While the use of a dc to ac inverter was addressed for a specific circumstance in a recent thread, I think it useful to expand this to the general case for those who want to or need to play off-grid.
There are a number of battery powered devices that convert the battery’s dc voltage to 120 V ac. The most common, least costly, and most readily available types work off of 12 V batteries. They typically have either a “cigarette lighter plug” to plug into a vehicle’s accessory 12 V power outlet and/or battery clamps to connect directly to the vehicle’s battery or to a standalone 12 V battery. Others have internal batteries. These latter include UPS systems for computers, the most costly type.
If you use a stand-along battery for one of those 12 V dc to 120 V ac inverters, always use a sealed lead acid battery for safety reasons. There are also “gel-cell” batteries but those are not addressed here as they are different animals being far more costly and requiring specialize chargers. You will also need an automotive-type 12 V charger to recharge it. Always use an “automatic” charger to avoid over-charging. Here’s how to select what you need
1. Select an inverter with at least 1.5 times the ac power input in watts listed on your amplifier.
2. Select a battery with the Ah rating you need for the hours of playing time using the calculations below by trying different Ah values.
3. Select a “automatic” 12 V charger based on how many hours of charging time you can tolerate. Calculate this as follows: Battery Ah rating / Charger rating = Charging time Hours. Add 10% of extra time to totally top off the battery.
There are several things you should know before powering your bass or guitar amplifier from one of these devices.
1. Most of the reasonably priced devices output a “modified sine wave.” This is essentially a square wave. In more costly devices it can be a multi-step square wave that better simulates a sine wave. Those that output pure sine waves, like most UPS devices, are even more costly.
2. A basic, modified sine wave inverter works best with amplifiers that have a switched mode power supply (SMPS). This includes most any Class D amplifier. If the power input says 120 V – 240 V or similar, like 110 V – 220 V, it has an SMPS. If it lists only one voltage there is no way to tell if it is an SMPS except from the specs or by contacting the manufacturer.
3. Amplifiers with linear power supplies (transformer / rectifier / capacitor) are generally not a good choice for modified sinewave inverters. One drawback is the transformer will likely run hot. The other is that some of the harmonics of the square wave input can get reproduced in the audio = buzz or hum.
Trivia: an SMPS can be run off an ac input or a dc input, meaning it doesn’t care if the input voltage is an ac sine wave, a dc voltage, or anything in between like a modified sine wave (= square wave).
The BIG question is how much playing time can you expect from the battery? Here is the way to calculate the worst case, meaning you’ll likely get longer playing times than this.
This calculation makes several assumptions.
1. The efficiency of the inverter is 75% meaning the power from the battery will be 1.3 times the power out at 120 V ac. This is a conservative value.
2. The peak to average value of the signal from your instrument is 10 dB.
3. The signal is continuous, i.e. you are playing constantly with no breaks and at close to the amplifier’s maximum output.
4. There is no accounting for the idling power draw for the amplifier when there is no signal.
5. At typically 100 mA each, the addition of stomp boxes with a 120 V ac to 9 V dc power supply (almost all use an SMPS) will only trivially decrease the playing time unless you use bunches of them.
To do the calculation you need to know only three things.
1. The voltage of the battery.
2. The Ah or mAh rating of the battery.
3. The rating for the amplifier’s ac power input in watts found next to the ac cord or socket for the ac cord.
So here goes and it’s simple math:
1. Amplifier ac Power Input in Watts / 10 = Average Power Input to Amplifier in Watts
2. (1.33 x Average Power Input to Amplifier in Watts) / Battery Voltage = Amperes from Battery
3. Ah Rating for Battery / Amperes from Battery = Hours of Use
Or
4. mAh Rating for Battery / (1000 x Amperes from Battery) = Hours of Use
So that’s how you can figure your playing time off-grid with a dc to 120 V ac inverter. Playing at lower volume levels and/or with breaks between songs and/or sets and your playing time will be longer. As I said, this is a conservative, worst case calculation.
As to cost, the least expensive and most versatile option is a 12 V to 120 V ac inverter and a standalone battery. Here’s an example for a 200 W ac input amplifier and 6 hours of continuous (i.e worst case) playing time with some typical prices found on-line.
