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Thread: Favor to Ask

  1. #1
    Registered User Mandobart's Avatar
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    Default Favor to Ask

    In the Equipment Forum, I've been discussing building a battery powered AC inverter to run my Carvin AG300 when there is no AC power available. I used a clamp-on a meter and pigtail to see the maximum current that either of my amps draws is about 0.6 A. Have any of you other electric mando enthusiasts measured the draw of your amp/PA at normal performance volumes? I'm curious to know since the output power ratings lead one to believe they would draw much greater current from a 115 VAC outlet. Thanks!

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    Registered User Tom Wright's Avatar
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    Default Re: Favor to Ask

    I have not measured, but the average draw is much less than full-power demand, and modern amps only draw power when needed. My Gallien-Krueger MB200 (200W into 4 ohms) says above the AC input that it draws 240W at full power and 48.8W average power. A Crate PowerBlock I had showed 35 VA draw, as I recall, for a nominally 100W amp, which is about 30W.

    I think you would only need that full-power capacity when playing bass near distortion level. For other instruments I think the demand is more just peaks, which get averaged over short intervals of power demand. That is my limited understanding. Tube amps need steady power to heat the cathodes, but solid-state amps with switching power supplies instead of transformers don't need the steady power.
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    Default Re: Favor to Ask

    Watt ratings on amp are mostly a marketing label. At best it's a theoretical capability for a brief microsecond of time. At worst it's a made up number for sales. There have been companies that tried to be honest with the rating, but who will chose a 60 watt amp that can produce 60 watts when you can get, for less money, a 300 watt amp(that produces 45 watts). and the 300 watt amp is perfectly fine as long as you don't measure it.

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    Registered User Tom Wright's Avatar
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    Default Re: Favor to Ask

    Not so much these days, most brands rate output at RMS, not peak power, but distortion level matters. Hi-fi looks for numbers like 0.01%, my cheap Crate head listed its optimistic 150W at 10% THD. It was more like 100W at best.

    Mandobart can take his 0.6 A power draw and conclude that he will need 69W average power, so from a battery system at 12V plus inverter, capacity ought to be 69/12 = 5.75 A times let's say three hours playing, so 18 amp/hours should do it. Important to know what is the peak throughput of the inverter, to handle transient peaks.
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    Registered User Mandobart's Avatar
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    Default Re: Favor to Ask

    Thanks everyone. You may want to read whats already been discussed on this thread.

    I'm well aware of the hype and hysteria surrounding mfrs stated power output. Thats why I measured the current draw of both my amps. This thread was not started to continue discussing the imaginative ways mfrs market their products, but these things take on a life of their own. I was hoping there were some others on the electric forum who had done the same thing to compare my findings with, players like me who have the curiosity, background and skill to see for themselves. So far no one has.

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    Registered User Mandobart's Avatar
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    Default Re: Favor to Ask

    After much searching I found this thread on the Acoustic Guitar Forum. It seems that others have done similar measurements with similar results - most amps, PA's and powered speakers don't draw that much power. So I ordered an inverter and small deep cycle 12 VDC battery. When it arrives and I set it up I'll report back with results.

  10. #7

    Default Re: Favor to Ask

    You could have done the same an appropriately sized UPS.

    Len B.
    Clearwater, FL

  11. #8
    Registered User Mandobart's Avatar
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    Default Re: Favor to Ask

    Quote Originally Posted by lenf12 View Post
    You could have done the same an appropriately sized UPS.

    Len B.
    Clearwater, FL
    That is an idea I didn't fully pursue. My experience with our industrial sized UPS's at work is they are for a short term infrequent loss of main AC power and are not designed for multiple deep cycles or to supply power for several hours. Do you have any recommendations for a small (~150 W) pure sine wave UPS capable of supplying 110 - 120 VAC for ~ 5 hours weighing 10 - 15 lbs? Thanks.
    Last edited by Mandobart; Apr-17-2016 at 12:43pm.

