AMP6-Basic backpack stereo units

[MickB]

Here's a pic of three AMP6-basic backpack stereo units I've built to link in with Londonskate's street skate sound system






The complete sound system consists of an 8 Freight cargo bicycle carrying the main amplifiers and speakers, plus a number of backpack repeater units carried by skaters which are linked by radio to the bike. There can be several hundred people taking part in the skates and the bodies tend to absorb the sound, so multiple synchronised units helps to provide a good spread of music throughout the skate.

I'm very pleased with the way the AMP6 works, it's excellent efficiency means we can get a good long run time with just 10 AA Ni-MH cells, and it sounds good. The units weigh only 2.5kg so they are easy to skate with. The speakers are Directed Audio SX-650 (6.5 inch) with neodymium magnets.

This forum has been a tremendously useful source of information, I'm no audio expert but thanks to all the helpful people who post on here I know a lot more now than when I started the project. Thank you all!

QUESTIONS:

I now want 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 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:

1. As I'm using 8 ohm load will it be better to use AMP4 or AMP11-LV?

2. Will either of those amplifiers be able to provide the full 100W output when powered by 24V.

3. Assuming it can manage 100W output, what is the average current draw from 24V at full power?

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 seem to be a reputable brand, and it's just about affordable.

Any help appreciated!

[mikechw]

If you are going to build mono...

Choose AMP11-LV...

AMP4 is already bridged and it can be parrallel.
Basically AMP11-LV is a parralleled AMP4.

With 24VDC ==> 17Vrms ==> 17 / 8ohm = 2.125Arms
2.125Arms x 2.125Arms x 8 ohm = 36.125Wrms

17 / 4 ohm = 4.25Arms
4.25Arms x 4.25Arms x 4 ohm = 72.25Wrms

If you need more power choose a 4ohm speaker driver ...

[MickB]

Thanks, mikechw. I hadn't realised it was that straightforward to work out. If anybody knows of a suitable speaker I'd appreciate any info. It has to have neodymium magnet to save weight and it has to be efficient.

Just out of interest, how do people manage to drive high power bass speakers in 12V car sound systems? Do they just use very low impedance speakers?

[mikechw]

In order to have a higher voltage supply in car systems,
they either use a charge pump voltage regulator to boost the voltage or
use an inverter to transform DC into AC, then through transformer to get a suitable DC voltage.

[Scratchy]

Yes, auto sub-woofers can go down to 1 ohm (dual 2 ohm voice coils run in parallel)
Here's a 4ohm neo woofer (most neos are 8 ohm):

http://www.parts-express.com/pe/show...number=264-833
Power handling: 120 watts RMS/240 watts max *VCdia: 2" *Impedance: 4 ohms *Re: 3.20 ohms *Frequency range: 30-800 Hz *Fs: 30 Hz *SPL: 84 dB 1W/1m *Vas: .66 cu. ft. *Qms: 4.26 *Qes: .31 *Qts: .29 *Xmax: 12 mm

You'd also get more power if your ran the direct audio 5.25" speakers at 4 ohms each - not in series.
The AMP6 can handle 4ohms just fine.
You should also look at some of krilli's posts on LiPo batteries in this forum (lighter and more power than AA NiMh).

***It's so easy, everyone should be doing it***

[gnome]

to produe sub bass, especially outside, you have to move a lot of air, and you're going to be pretty disapponted with any backpack sized sub, that 99dB efficency woofer is not going to be that efficent at sub frequencies (less than 100Hz) especially in an undersized box,
I think you're better off just placing a few folded horn subs in the middle and skate around them like these
http://www.billfitzmaurice.net/T39.html
at 14" width with a 1.8Kg Eminence basslite S2010
you could conceivably make a backpack that comes in at 10-15Kg, pity the bastard who has to skate around with that


or you could just fake it, with a sub harmonic harmoniser like this
http://www.waves.com/Content.aspx?id=337

[gnome]

really coo idea btw, and I wasn't expecting the lights :lol:
what sort of radio transmitter are you using? is it just FM?

[gnome]

ok I read the link, thats very very cool,absolutely love the mod bike, so scratch the subwoofer,
what you want is one or better two of these
http://www.billfitzmaurice.net/Jack.html
those car speakers can have very good sound, but they simply don't bring the SPL, what you really want is a PA type speaker, like the Jack10, it has really useful bass down to 60Hz, and wide dispersion, so in principle you wouldn't need the backpacks anymore, but they are so cool I'd keep them anyway, and you might find this hard to believe but you could actually comfortably power those with an amp6 thanks to the much greater efficiency, and you wouldn't have to cart around a huge battery either.

[Saturnus]

You don't actually need to produce frequencies below around 90Hz outside. If you have a slight rise in the midbassband around 120Hz the brain is automatically fooled to think there is subbass when in fact there is not. This trick only works outside. It doesn't work in a very large room, only outside.

