Reasons for the swap: the existing D2008/851200 are 35 years old and are a ferro fluid variant. Although there’s no increase in distortion indicating a big deterioration or a loss of fluidity in the ferro fluid, they are probably no longer performing at their best. On the other hand, the bass/mids of the mk1 SP2 are known to have very good longevity so it’s probably worth doing a replacement.
The question was to replace the original 851200 variant with the same or use the newer 852100 variant…
My understanding of the design optimisations for the original 851100 variant and the ferro fluid 851200 variant is that the 100 was more optimised for dynamics and the 200 for a smooth response with better control of sibilance; so my assumption was that the newer 852100 would be more like the 851100.
The crossover in the SP2s is very simple 2nd/3rd order circuit, so provided the impedance curve, electro acoustic parameters and acoustic interface are similar there should be no fundamental problem and the driver and crossover integration should be OK. Checking the data sheets suggest that the D2008/852100 variant shouldn’t present any real problems from these perspectives (although the cabinet changes would need some thought).
Thinking about what we wanted, and the different characteristics of the 851200 and 851100, we realised that we would now prefer the slightly brighter, slightly more dynamic presentation of the 851100, so we decided to try the new 852100 units.
Looking at things, we could make the cabinet changes such that either the newer tweeters or the older units could be used.
To do the cabinet modifications we designed and 3d printed a marking jig to cut the ‘ear’ slots for the new contact position on the sides, and a jig holding a pair of ball races to accurately guide the drill bit to position M4 threaded inserts in the cabinet. The same jig used to mark out the ear slots also held the M4 inserts in place while the glue set. These worked really well and the cabinet modification turned out to be a lot easier than expected!
So the result…
Well the dynamic gains and slightly brighter presentation are there immediately, but at the same time it gives less sibilance artefacts and more HF consistency than even the 851200 whilst maintaining the fundamental audio characteristics of the D2008 tweeters. Unexpectedly it also seems to give significantly higher resolution: after only 10 hours we are already hearing delicate treble details that we’ve never heard before.
So if you need to replace your tweeters and you’re no longer able to get the old D2008/851100, the new version does seem to not only be a suitable replacement, but also even a bit of an improvement over the original models To our ears, it appears to give the best characteristics of each unit rolled into one, and with some other gains added in.
If anyone need the designs for the jigs for the cabinet changes, we can certainly provide them.
Note that we can’t give any comment about the 851300 as to the best of our knowledge, we’ve never heard a speaker using them.