Subwoofer Project

Posted by (twitter: @_sorceress)
November 21st, 2016 3:09 pm

My First Hifi

hifi1

I have only ever owned one hifi system, which my parents bought for me one christmas around 1993. In hindsight it was not a very good system, but it played my tapes (twin tape decks!), it had a radio tuner, and standalone speakers, so as far as I was concerned at that age, it was brilliant! It couldn’t even play CDs, but as I didn’t have any CDs until I was a teenager that didn’t really matter.

After several years, the knobs and sliders (volume, equalizer) became super crackly, and the tape decks started damaging my tapes. Eventually, one tape got chewed up inside the machine so badly that the door wouldn’t open, even with excessive force. The hifi remained in that state for a couple more years before being scrapped.


Home-Built Amplifier #1

When scrappage day came, I reasoned that all of the electronics inside it still worked fine: the amplifier worked ok, as did the speakers. As I had an interest in electronics since childhood, I figured it should be possible to salvage the amplifier from the inside.

I should point out that this hifi was neither the first nor last piece of technology which failed me. And these experiences only led to greater wonder about how technology works, and cemented a belief that if something can break then it should be designed so as to be fixable. We shouldn’t have to lose the things we love.

The circuitboard inside the hifi was huge. I traced the circuit back from the speaker terminals, managing to locate the amplifier chip, read it’s model number, and look up the chip maker’s recommended circuit from that. Comparing components, I discovered the hifi was using exactly the recommended circuit, and it was a tiny part of this huge circuitboard. I carefully chopped out that small piece of circuit, and tested it to see if it still worked, which it did, somewhat to my amazement 😀

salvage-amp

I repackaged it in a small plastic electronics box, together with a new volume control and sockets for the speakers/dc-power/line-in. It was a tiny 2×1 watt amplifer, but it worked fantastic, still able to drive these big(-gish) speakers at a reasonable volume!

By this time, my PC had taken over the role of music player via it’s CD-ROM drive. I connected this amplifier to my PC. It sounded far better than the little multimedia speakers my computer came with. This would be my audio setup for the next ten years.

What’s more, it could run for a whole day on relatively high volume, from 4xAA NiCd batteries. I remember when the electrician came to work on the wiring in the house, I had my amp+speakers powered this way, driven by an ipod. Much confusion from the electrician when he threw the mains switch and the music didn’t stop… “where are you getting your power from?” so I showed him what I had made, and he thought it was great!

There was something very satisfying about it.


Home-Built Amplifer #2

Around 2012, I felt it was about time to replace my speakers. They were low quality when they were new, and by this time they were almost 20 years old. And I loved my music – both listening to it and composing it – so why was I still using this low quality stuff from my childhood? So yeah, it was about time. 😀

But I loved this setup I had – a tiny amplifier made from salvage parts – it lit me up inside each time I looked at it. So if I were to have a replacement for my speakers or my amplifier, I felt it had to be something home built, not a commercial product. And what’s more, my amplifer hadn’t broken down in all these years. It worked as well as the day I made it. So both emotionally and logically, this felt like the right way to go.

Remembering how simple the amp circuit was, I reasoned that I should have enough electronics skill by now to make something from base components, while building myself some decent speakers to go with it.

The point of this upgrade was to get a significant improvement over what I already had, so my guiding principle throughout would be maximum sound quality.

Over the course of several weeks I did some research on speaker and amp design, and taught myself a lot about audio tech. Then I began ordering parts online (things which I didn’t already have in my electronics parts box). I was slowly getting everything together which I would need. The 6″ woofers and tweeters I chose were each rated 80W, while the amplifier I designed was 2x25W class AB bridged. The power ratings between speaker and amp don’t have to be matched, and my choices ensured the amplifier couldn’t overexert the speakers, though I doubted I would run the amplifier on full volume anyway, because who ever does that?

    The 80W Speakers

I wasn’t very skilled with woodwork, but speakers are not terribly complex things being just boxes with holes in. So how hard could it be I thought? It would be good practice if nothing else!

I opted for sealed speaker enclosures (with no sound port), as these are supposed to produce less tonal distortion overall, despite being less capable of delivering power at bass frequencies.

