I now wanted to build a more powerful mono backpack unit capable of providing a lot more bass.
I'd like to use a 24V battery pack which means using either a bridged AMP4 (can't be bridged. ed.) or an AMP11-LV. Speakers will probably be a single Monacor SP-10A/250NEO 8 Ohm 10 inch bass/mid range speaker and a pair of 4 Ohm Directed Audio SX-525 (5.25 inch) speakers (connected in series to give 8 Ohm together) with a simple crossover.
Couple of questions I had:
1. As I'm using 8 ohm load will it be better to use AMP4 or AMP11-LV?
Decided on a AMP-11.
2. Will either of those amplifiers be able to provide the full 100W output when powered by 24V.
No, that would be with ~32V and 4ohm speaker. (laws of physics apply, unfortunately)
3. Assuming it can manage 100W output, what is the average current draw from 24V at full power?
At 24V and a avg. 8ohm, that can give peaks of 3A. But Pmax would be about 25W.
4. Am I being over-optimistic about powering a 10 inch speaker from such a modest power supply? I've chosen it because it's very light for it's size (neodymium magnet), it's efficient (99db 1W/1M), Monacor seems to be a reputable brand, and it's just about affordable.
25W with 99dB sensitivity would do great.
Well, the speaker box is built.
I'm sure there are lots of mistakes from an acoustic point of view but there comes a point where one just has to get on and build it. I'd be glad to hear any criticisms though as I may decide to build another one. I was tempted to go with gnome's suggestion of a pair of tweeters set at 90 degrees, but in the end decided to go with 3 splayed drivers to achieve wider dispersion at the expense of some efficiency. I've arranged them on a common arc to minimise comb filtering in the overlapping regions, does that make sense?
The 2 slightly larger holes at the corners of the baffle are for ports. The box has a slight "ring" to it when it's tapped and I wonder whether I've inadvertently constructed a bell with the ring of bracing around the woofer opening and too much symmetry. It is very rigid though.
Finished at last!
Well, it'll probably need a few tweaks but it's ready for it's first outing at Bognor Carnival this weekend. Huge thanks again to everybody on this forum for all the help and advice, it wouldn't have turned out nearly as well without your invaluable input. And of course to Jan for his excellent amplifier! Particular thanks to V-bro for his fine workmanship building the AMP11 and for recommending the tweeters and all sorts of other stuff.
I'll write up a proper article later but in the meantime here are some pics:
After much trial and error and frustration I've at last worked out how to paint things properly!
Amp bolted to heat transfer block and heatsink.
Battery pack and cross over. For reasons I'll explain in the article I've gone with 20 cells arranged as 2 banks of 10 cells in series for 24V nominal. Each bank will be charged separately.
These are all the parts made from various sizes and thickness of PVC pipe and gutter. It's incredibly useful stuff!
Small surface mount LEDs are glued to the underside of the grille with wires soldered directly to them. As their forward voltage is only 2V and I've got 24V supply they all run in series so total current draw is only 25mA. I've sprayed the speaker cone silver which acts as a reflector.
Bit of a squeeze for all the wiring as I decided to add a digital voltmeter after I'd built the box. One of the small boards screwed to the bottom of the recess carries the input transformers and the other is the low voltage cut-off which protects the batteries from being damaged by over-discharging. The cut-off consists of a MAX8212 voltage monitoring chip and a small relay controlled via a transistor.
When the voltage drops below about 21.5V the MAX8212 cuts the power to the transistor which in turn cuts off the relay. The chip is supplied via a diode with a capacitor across the supply which acts as a reservoir so that the cut-off isn't triggered by momentary voltage dips caused by big bass notes.
I added the voltmeter due to the necessity of charging the 2 banks of batteries separately. There's a 3 position toggle switch above the meter which selects the reading for the individual banks in the left and right positions, with the middle position showing the total supply voltage. It makes it easy to check that both banks have been charged before use, and the total voltage reading is useful for judging the remaining run time. The rotary ON/OFF switch incorporates a disk which covers the 2 charging sockets. The 2 holes ensure that the sockets are only accessible in the OFF position. The switch below the voltmeter is for the lights.