Power sag: Fact or fiction?

Discussion in 'Live Sound [BG]' started by AGCurry, Aug 6, 2022.

  1. AGCurry

    AGCurry Supporting Member

    Jun 29, 2005
    St. Louis
    I played an outdoor gig tonight.
    Four-piece band - drums, bass, two amplified guitars.
    The venue's "house P.A." was three Bose L1 Compact PA units.
    We also had a box fan running, as well as strings of little LED lights above the stage.

    Power was supplied to the stage by one 50-foot cord, probably 14 gauge.

    About three songs in, the band leader tells me I should turn down, explaining that when I hit strong notes it made his amp sound bad due to voltage sag.

    I was playing through my Traynor SB12, and the BL was using a little Fishman acoustic amp. The other guitarist was using a 1000-watt class D powered speaker.

    I've been playing for over 50 years, and I've never been asked to turn down for this reason, even when we used Twin Reverbs and Ampeg V4s.

    So: Is this "voltage sag" a real thing, or bogus?
     
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  2. It’s probably your volume is overpowering his crappy little amp.

    There’s a technical term for it which I’ve forgotten (masking?)…. like when a loud truck drives past and you can’t even hear the person speaking on the phone.
     
  3. I'm sure an engineer will be along shortly, but in the meantime, my vote goes to "this isn't really how voltage sag manifests itself with amps"

    According to the google, relevant to this, one thing voltage sag can result from is a sudden change in load, as with a motor starting. For the layperson (such as myself), this seems to create the possibility that whatever it was that your guitarist was experiencing was due to the sudden change in load produced by your big notes.

    BUT

    In order for that to be true, (very obviously) your "big notes" would have to be creating a sudden change in load of the type and sufficient to cause voltage sag.

    That seems unlikely because if that were the case, the two things I'd question are why would the only thing affected be his amp (i.e., it should affect everything on that circuit)? and at the frequency that there are similar circumstances, how is this possibly not a completely common occurrence for people who play electric instruments?

    I have a circuit in my house that apparently experienced voltage sag when a laser printer was plugged in and running a print job...the ceiling lights would dim and surge when something was printing. I'll have to see if a bass amp causes the same effect on playing with varied intensity. My strong suspicion is that it's not how this works.
     
    Last edited: Aug 7, 2022
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  4. Power sag due to an overloaded circuit is a real thing. Aka the laser printer example.
    Was that really what was happening here is hard to say.

    Long extension cords, lots of stuff on one circuit that you know of, and other stuff you might know about can contribute. You really need to insist that the venue does better than this. It is a safety issue, they would be liable.
    Modern amps with solid state rectifiers in the power supply however are less likely to be subject to being affected.
    Older amps were designed around a lower input voltage, 110VAC. Line voltages these days are running upwards of 125VAC, making sag less likely to affect the amp.

    Of course if you were getting power from a generator, all bets are off.
     
  5. MrLenny1

    MrLenny1 Supporting Member

    Jan 17, 2009
    New England
    IME never. If house power is on the low side all amps on that circuit would be affected.
     
  6. AGCurry

    AGCurry Supporting Member

    Jun 29, 2005
    St. Louis
    I made that argument: "Wouldn't my amp distort too?"

    BL's response: "It did - you probably just couldn't hear it."

    My thought: "Umm, pretty sure I would have heard it."
     
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  7. E2942

    E2942

    Jan 25, 2022
    All amps are not designed equally, even in the Power input portion.
    One amp will not rectify the input Voltage the same as another.
     
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  8. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    For old tube amps, especially those with tube rectifiers.

    Long extension cords with too many devices on one circuit can easily drop the AC a few volts. It can be even worse when using generators.

    I've experienced modern amps with voltage protection circuits toggling off and on outdoors. Just yesterday at a park gazebo gig, the inrush current on my Walter Woods amp was enough to trip a breaker when I powered it up (it's done this before). We had to run a 50 feet of extension cords to another outlet so I'd be on my own circuit from the rest of the band.

    If I was doing a lot of outdoor gigs, I'd be buying a voltage regulator like the Furman P1800. They handle up to 15A at 120V and will hold the voltage output steady even with input sagging as low as 97V!!! They are about $800, which is why I don't already own one. The older AR1215 sometimes shows up used in the $4-500 range.
     
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  9. E2942

    E2942

    Jan 25, 2022
    Remember, EVH used it for his "brown sound" by "tuning" a Variac to low voltages to his Peavey tube amp.
     
  10. BigCactus

    BigCactus Supporting Member

    May 1, 2020
    Richmond, Virginia
    Isn’t that what a compressor is for? And if you’re already using one, it sounds like your bandleader needs a better amp :p
     
  11. AGCurry

    AGCurry Supporting Member

    Jun 29, 2005
    St. Louis
    There was nary a tube (or "valve") to be found onstage last night.

