This project was a quick and down-and-dirty rebuild of a pair of Optimus Pro-77 speakers that I had laying around. I believe there were several versions of this speaker made, as it was a standard Radio Shack product for a good 15 years. The version I have has a ported cabinet, but I know that the older ones used a simple sealed cabinet. Either way, the bass on the old units was pretty one-notey thanks to using a large 5-1/4" woofer in such a tiny enclosure.

The one-piece cast-aluminum construction of the cabinet means that it is very difficult to find a tweeter to fit its odd-shaped opening. The only real option is to use the existing tweeter faceplate in combination with some sort of neodymium tweeter element. After briefly looking at the various neodymium 1" tweeters in the market, I decided to go with the Tangband 1" neodymium tweeter. The low cost of the unit plus the easy removal from the housing were the two main deciding factors.

#264-804 TB 1" Neodymium Tweeter

The woofer is another Tang Band unit. This woofer worked out quite well due to its small diameter, with a relatively large mounting flange. This enabled the use of a 4" driver with the opening that was once for a 5-1/4" unit. The overall enclosure space was much more suitable for a smaller driver, and it definitely gives this woofer more room to breathe. This Tang Band offers great bass response with good midrange clarity. Plus, it is magnetically shielded meaning that the new speaker system can be used right against CRT monitors.

#264-820 TB 4" PPM Woofer

The first step in converting your Optimus 77 is to remove all of the old drivers, terminals, and crossover parts. The old crossover consisted of a single coil and cap on my particular version. These were attached directly to the input terminal with some glue, and can be removed with a knife and some wire nippers. Its only a matter of removing 8 screws on the front of the cabinets to remove the woofer and tweeter. To further prepare the tweeter section, remove the inner screws that hold the tweeter magnet to the faceplate. This will leave you with a faceplate with a large hole in the center.

Remove the Tang Band tweeter element from its housing, by prying back on the snap-lock tabs and pulling the faceplate off. The element can further be removed from the back housing simply by pulling the two apart. Once you have just the tweeter element, it can then be secured to the old tweeter faceplate using some hot glue. To seal up the old holes that held the tweeter together, either fill with hot glue, or add some screw heads and glue in place. The new tweeter is now ready to be re-installed.

To secure the woofer into the hole, a different mounting technique had to be devised. I ended up using some small aluminum clips that I got from my local hardware store. This are intended for use with aluminum screen doors, though they work perfect in this application. Using the existing woofer screw holes, the clips secure the new woofer snugly in place. Note that new longer screws needed to be used.

The main goal in designing this crossover was to keep the parts count to a minimum for both cost and space savings within the cabinet. As such, some rather odd conjugate network values were used to help manipulate the frequency response of the system, without using costly inductors. This does drop the impedance down a bit, resulting in what I would consider a nominal 6 ohm speaker.

The crossover is nothing fancy, just a second order plus conjugate on the woofer, and a second order plus L-pad on the tweeter. The result however, is a somewhat low crossover point of 2,000 Hz. This is normally much lower than recommended for this particular tweeter. However, to get eliminate a large null in the frequency response that was a result of baffle diffraction, this low crossover point was needed. Also, this low point was needed to achieve a good phase relationship between the woofer and tweeter. With a higher crossover point, I was simply not happy with the sound.

The low crossover point does cause some slight distortion in the 2K to 3K region on some passages, but it is minimal and less of a sin than the other potential downsides. Again, this is an example of compromise in speaker design, and was a conscious decision. Since this speaker does only utilize a small 4" woofer, the maximum output levels will never be extremely high, working in favor of the tweeter.

Here is the overall frequency response of the system and the tweeter-reversed measurement. The axis was tipped several degrees below horizontal, but not too far off. You can see the effective XO point is just over 2kHz.

On and 30 degree horizontal off-axis response.

The impedance profile, notice that the minimum is about 5 ohms.

The reconstruction of the speakers gained several things in terms of performance. The first improvement is in the bass area. The new woofer offers more defined, clear, and accurate bass. It may not have the quantity of bass as the stock speaker, but it is definitely more refined and less muddy. The second is the midrange clarity. The Tang Band 4" PPM woofer definitely has a much cleaner and clearer midrange. The overall tonal balance of the speaker is a little better due to the more effective baffle step compensation.

Despite being a 4" driver, the Tang Band W4-656S offers good bass performance in a small enclosure.

This design was submitted by Darren Kuzma, he can be tracked down on the Tech Talk board with any questions.