Modernising old TL speakers (KEF B139-based IMF monitors)

[Paulthefirst]

Hi all,

As a newcomer to this site I've read some interesting stuff - but not the answer to upgrading old TL enclosures. Any thoughts are welcome!

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: http://imf-electronics.com/Promon/ontest/index.html http://www.marklev.com/IMF/Promon/promonmkiii.html 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???

And 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

[inertial]

Hi,

If you go with the DCX IMHO you need upgrade it. Look at
www.linearaudio.nl
About the drivers, they are good but very expansive stuff
What about the cheap TB oval woofer? Not sure it can be run to
400 Hz

Cheers,

Paolo

[Brett]

Try active with the DCX - it does not need to be modified as it's OK stock.

I wouldn't change out the drivers, just use the stock ones.

[Moondog55]

I'm with Brett on this one, but even cheaper should be the 2-way DCX 2310.

Upgrading the passive cross-over on the HF side with better caps may be worthwhile and tweeters are now much better but it is still a good speaker

Regards Ted

[Paulthefirst]

Further to my previous post and comments received, and much thought, I've decide not to tamper with the IMFs for now, but instead try to duplicate the performance in a different shape enclosure, intending it to be more flexible in placement, and hopefully also more 'spouse friendly'.

I've decided on these drivers: Peerless XXLS830843 bass, ATC SM75-150 and Scanspeak D3004-66000.

Using Martin King's software the Peerless driver models almost identically to the B139 in the IMF profile, subject to appropriate damping. My challenge is to make another TL enclosere that will be the equal of the IMF.

The Scanspeak I like because of its better off-axis response than the ring versions (and cheaper!), increasing the size of the 'sweet spot'. The mid & tweeter I'll put in a separate enclosure on top, which could avoid the need for 'toeing in' of the main enclosures. Also allows some styling variations.

The setup will be tri-amped - through ebay I've picked up an Alesis ra150 and an Alesis ra300, to complement my existing Musical Fidelity P170, and invested in a new Behringer DCX2496. Once its all set up I can consider whether to go for mods to the DCX to improve it, or go for an analogue active XO having found the appropriate parameters.

Also through ebay while looking for the DCX I've picked up a Behringer DEQ2496, so I can play with room EQ. Part of me thinks this is a good idea, spurred on by the realisation (through Martin King's software) of just how much the room modifies the sound, while part the purist in me - fed by years of 'HiFi' hype' is concerned about the adverse effect of all that electronic compensation. I'll find out which suits me best - and can always sell the DEQ on ebay again if it doesn't help! I just long for the system to sound like my IMFs did when played out of doors!!

I'll let you know how it goes, probably a few months though befor I get it done.

[inertial]

Wow , a XXLS up to 400Hz !! Braveheart !

Cheers,
Paolo

[GM]

Yeah, lots of EQ plus zobel required to make this scenario work, then when it starts pumping at high excursion down low all the $$/effort will probably sound like a big waste. There's a reason it's marketed as a subwoofer.

GM

[planet10]

Originally posted by Brett
Try active with the DCX - it does not need to be modified as it's OK stock.

The one i had for a while was far from OK... pretty bad IMHO. I could not live with a stock one in my system.

dave

[planet10]

Originally posted by Paulthefirst
As a newcomer to this site I've read some interesting stuff - but not the answer to upgrading old TL enclosures. Any thoughts are welcome!

My comments on the subject:

http://t-linespeakers.org/classics/T...LS80_mods.html

SInce i wrote that i've traveled a long way. Now i would consider FX120eN or FE127eN to replace the B110 and 2 tweeters. Maybe leave the supertweeter.

Does anyone have Thiel-Small data for the SP6171 version of the B139?

A friend gas a set of the racetrack B139 i could measure. (impedance curve attached from a pair i shipped to Taiwan from before i was calculating T/S)

The B139 has a nasty resonance at 1k and i'd recommend XO below 300 Hz....

dave

[Paulthefirst]

Yeow, some interesting responses there!

What's wrong with the XXLS up to 400Hz? Spec shows flat response to 600 where there's a dip - but a sharp roll-off would work, wouldn't it? Or is there something I should know? You're raising doubts!!

I tried modelling many drivers with Martin King's Mathcad program, with only two in the IMF coming close to the B139 - the other was the Scanspeak 26W8867. Both that and the XXLS 830843 were actually very similar - but the Scanspeak is approaching twice the price.

As for the DCX, I shall have to see. From what I've heard it seems to be a quality control issue, with some units good, others not. Once set up I suppose I have the option of designing an analogue XO to do the same thing, or modding the DCX.