The Speakerboxxx - Hi-Fi BT Boombox From Scratch!

I know what you're probably thinking, "Not another bluetooth speaker Instructable!"

BUT THIS IS TRUE HI-FI!*

*and not in the heavily abused, arbitrarily applied use of the term! Read on to see how :)

Step 1: Goals & Design Requirements

Goals

I love music. When you truly love music, you care about hearing it the way the artist intended. Accuracy in reproduction was my overarching design paradigm for this project. That drove me to build this Speakerboxxx (shout out to Outkast) from scratch. Engineering the enclosure, crossover circuits, zobel networks, and signal path to match those goals. I can happily say by the end of this project, the performance and accuracy of this box (even at great loudness) is beyond my expectations! To top it all off, this was achieved with the final ticket price of ~$150 USD, minus my labor of love :)

"User" Features:

• Portable ("small" was not the goal as much as "transportable")
• Great battery life (>15 hours @ medium volume)
• Bluetooth and 3.5mm Auxiliary input
• 5v USB 3A charger output (for phone or tablet charging)
• Detailed battery display (<1% accuracy, auto-calibrating)

Audio Design:

• 2x 5/8" silk dome Dayton Audio tweeters (6 ohm)
• 2x 6.5" poly woofer (4 ohm)
• 15w*2 Digital Tri-Path Amplifier (TA2024 chip)
• Bluetooth 3.0 w/AptX
• Ground-loop isolation on inputs

Out-of-scope

I had to draw the line in the sand somewhere, and for me that was saying "No" to active crossovers or digital signal processing. Those are out of scope for now! We're off to build the best audiophile Speakerboxxx we can, with the available applied analog engineering knowledge from the textbooks (read: internet).

Step 2: Bill of Materials

Speakers

• 2x - 6.5" Poly cone woofer 4ohm (Parts Express #299-609)
• 2x - 5/8" Dayton Audio soft dome tweeter ND16FA-6 (Parts Express #275-025)

Electronics

• 1x - 15W/ch TA2024 Amplifier board (Parts Express #320-330)
• 1x - Bluetooth Audio board (I repurposed an old RocketFish home audio BT board)
• 1x - Battery capacity display (eBay)
• 1x - Ground loop isolator (Amazon)
• 1x - 12v to 5v USB 3A DC/DC converter (had on-hand from DealExtreme)
• 1x - 12v 6,000mAh Li-Ion Battery

Crossover Components (These will change for different speakers)

• 2x - 0.33mH Inductors
• 2x - 0.47mH Inductors
• 2x - 5.0uF Film Capacitors (I had to use 4.7uF+0.33uF to approximate)
• 2x - 3.3uF Film Capacitors

Zobel Network Components (These are woofer specific)

• 2x - 3.9ohm Resistors (~5W)
• 2x - 27uF Capacitors, non-polar electrolytic or better (I had to use pairs of 22uf+4.7uf)

Connectors

• 1x - Power Switch (waterproof latching I had on hand from a previous Instructable!)
• 1x - 2.1mm DC barrel jack (charge port)
• 1x - 2 female 3.5mm to 1 3.5mm male aux splitter

Hardware

• 1x - 1/2" MDF (a single 2'x4' quarter-sheet)
• 1x - 8" Stainless steel drawer pull (Home Depot)
• 4x - Cabinet feet (Parts Express #260-7706)
• 1 roll 1/8"x3/8" Closed cell gasket tape (for woofers)

Step 3: Electronic Components Selection

NOTE
While the parts chosen for this project are presented below in a straightforward manner, this particular combination of electronics was the culmination of ~20 hours of research, pouring over the datasheets on many drivers and amps, while permuting an "optimal" solution.

Amplifier

I chose to go with a Tri-Path Class-T architecture amp board since they usually have very good "Audiophile" THD+N performance balanced with efficiency, since this is a battery-powered application where a higher performance linear or Class-A/B amp would be wasteful. I also recommend the Sure Electronics brand for their clean implementations of digital amplifier circuits, including the TA2024 board I used. Since these amplifier chips are so popular, you can find 100's of generic boards on eBay for a few dollars cheaper, but they usually have terrible heat dissipation design (no top heatsink or recommended copper PCB heatsink) and can overheat at even nominal volumes.

Woofers

These particular woofers were selected because they were available at Parts Express at a great price ($4.88/each) while having great reviews in performance. They lacked a full data sheet with frequency response graph, but there were sufficient Theile-Small parameters to optimize our design later. In addition, the info mentions a smooth low-end roll off which makes a low-pass filter on the woofer unnecessary. Saving a component in our crossover design.

