Boombox design and build thread (long first post)

Hi all,

After building a small chuck-it-in-a-backpack speaker, taking a couple of design hints from Saturnus' Boominator (bipole speakers were definitely a good plan), I've decided the next step is something bigger, something that I can carry to someone's house and use for a reasonably loud party.
Also be able to use it outdoors at a BBQ, etc etc.

I'm not thinking of building something as big as a Boominator. IMO, its overkill for what I want.

Besides, I already have some drivers here:
pair of these
3x KEF 3" coaxial satellites.
2x Fostex FE126eN
2x Tang Band W6-1139
a pair of 5" midbasses and tweeters from some (ex-pub-install) Wharfedale speakers.
a couple of Bose 2.5" FR drivers.

Having rigged up the KEF satellites with a 300Hz crossover to my Behringer monitors (B2031P, 8" woofer), they'll keep up until the woofer is moving ~4mm p/p, at which point the KEFs start sounding rather harsh.
As you can probably imagine, it was rather loud by that point.

The MCM woofers can stand more excursion than that and stay fairly clean, so I think they'll keep up for outdoors use.
I've been advised by the man himself that two of those woofers in a ~21L ported cabinet tuned ~49Hz will give a decent sound outdoors.

So, the KEF coaxials and the 8" woofers are, at the moment, favourite.


For the electronics, I think that, for a ~300Hz XO, active would be best. This would also mean I can have some kind of indoors-outdoors switch, to vary the woofer level for BSC.
I have an Amp6-basic from 41Hz, and have ordered another similar amplifier board off eBay. They'll both run off 12v and put out ~15w/ch each.
I did think for a while about going for a 4 channel car audio amplifier, but soon realised that, even if I got 200w total out of it, that'd empty a lot of batteries very quickly.

I'll likely go for a simple LR2 active crossover, but this leads to my first problem.
The amplifiers use a single 12v rail for power, but the op-amps will need a split supply. If I'm to run off one battery, I'll need to split it for -6v-0-6v. This, however, gives me a problem: there's a 6v DC offset between the crossover output and the amplifier input.

I suspect that a, say, 1uF cap in series would sort this out (might be a bit of a switch-on thump though), but would like a second opinion here.
I suppose I could use a smaller value cap to cut <40Hz bass out, but I'll need the input impedances of the amps for that. I can measure it for the Amp6b, but will have to wait until next month before the eBay board arrives.

I'm also planning on having blue-tooth connectivity, and possibly solar panels.
The battery will be a deep-cycle gel battery, as they can provide large instantaneous currents, but will last a while too. The price of Lithium based batteries is currently prohibitive.

I have a 12v 5A laptop-style power supply that won't be built in (weight), but I might be able to persuade it to charge the batteries. If not, I have some 15v 5A PSUs somewhere, though that'd need regulating down to be safe for the amplifiers.

I'll probably go with 9mm plywood for this build, to keep weight down. The KEF coaxials will of course need a seperate chamber each, but that shouldn't be much of a problem.


The eagle-eyed amongst you (if you've read this far) will notice I haven't mentioned anything to do with going bipole yet. I'm still mulling over whether or not its needed, and if so how best to do it with the stuff I have. Possibly the Fostex drivers running full-range with one of those tiny 10wx2 boards, and a switch to cut them off when on battery. Maybe. Those drivers would need a little molly-coddling, as they have phase plugs.


Comments and criticisms welcome - I want to keep costs down, but I'm happy to invest some time in this to get it right.

Chris
 
I'll likely go for a simple LR2 active crossover, but this leads to my first problem.
The amplifiers use a single 12v rail for power, but the op-amps will need a split supply. If I'm to run off one battery, I'll need to split it for -6v-0-6v. This, however, gives me a problem: there's a 6v DC offset between the crossover output and the amplifier input.

I suspect that a, say, 1uF cap in series would sort this out (might be a bit of a switch-on thump though), but would like a second opinion here.

Bumping this, as I'd like to order the op-amps, resistors, caps and stripboard soon.

A relay might do the trick - I don't think much delay between pre- and power-amp is needed to avoid a thump.

