Subwoofer project

Latest update: Sunday, 12-Jan-2003 02:59:30 CET

The history

I have always been interested in building things and one outlet for that interest has been that of building loudspeakers. Previous efforts have resulted in two quite different pair of speakers.

The first came in a complete kit, apart from the box, and resulted in two huge 100 litre 3-way speakers with 12" woofers that turned out to be well suited for disco music. (Too bad I mostly listen to rock/metal :-/) I also made a few errors when building the box, such as not making it sturdy enough which resulted in a sloppy bass. I managed to improve this later by adding extra reinforcement, but fixing in the field is never as good as fixing on the drawing board. They are light-driven and fill most rooms with sound quite easily though. My current use for them is as flower-pidestals and one of them is used as a speaker for my improvised practice-bass amp.

The second attempt, made as to be a pair of rear speakers to go with my dolby reciever (with the big ones as fronts), resulted in two 12-litre 2-way speakers with 6.5" woofers. These actually had a much better sound, mostly due to the superior phase-response. I made the filter myself (12 db/oct for both HP and LP) and voices, pianos and other acoustic instruments sound really great. I started using them as my main speakers instead, shifting to the previous ones only when the situation demanded it (party anyone? :)

Both above speaker pairs lack true bottom punch, the smaller ones the most, naturally. I decided that I would build something to give me that, i.e. a subwoofer.

The prerequisities

After reading tons of pages on the net about different speakers, different box designs, opinions and names of the dieties that should be called upon to build a worthy subwoofer, I decided upon a cheap, simple-to-build design that I had observed in real life. The box was a transmission line design with a 700 SEK car-speaker driven by a 2x30 stereo amp, currently at my (audiophilistic) teacher's home, where he had built it for a friend who was yet to come and fetch it.

This setting didn't produce an immense thumping, but rather it filled the low-end (below 60 hz or so) with a smooth sound that added enough to make the music fill the room, rather than being produced from a pair of speakers. It worked at rather high volumes too, as it was limited in frequency, but a 12" can only move so much air. Nevertheless, it felt a good compromise between box volume, frequency response and loudness, as I would probably (and actually have) stayed in rather small student-apartments for the following few years. (That will probably change during summer 2003, as I plan to have graduated by then.)

The box

I wanted something that I could store under my bed, behind a sofa or generally tuck away, while still having enough volume to reach low frequencies. I settled for a 120-litre design with the dimensions 120x50x20cm, yielding a single fold line with the speaker on on one side and the opening at the short end. As the standard dimension of particle board in Sweden is 120x200 cm, it made the cutting quite easy.

A drawing of the box will be up later.

The driver

Is a cheapo 300 SEK 12" car-speaker at 4 Ohms which had suitable characteristics for a transmission line mounting. Money is always a problem and I decided to go with something that works and see how that would sound before spendning any larger sum on a driver. This isn't ultra-hifi anyway, just DIY-save some money-have fun-hifi. :)

The amp

In order to power the speaker, I decided to go with a separate amplifier. Linking the subwoofer to my Sony-amp (which has 2x70 sony-watts) wouldn't be good enough. The amp is rather underpowered for any serious woofage. So, I bought a 220 Watt amp from BK Electronics, since I felt that I would probably make another, more powerful sub in the future and having a good amp was essential. It came for around 1300 SEK. I also needed a filter circuit that would sum the left and right stereo channels and filter out any unwanted frequencies. This I decided to design and build myself, as I couldn't find anything that looked good and was cheap enough. Also, the challenge intrigued me, since I had just finished a basic electronics course at school. With the help of Google, PSpice and good advice from a few friends, the circuit was designed, simulated and proved worthy of construction.

The schematics of the filter is available here. It holds two buffer stages, a 12db/oct adjustable LP-filter (40-110 hz) and a fixed 12db/oct filter at 110 hz and a 24 db/oct HP-filter at 15 hz. It is designed to be powered by a 12V power source provided by the amp (but needn't be), and thus need not +/- power as is common with op-amp circuits.

Current status

The filter circuit has passed computer testing, the amp is encased in a nice box with black aluminium casing and controls at the front. The loudspeaker is complete with driver mounted and everything. I have tested the amp and the speaker with my bass and it certainly goes all the way down to low-B, which is 30 hz. All that is left is to put the circuit on some PCB, mount it inside the amp casing and connect it to my stereo.

Pictures of everything will appear later.

My homepage is here and if you have any questions, feel free to email me.