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[Frank DeWitt]

Mike brought up Inverter vs conventional generators for running switching power supply powered speakers.  I thought I might learn something if we spread it out a bit.

The goal is to amplify the output of a mixer a lot so many people can hear it outdoors.  You must bring your own power.

It seems that there are three places that choices are needed.
Power source  Inverter generator,  Conventional generator,  Batteries

The amp or amps could be old school power supply. switching power supply or 12 volt DC input amp

The speakers could be popular speakers used today or some sort of high efficiency box.

For example,  Would it help to modify an amp by adding very large capacitors to the rails (In a professional manor, to a old amp that had no warranty and volunteered for the test.)

Would it be more efficient to use deep cell marine batteries and amps made for automotive use?

[Jordan Wolf]

Frank, I remember hearing that one of the Danley Sound Labs passive loudspeakers (Jericho?) has been driven by only an iPod.  Can't seem to find it via a web search at the moment...

[Mike Sokol]

Frank, I remember hearing that one of the Danley Sound Labs passive loudspeakers (Jericho?) has been driven by only an iPod.  Can't seem to find it via a web search at the moment...

On that note, when I was a wild child back in high school in the early 70's I had a pair of Altec Lansing Voice of the Theater cabinets for my band. Yes, these were the ones with the big 500-hz horns on top. I would leave them in the back of my van and hook them up to my Channel Master 8-track player with all of 5 watts output per channel. That small amount of power though a very efficient speaker cabinet was enough to get us kicked out of any McDonalds parking lot we wanted to terrorize. Sorry I don't have an official SPL reading, but it was loud enough to compete with the Thrush mufflers and cut-out pipes of the day. 

So to design your off-the-grid sound system I think it's best to pick a really efficient speaker then work your way upstream to the power source, not the other way 'round. Once you pick the initial crowd size and SPL level, you can predict just how many speakers you need for coverage. And yes, the music style has a lot of do with this since bass takes a lot of wattage from the power source.

[Frank DeWitt]

a pair of Altec Lansing Voice of the Theater cabinet hook them up to my Channel Master 8-track player with all of 5 watts output per channel.

So you honestly think that a pr of voice of the theater cabs with 10 watts in could compete with a Behringer B207 with "150 watts"?

I do, as well.

[Art Welter]

So you honestly think that a pr of voice of the theater cabs with 10 watts in could compete with a Behringer B207 with "150 watts"?

I do, as well.

The Behringer B207 is rated at 116 dB output from 150 watts, which implies about an 84 dB one watt one meter sensitivity.
An A-7 cabinet is about 103 dB sensitivity midband, so one could produce around 110 dB from 5 watts, a pair with 10 watts would do 116 dB.
The pair of A7 would require only 1/15(6.66 %) of the power to achieve roughly the same level as the little speaker.
Large, multi- driver horn loaded speakers can further increase sensitivity by another 10 dB, either getting twice as loud, or lasting 10 times longer yet on the same power. In the low frequency range, the sensitivity of large, multiple driver horns could easily use 100 times less power than the little speaker for the same SPL.

As far as capacitor storage, they can reduce peak power demands on batteries, which reduces the Peukert's effect which is a big problem with small lead acid batteries. Peukert's effect, simply put, is when too much amperage is pulled from a battery it's voltage will drop (causing a "brown out"), the voltage then slowly returns to it's former level after the battery rests. The battery "pukes out" in use, normally not a problem in a car application where power is being provided from the alternator. Still, even with capacitor storage, the average power used remains the same, and capacitors are not cheap.

I am using  GBS-40Ah batteries from http://evolveelectrics.com/GBS.html for my electric motorcycle.

Four in series are nominally 12.8 Volts,  weigh only 12.4 pounds, cost $248.
The amazing thing is you can pull peaks of 400 amps out of the pack with hardly any sag.
You can get by with much lower rated Ah (amp-hour) LiFP04 than lead acid because they have virtually no Peukert's effect, and the same battery weight gives like 10 times the performance.

As an example, 72 pounds of AGM lead acid batteries dropped to a 40% charge after only 4.2 miles (the bike started limping along), while 52 pounds of LiFP04  went 39 miles. Other than expense, the problem with LiFP04 is they can be permanently damaged from too deep a discharge, and because they exhibit almost no Peukert's effect, they perform almost the same until they are dead, with a much tighter range of voltage over the entire discharge cycle.

As far as whether to use amps made for automotive use, or an inverter and high voltage amps, it would depend on the output requirements. On the smaller scale, eliminating the inverter reduces one stage of loss, while on a large scale the loss of power in the inverter may be offset by efficiencies gained from higher voltage operation. That said, with a good battery bank providing power to some good high power car audio amps in to speakers with 110 dB sensitivity, a large space could be covered at concert levels for hours.

