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Class D modulation noise

Discussion in 'Amps and Cabs [BG]' started by micguy, Mar 13, 2018.


  1. micguy

    micguy

    May 17, 2011
    I'm posting this looking for help from someone who knows class D amps really well. I design audio gear for a living (but haven't done work on class D amps), so feel free to get a bit technical if need be.

    I have one and only one gig where I need an amp, and it's a quiet gig - I have to keep up with an acoustic piano, and that's about it. I have a little 110 cabinet that I built that works great - puts out plenty for this application, and fits easily in my trunk. The trouble I'm having is with modulation noise (or something that sounds like modulation noise) - it's a little like a speaker buzz, but not quite - more white in spectrum than that, and it happens over a wider dynamic range than a speaker buzz, though there is harmonic content related to the original signal as well. I'm nowhere near clipping or the physical limits of the speaker when this happens. There is a bit of a whine when I'm not playing, but that's very quiet. When I play, the fuzzy, whitish noise moves up and down with the signal. I don't think the audience hears it much, especially when others are playing, but I certainly do, being right on axis of the speaker.

    I think it's a power issue - at home, I can't replicate it _ I have literally turned the speaker upside down, and on its side to make sure it's not in the speaker. I've used 3 different amps, and a couple different basses trying to get it to misbehave at home, but so far, no issues at home with that speaker or either of 2 amps.

    At the gig, I've had the issue with 2 different amps - a Fender Rumble head and a TC Eleectronics BH250 - both use class D amps (they might enven use the same power module, for all I know). There's a power "snake" that gets used at that venue - a long heavy cable, with taps every few feet. A couple powered monitors get plugged in in addition to my amp.

    Does this kind of behavior suggest anything to anyone who's got expertise in this area?

    Aged Horse?
     
  2. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    You didn’t page properly. You do it thusly: @agedhorse :)
     
  3. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    These are very hard problems to diagnose, nut one thing that's pretty important is that the power system is properly grounded and bonded. The grounding is one method used to control EMF and its effects.

    It could be any number of things though, things like audio ground loops, which (can) behave differently when you add some small high frequency currents to the equation. In general, my experience with the Rumble class D amps has been that they are free from such problems. Their designer is well skilled in this sort of thing, so I wouldn't expect problems. I have no idea about your other amp.
     
  4. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    It could be a cell phone or wireless splatting out signal in the form of RFI that the amp is picking up. Turn off what you can and see if it helps.
     
    mogator, Al Kraft and agedhorse like this.
  5. basscooker

    basscooker Commercial User

    Apr 11, 2010
    cincy ky
    Owner, Chopshopamps.com
    This problem/diagnosis is out of my league, but one trick I try whenever I encounter a noise that may be RFI (sometimes it works, sometimes not):

    If you have a small patch cable handy, loosely tie or wrap it around your cables. Try the IEC cable first. If no change, try the instrument cable. If no change then try the speaker cable.

    I'm openly admitting I'm grabbing at straws and this is not likely to solve or eliminate the problem, but this has worked for me more than a few times.
     
    fleabitten likes this.
  6. micguy

    micguy

    May 17, 2011
    Good suggestion, but...I'm pretty familiar with what cell phone and RFI issues sound like (I've dealt with those in working on stuff at my workplace - they don't tend to modulate with the signal/go away when you stop playing like this does.
     
    beans-on-toast likes this.
  7. basscooker

    basscooker Commercial User

    Apr 11, 2010
    cincy ky
    Owner, Chopshopamps.com
    Could there be neon or fluorescent lights on the same circuit?

    Unlikely, but something glass nearby that is simply rattling? Or a loose something or another in the piano?

    Again, totally guessing. You said you can't duplicate the problem at home, so it has to be something in the environment.
     
    HolmeBass likes this.
  8. Is the idea that the cable that is being tied around the other cable(s) acts like an RF choke?
     