Tweeters

The Dayton Audio tweeters were selected for their good reviews and response graph, reasonable price, and my personal preference to the tone of soft-dome tweeters. As we will see later, their resonant frequency is 2,283Hz, which also works well with the "double tweeter resonance" rule-of-thumb for our intended crossover frequency of 4,000Hz.

Step 4: Designing the Zobel Network

"I thought I knew audio... WTF is a Zobel network" -Me.

THEORY
A Zobel network compensates for a property that occurs with every voice coil based speaker. A speakers voice coil exhibits inductive reactance at higher frequencies, and the impedance (i.e. resistance) rises as a function of frequency. This is usually observed the worst in the higher range of woofers. It also affects tweeters, but they are usually internally dampened against this impedance rise with the addition of ferrofluid (like the ones chosen for this project).
Learning Resources:
Zobel Calculator
Zobel Learning

Since we are running the 6.5" woofers all the way up to 4kHz (a relatively high crossover for that speaker size) a Zobel network will compensate by flattening that rise, stabilizing the impedance over the usable frequency range. Since the driver impedance plays a crucial role in higher order crossover designs, we make the crossover frequency and performance more accurate to the paper calculations when we stabilize the drivers impedance.

Zobel + Crossover Note:

The zobel network is installed after the passive crossover on a given signal path. Normally you design a crossover network with the given "nominal impedance" from the speakers datasheets. For the highest accuracy in design, one should design the Zobel networks to stabilize the impedance seen by the crossover network, then measure the actual impedance seen across the compensated driver, and use that data in the crossover design.

Step 5: Designing the Passive Crossover Network

THEORY
A crossover network filters frequencies, directing subsets of the musical range to the drivers that are best suited mechanically to reproduce the given frequency ranges. There are many crossover designs that range from a simple capacitor that filters out low frequencies that would harm a tweeter, to 4th-order networks with over 12 components designed to optimize every characteristic of the drivers choosen. This is one of the most important and yet overlooked (or underestimated) aspects of good loudspeaker design.
Learning Resources:
Wiki - Audio Crossovers
Comparing Crossover Slopes

Choosing a crossover Design:

After assessing the goals of this project, the capabilities of our given drivers and amplifier; I chose to implement a high-quality 2nd-order Linkwitz-Riley crossover network @ 4kHz. The 2nd-order Linkwitz-Riley crossover has a respectable 12dB/octave slope and gives a flat frequency power response at the crossover point, where both drivers will be -6dB, minimizing destructive overlap of sound.

Why not simpler? Simpler 1st-order crossovers will not suffice (at least, will not produce a "hi-fi" sound) for our given drivers and crossover frequency. It also is not recommended since the tweeters resonance frequency is ~2,200Hz where the power would be around -6dB if we were to implement a simple first order crossover at 4kHz. This goes against the recommendation and best practices of designing to be -18dB at the resonance frequency.

Choosing Components

After using the many crossover calculators available for the 2nd-order Linkwitz-Riley network, I found the values of capacitors and inductors needed for my given impedance drivers. Crossover component quality is actually very important and one should use at least non-polarized electrolytic capacitors with low ESR (or better yet, Film caps, which are used in this design) and preferably air or iron core inductors. The inductors are by far the most expensive component, and in part because and the hi-fi community loves their large gauge copper air core inductors. Those hi-fi air-core inductors are great for high power handling and stable performance when compared to ferrite core inductors when used in loudspeakers, but in our lower power application we can get away with using well spec-ed ferrite core inductors as long as they have sufficiently low DCR and a current saturation that we will not hit in our application.

Step 6: Testing It All! ... WHAT IS THAT NOISE

THEORY
*Engineering Disclaimer: This is paraphrase, as it applies to our given audio situation* Ground loops occur when there are multiple paths to ground. These paths usually have varying resistance and the actual measured ground voltage of varying components can differ (not =0V). This causes a lot of noise issues with the unbalanced audio components (amp & bt board) that use ground as a reference for the signal path. Ground loops can be broken by employing a ground loop isolator, which is a device that transfers a desired signal across a conduit(transformer) while not being actually electrically connected.
Learning Resources:
Wiki - Ground Loop
Audioholics - Ground Loops, Eliminating System Hum and Buzz

Basic Verification Test

Setup: After assembling the crossovers, zobel networks, and all supporting audio electronics I went about a basic verification test. In this test I had the amplifier being fed from the 12v battery pack, and in turn outputting to a single right(R) channel; a tweeter and woofer hooked up via the crossover network. The audio source in this setup was the recycled RocketFish BT home theater board which was being powered temporarily for this test by its included 5v AC adapter.