It's because we have a inbuilt filter that disguish sound differently when outside or indoors, just like the brain also filters the input from our eyes differently when we're outside. You won't ever notice this because evolution has perfected it so that we don't ever see or hear it happening.

[MickB]

Thanks for all the replies, they're really helpful as I'm still finding my way with this stuff.

quote:Originally posted by gnome

really coo idea btw, and I wasn't expecting the lights :lol:

Thanks! The complete system was designed by StevieB and Sparky of the LondonSkate. I just copied the ASBOX backpack repeater idea and made these AMP6 powered lightweight versions to link up with the rest. And yeah, I couldn't resist the temptation to use those silver speaker cones as nice shiny diffuse reflectors

quote:Originally posted by gnome

what sort of radio transmitter are you using? is it just FM?

It's a Trantec S4000IEM UHF system. The receivers are mounted on masts on the repeater units to get them above head height.

quote:Originally posted by Scratchy

You'd also get more power if your ran the direct audio 5.25" speakers at 4 ohms each - not in series.
The AMP6 can handle 4ohms just fine.

I think we're talking at cross purposes, the next version of the backpack is going to be a mono unit using the AMP11-LV. Originally I was thinking of 8 ohm load (2x Directed 5.25" 4 ohm in series plus 8 ohm woofer with crossover) but I've now decided to follow mikechw's suggestion of going with 4 ohm load so I'll need a pair of 8 ohm mid/high range speakers in parallel and a 4 ohm woofer.

quote:Originally posted by Scratchy

You should also look at some of krilli's posts on LiPo batteries in this forum (lighter and more power than AA NiMh).

I have looked into using LiPo/Li-ion batteries but I'm not entirely convinced that it's the way to go yet for a number of reasons. The AA Ni-MH I'm using are 2Ah Sanyo Eneloops which are robust and reliable, have a long life (if treated well) and have very low internal resistance so they can provide high peak current. Although Lipo would be lighter, the Tripath amps are so efficient that the batteries are already a small proportion of the total weight so we wouldn't save much. There's also the fire hazard when charging homebuilt Lithium battery packs. Laptops and their batteries have a lot of fail-safe protection circuitry built in but even they are known to catch fire occasionally. These units must be trustworthy and fool-proof, non-technical people must be able to just plug them into the charger and forget them without worrying about them catching fire if something goes wrong. And we've already got the peak detecting chargers for Ni-MH.

quote:Originally posted by gnome

ok I read the link, thats very very cool,absolutely love the mod bike, so scratch the subwoofer,
what you want is one or better two of these
http://www.billfitzmaurice.net/Jack.html
those car speakers can have very good sound, but they simply don't bring the SPL, what you really want is a PA type speaker, like the Jack10, it has really useful bass down to 60Hz, and wide dispersion, so in principle you wouldn't need the backpacks anymore, but they are so cool I'd keep them anyway, and you might find this hard to believe but you could actually comfortably power those with an amp6 thanks to the much greater efficiency, and you wouldn't have to cart around a huge battery either.

I'll pass that on to the bike guys, I know they're always looking at ways of improving efficiency, however I don't thing they're ready for a another full rebuild just yet.

The backpack units are definitely needed. The skate travels about 12 miles on average and although it's marshalled to keep everybody together, when there are several hundred skaters it can be 100 metres from front to back. Also, the skaters around the bike absorb the sound so the volume drops off quickly with distance. It's not like a stage setup where the speakers can project the sound above people's heads, we need the backpack repeaters to spread the sound throughout the skate. The present backpack units work very well, but it would be nice to have a couple more that put out more bass to fill out the sound.

DESIGN BRIEF:

With the 10x 2Ah AA cells powering the present AMP6 (2x25W) backpack we get about 6 hrs runtime which is more than enough. That means that 20x AA cells should be able to power the 100W AMP11 for a similar time, or maybe 21 or 22 cells to be able to drive 4 ohm load at full power. 22x AA cells weigh just over 600gms so that's not a problem. I'm aiming for a total weight of under 6 kg as that can be carried quite comfortably by a skater. So with those parameters fixed, I need a speaker/cabinet that can make the most efficient use of that power outdoors with a reasonable amount of bass. At a quick questimate I think I can build a sufficiently lighweight rigid cabinet with a net volume up to about 25 liters, although smaller would be nice, and preferably sealed as I think that generally means a smaller cabinet. I've used 9mm plywood for the present backpacks and it can be made very rigid with carefully designed internal bracing.

So it's a matter of finding a 4 ohm neodymium woofer that gives a reasonable compromise, and a pair of 8 ohm mid/high range that will be a good match for the woofer. There's probably no point in trying to get very low bass when there's less than 100W to play with, so Saturnus' idea makes a lot of sense. The Tang Band W8-1363SB Scratchy linked to looks interesting but unfortunately shipping is too expensive as I live in the UK, can't find anybody locally selling them.