After seeing the speakers, my first advice was to simply buy new speakers. The cabinets were decent enough for cheap cabinets, with about 6.8 litres each of available internal volume, but the cost of replacing the drivers was likely to be higher than the cost of a new set of the same speakers (CAN $169 according to their website). However, I was looking for another project to try out and this one seemed fairly simple to do (building the cabinets is always the most bothersome task in any of my projects, and I wouldn't have to do that in this case), so I decided to go ahead after he pushed me for a while.
From the image above, it's clear that the tweeter is one of those cheap Audax 3/4" models (or a copy), which made sourcing a replacement quite simple (just needed to find one with a 74 mm flange). I chose to go with the Audax TW01II.
Finding a replacement bass driver proved to be a bit more difficult. The bass driver's flange is 146 mm, but the cutout is 112 mm, and it appears that the only drivers on the market (well, the only ones that I could find!) were a couple of Seas drivers ranging in price from US$40 to US$130. I eventually chose the Seas CA15RLY (the cheapest of the lot) - spending a lot on drivers for this quality of cabinet is probably not a fiscally sound idea :-).
The only disadvantage with the proposed replacement drivers is that they will not be shielded like the originals, but my friend said that this isn't an issue, as they're not going to be used next to his TV.
The original crossover was quite simple and obviously wouldn't suffice, so I asked Madisound to design one for me. That boosts the cost by US$25, and of course there's the cost of the crossover components to consider.
This is the crossover proposed by Madisound:
So, the final tally was as follows:
A brief interruption..
My first impressions are the results are pretty good but a bit bass-shy, and
further tweaking may not be required (but I'll probably end up tweaking it anyway).
The finished speakers have a "live" sound, and sound best when mounted right up
against the wall (incidentally, I'd asked Madisound to design the crossover with this in
mind, but I don't know if they took this into consideration). Bass is lighter than
what I'm getting from my Mission 751 or Mordaunt Short MS3.20 speakers, but then again the boxes
are smaller too and I'm running them sealed. Overall I'm quite happy with the
results so far.
All things considered, I'm quite happy with the results so far. I expect that the bass response of the rebuilt speakers may improve with some further tweaking, but that really doesn't matter much in the long run, as I expect they will be used in conjunction with a subwoofer. BTW - notice the funky steel screws used in the rebuilt speakers? Well, there are no black screws available locally, so... In any case, I think I like how they look :-).
T/S parameter analysis
The parameters suggest that putting one of these drivers into a sealed 6.8 litre box would result in a somewhat bass-shy performance, which is exactly what I experienced. The original rear-mounted vent (1.25" diameter, 2.5" long) tunes the box too high for these drivers, so I may have to do something about it. I decided to try experimenting with a couple of options.
The first thing I tried with stuffing the box. The following is an impedance plot of the sealed box, with 100% stuffing:
Fb appears to be 71 Hz, instead of the 78 Hz expected from using this driver in this size sealed enclosure. This implies that the effective volume was increased fom 6.8 litres to 9.4 litres, an increase of approximately 38%.
The following is an impedance plot with the vent unsealed:
The impedance minimum appears to be at 56 Hz. The midpoint between the two peaks is 50 Hz. This is quite a close match for what I would expect to see if the box was 9.4 litres tuned with the same size vent. The actual frequency response doesn't match though - it approximates a 9.4 litre box tuned to 62 Hz with low Ql (2.5). The response is increased by about 3dB at 80 Hz though, falling off to no increase at 50 Hz..