12 V dc to 120 V ac 300 W modified sine wave inverter $30
12 V 15 Ah sealed lead acid battery $25
1.5 A automatic 12 V charger $30
Cheers,
Chuck McGregor
There are a number of battery powered devices that convert the battery’s dc voltage to 120 V ac. The most common, least costly, and most readily available types work off of 12 V batteries. They typically have either a “cigarette lighter plug” to plug into a vehicle’s accessory 12 V power outlet and/or battery clamps to connect directly to the vehicle’s battery or to a standalone 12 V battery. Others have internal batteries. These latter include UPS systems for computers, the most costly type.
If you use a stand-along battery for one of those 12 V dc to 120 V ac inverters, always use a sealed lead acid battery for safety reasons. There are also “gel-cell” batteries but those are not addressed here as they are different animals being far more costly and requiring specialize chargers. You will also need an automotive-type 12 V charger to recharge it. Always use an “automatic” charger to avoid over-charging. Here’s how to select what you need
1. Select an inverter with at least 1.5 times the ac power input in watts listed on your amplifier.
2. Select a battery with the Ah rating you need for the hours of playing time using the calculations below by trying different Ah values.
3. Select a “automatic” 12 V charger based on how many hours of charging time you can tolerate. Calculate this as follows: Battery Ah rating / Charger rating = Charging time Hours. Add 10% of extra time to totally top off the battery.
There are several things you should know before powering your bass or guitar amplifier from one of these devices.
1. Most of the reasonably priced devices output a “modified sine wave.” This is essentially a square wave. In more costly devices it can be a multi-step square wave that better simulates a sine wave. Those that output pure sine waves, like most UPS devices, are even more costly.
2. A basic, modified sine wave inverter works best with amplifiers that have a switched mode power supply (SMPS). This includes most any Class D amplifier. If the power input says 120 V – 240 V or similar, like 110 V – 220 V, it has an SMPS. If it lists only one voltage there is no way to tell if it is an SMPS except from the specs or by contacting the manufacturer.
3. Amplifiers with linear power supplies (transformer / rectifier / capacitor) are generally not a good choice for modified sinewave inverters. One drawback is the transformer will likely run hot. The other is that some of the harmonics of the square wave input can get reproduced in the audio = buzz or hum.
Trivia: an SMPS can be run off an ac input or a dc input, meaning it doesn’t care if the input voltage is an ac sine wave, a dc voltage, or anything in between like a modified sine wave (= square wave).
The BIG question is how much playing time can you expect from the battery? Here is the way to calculate the worst case, meaning you’ll likely get longer playing times than this.
This calculation makes several assumptions.
1. The efficiency of the inverter is 75% meaning the power from the battery will be 1.3 times the power out at 120 V ac. This is a conservative value.
2. The peak to average value of the signal from your instrument is 10 dB.
3. The signal is continuous, i.e. you are playing constantly with no breaks and at close to the amplifier’s maximum output.
4. There is no accounting for the idling power draw for the amplifier when there is no signal.
5. At typically 100 mA each, the addition of stomp boxes with a 120 V ac to 9 V dc power supply (almost all use an SMPS) will only trivially decrease the playing time unless you use bunches of them.
To do the calculation you need to know only three things.
1. The voltage of the battery.
2. The Ah or mAh rating of the battery.
3. The rating for the amplifier’s ac power input in watts found next to the ac cord or socket for the ac cord.
So here goes and it’s simple math:
1. Amplifier ac Power Input in Watts / 10 = Average Power Input to Amplifier in Watts
2. (1.33 x Average Power Input to Amplifier in Watts) / Battery Voltage = Amperes from Battery
3. Ah Rating for Battery / Amperes from Battery = Hours of Use
Or
4. mAh Rating for Battery / (1000 x Amperes from Battery) = Hours of Use
So that’s how you can figure your playing time off-grid with a dc to 120 V ac inverter. Playing at lower volume levels and/or with breaks between songs and/or sets and your playing time will be longer. As I said, this is a conservative, worst case calculation.
As to cost, the least expensive and most versatile option is a 12 V to 120 V ac inverter and a standalone battery. Here’s an example for a 200 W ac input amplifier and 6 hours of continuous (i.e worst case) playing time with some typical prices found on-line.
12 V dc to 120 V ac 300 W modified sine wave inverter $30
12 V 15 Ah sealed lead acid battery $25
1.5 A automatic 12 V charger $30
Cheers,
Chuck McGregor
