tweeter of which the Scanspeak R2904/710003 seemed the best compromise in view of its better dispersion than the ring version. Easy to assume that the best of modern drive units like these will be better than the aging B110/HF1300/supertweeter currently used, but the big question is, will they??? As for crossover, WA say they can design a suitable XO, however at ~£250 (parts only) I am tempted to spend only ~£200 more (“only” being in the context of the total cost of doing this right!) for a Behringer DCX2496 3-way active XO, and an extra pair of secondhand amps such as the Alesis RA300, and go for triamping with easy control over XO frequencies/slopes/EQ. I've no knowledge of these amps, but presumably they are not on a par with my existing Musical Fidelity P170, so it would be that vs passives that are less tweakable.
For reference my main use is music listening (from heavy/progressive rock, to classical, both chamber and dramatic orchestral, and opera, also playing a sampled piano.
All comments gratefully received!
Paul