Results: While only a single channel was hooked up, the Bluetooth connection worked perfectly and the audio sounded great! Keeping aware that the woofer bass will be lacking and overall performance will be lower since they are not made to operate in free-air. There was no audible noise floor when music was paused or during the quiet parts of high volume acoustic tracks. When placing my ear very close to the tweeter the slightest hiss could be heard (inches away its very quiet) which was very acceptable! :)

Full Integration Test:

Setup: The only difference between the basic test setup (above) and the full integration test setup, is that everything is now properly being powered from the single 12v li-ion battery. Removing the 5V AC wall adapter for the BT board and hooking it up to the battery as well.

Results: I pressed the now functional power button and everything lit up as it should! But a second later there was a terrible noise! Ok, terrible might be exaggerating a bit. There was a noticeable noise floor that was not present in the basic test setup. It was only noticeable when no music was playing and you were with a foot or two of the speakers. As soon as a normal music track would play, no one would be able to notice it. However, within a quiet acoustic track that needs the volume cranked up, the noise amplified with the volume and became distracting. Please take this with a grain of salt, because while this bothered me a lot since it was not present before in my tests, I believe this still would not have been noticeable to many casual listeners.

*INSERT CRAPLOAD OF RESEARCH*

Solution: Ground Loop Isolation!

Be sure to read the "Theory" section above! I found a great ground-loop isolator on Amazon for just under $10USD.

Step 7: Making the Box

THEORY
The best materials for building loudspeakers enclosures have low resonance. In terms of wood products, MDF (medium density fiberboard) is the best and most available choice. When you see more expensive cherry wood or walnut speaker cabinets on the market, they are almost always MDF construction with a nice hardwood veneer on the outside.
Learning Resources:
Speaker Enclosure Designer

From the inception of this project, I thought it would be cool to have the form factor close to that of the classic 80s-90s "boombox". The original big portable stereo that devoured D-Cell batteries, we all loved.

Outer dimensions: 22"W x 11"H x 7"D

I used the speaker calculator in the "Theory" section above to help tuning the overall size and it even makes cut size recommendations for all panels! I cut a spare end cap panel to use in the center as a middle support just for overall rigidity.

Construction

The general construction was done with wood glue and a few countersunk 1-1/2"-#8 screws for security. 90º angle clamps and other larger clamps will make this all much easier. Its important to have very smooth mating surfaces because we want this box to be air-tight in the end. Preferrably we will achieve this with the wood and glue alone, but just in case, we will take a pass with silicone on the inside to be absolutely sure we have made a proper sealed enclosure!

Sealing Outer ports (charging, USB, Aux)

I routed the openings for the ports to be as small as possible, in order to minimize the area we need to seal. After placing the cable or port, I used Sugru (awesome formable silicone) on both sides (inside and out) and allowed time to cure. If any more seals were needed, I patched with normal silicone II.

Step 8: Placing the Drivers

Placing the drivers was probably the most straight forward part of this whole process. I wanted the left and right sides to be symmetric, or at least equidistant from center. Mounting the tweeters close to the woofer, but farther from each other (instead of both in the center) will aid coherence and stereo separation. This will test your circle making skills, and I was very lucky to find a bowl in my house that was the perfect woofer cutout diameter :)

Step 9: Placing the Electronics

All electronic and wires within the box are mounted or tacked down (from vibration) with a Stanley high-temp glue gun (~410ºF). This glue is strong enough to hold wood and metal structural components together, so it will suffice for our needs. Be careful though, because this glue is bordering on the temperature tolerance of some plastics and wire jackets. I love using silicon jacketed wire for its high temp and current handling while remaining very flexible.

Battery

I had a natural inclination to mount the battery near the charging input port. While thats convenient for that charger connection, it will still need to have its GND connect to every other component and its VCC through the switch on the opposite side of the box.

Bluetooth

I mounted the bluetooth board to the roof of the box in hopes of optimizing its reception and distance, since it already has to penetrate the 1/2" MDF.

Amp

The amplifier was placed on the bottom of the box, opposite side of the battery. I placed it away from the walls since we need to connect wires to both sides for inputs and speaker outputs.

USB Charger Output

The USB Charger board was mounted to the top of the box as well, with its USB output port facing the rear of the box. It is mounted adjacent to the bluetooth module since it is being fed by the auxiliary 5V output lines from the 12v-5V USB adapter.

