Portable audio, a light weight boombox

[jjgoertz]

Hi guys, I'm new around here, and just wanted to share what I've been working on.


I'm building a small, lightweight boombox to provide portable audio around the house and in the yard, etcetera. In the interest of portability I've given up some low end extension to keep the efficiency at a decent level and keep the box small.

The drivers I chose for this set up are the B&C 6NDL38, and the Morel CAT 378. I'll be powering this system using the Dayton DTA-2 amp, running off of a pair of 3 cell lithium polymer batteries in parrallel, giving me 10,000 mAh, and hopefully a couple days worth of run time at reasonable levels.

I put the woofers in ~8.7 liters, with a vented alignment, tuned to about 63Hz. This ended up with box 20" long, 12" tall, and 6.5" deep externally.






Here you can see the additional compartments built into the back of the boombox, the wider compartment near the bottom will hold the two batteries side by side, and then the amp will be mounted in the vertical chamber.

The back panel is going to be split up into three pieces, two bolt on covers for the woofer compartments, and a removable cover for battery access, probably held in place with magnets.



To design the crossover I temporarily hooked up one half of the boombox in an active system with my MiniDSP. What I eventally settled on was a 1st order Butterworth on the woofer at 2200 Hz, a bsc comp filter with a 3dB cut starting at 500 Hz, and a 1st order Butterworth on the tweeter at 4000 Hz. This gave me a reasonable on axis FR, and fairly well behaved off axis response. The measurements below start on axis at the top, and then 15, 30, and 45 degrees off axis below that.



The passive circuit I came up with to try and create that same response is below. I'm waiting on parts to build the crossover now, but hopefully I get some similar results.

[thekorvers]

Re: Portable audio, a light weight boombox

Very nice! Welcome to the Forum. We will be following your progress.

One question: What for heaven's sake is that in your avatar? A doll without a head?

[Chris Roemer]

Re: Portable audio, a light weight boombox

How did you come up with that XO?

[jjgoertz]

Re: Portable audio, a light weight boombox

One question: What for heaven's sake is that in your avatar? A doll without a head?

Haha, not quite. It's just a little toy robot.

How did you come up with that XO?

The woofer crossover is just a Zobel, an inductor for my first order electrical roll off, and then the bsc circuit in series with everything else. The bsc circuit may need some work, I'm not quite sure.

For the tweeter I had a response plot of what it did with just the 5.6uF cap in series, and after emulating that response with my MiniDSP while testing actively it ended up playing nice with my woofer.

[Saturnus]

Re: Portable audio, a light weight boombox

Tune the bass reflex a bit higher, say 72-74hz'ish and you'll get better bass performance. It will sound deeper and more powerful.

[jjgoertz]

Re: Portable audio, a light weight boombox

Tune the bass reflex a bit higher, say 72-74hz'ish and you'll get better bass performance. It will sound deeper and more powerful.

I will do that when I pull it apart for finishing. When I modeled the driver in Unibox it showed me obtaining an f3 at around 60Hz, but it sure didn't work out that way in real life.

[Brian Steele]

Re: Portable audio, a light weight boombox

I will do that when I pull it apart for finishing. When I modeled the driver in Unibox it showed me obtaining an f3 at around 60Hz, but it sure didn't work out that way in real life.

What was the measured Fb? Sometimes the vent length to achieve a target Fb turns out to be quite a bit less than that predicted by software. With Fb too low, you'll end up with a drooping bass response at low frequencies.

Another thing - the FR graph suggests that it might be a bit too sensitive around 3kHz. Perhaps you could try dropping the response here a bit with MiniDSP to see if it sounds any better.

[jjgoertz]

Re: Portable audio, a light weight boombox

Here's a couple pics of the drivers mounted in the baffle. The tweeter is flush mounted and the woofer is rear mounted, all the edges have been hit with a 1/4" roundover. The box itself is made with a 1/2" aircraft ply front baffle, 1/4" for the top and bottom and 3/8" for the ends.

[joeybutts]

Re: Portable audio, a light weight boombox

Looks great. Nice craftsmanship. Should be bangin'.

[Saturnus]

Re: Portable audio, a light weight boombox

I will do that when I pull it apart for finishing. When I modeled the driver in Unibox it showed me obtaining an f3 at around 60Hz, but it sure didn't work out that way in real life.

