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Would bad power cause this distortion?

Discussion in 'Amps and Cabs [BG]' started by Fishheadjoe, Mar 27, 2014.

  1. Hey folks,
    I had a gig on Friday night at a bar that is shall we say, not the nicest place we play... old, beat up and showing signs of having been worked hard and put away wet for a number of years.
    So, we get ourselves set up, I have my Marshall VBA400 on stand by for a few minutes then engage the amp in order to do a brief sound check. (Cab is a NV610) As soon as I bring the amp up to performance level, I noticed that I was getting an audible distortion on the attack of the note, but only on the actual attack. It was not there after initial transient of the note diminished.
    I tried to replicate the problem here at home and have not been able to do so, even after an hour of running the amp pretty darn loud. (I know my neighbours were wondering why their stuff was sliding across the room!)
    So, my question again, does this "symptom" sound like it could be bad power at the club? No, as of right now, I don't have a power conditioner... responses here may well have me shopping for one though!
    Many thanks,


    *** bass used was a Pedulla Rapture with a fresh battery, so I assume that wasn't it and again, I couldn't replicate the problem at home!
  2. Have you tried new patch cables and a speaker cable? Those would be my first troubleshooting steps.
  3. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Sounds more like a bad tube to me.
  4. Could be caused by wonky power at the venue.

    Power conditioner won't help unless you are willing to spend hundreds. (the average "power conditioner" is a glorified power strip).
  5. I have not tried new/different cables as I cannot replicate the problem here at home. It works perfectly fine here and I have cranked the amp up substancially higher, then I would at a gig... still no peak distortion.

    Jimmy, that was my initial thought as well as my guitar players', who has significantly more experience then I with tube amps. It does however make it difficult to diagnose, if the problem isn't repeatable.

    Bass_pounder, yah I figured that a good one won't be cheap!

  6. cchorney

    cchorney Supporting Member

    Oct 21, 2010
    Meriden, CT
    Failing battery in one of your pedals? Even if the pedal is plugged in, a bad battery will announce itself that way.

  7. That's a good thought, but I run my bass direct, no pedals.

    Also, I have tried to replicate the problem with the Pedulla, my Roscoe and my RIC4003, can't get it to return.
  8. BurningSkies

    BurningSkies CRAZY BALDHEAD

    Feb 20, 2005
    Seweracuse, NY
    I've experienced distortion from bad power several times. I've had it happen in outdoor gigs with generator power a few times, and a couple hard wired gigs with dubious power. I've had it happen with both tube amps (Fender Twin, Mesa 400+) as well as solid state and class D amps (Markbass, Peavey IPR).

    So yes, very possible. Remember if the line voltage is dropping at the 'wall' your amp loses headroom as the 'rail voltage' lowers accordingly.

    There are a few places where I always 'bridge' my class D power amp because I know they have low wall voltage and the effect is pronounced with class D amplification due to how it works with a 1/3rd duty cycle. Bridging lessens the chance that I hit the 'rails' and clip.

    And as the attack of your note, that initial transient is the 'peak' that's why you're getting distortion on it...that's the part that's clipping.
  9. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    ^ This is a good possibility.

    As was mentioned, the attack is where the most demand is on the power supply. Also, you might notice this being more pronounced on the lower notes or if you crank the bass up more.

    If you can reproduce the problem at a vane and while you are playing, have someone measure the voltage at the power outlet, you'd know if it is sagging. Your amp can deal with a bit of sag. The internal voltages are lower and the power output section's bias will change. If the voltage is very low and it can damage your amp. On some amps, there is an operative range spec on the back of the amp or in the manual. Otherwise, the manufacturer can tell you what it is.
  10. Thanks again everyone...
    Yes, I recall now in my moment of panic, that the lower the notes I played, the worse/longer the distortion lasted.
    I'm going to try again here at home, for as long as my family can take it, to replicate it.
  11. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Well if you can't replicate it at home with any of your stuff, I guess it's less likely to be a tube issue.

    RE: power conditioner, no it won't help. Your basic power conditioners are glorified wall warts, and power conditioners that actually can alter voltages cost in the 4 figures.
  12. BurningSkies

    BurningSkies CRAZY BALDHEAD

    Feb 20, 2005
    Seweracuse, NY
    Yeah, power conditioner isn't going to help bad wall voltage. I ended up solving the problem myself after consulting with our resident Peavey engineer in regards to my IPR and he confirmed what I was thinking, saying that class D means low voltage reduces headroom exponentially...and I posited that having two amps in push/pull would mean reduced headroom, but since the headroom was so greatly expanded anyway I'd be less likely to get near the rail voltage.

    I don't think there's any 'magic bullet' for this.
  13. jj4001


    Dec 27, 2010
    Providence, RI
    Bad power can definitely cause this.

    I've had this problem at a couple of venues we play. I was freaking because I thought I had blown speakers after playing both of these places on back-to-back gigs. I couldn't re-create the issue testing the amp at home and it's been running perfectly everywhere else since then.