I showed my father my designs for the speakers, explaining what I wanted to make. He was keen to help with supplying the wood and doing some sawing, so I asked him to cut the pieces of 18mm thick MDF for the speaker sides, while I cut the front and back panels from 7mm marine plywood, which is a strong attractive plywood that doesn’t delaminate with age/weather. People make yachts with it :)

Assembly went well. The four MDF pieces belonging to each speaker were glued and screwed together with eight screws, making a very sturdy box frame. The front and rear panels were then screwed to the box frame, with the front panel additionally glued.

To cut the circles for the speakers, I had to make a special cutting tool which steered a knife blade in a circular path. This was very effective, and it made some clean circular cuts.

The speaker sides were painted matt black acrylic, while the front/rear panels were sanded and polished, to make a feature of the plywood veneers.

    The 2x25W Amplifer

Amplifier assembly also went well. The amplifier case was made in a matching style, from the same wood, but lined with aluminium foil for EM shielding. My regret here was not using a steel case, as this foil shielding was not very effective. The amplifier could pick up some mains hum if a power cable was placed alongside it, or if I rested my hand upon it. It was a minor complaint though, as it was barely noticable with sensible placement of cables.

This project took maybe 6 months from initial concept to finished product. I was very pleased overall. The speakers themselves are attractive, worthy of any living room. The sound quality is excellent, far better than what I had before: The vocal range is crystal clear, the treble much more airy, and the bass significantly deeper and more powerful. The speakers are however lacking some power at the lowest frequencies, especially noticable below 60Hz, and there is virtually no response below 40Hz.

So yeah, it was a huge improvement, but fell short of perfection. 😛


The Bookshelf Speakers

The next chapter has it’s origins in 2016. I thought it might be nice to make speakers for other rooms too, and an input/output mixer, so I can choose what sound sources to listen to, and what rooms I want to listen in, all controlled from my desk.

So that’s what I started making. I ordered some 5.25″ full range drivers this time, with coaxial “whizzer” cones capable of reproducing upto 20kHz. These seemed quite adequate for what I had in mind. I also built this speaker pair in the same style wooden cabinets as the previous, although they are physically smaller and what I’d call ‘bookshelf speakers’. I also bought some slider pots with the intention of making the mixers/faders.

Then I kind of lost interest… I felt it wasn’t a good idea anymore, and it wasn’t something I wanted to make now. So I had these two bookshelf speakers going spare.


Home-Built Amplifier #3

Some years had passed since building my (main) speakers and amplifier, and I was developing a more discerning ear. I had started noticing imperfections in the soundstage. For example, a speaker can have a tendency to appear as the origin of sound, instead of correctly projecting that sound into 3d space. I could close my eyes and be able to pinpoint where the speaker was, which is not how it should be. Closing one’s eyes, the sound should appear spatial, like live music, not like a recording coming from a speaker.

This came to a head early in 2017, when I figured that having more speakers might hide these imperfections, and might actually help project sound into 3d space, similar to how surround-sound systems achieve this.

So there was a growing list of reasons to act. This would be an opportunity to address multiple problems:

1) build ‘satellite’ speakers in an attempt to improve the soundstage.
2) build a subwoofer, to make up for the weaknesses of the main speakers.
3) build a new amplifier which can drive all of these speakers.
4) make sure the new amplifier is properly shielded this time.

Lucky for me that (1) was already done. I had just built some bookshelf speakers! (2) would be a big woodwork project, that I decided to delay until I actually had an amplifer capable of driving it. (3) Was something I could start work on now. And for (4), I had recently become the owner of a broken ATX psu, which would provide just the right size enclosure for this amplifier, and being made of moderately thick steel sheet, it would surely provide adequate shielding.

    The 7.1 Amplifier

The first task was to see if the ATX psu case was actually suitable. I removed the burned-out electronics, and looked at the interior space I had to work with. I also looked at the front and back faces where the cables and vents were situated. I imagined these two faces being dressed somehow to make them look nice, and how the various controls and sockets could be arranged there.

I felt there would be enough space to support 8 speakers, via 4 stereo amplifiers. There would also have to be a mono mix circuit somewhere, feeding a signal to an active low-pass filter for the subwoofer. ie, 8 channels = 3 stereo pairs + 1 subwoofer + 1 other. The other would presumably be a non-filtered mono mix for a center speaker. This describes a 7.1 amplifier.

