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A/C Current testing

Discussion in 'Amps and Cabs [BG]' started by konabass, Dec 9, 2003.

  1. Has anyone devised a good, solid test for the a/c current supply when you're setting up for a gig? I have had several places lately that have had poor power for my Mackie 1400i power amp. The last gig I did was an outdoor private party at a residence and it was an ordeal to find sufficient power. I borrowed a 1000watt theater-type light and hooked it into a Tripplite 20amp line conditioner that had a stack of LEDs indicating the "level" of current being used. It worked well, showing me the differences between 20amp and 15amp outlets 'after' the extension cord run, but lugging around this contraption is tough. What I was hoping to find (like the ESP Projects site) was a specific tester that would allow me to load test the a/c current supply available to the amp prior to sound check.
  2. notanaggie

    notanaggie Guest

    Sep 30, 2003
    'zactly what do you want to test?

    If you want to test the current capability, its easier to find the breaker, which you will need to do anyway after the "test load" trips the breaker.

    And the simplest thing to do is look at the outlet. If it is a house type outlet, meaning you can plug your equipment into it, and it is not on your breakout panel...it is either 15 or 20 amps capability. There is a detail difference between 15A and 20A outlets but that often gets messed up as people replace them.
    You also won't know that their mega refrigerator is also on that circuit ready to pop the breaker, but at least you know the capability.

    If you are trying to see if there is too much voltage drop, well... That isn't really very different, due to the electrical code requirements. But out in the sticks there is a lot of not-to-code wiring.

    In a bar, you may as well assume that if you overload the outlet, some pile of discarded buffalo wings freezer bags in a corner on top of that bad splice will probably ignite.............. That's not helpful....and is a bit nasty....oops....rotten day at work...my bad
  3. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    I keep one of those little radio shack plugs in my case, that has three lights to show whether an outlet is wired correctly with ground, hot and neutral. I plug that in my intended outlet first. If that checks out OK, I plug my rack line into the outlet, the rack contains a Furman PM-8 which shows voltage. If voltage OK, I fire up the master switch on the Furman. The PM-8 also has a Ampere meter, showing how much draw my gear has on that line. Its usually nominal, around 3 to 4 amps for a QSC 3002, Eden Nav preamp, Korg rack tuner, and my pedalboard power.
  4. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA

    Dude, you're scaring me.

    What kind of differences did the tester show you between 15 amp and 20 amp outlets?

    There should be no difference at a thousand watts.

    Ordinarily the only difference is in the gauge of the wire, either 12 gauge or 14 gauge. Both wires are capable of handling significantly more than the rated current, it's just that they tend to heat up when they do. :)
  5. Nightbass


    May 1, 2001
    Seattle, WA
    It sounds like you are concerned about the voltage delivered to your amp while under load. That is a valid concern. Quite frankly, though, after you measure it, what are you going to do about it?

    Perhaps you should look into buying/building a distro box like the pro sound guys use, to replace the extension cord idea. A good distro is made of large-gauge conductors and provides all the gear plugged into it a single ground point to eliminate ground loops. There won't be any appreciable voltage losses because the conductors are sized according to length. I'm talking 4 and 6AWG here, not wimpy 12AWG extension cords!

    You can also buy/build a distro with voltage and current meters if you are so inclined.
  6. The Furman PM-8 with the LED stack sound like the best way to gauge in real time what is available to the rig. The "distro panel" is a super idea, but on a smaller scale venue is not always practical. What I was trying to accomplish is to test the circuit AT the female end of the extension cord to verify it's current handling ability PRIOR to firing up the amp and banging on the strings. Trying to replicate the ultimate draw or load that a power amp will need during a performance and view that current using something conventional in a unit that combines all this is what I'm looking for. My test this last gig of hooking up a 1000 watt lamp through the power conditioner with it's LEDs seems like the best rig so far. Not all of us can depend on an electrician to have secured the needed current capability at the gigs we will have to travel to. And we all know the goofballs that will install a 20a breaker and then wire in a 15a receptacle with 14 gauge romex! I appreciate the replies to my post. I don't think there's really an instrument or tester or tool, however, short of my
    gerry-rig that is available to give me that current supply assurance. I wonder if a lot of problems with tone discussed here on TB really emanate from a current supply problem. I can just visualize the weekend warriors all plugging into the same wall socket in someone's garage or living room.
  7. bill h

    bill h

    Aug 31, 2002
    small town MN
    will a multimeter work for this? I know it will give you the volts and the amps, but will it give you the max draw?
  8. Oh, and also, I just bought two 10 gauge 100' 15amp extension cables from Home Depot at 73 coconuts apiece last weekend. Not that I'll always need 100' but I'll sleep easier knowing I have covered THAT base. BTW I personally don't need or would use 200' on one run. I'm not that much of a sixties casualty.
    Tough being the bass player that has to anguish over such issues. Guitar guys just bring lamp cord exentions. Where's the parity?
    I won't turn down either!
  9. Thanks Bill h. The multimeter will work to test volts, and if you have a clamp-on ammeter and can isolate the hot or neutral lead, you will be able to do volt and current testing--it's the loading AND simultaneous read out in one unit that I'm seeking. Or a full time electrician in my tool box that works for peanuts.
  10. Nightbass


