If you bear with this post you’ll eventually find it relates (loosely) to the JR149.
I finally dug my IMF MCR2A mini monitors out from the back of my wardrobe to measure; they haven’t seen daylight for about a decade! I’ve mentioned the MCR2A on this thread before, it’s one of the last models IMF ever produced and is their smallest, measuring 382mm x 198mm x 184mm (HxWxD), with an internal volume of 8.4L. It’s supposed to use a 5/36 mid-bass driver (IMF’s answer to the B110) and a TW2100F tweeter, but my pair are one of a handful that were fitted with KEF B110 and T27 units. This was done around the time IMF were being wound up, circa 1986. (It took me a few years to piece together the full story behind this and I don’t really want to go into the somewhat unsavoury details here).
I bought them in the late 2000’s and they arrived in NOS condition, complete with original box and packaging. They even still had their original, laser-cut brown foam grilles that still looked brand new (I’ve stored these in a dry, dark cupboard for the past 12 years in an attempt to preserve them!).
They have a similar aesthetic appearance to the KEF Reference Model 101. The 5/36 and B110 share the same PCD so the B110 fits into the MCR2A’s rebated baffle cutout perfectly. The TW2100F was flush-mounted and screwed in from the rear (like the HF2000 in the RSPM), but the T27 is much larger than the TW2100F so the countersunk pocket was drilled out and the opening enlarged to accommodate the T27’s magnet. No rebate was routed, this means the T27’s face plate sits proud of the baffle. (I also have a pair of unadulterated MCR2A enclosures in my cupboard that I bought a couple of years later which are fitted with the correct MCR2A crossovers and 5/36 drivers but no tweeters. I've been looking for a pair of TW2100F's to install in them for 10 years without success; they are evidently rarer than rocking horse sh!t...)
When I originally received the T27/B110-equipped MCR2A's I removed the drive units to have a nosey inside at the crossover. I had great difficulty finding it as the enclosure is crammed full of long hair wool and I would’ve needed to removed a lot of it to create some space, and I decided against doing this at the time in case I disrupted anything. All I managed to eyeball was a small blue ALCAP, nothing resembling the proper MCR2A crossover. That’s not to say there weren’t more components inside, just that I did not see any more at that time. I could remove the drivers again and do a deeper dive, this time removing all the wool, but I’d rather not disturb them again as the solder tags on the B110 have a habit of breaking off with age. Besides, I was hoping you might be able to tell me something about the crossover from my measurements!
My original impressions upon first auditioning these T27/B110-equipped MCR2A were that they were very dynamic and revealing but rather forward-sounding and bass-shy compared to my JR149. I also noticed a ‘lispy’ quality to the treble. I don’t recall what made me try it but I fitted a pair of LS3/5A tweeter grilles onto the T27 domes and this had a marked effect on the treble, removing the ‘lispy’ character and adding more incision, detail and air higher up the spectrum (note this was all subjective impression, I had no measuring kit in 2008!).
I’ve been meaning to frequency sweep these speakers for years but never got round to it, until yesterday. I set each enclosure on a 60cm high stand and placed it hard against the wall to give it the best chance possible, and then proceeded to take measurements from 1 metre distance and at three heights: T27 axis, B110 axis, and the midpoint between the two. I set the volume for 80dB SPL at 1kHz.
IMF MCR2A measurements:
IMF MCR2A commentary:
- Pair-matching looks very good to me for aged KEF drivers that presumably were not hand-selected to go into these enclosures.
- LF starts to shelf down from 400Hz and ends up around -7dB down in level from MF and HF.
- The most linear HF response is obtained when on-axis with the T27. As you move toward B110 axis, a 5kHz cancellation emerges. What does this reveal about the crossover design?
- The distortion plot looks fairly innocuous, though there is a slight in increase in 3rd order distortion around 1.5kHz. This may simply be due to running the tests at 80dB instead of my usual 70dB-75dB or it could perhaps reveal a low-order crossover or low crossover frequency is being used?
For a comparison I also repeated the tests on my JR149 mk1 and JR149 mk2. As luck would have it, the driver height and spacing in the JR149 mk1 and mk2 is exactly the same as the MCR2A. Note I did NOT change the volume on the amp for the JR149 mk1 and mk2 tests so any difference in sensitivity you see vs the MCR2A is due to the JR149 crossover.
JR149 mk1 measurements:
JR149 mk1 vs MCR2A commentary:
- The LF response of the JR149 is very similar to that of the MCR2A (similar dB output level), but the JR149 crossover’s baffle-step correction is attenuating the MF and HF by almost -5dB to achieve a more balanced frequency response.
- The JR149 MF/HF is far less affected in the vertical axis than the MCR2A, there is no cancellation at 5kHz.
- The JR149 has a less-forward response than the MCR2A between 800Hz and 3kHz, and also has less energy between 5kHz and 12kHz (though the 5kHz-12kHz difference is greater when the JR149 is measured on T27 axis and lesser when the JR149 is measured on B110 axis).
- The JR149 does not exhibit the 3rd order harmonic distortion at 1.5kHz that the MCR2A does. This may simply be due to the lower SPL, or it might indicate that the JR149 is crossing over to the T27 at a higher frequency or with a steeper rolloff than the MCR2A crossover?
JR149 mk2 measurements:
JR149 mk2 vs MCR2A commentary:
- The LF response of the JR149 mk2 is very similar to that of the MCR2A (similar dB output level), but the JR149 mk2 crossover’s baffle-step correction is attenuating the MF and HF by around -3.5dB to achieve a more balanced frequency response.
- The JR149 mk2 MF/HF is far less affected in the vertical axis than the MCR2A, there is no cancellation at 5kHz.
- The JR149 mk2 has a more forward response above 1.5kHz that's quite similar to the MCR2A but does not exhibit the resonance at 1.1kHz.
- The JR149 mk2 does not exhibit the 3rd order harmonic distortion at 1.5kHz that the MCR2A does. This may simply be due to the lower SPL, or it might indicate that the JR149 is crossing over to the tweeter at a higher frequency or with a steeper rolloff than the MCR2A crossover?
Further thoughts:
I’m going to try listening to the MCR2A with a few dBs of bass boost and see how they compare with my JR149s.
I’d also like to repeat the measurements with the metal grilles removed from the T27 domes, but I’m not sure if I’ll be able to remove them safely (I stuck them on with blu tac in 2008 and haven’t touched them since!).
So what do I have here, folks, something that's better or worse than a mk1 Kan? You have to admit they do look rather beautiful in their American Walnut veneer!...
PS - If you feel this strays too much off-topic Tony please move it to another thread, I was reluctant to create a specific MCR2A thread for this as they're not MCR2A's as originally designed.