After careful consideration, I decided on the following:

– A 2x25W class AB to drive the main speakers (taken from Home-Built Amplifier #2 which I’m replacing)
– A 2x15W class AB for the small bookshelf speakers.
– A 2x25W class D for the subwoofer (+ center)
– A 2x3W class D for additional satellites, and as a low voltage option for if I ever needed to power speakers from AA batteries.

Class D amplifiers are quite an innovative design. These work via high speed pulse-width-modulation, allowing much greater efficiency over the resistive techniques of linear (AB) amplifiers. Class D are >95% efficient, while class AB dumps at least 50% of the power it sources as heat. This means that class D amplifiers are typically much smaller and more powerful than their linear counterparts, and need little in the way of a heat management.

On the down side, Class D amplifiers can introduce artifacts when reproducing high frequencies, where the PWM pulsetrain produces a cruder approximation of the waveform. So while class D can underperform at above 10kHz, they are perfect for driving a subwoofer. Their efficiency also makes them a great choice for battery powered devices.

The Subwoofer

    The Design…

I began the subwoofer project in the springtime 2017, by looking at what woofers were available, and simultaneously chuntering Thiele-Small parameters through a speaker cabinet simulator. These parameters are carefully defined for any loudspeaker you may wish to purchase, and they encapsulate the precise dynamics of the speaker cone, allowing it to be modelled inside a cabinet.

speaker2

There are countless combinations of speaker and cabinet designs possible, and most of those I tried didn’t show very good results, with lower cut-off frequencies around 35-50Hz. Discussing these options with my good friend Tijn, a learned that a subwoofer should be able to comfortably go down into the 20-30Hz range, although there isn’t much musical quality in that range, just rumbling. (The threshold of human hearing is about 20Hz)

One design I eventually stumbled upon did exactly this, but it required a 62 litre cabinet, which is pretty much a 40cm cube – about the size of a small cupboard! My heart quickly became settled on this speaker + cabinet design.

I ordered the speaker and some wood, but they remained untouched for some time. At this size, I realised it was not a project to be undertaken lightly, and so I felt a little reluctant to get started.

    The First Cut…

01-wood-raw

00-wood-plan

The 62 litre figure is the volume of the air inside the cabinet, once the volume of the speaker and port are subtracted. I estimated each of these to be about 1 litre, so I felt that a nice round 64 litre would be close enough to perfect, meaning I could go with a 40cm cubic cabinet.

Then came the puzzle of working out wood panel sizes to make a box this size. The wood sheet I had purchased was 122cm square, 9mm thick plywood. Taking into consideration the wood thickness + saw thickness + excess for sanding/tidying the edges, I realised this wouldn’t actually be big enough to fit the 6 panels, and I’d need at least 125cm width. With help from my good friend Wan one morning, we worked out an almost-cube shape that worked, and curiously all 6 of the panels would be exactly the same size!

This happened next:

02-wood-sawn

03-wood-block

04-wood-balance

The six pieces each have their own unique grain pattern. It was time to select the nicest two for the top+front faces of the cabinet, and the least attractive two for the base+rear.

I felt that 9mm plywood was not really thick enough to take a screw into it’s edge, so I cut these eight triangular corner blocks and glued them to the top+bottom panels, to provide a sturdy fixing point for the side panels. You can see in the third picture, there is a intentional excess on the edges to allow for sanding/finishing.

Next job was a combination of sanding to size, drilling, and screwing the pieces together. Even with careful measuring down to half millimetres, and similarly careful cutting and drilling, it’s somehow impossible to get precisely engineered bits of wood. After screwing and sanding the edges smooth, the separated pieces will only go back together correctly one way – any other way and there would be small but visible misalignments. There is a slight curvature to the panel surface, so maybe that is why. Either way, care must be taken to remember where each panel goes, and it’s correct orientation. They’re all marked :)

05-wood-screwed

Now that all of the pieces were the right size and sanded smooth, I gave each panel a first coat of polish. While giving this treatment, I felt that I wanted to change my design slightly. Instead of a front facing speaker, I opted for a downfacing speaker. There is apparently no difference in sound with either subwoofer configuration, but downfacing is structurally better (centrally balanced and no hanging stress), as well as reducing the risk of the speaker cone being bumped and damaged.

I also opted to rotate the cabinet about it’s diagonal, to switch the front and top faces, as this would make the top face be without any joins showing, and look more like a piece of furniture. The base with the legs and speaker would then be a separate component, with the “box” part sitting on top.