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    Registered User Mandobart's Avatar
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    Default Re: Favor to Ask

    Turns out the UPS is a non-starter (so to speak ). No one I found makes a UPS to supply a load for more than a few minutes - nothing approaching a 4 hour gig.

    You can read the results of my inverter design here.

  13. #10

    Default Re: Favor to Ask

    Have you checked out APC UPSs. They have models designed to supply pure sine wave AC power for banks of file servers through a multi-hour power outage. I can't believe that your Carvin amp pulls that much power. Their UPSs can get kinda pricey however. I will check out your inverter design results over the weekend. Cheers!!

    Len B.
    Clearwater, FL

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    Market Man Barry Wilson's Avatar
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    Default Re: Favor to Ask

    I replaced my battery in my eliminator with one from APC. Their price for replacements was awesome and shipped to me in 4 days (warehouse in Ontario)
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  15. #12
    Registered User Mandobart's Avatar
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    Default Re: Favor to Ask

    If interested, you can read about my prototype inverter here. 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.

  16. #13
    Registered User Mandobart's Avatar
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    Default Re: Favor to Ask

    Quote Originally Posted by Mandobart View Post
    If interested, you can read about my prototype inverter here. 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.
    Sorry, I meant here post #48.

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    coprolite mandroid's Avatar
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    Default Re: Favor to Ask

    Mostly I'd just look up the current draw Spec as written in the owners manual.

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    Registered User Mandobart's Avatar
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    Default Re: Favor to Ask

    Quote Originally Posted by mandroid View Post
    Mostly I'd just look up the current draw Spec as written in the owners manual.
    And you'd be still looking - it's not there. Max draw designed for is 15 A since it doesn't have a 20 A plug. Carvin "tech" guy says 7 A (AC) is the max to expect. Input power it is rated for is 100 VA. So if your volts are on low end of the US standard (110 VAC) it is rated for 0.91 A (AC from the wall socket).

    Being a trained electronics tech with test equipment I fell back on the "let's test it and find out" routine. I found the most AC current I could pull out of the wall without breaking eardrums or windows was 0.6 A. Then I erred in using the rated AC current value (0.91 A from 100 VA) divided by an efficiency of 90% to estimate DC current from the battery at 1.01 A DC.

    What I should have done is take the rated AC input power (100 VA) divided by the 90% efficiency to get 111 W DC. 111 W ÷ 12 VDC = 9.3 A DC, the maximum battery current needed to supply maximum AC input apparent power. OR take my max tolerable AC current of 0.6 A x 120 VAC (that's what it runs at my place) = 72 VA. 72 VA ÷ 0.9 = 80 W DC from battery. 80 W ÷ 12 VDC = 6.7 A supplied by battery. This matches more closely to the actual current my battery needs to supply my amp at normal practice level (I measured 3 A DC).

  19. #16
    Registered User Mandobart's Avatar
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    Default Re: Favor to Ask

    So far I've run my Carvin for a total of 4-1/2 hours (2-1/2 hours at an outdoor gig tonight) off a single charge on my inverter battery with plenty of juice left. Looks like this little project will do the trick.

  20. #17

    Default Re: Favor to Ask

    Congratulations MandoBart!! When can we expect the commercial version to be available on the market?

    Len B.
    Clearwater, FL

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  22. #18
    Registered User Mandobart's Avatar
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    Default Re: Favor to Ask

    Well I've no intention of going into business. This would be a niche within a niche market - musicians who want/need sound reinforcement when there is no AC power access, who are willing to lug another piece of gear. Cost to make an ugly prototype was ~$250 USD. To custom build a unit to meet each individual customer's needs, make it look professional, offer a meaningful warranty, shipping etc. would likely drive the cost up to where many people would just go for a DC powered amp like one of the Roland cube models. I think I'll stick to going broke playing music

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