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[kipchoge spencer]

I have been touring with my band by bicycle for five years, carrying our instruments and pedal-powered sound system with us on our bikes. We've been co-producing little festivals along the way, and are now partnering with a production company to grow the festivals to 2000-2500 people.

I'm looking for advice on directions to go in designing the new sound system. Criteria in rough order of importance (from high to low):
 
- efficiency at converting bike power to sound, which relates to
- the total number of bikes required for the system (each bike produces about 80w)
- sound quality
- volume (feel the lows but not shaken by them)
- weight and size of components (40-50 pounds is an optimal weight for a component; 90 pounds is max for a component); total weight is also important; this system will be transported by bicycle, too
- cost
- appearance (does it look like a toy or like "damn, I'm gonna get rocked")

What we have now is good for about 400 people outdoors, what I'd call medium listening levels. It's two JBL PRX 512s or 615s for mains and different monitors depending on the situation (maybe an Eon 510 or PRX 612).

A great sales engineer at Sweetwater Sound really took a genuine interest in helping us design a bike powerable system. He's proposed a "mini line array" called K-array (2 x KR-200), with its own subs (2 x KL18MA) and then some (6) satellite Class D powered QSC 8" speakers on an arc 100' out in the audience. And an Allen and Heath iLive mixer and mix rack with the ethernet "snake", and in-ears. He ran the numbers (the manufacturers numbers) and said it would draw about 25 amps all told. I like the thinking, but I'm a little worried about the K-arrays because I can't find anywhere to test them before buying and because there's so little field experience to read about online.

Our power system has capacitors for the peaks, so we can generally get away with producing on average about 15-20% of the rated power of the sound system.

We've experimented with modifying the JBLs to run on direct (split rail) DC, instead of generator-capacitor-inverter and then conversion back to DC by the amp. This saves us A LOT in efficiency (maybe 30-40%) but sacrifices max SPL (I think) and the over/under voltage safeguards of the amps. Also, when the voltage gets too low, the speakers still work but sound quality suffers (we are able to build a bike power system that maintains constant voltage, though).

Advice much appreciated.

[kipchoge spencer]

Anyone have any thoughts about this, even if not the whole thing? Is there a better place to ask?

thanks!

[Christian Tepfer]

I still am puzzled by the idea itself. Takes a while to embrace the "all bike" thinking. Did you tour Germany already? We love green energy :-)

First of all, what voltage is the bike power system putting out? You wrote that you can maintain (relatively) constant voltages with a bunch of bikes.

I would definitely power the system with DC, there are DC powered systems but I don't think they are sufficient for the audience count.

Depending on the sub level required the current draw can be pretty heavy (and the sub amp's want the highest rail voltage), so that's your biggest concern, overall power and power shaping wise. A low cut higher than usual may be of help here.

I don't know the K-Array stuff. So no comment on that.

Line array elements have the upside to be portable (within you weight limit), as opposed to a bigger trap box system. Still, the subs are your problem.
If you can go the In Ear route for monitoring you will be save on that side.

All in all, more information needed, what kind of power system can you create (voltage, average current, peak current how high for how long)

[David Sturzenbecher]

Anyone have any thoughts about this, even if not the whole thing? Is there a better place to ask?

thanks!

I know Brian at http://www.speakerpower.net/ can make you an amp that runs on DC and are quite efficient. Also, most Danley speakers have cutouts for these amplifiers.

[Peter Kowalczyk]

The bike powered PA meme seems to be spreading.  As an avid cyclist and musician with tree-hugging tendencies, I think it's an awesome idea!  I've seen one in my area that has a high 'cool factor', but (in my opinion) leaves much room for technological and sonic improvement.

I've not implemented any such system myself, but from observations, I think a well conceived and executed power supply scheme is the crux.  As Christian said, an entirely DC-powered system will be more efficient.  While few self-powered speakers accept DC power, all amplifiers operate on DC rails.  Depending on your engineering comprehension and ambition, i suspect that connecting your bike-powered DC power system directly to the amp rails will be the most efficent and electrically simple approach.

I'd love to hear more about the generator system itself.  To implement the scheme above, you'll need to figure out a way to store and regulate the (presumably variable) output of your bike generators.  A capacitor bank may be sufficient, but depending on your output power requirements and the variation you'll see from the bike generators, an intermediate battery system (with associated charge / discharge control circuitry) might be required. 

Also, to properly power the DC amp rails directly, you'll probably need some sort of DC-DC conversion circuitry to step between the output voltage of your generator and the proper rail voltage of the amplifier.

It should go without saying that Class-D amps and switching power electronics are your friend, especially since you're transporting the system by bike as well.  I'd  hate to deal with external amplifiers and their associated heavy-gauge cabling.  There are a variety of relatively light-weight self powered speakers available that would be well-suited to this application.

Good luck!

[Jerome Malsack]

You should consider a battery system and charging control.
That way if the power has a fall off the battery can back up some. 

Add in some spin stationary bikes for the fans to user to help generate the power. 

[Jon C Thomas]

The main problem is going to be transporting the subwoofers.
I would consider a pair of dual wheel trailers with 4ohm neo 18" drivers in sonotube enclosures to be the minimum for outdoor events of that size. Anything else is going to be lacking low end or heavy. Pro subs of good quality are designed for nuclear abuse and weigh accordingly, (The K-Array subs are small and light but $2500 each )

Some of the better designed car amps will be efficient and affordable/replaceable. They can run off decent lithium batteries for hours and put out the 1000-2000 watts you will need.   