Help!!

TIA,
Chris
 
Still struggling on what to do for the pre-amp.


I've gone for the KEF coaxials, and the MCM 8" woofers, bi-amped, and have got some bits of wood cut up.

Note that all holes have been made by drilling through by a 2mm, then 4-8mm (bigger for the bigger holes) lots of times, then joining the holes with a penknife.
I'm using a hand-drill.

[IMGDEAD]https://fbcdn-sphotos-f-a.akamaihd.net/hphotos-ak-prn1/941678_10200313530495698_417853605_n.jpg[/IMGDEAD]

I also found a good deal on some batteries: 6v @10Ah for £13 each. I bought a pair, which means I have a split supply if I want it.
[IMGDEAD]https://fbcdn-sphotos-b-a.akamaihd.net/hphotos-ak-ash4/401832_10200337022162975_256881414_n.jpg[/IMGDEAD]

Here's where I'm at so far. A couple of pieces need sanding to size, then they'll all fit together.
[IMGDEAD]https://fbcdn-sphotos-d-a.akamaihd.net/hphotos-ak-ash4/408419_10200337022962995_2023604554_n.jpg[/IMGDEAD]

I also cut and glued a load of small (~30mm "square" - my definition of square is as loose as it gets) bits of MDF together. Can anyone guess what they're for?
Hint: on most of them, only one edge is lined up.

[IMGDEAD]https://fbcdn-sphotos-f-a.akamaihd.net/hphotos-ak-ash4/270893_10200337023683013_683830725_n.jpg[/IMGDEAD]

Chris
 
Capacitive coupling between the preamp and power amp ought to work. But you might also be able to use the power supply for the power amp for the preamp also. The power amp most likely has a reference or quiescent voltage of +6V with respect to ground. Using that to power your opamps, ground becomes V- = -6V and +12V becomes V+ = +6V.

Not knowing anything about the power amp that you plan to use, I can't really say, but in essence in one way or another the power amp must have the circuitry to effect a bipolar supply.

Regards,
Pete
 
Here's one of the amps:
NEW MKll TA2024 Fully Finished Tested PCB Power Digital Amplifier Board 2x 15W | eBay
Its a simple switching amp operating BTL to give ~15w/ch.

The other amp (amp6-basic from 41Hz) is similar.

The former will probably drive the mids, the latter the bass drivers, as I can attach a small heatsink to that one. Realistically, I don't expect heat dissipation to be a problem as there's gonna be a couple of 40mm fans keeping the air circulating. Even so, its a small peace of mind measure.

I was going to split the supply at the batteries to get 6-0-6 for the pre-amp and 0-12v for the power amp.
The problem is, so far as I can tell, the 6v difference between the pre-amp output and the power amp input. A capacitor will block it, but won't stop the loud transient upon switch-on.

If I switch the pre-amp on, wait a couple of seconds, then switch the power amp on, I suspect that'll prevent the switch-on thump. It'd be nice to do it with a delay circuit and a relay, but I was just thinking of using a pair of switches, one for the pre-amp, one for the power amp.

Thoughts

Cheers
Chris
 
Ordered lots of T-nuts and some other bits, should be here tomorrow.

The T-nuts will go into the squares that I cut out, which will then be glued around the inside edge of the boombox. Then the screws go through the back panel into the T-nuts, giving me a removable back panel for accessing the drivers etc easily.

I've been routing wires and things while waiting for these parts to arrive. Just threw the drivers in to see what it'll look like...

[IMGDEAD]https://fbcdn-sphotos-f-a.akamaihd.net/hphotos-ak-frc3/971963_10200346914890287_601674307_n.jpg[/IMGDEAD]

Its currently upside-down as some glue is drying, but I'll rectify that later.
DVD included for scale. Its not tiny, but not massive either.

Chris
 
There is no information about how to interface to the input stage of the TA2024, so your guess is as good as mine, unless you have used that power amp before. Perhaps minimizing capacitance of the coupling capacitor would be the way to eliminate a large turn-on thump? To do that you would need to know input resistance of the power amp.