  9. basscooker

    basscooker Commercial User

    Apr 11, 2010
    cincy ky
    Owner, Chopshopamps.com
    Yes

    I actually keep a little length of copper wire in my gig bag for this. It has worked for me a few times and takes up basically no space.
     
    Last edited: Mar 13, 2018
    Old Garage-Bander likes this.
  10. micguy

    micguy

    May 17, 2011
    Agreed, it is definitely related to something in the environment. The last couple of times, we didn't actually have a pianist (we did have a harpist), so the piano was very far away, and the noise was definitely coming only from my speaker - i'ts not acoustical rattling, its something in the electronics and how they're interacting with that environment that's coming out of my speaker. I don't think we have any flourescent lights in the space, and I'm pretty certain there's no neon, but I do wonder if there is anything on SCR dimmers on that circuit.
     
    basscooker likes this.
  11. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Or... the circuit itself is dimmed (with the dimmer rigged at 100%, there will still be some feed through from the phase control firing at almost 360 degrees but not quite). This high rise time feed through may be more easily coupled to the power supply rails, and once there can cause problems due to very little power supply rejection by the class D output stage.
     
    HolmeBass and Al Kraft like this.
  12. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    What you are describing could be an intermodulation distortion. Your signal being combined with a high frequency.

    Since it can’t be reproduced at home, there is a good chance that it is due to an external influence. No matter how well an amp is shielded, measures can be overwhelmed with a strong enough signal.
     
    Bflat and Al Kraft like this.
  13. bucephylus

    bucephylus Supporting Member Commercial User

    Aug 18, 2002
    General Manager TecPadz LLC
    What kind of bass/pickups are you using?
     
  14. Al Kraft

    Al Kraft Supporting Member

    May 2, 2016
    Northern Virginia
    I agree that this is a very likely candidate. Many folks when they are home have their phone on a table, but when they are out (at a rehearsal space perhaps) will have it in their pocket or in a case on their belt. Issues like this are not uncommon IME.

    It's also possible you are close to a relay/hub point for something wireless which may be in or outside the building. I would try some different power outlets (not on the power snake) and maybe even position your amp at different places in the building and see if that reduces the noise.

    Last thing, and you may have already done it, is see if the noise if there if the bass is not plugged into the amp. If it is not, you could try a different instrument cable (and different lengths). Best of luck with your trouble shooting.
     
    beans-on-toast and Bflat like this.
  15. If your bass and pickups are not well shielded, they could be picking up interference which is just being amplified by your amp. Or, it could be in the power line. If your amp has a ground-lift switch try it. If not, try using a power conditioner between your amp and the power source. If that doesn't cure it, it's most likely your bass and/or your cables.
     
  16. Have you ever tried putting several loops in the instrument cable to make the choke?
    Or ever tried ferrite beads?
    I wonder if those might be two more tools for your bag of tricks?
     
  17. Ground lift switches are useful for dealing with ground loop induced hum when feeding an external mixer or PA from your D.I.

    I would not dump money into a power "conditioner" right off the bat without knowing more about what's happening. Most of them don't really "condition" the power beyond providing some surge suppression and maybe a little noise filtering (which may or may not help the OP's problem). They can't even out voltage fluctuations or fix over/under voltage conditions, unless you're spending big $$$.

    One would need to understand both the problem and how the conditioner works to say for certain it would be effective. They certainly won't do any harm, but they are not the magic bullet some beleive them to be when they hear the word "conditioner."
     
  18. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    They in fact can cause problems that otherwise wouldn't be an issue. At RF frequencies, nothing it terribly predictable.
     
  19. Now that's a big 10-4 good buddy. :woot:
     
    Al Kraft, Bflat and agedhorse like this.
  20. Bflat

    Bflat

    Feb 5, 2008
    You know anyone with a rf spectrum analyzer, otherwise it could take a while, sounds like 2 asynchronous signals mixing.