    No generator, just a long and quite possibly inadequate extension cord.

    Nothing shut down. My question was more related to whether a voltage sag could cause distortion.

    I don't think we can justify that expense, based on the little we're paid. Would it not be preferable to ensure at the start that adequate power is supplied to us?
     
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  12. AGCurry

    AGCurry Supporting Member

    Jun 29, 2005
    St. Louis
    What does using a compressor have to do with it?
    I'm not about to tell a guitarist what amp to use. I did suggest that he raise it off the floor so he could hear it better, and, therefore, possibly be able to turn it down. He said no because he was afraid it might fall.:rolleyes:
     
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  13. Aloe

    Aloe

    Apr 10, 2016
    Ukraine
    I believe, those Fishmans have universal power transformer (one size fits 100 to 240V, i.e. they should be guaranteed to work perfectly from 90V to 260V). 120V to 90V is 25% difference and it's a lot.

    all my amps that have a switch for 230V are totally fine at 200V or slightly below, that's like 15-20% drop. +-10% is considered normal. and Fishmans could withstand 25% with no problems, maybe even more.

    I think what he really meant was "you're too loud, my amp doesn't keep up and I cannot hear myself"
     
    Last edited: Aug 7, 2022
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  14. Snaxster

    Snaxster

    Nov 29, 2008
    TL;DR, but about this:

    It is real.

     
  15. BuddhaFingas

    BuddhaFingas

    Aug 16, 2019
    In an AC power feed, loads on the same brach are in parallel.
    Voltage is common across parallel loads.
    Current is additive.

    If your load was enough to sag the line voltage at the stage end of the power cord, voltage for everything powered from it would have sagged.

    If you were causing enough of a voltage drop that it was 'distorting' (but not shutting down) his amp, you'd likely hear/see it across everything on that circuit... PA, lights, etc.

    It's not impossible that his amp is hypersensitive to power fluctuations, in which case he needs better power regulation to make up for the lack, but odds are what he heard was an acoustic rather than electrical effect.

    Tube-amp "power sag effect" from what Ive read was an internal-to-the-amp issue... the amp itself drawing suddenly more power faster than its own power-supply's DC section could react. It wasn't an input-power issue.
     
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  16. Basslice

    Basslice Supporting Member

    May 11, 2008
    Western Massachusetts
    We play a lot of swamp and pond gigs and have to use generators. Every now now and then we overload the power and get "sag". The generator doesn't stop working, but some of our equipment gets wonky and sometimes our PA will shutdown. And turning down does help. Don't know if this answer does.

    Edit: We use two Bose L1 towers. I have tripped the PA just with my strong Vox bursts! (only when on generator). As someone else said, each thing that is plugged in has its own power management and when all on one line one amp can certainly respond differently than another.
     
    Last edited: Aug 7, 2022
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  17. mwbassace

    mwbassace

    Jul 26, 2010
    N.W. Ohio
    First let me say I am an IBEW electrician with 25 yrs in the trade working in commercial & industrial sites. That being said & from the info on the quoted post, along with the OP I feel that your band leader is wrong.

    Your cord could have been a size larger gauge wire, or have another one (preferebly fed from a different circiuit) & 14 AWG is good for 15 amps. And from your description of gear I'm betting your current draw wasn't huge. Even if the cord had 20A running thru it, it would be ok. Not ideal & for sure warm/hot to the touch, but would be ok for a 4 hour gig. But going to a larger wire size has to do with ampacity & is to keep the wire from heating up, melting & eventually causing a fire. In this case the cord had nothing to do with voltage, as i'd be shocked if it wasn't 600v, 90 degree rated wire. Voltage drops caused by the resistance created by the length of wire don't become an issue for us in the field until 100' or longer.
     
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  18. SLO Surfer

    SLO Surfer

    Jun 3, 2009
    Los Osos, CA
    How well could you hear his little guitar amp? Because,
    Also, I think someone mentioned compressors because they can help limit any louder notes, preventing volume spikes.
     
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  19. AGCurry

    AGCurry Supporting Member

    Jun 29, 2005
    St. Louis
    His amp was plenty loud! I wish he had taken my suggestion to raise it off the floor, because I don't think HE could hear it as well as I could.

    My technique is adequate to play any note as soft or as loud as needed. I like dynamics.
     
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  20. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator Gold Supporting Member

    Oct 19, 2000
    Louisville, KY
    It would be infinitely preferable. Years ago i played a gig at some ultra-wealthy party where they had bells and whistles for everything...except the band. They daisy chained a bunch of lawnmower extensions together and then stuck power strips on the end. Amps were screwy all night, and the preamp on my PJB combo was never the same after that. Fortunately the power section did not seem to have sustained any long term damage.
     
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