It would be better if the coaxial drivers were mounted on the outside surface of the baffle board rather than inside, but you probably know that and have a reason for doing it as your photo shows.

-Pete
 
There is no information about how to interface to the input stage of the TA2024, so your guess is as good as mine, unless you have used that power amp before. Perhaps minimizing capacitance of the coupling capacitor would be the way to eliminate a large turn-on thump? To do that you would need to know input resistance of the power amp.

It would be better if the coaxial drivers were mounted on the outside surface of the baffle board rather than inside, but you probably know that and have a reason for doing it as your photo shows.

-Pete

There's a big red 1uF cap on the input at the moment, and the input impedance is measurable from the other end of the cap to ground. Just can't remember what it is, off-hand.
I'll do some maths and figure the smallest caps I can get away with for each amp, and see how that goes.

If it needs it, I'll go with a 2-stage switch-on.

There's a couple of reasons for mounting the coaxial that way...
1 - it has a crossover attached that won't go through the mounting hole
2 - it has a nice rubber sealing gasket for rear-mounting, as KEF initially intended
3 - time alignment

I'll take a file to it to remove as much wood as possible around there, as I know the close-by boundries will create lots of reflections and frequency response anomalies.


After a very productive afternoon, its making sound.
The batteries are, for the moment, cable tied together, and the amps have half a 3.5mm jack lead soldered to each.
I'm using Foobar2000 via a cheapy 6-channel soundcard to crossover around 300Hz 2nd order, and I must say that, on first listen, it sounds rather good.
I'm currently using a finger-twist fuse holder as an on-off switch, so its a bit of a kludge (for lack of a better word) at the moment.

I've bought a couple of recessed speaker panels for mounting controls on the front and connectors on the back.

Photos soon,

Chris
 
You could put a strip of 3 mm (1/8") masonite (also called hardboard) along the bottom of the baffle, widen the existing holes in the baffle for the coaxial drivers for them to fit thru, and attach the coaxial drivers to the masonite from behind.

Not having a "wave guide" in front of the coaxial drivers I think would be of more benefit than improved time alignment.

Probably the appearance would be better if you covered the entire baffle with masonite or some other thin wood, but you are probably not very keen on making some more thru holes with your drill.

How did you go about measuring input impedance of the power amp?

-Pete
 
I measured the other end of the input cap to ground.

Decided I'll be going for active crossovers, anyway, as I'm finding the Kef amp (as in, the amp driving the kef speakers) clips too early with LR2 at 300Hz. I'll be taking the crossover up, perhaps to 600Hz, so that I get maximum output before distortion hits.

Still playing with Foobar2000 at the moment.

I'm trying to get the outside closer to finished, as I haven't been able to do much listening to decide what to do for the electronics.
Chances are it'll have a master volume and a LF attenuation dial, as the woofers come in ~4dB too hot with the gains leveled on Foobar.


Today I've mounted an XLR and SpeakOn connector plate to the back. There'll be an XLR input, and the 4-pole speakon will function as a power-link/charger/mains power connection.
There are DC sockets that'll do what I'm after, but none of them were up to 12A currents (theoretical peaks, I know, but even so...) the battery might well see.

Chris
 
With it in a half-finished-but-working state, I took it to the park yesterday afternoon, and stayed until around 10.30pm.
Turned up, it was reasonably loud until past 50m listening distance. I hadn't expected the sound to go that far, to be honest.

The 4-pole speakon power link I made up worked nicely, and no fuses were blown.

Turns out I need between 6dB and 0dB of attenuation on the woofers, depending on circumstance, so I'll probably put a resistor underneath a pot so that the level can be varied once the pre-amp's done.

For now, 600Hz 2nd order seems to keep everything running nicely, and I've decided that the co-axials definitely need something doing to get the upper midrange a little nicer. As Pete mentioned, the miniature waveguide in front of the coaxials is doing weird things to the sound.

I think I'll just strip everything out and put a nice big chamfer on there, which of course means a lot of work with a file. I think the improvement in sound will be worth it, though.

Hoping to get photos up soonish

Chris