Portable Amp Power Supply (Inverter)

Mandobart

I've been asked to play at a friend's wedding in the mountains with no AC power, which got me thinking about a battery powered acoustic amp. I know there are a few decent battery powered amps out there. I like the Roland Street Cube, runs about $500 USD. So I was thinking I really like my Carvin AG300, what If I brought my own power for it?

A small gas generator is one option, but then I looked into DC - AC inverters. We use them all the time at work (pretty high powered ones) for UPS's. For my amp I plan on using a pure sine wave inverter rather than modified sine wave. The big question is how much power do I need? Amplifier mfrs are notorious for use of fiction, folklore and fantasy in this regard.

So I went the simple route and measured the actual AC current draw of each of my amps at normal performance levels. I used a clamp-on ammeter and pigtail to see the maximum current that either of my amps draws is about 0.6 A. I've looked around and found I can get a good pure sine wave inverter and deep cycle battery to put together a fairly lightweight system able to power my amp for ~4 - 5 hours for a lot less than $500. I haven't ordered all the parts yet as I'm still looking to see if anyone else out there in the web world has done something similar yet. So now we come to my question(s):

1. Has anyone here done a similar measurement on their amp or PA to determine the actual current draw at normal performance levels? If so what did you find?
2. Has anyone else bought or built an inverter power supply for busking or other such use when AC power is not available?

Thanks everyone!

Buco

I only borrowed a friend's Roland cube to play at friends wedding and it worked out great. My only cost was 6 AA lithium batteries.

Sounds like you're going the right way about it. What capacity battery are you looking at?

Mandobart

I'm planning on using 12 VDC AGM or gel battery. My max draw I measured from the amp is under 1 ampere. The Carvin is rated at 100 VA, so plugged into a 110 VAC outlet max rated draw is 0.91 A. Most pure sine wave inverters have about 90% efficiency (hysteresis loss, cooling fan, etc.) so the battery needs to provide 0.91/0.9 or ~ 1.01 A at most. Most gigs I do are 5 hours or less, so I need to supply 1.01 A x 5 hrs = ~5.05 amp-hr (at most). But, even on a deep cycle battery its best to only draw it down to 50% capacity or more, so call it 10 amp-hr. There are several deep cycle cells out there that can provide this weighing 8 - 20 lbs (which is also a factor for me).

Mandobart

I finally turned up this thread at the Acoustic Guitar Forum. Seems that others have done similar measurements and arrived at similar conclusions - most modern amps, PA's and powered speakers don't draw that much power.

Buco

This is very useful, thanks. My band will play an outdoor gig in June.

Mandobart

I just ordered an inverter and small deep cycle 12 VDC battery. When it arrives and I set it up I'll report back with results.

Buco

I found a friend of mine has a generator, Honda 2000W, he says it's very quiet and will run entire day at half load with less than a gallon of fuel. That'll be our power source. We use Bose L1 PA system which shouldn't draw more than 0.5A or so at full power.

Mandobart

Here are the preliminary results of my inverter experiment:

I purchased a Cotek SK200-112 Pure Sine Wave inverter rated at 200 W continuous duty for about $127 from Amazon (they have since gone up about $20). It is powered by a Power Patrol 12 VDC, 12 amp-hr sealed AGM battery which cost about $40 with shipping.

The whole system fits in a shoebox and weighs about 8 lbs. Total cost so far about $170 USD. I haven't built a rack to carry it around in yet. I plan to add a voltmeter and maybe ammeter that can be switched on or off (no need to constantly draw power to check parameters). The inverter works great to power my Carvin AG300 amplifier. NOTE: current is measured in amperes, abbreviated "A" and frequently spoken as "amps." Not to be confused with the term "amp" as a contraction for "amplifier." An ampere-hour, A-H, amp-hr is a battery rating, meaning how many amperes it can supply for a given time. It is not a linear relationship. The higher the discharge rate, the less time the battery can supply useable current. A 100 amp-hr battery may supply 1 amp for 100 hours. It would not be able to supply 100 amps for 1 hour; it would be considerably less.

Lessons learned - the battery supplies (alternately the amplifier and inverter draws) more current than I calculated based on a 90% inverter efficiency. I calculated I would need to supply a little over 1 amp from the battery. I figured I needed about a 9 amp-hr battery to supply my amplifier for a 4 hour gig without discharging the battery more than 50%. So I ordered a 12 amp-hr deep cycle battery to have some margin but still be small and lightweight.

For anyone doing the same thing, remember that a clamp on AC ammeter may read current in a DC circuit but it will not be accurate. Since you cannot have transformer action with DC, the sensor for a DC clamp on ammeter is different than for one that measures AC. An in-line DC ammeter (requires you to break the circuit and insert the ammeter in series) will work IF your meter is rated for the current you wish to measure. Most bargain multimeters only go up to 200 mA.

I measured the following with an in-line DC ammeter:

Battery supplies 0.75 DC amps to power the inverter just idling (supplying no load, just running the inverter). When I plug in and turn on the amplifier the battery supplies 2.28 DC amps. If I use the USB charging port on my amplifier the battery current rises to 2.86 amps. If I then play music through the amplifier at pretty loud volume (not maxed out, my ears can't handle it) the total battery current goes up to 2.9 - 3.5 amps. Without using the amplifier's USB port for charging a phone/pad/etc. the current drops down to 2.3 - 2.5 amps. So a 4 hour gig at fairly high level will deplete 10 amp-hr which is a lot to drain out of a 12 amp-hr rated battery, even designed for deep cycles.

Options are add a second identical battery in parallel with the first to double the capacity (to 20 amp-hrs) OR buy a larger, heavier battery rated at 20 or more amp-hours. Or just see how long I can cycle the battery like this before it dies. There are pros and cons each way.

Mandobart

Okay I warned you all this was coming. It ain't pretty....

Here is my first prototype inverter supply for powering my Carvin AG300. Overall dimensions 8" wide x 8.5" deep x 14.5" tall. Total weight 29.8 lbs, nearly all of that the battery, a 12 VDC 35 amp-hr deep cycle I got from Harbor Freight for ~$73. The black switch is a DPST to isolate the battery from the inverter. The inverter also has its own ON/OFF/REMOTE toggle. It should be able to power my amp for 5 hours or more. I'll give it a real world test on that soon. For now it runs my amp for an hour practice session with no drop in terminal voltage.

Total approximate cost (so far) about $250.

Scoredog

watching to see where this lands...good luck!
I am thinking a used Roland cube may be a simple answer if you just need a guitar amp.

Mandobart

I need vocals as well. I demo'ed several Rolands (and other battery powered amps in the under $1000 bracket) and the only one that worked well enough for me was the Street EX at $500, and it still didn't sound as good as my Carvin AG300. This was the impetus for my project - to see if I could build something cheaper than the cost of a "good enough" battery powered amp that was also small and light enough. Everyone has a different threshold of sounding "good enough", "light enough", "small enough", supplies power "long enough" and affordability.