I think Dan at monkeyelectric modifies Peavey and JBL PRX boxes for DC,http://store.monkeylectric.com/product_p/msnd15.htm, and Saturanus (designer of the boominator) at DIYaudio and Speakerplans 12v forum can give some help to this project. Surprisingly 12v festivals are popular in Europe.

[kipchoge spencer]

Thanks so much for the replies, Christian, and everyone else. Sorry it took me a while to respond, I've been away at a festival.

Yes, we did tour Germany, and most of the rest of western Europe, and will be back before long. I agree, you do love green energy !

We have the capacity to put out a wide range of voltages, both by choosing different voltage generators and/or capacitors. And we can of course do direct DC-DC conversion, too. So, we can put out anywhere from 12-50V, and could probably do more if it made sense. The capacitors give us lots of storage and all the regulation we need. There's a dump circuit that activates when there's too much system voltage and we need to put it somewhere.

In my dream world we'd have about one rider per 100 people in the crowd. We average about 90 watts out of an average pedaler. So for a 2000 person show, that'd be 20 pedalers and 1800 continuous watts. I would add ten pedalers if it would make a big difference in sound quality, or was needed just to make it loud enough.

An engineer buddy of mine recommended the EAW LA-400 folded horn subs for their extreme efficiency. They're a bit on the extreme side as far as weight and size, though. Anyone know of anything similar but smaller?

Yes, Jerome, the fans do all of the powering.

I'd  hate to deal with external amplifiers and their associated heavy-gauge cabling.  There are a variety of relatively light-weight self powered speakers available that would be well-suited to this application.

Good luck!

Thanks, Peter. I was wondering about this. On the one hand, it seems conceptually lighter to have just one amp doing all the work, and not having to send power to each box. But are you saying that the cabling that passive units require will end up weighing more than the active amp modules that they eliminate?

Jon, I'm guessing that Dan is doing a similar mod to the PRX as we are, and it sounds good as long as you keep the voltage high enough. I've yet to hear any 12v systems that sound good with live music. Have you heard otherwise?

Thanks for the link, David. I'll get in touch with Brian.

I still am puzzled by the idea itself. Takes a while to embrace the "all bike" thinking. Did you tour Germany already? We love green energy :-)

First of all, what voltage is the bike power system putting out? You wrote that you can maintain (relatively) constant voltages with a bunch of bikes.

I would definitely power the system with DC, there are DC powered systems but I don't think they are sufficient for the audience count.

Depending on the sub level required the current draw can be pretty heavy (and the sub amp's want the highest rail voltage), so that's your biggest concern, overall power and power shaping wise. A low cut higher than usual may be of help here.

I don't know the K-Array stuff. So no comment on that.

Line array elements have the upside to be portable (within you weight limit), as opposed to a bigger trap box system. Still, the subs are your problem.
If you can go the In Ear route for monitoring you will be save on that side.

All in all, more information needed, what kind of power system can you create (voltage, average current, peak current how high for how long)

[Riley Casey]

Thirty years ago, when amplifier power and speaker driver power capacity were both limited and expensive, all speakers were horn loaded.  Your limited electrical power supply drives you to this same need for efficiency but without the capacity for the large size horns required for low frequency transducers.  It's time to think outside the box - literally.  This may require some trade offs in fidelity and program. Let's face it, loud rock music is born of the industrial worlds' supply of cheap and plentiful electricity.  Some options are probably not going to be off the shelf products. You may need to build horns onsite from smaller components that bolt together.  Google Community Light & Sound Leviathan Fiberglas bass horns from the 1970s for some ideas.  Built in power amps may or may not be the most efficient solutions when you do the math.  If a speaker requires 1000 watts you will need the copper wire nessesary to deliver that power regardless of if its 24 VDC or power amp output.  This is very literally a math project as much as anything else as you are working outside the box of off the shelf solutions and inventing as you go along.

An engineer buddy of mine recommended the EAW LA-400 folded horn subs for their extreme efficiency. They're a bit on the extreme side as far as weight and size, though. Anyone know of anything similar but smaller?

Yes, Jerome, the fans do all of the powering.

Thanks, Peter. I was wondering about this. On the one hand, it seems conceptually lighter to have just one amp doing all the work, and not having to send power to each box. But are you saying that the cabling that passive units require will end up weighing more than the active amp modules that they eliminate?

Jon, I'm guessing that Dan is doing a similar mod to the PRX as we are, and it sounds good as long as you keep the voltage high enough. I've yet to hear any 12v systems that sound good with live music. Have you heard otherwise?

Thanks for the link, David. I'll get in touch with Brian.

[Peter Kowalczyk]

On the one hand, it seems conceptually lighter to have just one amp doing all the work, and not having to send power to each box. But are you saying that the cabling that passive units require will end up weighing more than the active amp modules that they eliminate?

As Mr. Casey noted, it depends on your specific application.  Either way, cabling should not be ignored.  The impedance of a run of copper cable isn't neglible, especially when the total load impedance (of your drivers) is nominally <10 ohms.  Hypothetically speaking, if your round-trip cable impedance is 1R, and your driver is 4R, then 20% of the voltage generated by your amplifier is dropped across the cable, not the driver - not very efficient.  If you need a long run between amp rack and speaker, then you need heavier gauge cable to mitigate the loss, and this adds up in weight and cost. 

Self-powered speakers mean that the cable between amp and driver is short, and cable losses are much less significant.  That said, you then need to deliver power to the speaker, and the same considerations apply.  If you're distributing low-voltage DC across any distance, then voltage dropped in the cables could be significant...