For a long while, my main set of speakers have been a pair of Mission
751s that I purchased in England in 1993 while I was there on a business
trip. Unfortunately, while the boxes are in near perfect condition, the
time and tropical climate have not been kind to the drivers. The
tweeters were the first to go, with the coating disappearing from the
domes and the domes themselves developing kinks that resulted in
increased distortion at higher frequencies. Then the rubber
surrounds for the bass drivers started to go (which was a bit unusual
So, a project that was geared toward just replacing the tweeters turned
out to be a full driver and cross-over replacement, with the only thing
being kept being the cabinets. So officially these are no longer Mission 751s,
but a new speaker design using the Mission 751 cabinets.
It turns out that the choice of drivers available to me was extremely
limited, if I didn't want to make any visible modifications to the
Mission 751 cabinets. The only bass driver that I could find that
both physically fit and had t/s parameters that suggested that they
would work decently well in the Mission 751 cabinets were the Dayton
Audio DC130A-8 drivers. Luckily these drivers are also pretty
cheap (available for about US$20 each when I purchased them). As
for the tweeters, the best match for the Mission 751's OEM tweeter
faceplate that I could find turned out to be the Peerless OC25SC65-04 1"
dome tweeter, available from Parts Express for about US$8 each.
This means that the total driver replacement cost turned out to be
US$56, not too bad for a pair of speakers that cost me around US$500
when purchased new. Are there better drivers available?
Sure, but they would have required visible modification of the cabinets,
which I wanted to avoid.
Mission 751's OEM driver was pretty simple to do - just remove the bolts
holding the metal bracket over the OEM driver, replace the OEM driver
with the new bass driver and then replace the metal bracket and bolts. I
was planning to remove the paper bracket around the surround of the the
DC130A-8 drivers to allow the bracket to mount a bit more flush to the
front baffle, but decided not to in the end, and I'm glad that I chose
not to, as I think the change would have very little difference. The
tweeter on the other hand took a little more work to install. I
removed the OEM tweeter faceplate from the tweeter, remove the metal
protective mesh and basically hot-glued the new tweeter into place (it's
a small neo-magnet based tweeter, so the hotglue would be more than
enough to hold it in place). I applied hotglue liberally along the
sides if the tweeter as well to ensure that there would be no air leaks
around the tweeter when the OEM tweeter faceplate was reinstalled.
I took the same approach designing a new x-over for the chosen drivers
as I've done in the past - I measured both the impedance and frequency
response curves for the drivers, measured the z-axis offset for the
tweeters, and then used XSim to come up with a suitable x-over for them.
Originally I planned to try and match the original response of the
Mission 751s as closely as possible, but subsequently decided to tweak
the response a little bit to remove a little of the "sizzle" at the top
end of the response curve. Illustrated below is the current
version of the x-over that I'm using at the moment. The parts cost
worked out to about US$50, bringing the cost of the whole exercise to
What's that 0.12mH inductor doing in the bypass
leg of the woofer circuit? Well, that's there to add a "notch" filter
that trims the DC130A's response at higher frequencies, making it a
better match for the Peerless tweeter through and above the x-over
Another note: while the x-over diagram shows a common ground, the
Mission 751 is configured to support biamping, so in the actual build I
kept the ground path for the tweeter separate from the ground path for
I think that the results
I've achieved seem to
be quite successful. And the rebuilt speakers look like they came from
the factory that way.
The grille for these speakers really does a number on the frequency
response (see chart below). When I have a chance, I'm going to look into
seeing of the grilles can be modified (possibly by adding a felt panel)
to reduce or eliminate their impact on the response curve.
After I started rebuilding these speakers, my attention was drawn to
another tweeter that just might fit - the Dayton Audio ND25FN-4. Those look like a copy of the
Peerless tweeter that I used, except that they also include a heatsink.I think if I was going to do another
rebuild, I'd probably try those instead, to see if they fit.