Portable audio, a light weight boombox

[Jbarnes]

Re: Portable audio, a light weight boombox

I agree with Paul, nice form. The white grill would look really good with a light wood, like maple or birch with clear only.

Jay

[Paul Carmody]

Re: Portable audio, a light weight boombox

That looks great! Very practical in size, looks easy to transport, yet the finish and grill cloth make it look classy. I like it.

[jjgoertz]

Re: Portable audio, a light weight boombox

Got around to building a new grill frame and wrapping it with black grill cloth, I think it looks much better this way. I've really been enjoying the way this thing sounds, the best balance I've found so far has been playing from atop a short table, about 18 inches from the wall while indoors. Efficiency turned out well, in about 6 hours of indoor and outdoor listening at moderate to high volumes I used about 400 mAh, and having 10,000 mAh onboard means I should get days of play time.

[jjgoertz]

Re: Portable audio, a light weight boombox

The boombox is operational.

The crossovers got mounted to the sidewalls:



The amp was wired and installed:



And I made and installed a grill. I'm not thrilled with how the white grill ended up looking, may do another in the future with a course black cloth, or possibly just a piece of expanded sheet metal.



That just leaves a cover for the electronics to build, but for now I'm just having fun listening to this thing.

[jjgoertz]

Re: Portable audio, a light weight boombox

Wow, that's a pretty deep notch in the woofer's response just above 5kHz. What does the combined impedance curve look like?

Well, that's where my notch filter is. In theory the woofer impedance there would be infinity-ish, but I don't have any way to measure it.

[Brian Steele]

Re: Portable audio, a light weight boombox

Wow, that's a pretty deep notch in the woofer's response just above 5kHz. What does the combined impedance curve look like?

[jjgoertz]

Re: Portable audio, a light weight boombox

Got the crossover design done. On the woofer I went to the 8ohm 6uF zobel, got rid of the BSC compensation completely and used the coil from the BSC parralleled with a 1.2uF cap for my notch filter. On the tweeter I used a 6.5ohm series resistor ahead of the cap.

Final crossover looks like this:



And the individual and summed responses with the new crossover:



And finally the magazine approved frequency response, 1/6th smoothed, which would probably be called +-2.5dB from 100Hz up.

[Chris Roemer]

Re: Portable audio, a light weight boombox

I know the peak is near 4k, but I was notching a bit above it at closer to 5k, so yeah, a 2.7uF should work for you.

[jjgoertz]

Re: Portable audio, a light weight boombox

Just sayin', the tiny cap on my notch is picked to resonate with my stated LP coil. If you use a different coil, I can model the proper "tank" cap value to notch out that 4k peak (if you don't have the software to do that).

Do you have any passive elements on it now (for this measurement)? Zobels are cheap (REALLY cheap using cheap resistors and npe caps) and will help pull that woofer down.

I have all the passive elements on it now that we're in my crossover drawing. The low pass coil is a 0.44mH 0.31 Ohm. I don't have anyway to simulate it, but it looks like I need about a 3.5uF capacitor in parrallel to resonate at 4k?

[Chris Roemer]

Re: Portable audio, a light weight boombox

Just sayin', the tiny cap on my notch is picked to resonate with my stated LP coil. If you use a different coil, I can model the proper "tank" cap value to notch out that 4k peak (if you don't have the software to do that).

Do you have any passive elements on it now (for this measurement)? Zobels are cheap (REALLY cheap using cheap resistors and npe caps) and will help pull that woofer down.

[jjgoertz]

Re: Portable audio, a light weight boombox

Here's the frequency response with the current crossover. I probably won't bother messing with the bsc, as I don't want to give up any more efficiency, but I will need to pad the tweeter about 3dB. And then that 4khz hump from the woofer is kind of nasty so I'll probably use Chris' idea for a notch at 4khz.

The cap on the tweeter probably doesn't need to change tho, it's already down 25dB at Fs.

[jjgoertz]

Re: Portable audio, a light weight boombox

Chris, that analysis was helpful, and I'll be sure to use some of your advice if I need to make adjustments after my I take some measurements.


I got some more work done on this thing, the box itself is pretty much done with the hardware mounted, and I wired up one of the crossovers. I should be able to get some measurements tonight.

[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).

[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.

[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.