    May 1, 2001
    Seattle, WA
    I suppose those 10AWG extension cords are a good start. A 1000W load (8.3A) will lose 2VAC over 100FT of 10AWG copper, so you'd still have a nominal 118VAC. Of course the house wiring may drop more than that, so that's why we need to minimize the losses as much as we can on our end.

    As far as a test load, the Second Law of Thermodynamics says that you're going to have to convert 1000W of electrical energy to some other form of energy, such as light, heat, motion, or magnetic force. If you don't want to use your 1000W theater spot, you could use a 1000W heating element, but you know how well that would go over. So the lamp may be the best load.

    If you play a lot of places with bad power, you might want to look into a power amp that draws less current and is able to tolerate voltage brownouts a lot better, such as the QSC PLX or Crown K series, which will run all the way down to 90VAC and draw less current than your Mackie.
  11. Nightbass, thanks for the number crunching. You know, you bring up a good point on the Mackie power consumption. We use a Mackie 808s to rehearse with and it has had problems with current when we crank it up to full throttle. I am concerned that it's a Mackie-specific or actually power supply problem. Would be interesting to use a QSC although, my audio service guy down here says the QSCs have a humidity and corrosion problem with the weather we have. He has repeated this about the QSCs more than twice to me. I have viewed a ton of positive info on QSC,Crest etc. Maybe it would be worth it to test drive some other kind of power amp (behind the Ampeg SVT Pro preamp). Sure enjoy the Mackie, though.
  12. Steve


    Aug 10, 2001
    made by Amprobe, works with any clamp-on type ammeter. It will give you real time voltage/amperage readings at the cord end or receptacle
  13. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    One thing to be aware of is that amplifiers with switching power supplies draw current on a "power" basis.

    Yes they will run on 90 volts, but the current they pull goes UP as the voltage goes down. So at 90 volts they are pulling around 30% more current than at 120V. This can cause a sort of cascading overload that pops breakers.

    An amplifier with a standard type power supply will drop in power capability as line voltage drops, and so stays at a relatively constant current draw as voltage drops.
  14. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio!

    Jul 3, 2001
    Santa Ana, Calif.
    Former Technical Communications Developer, QSC Audio
    That would be if the switching power supply was hard regulated. Most power amps I know of with SMPS use a soft regulation, if any.

    Actually, the current draw will drop, too. The maximum output voltage will drop somewhat proportionally to the mains voltage, so the amp's maximum power capability will drop with the square of the output voltage reduction. For example a 10% drop in the maximum output voltage amounts to a 19% drop in maximum output power. If you don't hit the clipping point, though, there should be no significant effect on performance. Ironically, the amp will be operating more efficiently.

    Amps with unregulated SMPS will behave this way also, and amps with soft-regulated SMPS will be somewhere in between.
  15. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    I admit to describing the worst case....but...

    One of the touted advantages of SMPS is s'posed to be relative insensitivity to line voltage changes.....brownouts etc.

    The more insensitive to line voltage the supply is, the more it conforms with my statement of "constant power" draw.

    If the output power remains substantially constant, or even if it drops a bit, the input current must go up as voltage declines, that's physics. As you say, it depends on the design, which usually isn't specified in the sales literature.

    Yes, a plain unregulated switcher will behave same as a regular supply. With either, power voltage goes down, current goes down also. But since currents in the amplifier reduce only as the square root of output power, the current tends to drop along with voltage. The squared terms "cancel", leaving a straight proportion.

    The modifier to that is that clipping percentage may increase, and then power doesn't go down as much, as you mention.

    I assume we are talking about situations where the power is "edgy" anyway, so we are presumably pulling a lot of power to start with. A little extra may make the difference.

    Since most folks don't cut the signal immediately when the power drops, it is pretty likely that clipping will increase during voltage drops. So counting on a proportionate power reduction may not pan out exactly.

    I think we agree...and anyway it isn't that big an effect in either case unless you are really pushing the outlet capability. That isn't a really good idea unless you like finding circuit breakers.
  16. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio!

