by Jon Chappell
(Click images to englarge.)
When I’m not in my studio hunched over musical instruments and audio gear, I escape to the great outdoors and look skyward while engaging in my other hobby, flying radio-controlled model airplanes. I fly both big and small craft, gas- and electric-powered, but what has really taken off in the hobby lately is the advancement of electric flight, largely due to the development of better and better batteries (very similar to what’s happening with automobiles; it’s not the motors holding us back, it’s the batteries). Anyone in the aeromodeling hobby knows way more about battery chemistries, DC motors, and Ohm’s law than is probably healthy, and certainly more than the average musician—even ones that are technically inclined. They also have many more metering gadgets and diagnostic tools available to them that measure current, voltage, watts, thrust, and torque.
I used to keep my two passions separate, until I realized that the cutting-edge battery technology that was hitting the aeromodeling market first (and making my life easier on the flying field) can easily be repurposed for mobile music performance applications. If you don’t want to bother with the hassle and expense of renting a generator for an outdoor gig, or if it’s a power-less indoor gig that obviously precludes running a gas generator in the corner (brides' mothers tend to frown on this practice), consider the following inexpensive, simple, and lightweight (no marine or tractor batteries here!) way to construct and power a guitar amp that runs on batteries.
Though I had experimented with using my newer airplane batteries for music gear, it was my friend and fellow musician and aeromodeler Jim Molwitz who gets the credit for realizing the actual solution seen in the photos. With the help of another carpenter/modeler friend of ours, Tom Warner, who made the cabinet and installed a 15" EV speaker (see Fig.1), we managed to get a completely gig-worthy setup very inexpensively by cobbling together some components that don’t normally share the same environment.
Fig. 1. The back of our project, with the parts exposed. You see the blue battery just under the cabinet and below that the black watt meter we used for testing. Note the nice pine cabinet built by our friend Tom.
To make a completely self-powered self-contained amp—one that you can bring on a gig and play in a real-world situation—you need four things:
The above four elements are the minimum requirements for getting a sound you (and your bandmates) can live with. A combo amp (true to its name) builds those four components into a single enclosure, of course, and many people may not realize this when they use a combo. Nor should they have to. Taking for granted the separation of these stages is ultimately a good thing, because it keeps the musician’s mind focused more on the music. And when done well, as has been done in the classic combos of yore, it’s a testament to the amp designers’ success at integration.
Keep in mind the above setup does not include any effects nor tone-shaping circuitry. But most preamps include tone controls, and the most logical (if perhaps not ideal in all cases) place to incorporate effects—without increasing the number of components listed above—is also at the preamp stage.
A multi-effects processor neatly dispatches all the needs of a preamp and an effects chain in one box. With something as simple as, say, a Korg Pandora, you will have met all your power amp’s electronic requirements with a gadget the size of a cellphone and that clips to your guitar strap or belt. If you want something more, you can go with, say, the Vox ToneLab SE (which is what we used for our project, see Fig. 2). It’s the perfect match of portability and real-time functionality. It’s easy to see and stomp on, and it has an assignable pedal for wah, volume, and other expressive use.
Fig. 2. We used a Vox ToneLab LE which acted as a preamp, tone shaper, and effects chain. Plus, it’s DC-powered, which is critical to our project.
Once you choose your multi-effects, you have to plug its audio output into a power amp. A power amp? Here’s where most people would throw up their hands. While many effects pedals can be powered with batteries, most people think of “power amps” as behemoths the size and weight of cinderblocks powering stadium speakers, and with those fat cables carrying what is surely lethal doses of electricity. How would you ever meet the needs of such a monster? The answer is simple: those are AC-powered amps. You want a DC-powered amp. DC power amps are scarce in the musical instrument and pro audio arenas, but huge in another industry that's bigger than music and pro audio by magnitudes: automotive audio. So we’re going to use a power amp that’s supposed to go in a car as part of its audio system.
Of course, we have to supply power to two items in our rig, the multi-effects processor and the amp. The multi-effects is easy, because it’s already DC-powered. (When you plug your MFX into the wall, you’re converting AC to DC.) Warning: Make sure your unit is DC and not AC powered. Sometimes this info is not on the back of the unit, but in the manual or the power supply. A DC-powered unit means you can hook an appropriate battery to it directly, assuming of course you wire up the correct connectors. You can buy any rechargeable battery that meets the voltage and minimum current-draw requirements, but for this project we went with lithium polymer batteries, described below.
An amp designed to run inside an automobile (or other moving vehicle) runs on DC—meaning directly off a battery, not an electrical outlet like the ones found in your home. These amps are designed smaller, lighter, and easier to power than an AC amp of equivalent size. They’re also cheaper—just the ticket for our project! By simply connecting the two power leads from the auto amp to the battery, you’ve avoided a lot of intervening circuitry. It does mean that you’ll be relying on your preamp/multi-effects for almost 100% of your sound. But in these days of increasingly popular modeling and speaker emulation found within guitar processors, a clean power amp can be just the ticket.
In our project, we used a 40-watt Kenwood amp cannibalized from a customized 2002 Ford Mustang (see Fig. 3). It was the just right size for a small club gig, or in a larger venue where the amp would be miked through the P.A. and only needs to be loud enough for stage monitoring. The amp has two 12-gaug-wire power leads, with standard 3.5mm bullet connectors. We outfitted our batteries with the corresponding female receptacles (batteries should always have female connecters so the exposed leads can’t touch, which will instantly short circuit the battery and possibly cause a fire). We had a hookup.
