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Stax F-81: Make a great speaker better?

Just bought a pair (second time). They sound glorious with Manley 200's. First time around amplification was not up to par(Moscode600).Looking for intelligent opinions on ways to make this incredible speaker better. Power cords(so far MIT Z-cords have made a significant if not overwhelming improvement, speaker cable(Siltech, power conditioners(they do sound a bit more refined plugged into a Power Wedge). I am most curious about a mod that I read about and can't remember where: the removal of the RC network in the power supply. Thank you all ahead of time.
frogman

Showing 3 responses by kr489ec

kr489ec
12 posts
 
Don't do it. The only mod I found effective was a compensation filter for the mid-bass suck-out. The filter makes the F81 quite flat but power handling is compromised. Use the filter plus a sub to solve that.
kr489ec
12 posts
 
The problem with using any sub is the interface. Since the F81 has a significant amplitude suckout in the mid-bass and peak in impedance at 200Hz, one must compensate for both of these in order that adding a sub does not make things worse, as it usually does with F81s. The circuit is a 6dB/8vo boost from 500Hz down to 100hz. Below that, I ran it flat (w/o sub) or rolled off at 24dB/8vo (with a sub). Kal
kr489ec
12 posts
 
I wish I could find my original notes. I have the formulas but would have to run the calculations all over again. Basically, it takes an opamp (per channel), 3 resistors and a capacitor to set-up the bandwidth limited boost to match the amplitude traces provided with the speakers. I set the filter to start the boost at 400Hz and run it for an octave to where the response curve begins to recover. A steep roll-off at 100Hz is needed for use with a sub or a slower one (just a series cap) if there's no sub. E-mail me for a diagram.
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