58.175.51.30
I am in the process of helping an inmate rebuild the passive XOs on his 3.3s, by providing my recommended circuit schematic (as the stock XO delivers a rather woeful overall (electrical!!) frequency response :-)) ).I started off using lspCAD to model the ribbon highpass filters of nearly all the true-ribbon Maggies - to whit, the IIIa/3.3/3.5/3.6/20/20.1 and T-IVa.
These schematics are all listed under "Tweaks" and looking at them, I am fascinated by the fact that every bloody model has a different -12dB HP roll-off ... yet they all have the same 2 ohm ribbon - except the 3.6 which has a thinner ribbon (which is 3 ohms). This is taken down much lower than all the other ribbons - I guess because the thinner ribbon is more robust and can take the increased current flowing at the lower frequencies?
I would've thought that Magnepan would've learn't during their product lifecycle that the ribbon was strong enough to take lower frequencies (without burning out) so, once they developed the 3.5 ribbon HP filter, they would've used this on the the 20. :-)) But no ... the -3dB point for the 3.5 is 3,900Hz while the -3dB point for the 20 is 7,500Hz!!
Anyway, for those of you who are interested, the -3dB figures are as follows:
* IIIa: 6,200Hz
* 3.3: 4,500Hz
* 3.5: 3,900Hz
* 3.6: 2,300Hz (thinner ribbon)
* 20: 7,500Hz
* 20.1: 7,400Hz
* T-IVa: 5,600Hz.IMO, the optimal setting for the ribbon in a IIIa/3.3/3.5 should be set around the 3.5 point of 3,900Hz. A bit lower is not a problem - I run mine at 3,400Hz - but certainly not as low as the (thinner, 3ohm ribbon of the )3.6 at 2,300Hz.
Regards,
Andy
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