Looking for generator, or battery options for amp on the go

bluesman39 · #1 07-18-2010, 06:15 PM

I sometimes play guitar w/ brass bands in outdoor situations where plugging in is impossible, sometimes even marching in parades. I've been using a 15 watt Traynor portable(battery-powered) amp which gives 10 hrs. of amplification, but it's not loud enough. I've been told that using a car battery and a converter, then plugging a regular amp in, is a good option. (I'm not worried about transportation or size, I can work that out on my own.) If anybody has details on that method, I would greatly appreciate their sharing. Also, if anybody has generators for sale, or knows where to find them and more specifics, I would love to know.

randyc · #2 07-18-2010, 06:29 PM

That would be practical (BTW the correct name for a device that converts D.C. to A.C. is "inverter"). There are many inexpensive models available. You'll need to determine the power consumption (not power output) of the amplifier you intend to use in order to select the proper inverter.

Since most amplifiers, especially vacuum tube models, don't work well at lower than their 120 volt A.C. design value, you'll need to add some safety margin to the inverter capability, otherwise the inverter output voltage will "sag".

As an example, if you've determined that the consumption of your amplifier is 85 watts, I'd select an inverter with at least 50% power margin; multiply the power consumption by 1.5 to determine the inverter capability. In the example of 85 watts, the inverter should be rated at 125 watts, minimum.

Good luck

PS: since inverters frequently produce electromagnetic interference, you may be advised to look for models that are specifically designated for stereo equipment, sound reproduction equipment and the like. These are likely to be more appropriately filtered and satisfactory for your purposes.

TieDyedDevil · #3 07-18-2010, 07:17 PM

To add to what Randy said, you'll also need to size the battery according to how long you'll need to operate the amp. Batteries are rated by amp-hours. For example, a 5 amp-hour battery could deliver 1 amp for five hours or 0.1 amp for 50 hours. You need to be cognizant of the maximum continuous discharge rate, which is spec'd as a fraction of capacity (C). For example, if the battery above was spec'd at a maximum continuous discharge rate of 0.2C, you'd be able to draw 1 amp continuously.

To put this all into perspective, let's say that you find that you'll need a 120-watt inverter to run your amp. A watt is simply power times voltage. Since the inverter puts out 120 volts (nominally, at least for the US market), it can deliver a maximum of 1 ampere at that voltage.

At the *input* to inverter (i.e. where the battery connects), you still need 120 watts. (Actually, the input power will be a bit more than that because the inverter isn't 100% efficient. But I'm going to ignore that because most modern inverters are in the 90% range for efficiency. In fact, I'll be approximating some other numbers as well. You have some wiggle room since your amp rarely draws the full rated current.)

Now since the nominal input voltage to the inverter is 12 volts, the battery must deliver (approximately, see above) 10 amperes to produce the inverter's full 120 watt output. If you need to play for an hour, you'll draw 10 amp-hours from the battery. Play for three hours, and you'll draw 30 amp-hours from the battery.

Remember what I said about the maximum discharge rate? You need to divide the expected consumption by the battery's maximum discharge rate. If the batter is rated at 0.2C continuous discharge capacity, and you want to be able to play for three hours using your 120-watt inverter, you'll need a 150 amp-hour battery. (That's 30-amp hours divided by 0.2C.)

You can look up specs on batteries and plug in your own numbers.

I suggest that you look into so-called "marine" batteries; these have a higher continuous discharge capacity than typical lead-acid batteries, making them better suited for use with an inverter.

It has been a while since I've looked into this, but you should be able to find an all-in-one unit with an inverter and a battery in one easy-to-carry package. Poke around on Amazon; something should turn up.

If your engagements run more than a few hours, I'd suggest looking into a small gasoline-fueled generator. The small generators runs run quietly enough that you could tuck one away behind the bandstand and run an extension cord to your amp.