58.175.51.30
OK, thanks to information from several inmates, I can now present a slightly different table of true-ribbon roll-off points - ie. the "@ frequencies" ... as in "12dB ribbon highpass @ XXXHz".
This is much more consistant than my previous table ... so, my apologies for my previous wrong assumptions as to which Maggies had what ribbon resistances. And a small apology to Magnepan for implying that they had no idea what they were doing!! :-)) (Although, they haven't been totally consistent, IMO!)
From modelling the stock 12dB ribbon HP filters (shown under "Tweaks") in lspCAD, I present the following table for the -3dB points (ie. the "@ frequencies"):
* IIIa: 2ohm ribbon - 6,200Hz (NB: this is the later IIIa)
* T-IVa: 2ohm ribbon - 5,600Hz
* 3.3: 3ohm ribbon - 2,600Hz
* 3.5: 3ohm ribbon - 2,300Hz
* 3.6: 3ohm ribbon - 2,300Hz
* 20: 3ohm ribbon - 4,700Hz
* 20.1: 3ohm ribbon - 4,500Hz
* and my own active IIIas: 2ohm ribbon - 3,400Hz.
IMO, the true-ribbon is a magical driver ... the more frequency range that you can cover with this, the better! :-)) I suggest Magnepan believes this also, so they changed back to the thinner, 3ohm ribbon with the 3.3 - and subsequent models - so they could bring it down lower. (I presume the thinner, 3ohm ribbon is more robust?)
Then they experimented a bit more and brought it down to an even lower frequency with the 3.5 & 3.6.
So I suggest to any 3.3/20/20.1 owners who are contemplating rebuilding their passive XOs that they should use the 3.5s'/3.6s' 2,300Hz -3dB frequency point for their ribbon HP filter. Of course, this means also reducing the mid LP roll-off frequency, to compensate.
But why didn't Magnepan use this same frequency for the 20s and 20.1s? Why start rolling the ribbon off at 4,700Hz/4,500Hz for these premium speakers?
WRT the (similar vintage) IIIa/T-IVa, Magnepan brought the wider, 2ohm ribbon down a bit lower with the T-IVa. However, as I didn't know that the 3.3 and the 3.5 had a different ribbon, I chose a -3dB point of 3,400Hz for my current 3-way active IIIa XOs as I wanted a 2,600Hz "crossover frequency" (-6dB point) between mid & ribbon. I've been running this way for at least 8 years so I'd have to conclude that the ribbon can cope with this lower roll-off point! :-)) And I - nor friends who come round to listen - certainly can't hear any negatives.
Regards,
Andy
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