Art

[Scott Helmke]

Frank, I remember hearing that one of the Danley Sound Labs passive loudspeakers (Jericho?) has been driven by only an iPod.  Can't seem to find it via a web search at the moment...

That first watt is always the loudest... after that it's diminishing returns.

Danley speakers are *very* efficient, though. Well worth a look if you're trying to get the most out of limited power.

[Mike Sokol]

That first watt is always the loudest... after that it's diminishing returns.

Danley speakers are *very* efficient, though. Well worth a look if you're trying to get the most out of limited power.

We often forget that early sound systems from the 60's and 70's had very small amplifiers, with a pair of 6L6 tubes pumping out all of 40 watts. And an Altec A7 VOT cabinet driven by a 40 watt amplifier was significantly loud. Once big amplifiers became relatively cheap (Phase Linear was one of the first IIRC), then speakers became less efficient (Bose 901s and their spawn). So comparing watts to watts on speakers is pretty useless unless you consider speaker efficiency as well.

I also built a pair of JBL double 15" scoops around that time, and drove them with a Dynaco 400 amplifier rated for 300 watts per channel at 4 ohms IIRC. That made a LOT of bass in small clubs which I needed for my Mini-Moog's low notes. Those were the days...

[Mike Sokol]

The goal is to amplify the output of a mixer a lot so many people can hear it outdoors.  You must bring your own power.

I'm going to up the challenge a bit and suggest that you also need to consider stage lighting as well for your OTG (Off The Grid) system. So let's say you wanted to create a weekly "green" outside concert series that was only powered from the solar cells you use for recharging your batteries during the week. Just how many solar cells  and batteries would you need to have a decent concert on Saturday night? Limiting the musicians to low power backline gear such as Line-6 guitar and bass pedals would be a start. And certainly LED lighting for a 3-hour show wouldn't draw too much power. Efficient speaker and amplifier selection would be very important as well. Could you get a decent 100 dB SPL concert mix for a few thousand people in a "Green Shed" powered by nothing but sunlight?

[Cailen Waddell]

I'm going to up the challenge a bit and suggest that you also need to consider stage lighting as well for your OTG (Off The Grid) system. So let's say you wanted to create a weekly "green" outside concert series that was only powered from the solar cells you use for recharging your batteries during the week. Just how many solar cells  and batteries would you need to have a decent concert on Saturday night? Limiting the musicians to low power backline gear such as Line-6 guitar and bass pedals would be a start. And certainly LED lighting for a 3-hour show wouldn't draw too much power. Efficient speaker and amplifier selection would be very important as well. Could you get a decent 100 dB SPL concert mix for a few thousand people in a "Green Shed" powered by nothing but sunlight?

It would be all about the solar array and batteries then....   Give me an acre of solar panels and we are good to go .   

Seriously though - would you give thought to a grid tied system to eliminate the batteries?   With appropriate solar cells, you could potentially make the concerts free with the electricity offset....

[Chris Hindle]

I'm going to up the challenge a bit and suggest that you also need to consider stage lighting as well for your OTG (Off The Grid) system. So let's say you wanted to create a weekly "green" outside concert series that was only powered from the solar cells you use for recharging your batteries during the week. Just how many solar cells  and batteries would you need to have a decent concert on Saturday night? Limiting the musicians to low power backline gear such as Line-6 guitar and bass pedals would be a start. And certainly LED lighting for a 3-hour show wouldn't draw too much power. Efficient speaker and amplifier selection would be very important as well. Could you get a decent 100 dB SPL concert mix for a few thousand people in a "Green Shed" powered by nothing but sunlight?

Sure. What's the battery budget.... Hope you have deep pockets....
I participated in an experiment a few years back. Before LED lighting really took hold.
There's gonna be some big-ass solar cells to get those babies charged up. Around here, I would count on 3-1/2 to 4 days of full sun per week. Just means more cells, and more batteries.
"Green" and "Off the Grid" all sounds fine and nice on paper. Then the realities sink in..... Nothing beats a 25KW sitting behind the charger shed, just in case...
SunSparky estimated our loads, and figured we'd be good for 4-5 hours. (sorry, don't remenber the VA he came up with)
After an hour's worth of showtime, squints went to Jenny. After the next band changeout, so did audio.
SunSparky was, ahh, "puzzled" to say the least.
I'm guessing he calculated based on a steady state draw. A live show is anything but.
The sun/battery rig was "retired" from R&R and did sponsered dinner party / background canned music type stuff, where it worked very well.

[ProSoundWeb Community]