    Jul 3, 2001
    Santa Ana, Calif.
    Former Technical Communications Developer, QSC Audio
    No, there is nothing to cancel. When the AC voltage drops, the amp's headroom sags also, and there is less waste heat because below clipping the power out is not affected, but the amp is taking in less power from the mains. As long as the amp remains out of clipping, it is simply operating more efficiently, drawing less current as the AC voltage dips. But, as I mentioned, at the increased risk of clipping due to the lower maximum output voltage.
  17. notanaggie

    notanaggie Guest

    Sep 30, 2003
    Mr Lee:

    I think you might have missed the point of what notanaggie wrote. He's assuming that a lot of power, near or at clipping, is being used. So if you have a brownout, its likely to clip or clip more. Since this thread was about max power capability of outlets, its a fair bet that things are cranked.

    Your point was that power capability drops as the voltage drop squared. But power isn't current.

    He said, and he's right, that the change in current is about the same as the change in voltage unless clipped. I think you said that too, actually.

    I think the "canceling" thing was about the math, since the current relates to square root of power, as does voltage. You worked from voltage to power, squaring the voltage change (the 10% to 19% thing). He was working back from power to current, So your 10% voltage change to 19% power change is reversed going from power to current, which was the original topic.

    If internal voltages drop 10% and the power capability drops 19% (I didn't check the math) the maximum currents drop only about 10% also, not 19%.

    So what I got out of it was that maximum line current of a conventional amp drops generally in proportion to line voltage, but maybe not that fast, if clipping starts or is increased due to the lower voltages. That seems to go along with what I have seen.

    For sure the soundman doesn't turn down for a brownout, they never turn down anything for any reason!

    And there are some amplifiers with switching power supplies that regulate more closely, like some of the Asian ones. I saw one a while back, I forget the brand, but I think it was a Korean import.
    The output power stayed fairly constant down to 85 or 90 V, and the line current did go up, pretty much like notanaggie said.

    I know, because I was curious what the switching supply would do, and I cranked the line voltage up and down to see with it at full power. Power didn't go down much, and also didn't go up much at higher voltage.

    Personally, I'm with you that close regulation isn't better, seems like you would lose the peak headroom that transformer type amps have.
  18. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Well, this morning I decided to make a practical test to "tiebreak" and see if I was off in the weeds....

    I got an amplifier with a conventional power supply, and set it up (sine wave input, its easier to measure) just under clipping. I checked the line current.

    Then I reduced line voltage 10%, which caused the amp to clip. Internal voltages dropped a similar amount to the line voltage reduction.

    Sure enough, the line current went UP about 4 or 5 percent.

    Yes the unclipped power capability goes down as line voltage falls.
    Yes there isn't much difference if the amp isn't near clipping, other than the possible efficiency improvement.
    (BTW, that is assuming you are moving away from the point of least efficiency, not towards it. If the amp is closer to the point of least efficiency at the lower voltage, efficiency goes down)

    But, if the voltage drop increases clipping, the test confirms it is possible for a conventional amplifier's current to go UP when line voltage falls.

    Since, as pointed out elsewhere, the question referred to maximum current capability of outlets, it seems relevant to look at what happens when amps are run near full power.

    Of course, pure sine waves don't get a lot of use in bass amplification, but the effect should be fairly independent of waveform as long as there is significant clipping as a result of lower line voltage.
  19. I've been with TB for about two years now and always find it fascintating how the initial post and some of the replies end up being tangential. This discussion into power supplies is slightly off my original target but is educational, as they always are, none the less.
    I did look into Amprobe's # 480172 which is a line splitter, a 120v adapter for clamp-on amp meters. At about $15, I will hunt one down. Next would be a heating element or lamp that can generate upwards of 1000 watts to load my potential circuit. My search for an application-specific tester will continue but at a less frantic pace.
  20. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Sorry for wandering off.....

    But back to your test issue, I don't really see a way to test the max current capability without essentially loading it until the breaker goes. And once you have done that, as Nightbass said, then what?

    Short of demanding that the venue rewire before the gig, you are probably gonna use that outlet anyway.

    If you have a switchable voltage booster, or "Variac" , etc, I guess you can kick it up a notch or two if voltage is low. But be aware that if you do that, line current draw from the outlet will go up in proportion, and so voltage drops might increase a bit also. That partly undoes your boost setting.

    If the question is whether a 3-wire outlet is properly grounded, the little plug-in testers are not perfect, but they will tell if there is a gross problem of the third wire not being grounded at all.
    They won't tell you what happens if a lot of current is sent through the ground wire by a fault. So they are not an iron-clad guarantee.

    As far as the circuit being safe to draw significant current from, you are sort of stuck. Any test you do to draw current would likely be at least as safe or unsafe as just using it normally.

    I would suggest using the outlet tester, measuring voltage maybe, and looking to see if the outlet looks really funky. Assuming nothing too bad appears, you are probably OK to plug in and not worry.
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