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Voltage drop cause intermittent amp problem

Discussion in 'Live Sound [BG]' started by rapidfirerob, Feb 9, 2013.

  1. rapidfirerob

    rapidfirerob Fusion rules!

    I just played an outdoor festival today. We had to run all our instruments, plus one mic, and their PA off one line of electricity. I've had to do this before without issues, luckily. Today the keyboard player and one of the guitarists had an issue right from the start with their equipment losing power. I had an issue some time later where my Genz Benz Shuttle 6.0 lost power for a second, then would be fine. This happened fairly often. I assume this was caused by the voltage drop. The other guitarist has a nice power strip that tells the voltage. It was around 100 and sometimes in the high 90s. I changed my cable and that made no difference. Could you please explain how this works? I assume this would not damage the amp? I've had no problems with the GB before today. Thanks. It's an Allman Bros. tribute band.
  2. ggunn


    Aug 30, 2006
    Austin, TX
    Voltage drop happens when too much current is being pulled through conductors (wires) which are too small in diameter, too long, or both. Some gear will overheat under low voltage conditions because in order to try and produce the same power at the lower voltage, it draws more current. Current = heat.

    Some gear also has protection circuitry which will take it off line if the voltage drops too low.
  3. Johnny Crab

    Johnny Crab ACME,QSC,Fame/Hondo/Greco user & BOSE Abuser Gold Supporting Member

    Feb 11, 2004
    South Texas
    Wallwarts do not like low voltage and some will even go BANG when they fail due to this.
    Ask me and our keyboard player how we know.......
  4. Medford Bassman

    Medford Bassman Supporting Member

    Aug 8, 2007
    Medford, Wisconsin
    I had that happen once at a gig with my Shuttle. Outdoor wedding, poor power to the band.
  5. ggunn


    Aug 30, 2006
    Austin, TX
    That sounds plausible for a transformer type wall wart. A regulated power supply is going to try to deliver the same output voltage to a load no matter what the input voltage. The lower the input voltage, the more current the transformer has to pull to make the output voltage what it needs to be. Too much current, and, as you so eloquently put it -- BANG.
  6. LongHairFreak

    LongHairFreak Insert cool nickname? Nobody's given me one yet. Supporting Member

    Aug 7, 2006
    Twin Cities - MN
    Interested party here...
    So, aside of making sure supply meets demand (however that may be accomplished) from the start, is there anything that can be done after low supply is discovered?
  7. ggunn


    Aug 30, 2006
    Austin, TX
    Splitting up the power requirement over more circuits is about all you can do. If there are lights, get them onto their own circuit. If you are running a single long extension cord to the stage, it would help to run another one if you have access to another outlet even if it's on the same circuit.

    Here's another thing: if you are running long extension cords to an outdoor stage, you are likely to have grounding problems as well. If you are holding a guitar or mic and touch something that is solidly grounded, you may get shocked.

    And ALWAYS (even if you are inside but especially if you are outside) check the power for hot-neutral reverse, open ground, etc. Something amiss in the wiring could give you more than a tingle if you touch a grounded object.
  8. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    This issue comes up from time to time, so let me add a little information so you understand what is happenig, why it's happening and why the protection is important as well as ways to circumvent problems whenever possible.

    Low line voltage is typically caused by too much resistance (actually impedance) in the conductors feeding the power source that the amp is plugged into. The voltage drops as the load (current draw) increases. The two most common causes of this are the power source's wires being too small for the load and the distance of the power cable (including inside the walls and extension cords) causing the voltage drop.

    In the "good 'ol days", amps were not terribly powerful, efficient, or high performance. They were also not terribly reliable, so while low supply voltage certainly caused performance reduction the impact on reliability was not as easily seen because amps failing was pretty common. As power amps became bigger, more sophisticated, higher performance, reliability had to increase otherewise the cost of ownership was be a deal killer for this better technology. As general reliability increased, the failures due to low line voltage became more obvious. Protection circuits were developed to prevent the damage due to such problems and were infinately better than a failed amp and costly repair.

    Currently, with switchmode power supplies and class D amplifiers, the performance bar has been raised again and it because of the nature of the new topology additional attention has been put towards improving the reliability even more. In quality SMPS/class D products (especially the pro level PA amps and bass amps), a lot of thought has gone into studying all of these failure mechanisms and by properly designing around and protecting against these (including low line voltage), reliability has been improved significantly which is why you are seeing longer, transferrable warranties as an end user. It's up to the designer to choose a design that protects against damage but is not unnecessarily sensitive to unimportant conditions. Some companies do a much better job than others.

    In general under normal operating conditions, our amps operate down to ~95 to 98 volts, though if there is a lot of power line distortion (looks like a clipped voltage waveform) which can be especially damaging, this threshold will effecively shift up to around 100 volts. In any case, these are pretty severe conditions, not something that I would want to operate any piece of costly elctronics on because of the very real potential for damage.

    How can you avoid this problem? The first step is to insure that the wiring (including extension cords) is sufficient for the load and distance. In the USA, with 120 volt mains circuits, either 15 or 20 amp circuits are available. 12 gauge extension cords will minimize the voltage drop due to load, and if the problem persists the only pracical solution is to reduce the load on the circuit. This means either using another available circuit or removing load (such as lighting).
  9. ggunn


    Aug 30, 2006
    Austin, TX
    Didn't I say that? :D
  10. rapidfirerob

    rapidfirerob Fusion rules!

    Thanks for all the information. We're discussing what we can about it in case this comes up in the future.
  11. ric stave

    ric stave

    May 6, 2006
    Buffalo, NY
    I did a show a few years ago where we ran into a problem like this - it was an outdoor/parking lot/vendor booths + band thing - I had advanced our requirements - a minimum of 3 (usually 2, but I was playing it safe) separate dedicated solid 20 amp circuits. When I arrived, I found our 'tent', and saw 3 orange extension cords at the back of it.... I followed them back... they were each easily 75' long, and 2 were plugged into the same outlet (an outdoor outlet along a path about 150' from the closest building) and the other plugged into the NEXT outlet (prob the same circuit)

    I set up our PA and put some iPod music on, at a low level, waiting for someone with the organization to show up. After about 5-10 min, the circuit popped. Ended up having to get a separate line run from one of the booths for our lights, and on the other circuit, cut our subs, ran the bass only thru my amp, and try to explain to the idiot (who claimed he was the town electrician) WHY the power was inadequate... but he was hammered already, so it really didn't matter.