It rarely does behave as modelled unless all parameters are accounted for and the simulation software is professional enough.

Btw, I'll add that the passive filter will naturally change the driver Re. And thereby most other electrical parameters such as Qes (and thereby Qts) and sensitivity.

[jjgoertz]

Re: Portable audio, a light weight boombox

What was the measured Fb? Sometimes the vent length to achieve a target Fb turns out to be quite a bit less than that predicted by software. With Fb too low, you'll end up with a drooping bass response at low frequencies.

I don't have a DATS or an impedance meter, so I can't directly measure Fb. On my subwoofer builds I've watched the cone while playing sine waves and looked for the minimum amount travel as I varied the frequency, but the cone movement is small enough and fast enough that I can't really do that here.

It rarely does behave as modelled unless all parameters are accounted for and the simulation software is professional enough.

Btw, I'll add that the passive filter will naturally change the driver Re. And thereby most other electrical parameters such as Qes (and thereby Qts) and sensitivity.

I had pretty much just decided that the T/S parameters supplied for the woofer weren't accurate. It would probably be worth picking up measurement equipment at some point.

[jjgoertz]

Re: Portable audio, a light weight boombox

What was the measured Fb? Sometimes the vent length to achieve a target Fb turns out to be quite a bit less than that predicted by software. With Fb too low, you'll end up with a drooping bass response at low frequencies.

Another thing - the FR graph suggests that it might be a bit too sensitive around 3kHz. Perhaps you could try dropping the response here a bit with MiniDSP to see if it sounds any better.

Just finished finding Fb by measuring the current through the speaker while holding a constant voltage as we played sine waves, and it was 63-64Hz. I'll probably try raising the tune to the low 70s and go from there.

[jjgoertz]

Re: Portable audio, a light weight boombox

Took the cabinet apart and started getting some color on the wood. The color is Antique Cherry Red by Transfast. A bit more bold than the average color you see in a wood stain/dye.






And got the holes drilled in the top panel for the IO stuff. Recessed mounting for the volume control that comes with the DTA-2 amp, and a nice Switchcraft jack for the 3.5mm stereo input.



Also cut the ports down to 1.25", which should give me a 73Hz tune.

[Chris Roemer]

Re: Portable audio, a light weight boombox

A passive XO sim suggests that your topology nets an Fc at 2kHz, maybe 1-2dB of BSC (if any - the driver with your LP circuit shows about 89dB (peak) near 400-500 Hz, dropping to a nominal 87 or so by 2k, and dropping to around 87dB near 200Hz. The CAT looks to be running 3-5dB too hot. 4-5 ohms of series resistance should tame it. It's only down -10dB at its Fs (700Hz).

The woofer (w/your filter) runs about 86dB @ Fc (2k), but actually rises (instead of rolling off) a dB or 2, peaking near 4k.

A little flatter response (not that you asked) might be had by using a 3uF series cap and 3 ohm resistor on the tweeter, and for the woofer I'd use an 8ohm/6uF Zobel, with a 1.5mH series coil (1ohm DCR) with a small 0.68uF cap parallelled with that coil for a "notch" (to push the 4k peak down). This brings the woofer's peak at 4k down by -16dB (compared to your filter) and the CAT ends up -15dB down at Fs.

[Brian Steele]

Re: Portable audio, a light weight boombox

A passive XO sim suggests that your topology nets an Fc at 2kHz, maybe 1-2dB of BSC (if any - the driver with your LP circuit shows about 89dB (peak) near 400-500 Hz, dropping to a nominal 87 or so by 2k, and dropping to around 87dB near 200Hz. The CAT looks to be running 3-5dB too hot. 4-5 ohms of series resistance should tame it. It's only down -10dB at its Fs (700Hz).

FWIW, considering how boomboxes are typically used (placed on the ground, out in the open, listener vertically off-axis, sometimes pretty significantly), it might be best if the boombox runs a little "bright", though not in the 3kHz area. Years ago I used to own a Sharp QT-37 boombox - the little thing was BRIGHT when you faced it head-on, but it actually sounded pretty decent when used as described above. It also had a tone control to control that brightness when necessary . When I finally do my own boombox build, I'll be following that model (two small detachable speakers and a central control box that I'm going to use for bass unit duties as well).