06-legs-screwed

07-hole-cut

I anticipated that the legs could be under some stress if the cabinet were pushed along the floor, so the legs needed to be very sturdy. These are deep screwed from the base panel through these wide washers which prevent the screw (+leg) from twisting out.

A cutting tool was then made to create the circular hole in the base for the speaker. This consisted of a sharp blade fastened to a block of wood, with a pivot pin that fits through a pilot hole in the base panel. It took several minutes to drive the blade around enough times around to cut through the wood. But cut was very smooth – much better than we’d hope to get with a jigsaw – so it was worth the time, even though it did result in some blood.

08-line-up

15-solder

The speaker lined up very nicely with the hole. I was very happy with how that went. I decided to solder on the connecting wires at this time. I know spade/socket connectors are normally used here, but I like my soldering.

Despite both wires being red, care was taken with the polarity. The other end of the wires went to a terminal block, which the back plate would later connect to.

    The Port…

The next job was construction of the port:

09-flange-rough

10-flange-black

A port is a carefully designed piece of hardware. It is not there simply to allow sound out – no cabinet at all would achieve as much. Let me explain them…

The air inside of a cabinet is compressed when the speaker cone moves in, creating an overpressure. Meanwhile, the air outside the cabinet is expanded, creating an underpressure. If permitted, these two motions would cancel out and destroy the sound waves via destructive interferrence. The cabinet walls normally prevent that from happening, so only the intended outside motions propagate to the ear.

Any “sound leakage” (via holes or flex in the cabinet walls) will cause some of these internal motions to get outside. Ideally, cabinets should be perfectly sealed and sound proofed, except for the speaker hole.

The problem is that air inside a sealed cabinet becomes very springy, and opposes compression/expansion like a thick rubber band. This is particularly problematic at low frequencies where the speaker cone must fight this resistance harder… So ports were invented.

11-tube-cut

12-flange-tube

The port is a carefully designed hole. It’s specific geometry (area, length) creates what is known as a helmholtz inverter. The sound waves undergo a phase inversion along the length of the port, turning overpressures into underpressures and vice-versa.

This means that instead of destructive interferrence, there is constructive interferrence. The port sound reinforces the speaker sound. Win Win \o/

Though it is not all win win. Ports only behave this way at specific frequencies, which they are tuned to. At much lower frequencies, the phase inversion doesn’t happen, and nasty destructive interferrence happens instead. At much higher frequencies, the port becomes invisible to the sound, and you might as well have a sealed cabinet.

The overall effect is that a carefully designed port can greatly improve the bass response of a cabinet. They are typically able to reduce the lower cut-off frequency by several Hz – possibly halving it – at the expense of the even lower frequencies.

13-port-cradle

14-port-fitted

Additional considerations:

– The port needs to be fat enough to allow sound to move through. Too thin and the air will be forced to move too quickly, become turbulent, and make chuffing noises.

– The port needs to have the correct length. Too fat and it’s impossible to create the correct length (negative). Too thin and the length may be too long to fit inside the cabinet.

– Port diameters between 1/4 and 1/2 of speaker diameter are usually good. With large speakers, this diameter can be split between two or more ports if desired.

The port I’ve designed has a flange on the inside end. This further helps to prevent the air becoming turbulent when reciprocating through. The funny construction is a cradle, which holds the port on the inside. It is glued to the cabinet panel, tight fitting and allowing no wobble or vibrations.

Completion…

A slow start and a few pauses meant this project took longer than it really should have. But it eventually came to completion, about 6 months after the initial idea.

18-polish-3

16-polish-1

19-polish-4

These last images show the subwoofer after reassembly and a couple more rounds of polishing.

I may do more polishing at a later date if I feel I need to. It is a quality one builds up over time :)

There are a few stages of the project which I don’t have photos of, sadly:

– Cutting the holes for the port and terminal connector, using a similar tool as used for the speaker hole.
– Bolting the speaker to the base, with 4x M4 nuts, washers and vibration-proof nyloc nuts.
– Adding reinforcement strips along all of the internal edges, which should help to prevent flexing/leakage.
– Reinforcement of the top panel with bracing. It should be strong enough now to stand on, though I won’t risk a test 😛
– The base being secured to the top/box part with 8 screws, and edging around the box part for these screws to fasten into.

20-finished


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