    Good luck. Hope it's just this one isolated incident for you.
  14. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Since it works at home with a good power source, the amp appears fine. If it happened at home, I would suspect the power supply electrolytic capacitors are ready to be changed. With both bad power or aging caps, loss of headroom and early onset of distortion are indicators.

    One thing that can help at a gig is to residtribute your power. Don't put all the gear on the same circuit breaker. This might help. If all the power lines at the venue are being taxed too much, it may not help much.
  15. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    As usual David beat me to the punch!!

    If the wall voltage is low then all the voltage supplies will be lower too. Less positive B+ and bias closer to ground potential. This can play havoc with the amps output stage and cause distortion on the attack.
  16. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Sorry...I said power conditioners were glorified wall warts, but I meant to say they were glorified power strips. Oops...
  17. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    You mean Classes A through H.

    If the power is "bad" enough to cause what you describe, your amp's performance is not your biggest concern. What could be dropping the voltage that badly is your concern, because it's a likely fire hazard.

    I'm sure you have a halfway decent voltmeter, right?
  18. BurningSkies

    BurningSkies CRAZY BALDHEAD

    Feb 20, 2005
    Seweracuse, NY
    It only happens in a handful of rooms. One of which is a place with a kitchen, where I'm sure the fryers or some other high wattage implements are on the same lines...sharing a common wall.

    Other times as I said, it's happened with tube amps in places with bad stage power or generator power. Usually its not a problem, certainly not our practice room and I don't use amps at home. :)

    Edit: And I did some digging, and Bob the original post I was referring to from several years ago was this (its obvious that I didn't retain all of what was said, and it was Aged Horse from GB, not Peavey who replied):

    Let me make a few general comments about this specific topic that you have experienced because it's responsable for a lot of misunderstandings even from people who should know better...

    1. What you are experiencing may certainly be the symptoms of low AC line voltage. This applies to all amps, with either traditional line frequency or SMPS power supplies (except regulated), class AB/G/H/D.

    2. The output power of any amp is very sensitive to the power supply voltage because as the voltage sags (or enters low due to indaequate supply wiring) the audio output power available decreases by the SQUARE of the voltage drop. The square factor is an incredably important detail in understanding why amp output power is so greatly diminished by falling AC line voltage.

    3. As a numerical example, with an amp rated at 900 watts into 4 ohms, that would be an output voltage of 60 volts RMS and more importantly 85 volts peak. The peak voltage is responsable for the maximum undistorted output. Now, say the input voltage falls from 120 to 105 volts RMS (not uncommon where power distribution issues are present)... that is a drop of 15 volts or only 13%. This voltage drop applies proportionally to the RMS voltage of the audio output signal but due to how power supplies are a non-linear load the peak voltage will fall even faster (technically flat-topping of the AC line voltage waveform and increasing the THD of the incoming power) and may fall 18 or 19%. Since the clipping point is defined by the available peak voltage, take 18% from the peak voltage of the (120V RMS condition) 85 volts and now it becomes 69.7 volts, convert that back to the RMS equivalent which is 49.3 volts. This represents an undistorted power of 607 watts or a 33% power loss for 13% RMS (18% typical PEAK) AC line drop.

    I know some of you guys are intersted in this stuff, it's an advanced topic but one that plays so directly into an amp's performance and one that affects larger amps more than smaller amps since the current draw (which is tha major factor responsable for voltage drop) is proportionally larger on a larger amp. BurningSkies experience is a GREAT real world example of what is happening and why amps must be rtated with a fixed and unchanging input power voltage for making comparisons between amps, and what happens to the power output of ANY amp when AC line voltage falls.

    The only exception to this is amps that use a PWM regulated SPMS... but these are very, very rare in the audio amplifier world.
  19. Bim1959


    Apr 15, 2009
    Naples Florida
    Sales and electronic tech/piano tech: England Music Center - Clinton IA - now closed
    I've had problems with funny power before (ever try to play off a generator before when you have a Hammond organ? Not pretty) so at places I've never played before I have a Tripp Lite power stabilizer/surge suppressor that goes into the wall first, then I run my bass rig out of it. For a good one you're gonna shell out $600 or more but if your equipment is worth it I wouldn't run without it. Im not sure which model mine is (it's a good sized box with 6 outlets on it) but it had a $25k insurance policy that came with it - that if it didn't do it's job and die first then Tripp Lite would cover your stuff up to $25k.

    With the way a lot of venues are anymore, I'd rather play safe than sorry.....
  20. Put some more time into running the amp over the last couple of days... it's still not performing as it should.
    It no longer distorts on the transient, but not I am getting "fuzz" as the note decays. Almost a "breath" like sound.
    I switched out cables, tried three basses, hooked up two different amps to the cab, to ensure the cab wasn't the problem... no dice.
    SO, I dropped the amp off at my tech last night, kinda hoping he'll have it up to snuff by my gig on Friday night.
    I'll update the thread once I find out what's gong on.
    Again, thanks to all who offered advice.