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.
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I never found a sub that could keep up with this speaker, I tried, Muse 18, Janis, and other less notable. The best advise I got was from the seller who said dont try to add bass as you will ruin the qualities of this fine speaker. I ran it with OTL's Counterpoint SA4's and the sound was glorious. I regret to this day I sold them.
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
Kal, thanks again. You obviously have good ears and know what you are talking about. That is exactly what I hear. Could you describe this circuit from the standpoint of constructing it? Component values, schematic etc. I am handy with a soldering iron and maybe a bit too daring when it comes to playing with the guts of a component. I follow directions well if told what to connect to what. Any experiene with "augmenting" subs. No interface. REL? Thanks again for your time. Lino.
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.