Fig. 3. A 40-watt Kenwood power amp mounts easily on the inside of the cabinet and has no controls; just two 12-gauge-wire power leads.
The battery we choose in our project was the type known as lithium polymer. This is not a well-known chemistry outside of RC modeling, but the cells have better energy density than lithium ions (the kind used in everything from cellphones to laptops to cars), and just a little better than the new lithium phosphate (a.k.a. A123) batteries, which are appearing now in power tools. Li-po’s are more expensive and require special charging gear and more care, but for modeling—where fighting weight is a constant battle and light weight is a valuable attribute—it’s worth it. Each cell is rated at nominal (working) 3.7V, and since our pack was a 3-cell, we could produce 11.1V—enough to power our 12V automobile power amp.
A battery also is rated by two other factors, its capacity (measured in milliamp-hours, or mAh) and its discharge rate (called “C”). Our battery, made by Turnigy, is a 4,000 mAh battery, meaning it will run at 4.0 amps for an hour before it’s exhausted and needs a complete recharge. If you double the current draw to 8 amps, it will run for 30 minutes, 16 amps for 15 minutes, etc. Each time you double the discharge rate, you halve the time. Of course, you can use other multiples as well (e.g., 3 times the C rate yields 12 amps for 1/3 hr., or 20 minutes). A glance at the battery’s label in Fig. 4 shows that our battery has a “30-40C Discharge” rate. Let’s split the difference at 35C, which means you could potentially draw 140 amps without damaging the battery. That’s a whole lot of amps, and way more than we need to power a guitar amp.
Fig. 4. This rechargeable lithium polymer battery pack by Turnigy (containing three 3.7V cells) is rated at 11.1V, 4,000 mAh (4 Amp hours capacity) and a discharge rate of 30-40C (meaning it can safely draw between 30-40 times 4 A, or up to 160 amps, if only momentarily).
Electrical power is rated in watts, which is just Volts x Amps. So 11.1V x 4 amps = 44.4 watts, exceeding the limit of our amp. So with, say four batteries, we could play four really loud one-hour sets. That’s enough for a gig. And the batteries are only about $34 apiece, and will last upward of 1,000 charges (though they do lose capacity at the end of their life). We are more than covered in the power department. But as we'll see below, the reality is that we really need only one or two batteries. And the best part: this pack weighs about 12 ounces or just 3/4 of a pound.
For the multi-effects, we use a 3-cell battery with the same voltage (11.1V), but much lower capacity and C rate, as the current draw is so much smaller. In our case, it was a 1,320 mAh pack with a 13-20C discharge rate (see Fig. 5). Because we’re “driving” microcircuits on the level never exceeding 300 mA (0.3 A), this battery will last more than four hours before it needs recharging.
Fig. 5. To power the multi-effects we used a small lipoly battery: an 11.1V (3 cell) 1,320 mAh pack.
After hooking up the batteries to power the multi-effects and amp, and making the audio connections, we wanted to test our system. We took the additional step of inserting a watt meter between the battery and the amp. This gave us a realtime view of our energy use on several levels. The voltage stayed a consistent 11.1V, but the louder we played, the more amps we drew. We then watched the watt display, which is really just a calculator between the two dynamic elements (voltage and current), but that’s the reading we’re interested in. We were pleasantly surprised to see that even at loud volumes, we were way under the maximum rated output. This meant our batteries would last much longer than the “worst case” we predicted, and that we could do an entire gig with two batteries.
If you look at the watt meter below you see four display fields (see Fig. 6). Clockwise from top left: amp draw, voltage, capacity used in amp-hours, and watts. Note that in this photo, the math doesn’t work out among the amps, volts, and watts because the display constantly changes, and not all displays in sync (at least not quick enough for a fast camera shutter). But eyeballing the meter for a minute or so gives you a pretty good idea of your energy consumption.
Fig. 6. A watt meter measure amp draw, voltage, capacity used (in Ah), and watts. (Note that the math doesn’t work out among the amps, volts, and watts, because the display constantly changes, and the camera caught this photo in mid-transition.)
The only mod we added was an intervening potentiometer between the multi-effects and the Kenwood amp (see Fig. 7). This wasn’t essential, but we liked having the additional flexibility of controlling the volume from the speaker cab rather than the ToneLab.
Fig. 7. We added a volume pot between the multi-effects and power amp input for additional level control.
A short video watching the watt meter respond to some blues guitar playing.
If all you’ve ever thought about self-powered rigs conjured images of gas generators or heavy car batteries, think again. A car battery is as heavy as carrying an additional amp! Our battery weighs less than a pound. You owe it to your back muscles to investigate the smaller, high-density batteries that are coming onto the scene seemingly every week. You may have to research your connectors, bone up on your soldering, get some metering gear, and even re-acquaint yourself with Ohm’s law, but this is a perfect project where all those skills will come together. And it will make you more independent from the tyranny of "bad club power" and free you up in mobile performance situations that require portable and unplugged power.
You must be a registered user to add a comment here. If you've already registered, please log in. If you haven't registered yet, please register and log in.
HarmonyCentral.com is the leading Internet resource for musicians, supplying valuable information from news and product reviews, to classified ads and chat rooms.