I thought it would be pertinent to chime in on this thread given the interest!!
Danny does give praise to the overall performance of the speaker, so my design can't be all bad......
It would seem that my designs are the target of aftermarket upgrades, so that’s either praise of the basic design being worthy of spending time upgrading, or an indictment of the design that it needs upgrading
I'll give you some of my thoughts on the mods that Danny proposes and also some warnings where necessary....
Firstly, let me say that I am required to point out that any modifications that you make will obviously void the warranty, so be careful!
Now this is not the first time that one of my designs has been the subject of a long thread of how to modify it to get better results. I will say now what I have said before, and I know that Danny doesn't deny this: The reason why it is worth even thinking about modifying it is that it begins as a tremendous value product for the money.
When I designed this product, I had to meet a certain price target. It's nice to think, as an audiophile, that price points are somewhat negotiable: $199, $209, $219, $229...they are all close enough aren't they? The actual answer is no. Price point is critical in retail sales. Miss by a little and sales can go to virtually zero as the large box retailer totally rejects the product when it is the wrong price point. This becomes a major challenge in the design process. Every part that adds cost to the bill of materials has to be very carefully evaluated as to its impact on the performance.
Let me go through some of these mods and look at the cost/performance issues and discuss why I chose the path that I did.
I note that Danny chose to lift the extreme top end of the response by bypassing the series resistor on the tweeter. That's nice, if that's the response that you want as opposed to the sound that has received such praise, but it adds cost. That option would have had to be paid for by taking it out of something else. Likewise the substitution of the sand cast resistor with a Mills resistor would have taken the cost way up, as would substituting the film cap with a Sonicap. I doubt that paying for these alternative caps and resistors by taking away from the overall design would be a good trade-off in the original. (As an aside, the added cap across the resistor does not work because of a supposed inductive component of the resistor. It simple reduces attenuation of the tweeter above a certain frequency).
Danny comments on the quality of the cabinet MDF and the thickness of the baffle and wrap. The cabinet is the major cost in a speaker. It would be nice, wouldn't it, to have thicker cabinet walls, internal bracing, more rigid MDF, more internal damping etc. However, all of this would greatly increase the cost, and for what? Let’s think about cabinet vibration. The major excitation source if the reaction force imparted to the magnet and hence driver frame that feeds mechanical energy into the cabinet causing it to vibrate. Now, measures to thicken up the walls, or use a higher grade of MDF, absolutely do not remove the resonances. They move them up in frequency. In fact, they can become more audible!
The grade of MDF I chose is softer and so has better internal damping, and can actually sound better than the "better" grades. In fact my early prototype cabinets were better built, with internal bracing and thicker walls. When I looked at the cost I realized that this cost would be better spent on improved cone materials, larger voice coils, larger magnets, better-than-standard xover complexity and parts etc.
Regarding the internal damping material, the cabinet is not stuffed full. This is deliberate. Besides costing more for a higher grade and more of a better damping material, there is a fallacy involved in the effect of damping material. In theory, a heavily stuffed cabinet changes from adiabatic expansion to isothermal and as such appears to be up to 1.4 times larger. This effective increase in cabinet size is offset by the greater resistive losses and so the performance gain is not that great. In the practical quantities that we are likely to see used, the effective increase is negligible. However, in a vented box system, putting in too much damping material will severely reduce the output of the vent and greatly reduce bass output. The choice I made of damping material was optimized between damping to some degree the standing waves while not impacting bass performance.
Danny shows a measurement of the impedance of the speaker that shows some apparent resonances around 25Hz and 60Hz. He attributes these to internal standing waves. This is erroneous. The cabinet cannot support standing waves at these low frequencies. If they exist they are more likely to be mechanically coupled. However these blips in the impedance curve do not show up in my measurements. I suggest that they are due to something in the measurement procedure. I know the repeat measurement after the modifications shows they are gone, but I wouldn’t attribute this to the added stuffing reducing internal standing waves. I would however encourage Danny to re-investigate these measurements.
On the subject of the trim rings, removing them will change the response measured on certain axes, but will have little effect on the frontal average response, since the effects changing at different measurement axes because they are due to diffraction, You may notice subtle changes depending on your room acoustics, and very large changes depending upon your eyesight...LOL. I designed the speaker to have the trim to improve the looks by covering the chassis and screws. A worthwhile trade I believe towards getting a successful looking visual design that will appeal even with the grille off.
Talking of eyesight, no wonder Danny is happy with the trims removed, as he could not see that the inductor he discusses is not actually connected at both ends to the same trace. The break in the trace is hidden under a component, but a simple test with a component bridge or multi-meter would have confirmed this is.
So, based on the first modification offered, you will have a speaker that looks uglier and sounds different. Your choice.
However, if you go ahead be very careful. Danny does warn about the difficulty of removing the trim. Using a flat bladed screwdriver to prize off the trim very close to the driver surround requires careful dexterity. One slip and you will tear the surround. How do I know I hear you ask? Perhaps because I’ve done it myself? At least I have lots of spare drivers at my disposal.
Also, removing components from the xover circuit board is not easy. They are all epoxied to the board. Don't expect to remove them and restore it back to original if you decide you need to.
Next, the level 2 mods. Be careful with this regarding the change in xover frequency. The change to the tweeter puts more power into it at the lower frequencies. Potentially as much as double the power. I am sure that Danny hasn't done long term power handling tests on multiple samples to make sure that it can still meet its specifications for max power. I know I have for the original design, but not for the modified version.
Additionally, during manufacture the tweeter is measured and tested to performance limits within its expected working frequency range. The spread in response away from the reference is holding within +/-0.75dB during production over the range from 3kHz to 10kHz, which is quite impressive for a budget speaker. Using it down to a lower frequency will take it into a range where I don't guarantee such tight tolerance, so your expected results could vary. I normally design around drivers that are selected to match the mean of production, not around a single sample.
Also, changing all the inductors to Erse 16 gauge air core will likely change the effective resistance of the inductors. I accounted for inductor resistance in my design, and in fact take this into account for the bass tuning. So changes in this will also alter the bass respone.
In conclusion, for the cost of the level 2 upgrade, you will get a different sound, hopefully one you like, because there is no going back if you make the changes, and remember, no warranty.
For that much extra money on top of the purchase price, you could get a pair of the F5 speakers, or wait for my next series of speakers....
I don’t want to sound like a Debbie Downer over this. Ours is a great hobby. I started as a hobbyist. It’s fun to experiment. It’s even more fun to feel accomplishment from your experiments. Just be careful. Be sure that these mods are going to give you the sound you want before you commit to making changes.
I’ll finish with a great quote. This is taken from one of the best textbooks on Acoustics, written by Leo L. Beranek. “It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else’s loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in performing a person’s opinion.”
Above all, Have fun!