2 Way Speaker Project

 

 

 
Sometime in high school I developed a fetish for designing and building audio speaker systems. My very first effort was a 3’ high, 2’ wide, 1’ deep plywood box with one 10” driver bought from a second hand store. I cut a randomly sized bass reflex port in the front panel and covered the sides with fake naugahyde. It wasn’t very high fidelity and was really ugly but it sure could pump out the bass frequencies.
 

Since then I’ve been tinkering with new and used speakers whenever I had the time. For awhile back in the 70s, I was SpeakerLabs most frequent customer that never bought anything. I was too poor.
 

Sometime in the early 90’s, while living in Boston, I bought a pair of Cambridge Soundworks Model 6 speakers from their factory store. They were traditional 2-way bookshelf speakers with an 8” woofer and a 2” tweeter. Nothing exotic but they were sufficient for the studio apartment we were living in at the time.
 

Over the years these speakers were moved around a lot and occasionally abused with too much power. Recently I pulled the grill cloth off and found that the woofers were seriously deteriorated. There are a lot of 8” woofers on the market. Just replacing these would be quick and easy, but that wouldn’t be any fun.
 

 

So with time on my hands at the moment, I indulged my latent speaker builder obsession, shopped online for new components and gutted the old speaker cabinets.

 

 

 

 

 

A speaker’s crossover network is a simple circuit of inductors and capacitors that filters the incoming audio signal into two separate circuits, one each for the woofer and tweeter so that neither driver will be exposed to audio frequencies outside of their intended range.

The original crossover network was shockingly inferior (read “low cost”) with iron core inductors and light weight wire. Higher grade crossovers use at least 18 gauge magnet wire wrapped around an air core. About 75% of the power from the amplifier is channelled through this inductor to the woofer.  It needs to cleanly filter the signal without any extraneous impedance or resonance.

 

 

 

 

The old crossover was permanently attached to the speaker wire binding posts on the back panel. I removed the whole assembly and built a replacement panel to support new binding posts and an L-Pad to adjust the tweeter volume.

 

 

 

 

 

 

 

 

 

 

 

 

I then mounted the new crossover on the inside back wall of the cabinet using screws and silicon seal to avoid vibrations.  It was difficult to reach inside of the cabinet via the woofer opening and drive the screws in straight.

 

 

 

 

 

 

 

 

Recreating the wiring harness was a tedious job of cutting wires to the correct length, stripping and hand crimping terminals on each end.   I labeled the ends of each wire with a two letter code to identify its intended location and polarity.

 

 

 

 

The new woofers were selected based on a review submitted by another hobbyist that had successfully used them as replacements in Cambridge Soundworks Model 6’s, exactly this same speaker system.

 

The tweeters were selected for their broad range.  They are called “phenolic ring” drivers and are used in several commercial products including Marantz.  They were the go-to tweeter in Speaker Lab systems that were not horn loaded.

 

 

The new crossover is a generic brand but made of high grade components. I chose a crossover frequency of 3000hz for the high end though I had many options from which to choose. Thinking about it later, I might have done better with a 2500hz crossover frequency given the wide range of these new tweeters and the fact that I can control their presence individually with the LPad volume control.

 

 

 

Back in the shop it was time to assemble all of the parts. The new woofer and tweeter were glued into place with silicon seal and screws to keep the cabinet air tight. This is an air-suspension design that depends on an “air spring” in the cabinet to control the woofer cone movement. Not shown here is the poly-fill batting stuffed into the cabinet at the last moment to dampen standing waves.

 

 

When it was all done and dry I carried the rebuilt speakers up to the loft overlooking the living room. These would serve as the rear two speakers of a six speaker surround sound system. The results are not bad but maybe a bit lacking in the midrange. This is partly due to the chosen crossover frequency and partly the acoustics of the room and the intrinsic inefficiency of an air suspension design.

I dedicated a stereo power amplifier just for these two speakers and can overwhelm everything else without taxing the system.

I’m still shopping for a pair of good used Speakerlab Super Sevens with which I will gladly replace these. Until then, these will have to do.

 

 

 

The old woofers were put to good use. I nailed them to the wall of my shop and use their magnets to hold tools.