Remainder

The remaining items (crossovers, ground-loop isolator, and cables) were placed in logical places where room remained. Try to make an effort to space the inductors of the crossovers apart from each other and the woofer magnet to avoid inductive coupling.

Step 10: Coating the Box

DuraTex Speaker Coating

Looking around at potential protective outer coatings for the box, I found many different classy wood veneers and basic paints, but wanted something more durable against wear. I quickly found (and kept finding) DuraTex as the most used and recommended coating. Chances are you've probably seen it at a concert on a touring amp, speaker, or equipment cases. Its self-priming and adheres well to wood and wood spackle. It dries to the touch within an hour or two (I waited ~30 min between coats), but takes up to 7 days to cure to full protective hardness.

I love the way it turned out and highly recommend DuraTex as any protective durable wood coating. Due to its high viscosity you can achieve many different textures by simply switching up your roller or brush type, or even getting a "leather" look by blotting with cellophane (instructions available from DuraTex!).

Speaker Cabinet Feet

Cabinet feet are very important for at least 2 reasons: the first reason is they help compensate for mating uneven surfaces (including the small unevenness of the bottom of the box) to the surface the box sits on. The second reason is to isolate the box and the massive vibrations it can make from transferring to the surface it sits on. If the completed box were to sit on top of a semi-resonant surface with no isolating feet, we would introduce preventable noise from the environment.

Step 11: Putting It All Together!

The box is now all sealed up with the exterior coating and all electronics are mounted securely inside. I performed one last sanity check and everything is still sounding promising.

Tweeter Mounting

Since the tweeters are flush-mount with a very small ~3mm flange, at least one of my hand-routed holes was not going to be airtight by nature. So after flush mounting the tweeters, I sealed the back of them with Sugru and silicone for structure and airtightness, all through the exposed woofer holes.

Woofer Mounting

The woofer mounting was made much easier by the great closed-cell speaker gasket tape I picked up from Parts-Express. I carefully lined the back of each basket with the gasket tape (as the photos show), hooked up the wires and dropped them in! A few pan head screws into fresh pilot holes around the edge and the woofers were secure and airtight as could be!

WE ARE DONE! Please join me as I grab a glass of wine and have a listening party in my next step :)

Step 12: Results: Listening Test and Project Retrospective

As I sat down to do my initial listening test, I tried to remind myself to be objective and to have reasonable expectations for my first true hi-fi loudspeaker design. Boy, was I really blown away by the results! This Speakerboxxx was also maxing out my living room at half volume, so the loundness was great. I personally listen to a wide gamut of musical genres and also tried some staple tracks that test speaker range, speed and detail. After a few hours spent listening from Hendrix to Winehouse to Dire Straights back to Drake and Klingande with everything in between, I had my "objective" listening results.

My Snooty Hi-Fi Reviewers Review™

The high range is very crisp with great detail but without harshness or tininess I usually attribute to the mylar or titanium drivers. The crossover is completely transparent (which is exactly as it should be) and any brightness that was present in the higher range of the woofer was tamed by the zobel network and now blends beautifully into the lower tweeter range. The place that really shines though, is the tone and quality of the vocal midrange stemming from those full 6.5" woofers that are rarely seen on portable systems. The woofers have a natural low-end roll off which I would peg subjectively at about 80Hz. While they could have theoretically been optimized for a bit lower f3, they keep a tight sound (and impact) through their audible range and roll off cleanly before hearing any ringing or oddness around the 52Hz resonant frequency of our particular woofers.

Retrospective

Well I am very happy with the results and I hope that this instructable was as interesting and informative for you as it was for me while creating it. If given the chance to do this Speakerboxxx again, the only thing I might have changed is to select a woofer that provides more technical data, so the design and expectations can be further optimized. Of course this entire process could be much more scientific if I possessed the advanced measurement equipment of those in the audio field, I hope that my ability to create this level of performance in spite of that leaves the home Audiophile optimistic!

Step 13: P.S. Creating a Protective Grill

This is labeled as post script to the project because while I did not originally intend on making a protective grill, apparently even grown adults enjoy poking at exposed speakers :/

Here is my quick and effective, mini-instructable "Grill in an afternoon":

1. Cut a piece of MDF equal in size to the face of the speaker box.
2. Rough draw a 1" border around the inside of the MDF and at least 0.5" around the outside of the tweeters.
3. Route out the frame (I hand routed with a Dremel) and sand corners smooth.
4. Spray a quick 2 layers of black laquer spray paint.
5. Mount 4 magnetic cabinet door mounts (Home depot)
6. Stretch black grill fabric (acoustically transparent) around the frame, glueing down the fabric with tension, with a low-temp glue gun.