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Making the subwoofers I inherited useful

Among the stuff we removed from our late son's apartment were two subwoofers without the rest of the systems they belong to. I could have bought complete systems for less than the original parts to go with these but they were too good to throw out...

Project Log

Your brain determines the direction that a sound is coming from by comparing the how far out of phase the waves reaching the left and right ears are. Since the wavelength depends on frequency this means that the lower the frequency the harder it is for the brain to determine the direction and for a human with an average width head you can't tell where anything lower than about 100 Hz is coming from.
"Wait a minute" you are saying "when I listen to my stereo I can clearly tell where the kick drum is and it's frequency is lower than that!"
You're right but that drum (or bass or any other low sound) doesn't produce a pure sine wave and it is the harmonics (multiples of the fundamental frequency that combine with the fundamental to cause the non sinusoidal wave form) in the sound that your brain determines the direction from.

Back in the 1970s a fellow named Floyd Toole (head of the physics department at the National Research Council's lab in Ottawa) was thinking about that and came up with the concept that if you played the frequencies below 100 Hz through one speaker (which could be anywhere in the room) and everything above 100 through smaller speakers your brain wouldn't notice that the fundamentals weren't coming from the same place as the harmonics. He called this a "subwoofer" and his research was the basis of the field of psychoacoustics.
Many years ago while I was working as a speaker designer I had the opportunity to spend a few hours with Floyd and shortly after I designed a subwoofer system (unfortunately it never made it to production).

In the ensuing years subwoofers have become fairly common, especially in car audio, home theatre sound systems and computer speakers.

Our late son was a bit of a hoarder when it came to electronics, often hauling home things he thought someone might need one day (and usually giving it away when he found someone who could use it). When we cleaned out his apartment (a few years ago) there were 3 subwoofers, a Coby system (complete & with original packaging), an Altec Lansing VS4121 and a little gray one marked only "CA" (neither of which had the satellite speakers).

Because the Altec system's controls are in one of the satellite speakers I couldn't even test it without them; Maybe if I shopped long enough I might have found a set of the satellites but that would hardly be worth it when I can buy a complete system for about $50.

All I could find out about the little gray one is that CA means Cyber Acoustics; I couldn't find anything that looks exactly like this one online so I had no other information. It looked straightforward with stereo 3.5mm jacks for input and output and a clearly marked power jack but when I hooked it up it didn't work.
But the cabinets and the actual woofers were in good shape so I felt they were too potentially useful to throw out too....

Part 1 (Altec)
A while later I came across subwoofer amplifiers on eBay for quite reasonable prices so I ordered one to try and decided to install it in the Altec
Note that whoever prepared the drawing marked both controls "Bass volume". The left one is bass and the right is volume.


Here's what the Altec looked like when new


A couple of months ago I took it apart and stripped out the original electronics. I wanted the controls to be on the front of the cabinet, which is a plastic casting so that was pretty straightforward (the best location was right where the silk screened logo was so that had to go first). That meant that the power jack and the inputs would have to be extended to the panel on the rear that the original electronics was mounted on, as well as speaker terminals for the wires to the satellite speakers; Fortunately the pair of speaker terminals I could get to without getting too frozen (the speaker parts are stored in the garage attic) were just the right size to fit while covering all of the original holes. I also added a pair of RCA jacks for the inputs and a panel mount power supply jack.


Once the subwoofer was assembled it was time to deal with the satellite speakers. The ones I chose (also in the stuff from our son) are from one of those home theatre sound systems that have a bunch of small speakers and a subwoofer but one was a front speaker and the other an "ambient" (rear?) so I opened them up to make sure they were the same inside and found that the "ambient" one had a capacitor in series with the speaker (it's gone now). The grill cloth on them was a bit ratty and one was stained so while they were apart I gave them a quick spray with black paint to freshen them up a bit


And from the rear


I didn't take pics of the system installed but the subwoofer is on the floor under the electronics bench and the satellites are sitting inconspicuously on shelves at opposite ends of the room. I've been listening to music from the shop computer through it it for a couple of months and I'm very pleased with the result.

After that worked out so well I tried to order another if the same amp but I couldn't find one for sale anywhere so I ordered a different one for the CA and put it aside while I waited for it to travel halfway around the world.

Part 2 (CA)
The second amp arrived a couple of weeks ago so it was the CA's turn

Here's what it looked like before (the other 3 sides are plain)


The driver & port are on the bottom. I know this driver is designed for low frequencies but I still have a hard time calling a 4" speaker "woofer".


The original electronics. That heat sink is a big, thick piece of aluminum so I salvaged it as well as the power supply jack and the 3.5mm stereo jacks that I re-used for this project


Temporary connection to test the new amp. I wanted to find out what the two red 5mm LEDs were for (overload indicators? flash with the music?). When they turned out to be simple power on indicators and both connected to the same part of the circuit I removed them (before this pic) and replaced one with a blue 5mm LED
This one didn't come with the connectors to match the ones on the board and I only had one that would match so I used that one for the power supply and removed the connectors for the input and speakers from the board so I could solder wires into it more easily.


The original panel was just about a perfect size to mount the amp to so I peeled off the original overlay and cut a new one from some 0.030" plastic sheet. The hole for the old input jack was in a good location for one of the pots so I drilled another in the right location for the other pot plus one for the LED. It didn't really matter what colour LED I used (or even whether it had one - I didn't put one in the first subwoofer) but Matt liked blue so much he bought a bunch of blue LEDs and changed the LEDs in some of his stuff so I thought that would be fitting to use one of those for this.


That meant the jacks would have to go somewhere else. I was about to cut a piece of that yellowish plastic I used for the front of the portable stereo when I remembered that I had the cover from an eBay project box that was about the right size and (bonus!) was the right colour too . You've already seen my method of covering the panel with masking tape for laying out where the holes need to go so I'll just show the finished panel and the hole it covers this time


I used a piece of perf board to mount the two 3.5mm stereo jacks and the power jack that I salvaged from the original board on and came up with a pair of little plastic brackets to attach it to the panel but I forgot to take a pic of it.
Here it is all put together.


And the front.


I bridged the 2 jacks because I was thinking about using it in the garage shop and that would allow me to plug the amp for the speakers on the other side of the wall in (sort of a daisy chain connection) but I'm on the verge of changing my mind about that.
The garage is divided into 3 rooms, the space where I park the sidecar outfit I'm currently driving (I have one for summer and one for winter), the garage shop and the big space (my wife parks her car there in the winter but it is shop space in the warm half of the year). Audio from the computer in the shop is fed to a small stereo amp in the shop connected to a pair of small wedge speakers made for sitting on the rear deck of a car (hung from the ceiling) and another small amp with a pair of Poly-Planar P20 styrofoam speakers in the big space. The Polys aren't bad (good upper bass, mids & treble but not exactly boomy) but the wedges have 3.5" "woofers" so this little subwoofer would probably make them sound pretty good but it wouldn't do much for the other side of the wall.....
And I've been thinking about what Floyd told me about his subwoofer. He lived in a split level house so he mounted the sub in the short wall between the living room and the rec room so it would work with satellite speakers in both rooms.....

So I've made an offer on another subwoofer amp. I don't think I have a woofer suitable an in wall sub but they always have used speakers cheap at the Re Store that I can salvage one from.

And the 5" 2 way speakers in the room where my main computer is are good but they could use a bit more bass and if I move a few things around there should be room for that little gray box.....

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