Why are my amps making this noise with reactive loads?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Bluesful, Sep 6, 2019.

  1. Bluesful

    Bluesful Supporting Member

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    Hi All,

    Seeking some advice.

    I have a couple of NMV amps that when cranked into a reactive load exhibit this weird, grainy, almost harmonic that sits on top of the notes/chords played.

    Is this just ghosting and because I'm so close to the speakers it's really prominent to my ears? Or is there something else going on?

    I've tried to provide some examples below. I used a looper to ensure that the exact same thing was played in each example.

    The first example is my Power Station with a mic used on my speaker cab:



    The second example is using my Suhr Reactive Load and IRs in my DAW:



    On the chords you'll note that graininess sitting on top, and when I play the single note you'll hear that whiney, almost harmonic sitting there.

    I've noticed that it is more prominent when I'm using the neck pickup and less prominent when switching to the bridge pickup.

    What I've also captured for reference is the noise that happens from the tubes on the amp. In this clip I'm using the Suhr Reactive Load and my studio monitors are off, so all you're hearing is the noise coming from the tubes/back of the amp (the Suhr Reactive Load also makes noise, but this clip is mostly the amp).



    Am I going crazy hearing this noise? Is this normal, or is there something else causing this?

    Any thoughts would be most appreciated.
     
  2. 79Stone1

    79Stone1 Member

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    Do you notice it when playing through a speaker cab and nothing else? In the audio clips i can hear a lot of noise and ringing when you're not even playing. When you play chords to me it sounds similar to what can happen when a filler cap has bitthe dust. The power supply can no longer supply the current demanded by the tubes and you can get harsh clipping and noise. Ghost notes are sometimes associated with a lack of filtering and blocking distortion that would occur with a failed filter cap. Id assume this problem would be evident regardless of what type of load the OT sees, and i have no idea why it would be audible in the tubes themselves.
     
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  3. Bluesful

    Bluesful Supporting Member

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    I've only ever been able to test it at that volume for a very, very short time (we're talking a cranked amp here). My recollection was that the noise wasn't present with no reactive load in line.

    The reason I was thinking that it wasn't the filter caps is that this particular amp is basically brand new. It was built in February/March of this year.

    You can hear and feel it change at a certain spot on the MV pot too. Once it clicks passed it certain point you'll get the noise.
     
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  4. Oldschool59

    Oldschool59 Member

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    My amps do the same, at least as far as the acoustic noise. I don’t get the « ghosting » harmonic through the speaker, but the OT will « sing » and play out the guitar part. It is my understanding that some amp architectures are more susceptible to this behaviour under attenuator load, particularly M-types.

    I don’t want to open a can of worms, but I hear some amp makers will void the warranty if attenuators are used. For the record, my attenuator is the Ox. Cheers.
     
  5. Mike Lind

    Mike Lind Member

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    Amp transformer and coils in a reactive load will sing when enough current is flowing through them.

    Users sometimes report this as being a fault with their attenuator/loadbox but it’s perfectly normal as it’s kind of difficult hearing it with only a speaker connected.

    However, there could still be an issue with your amp that manifest itself when driven hard.

    If you have an amp that is prone to self-oscillation it will appear much sooner with a speaker or reactive load in comparison to a resistive one. I believe that the Power Station can be dialed in so its load is resistive so that would be something you could try. Maybe someone can verify this?

    A properly designed load used correctly should not be any more dangerous to your amp than a speaker. The problem usually comes from driving your amp harder than before.
     
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  6. Husky

    Husky Gold Supporting Member

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    Filtering can be low out of the box, maybe you should add some if you don’t mind the stiffness it will add, many people find the sag and a little ghosting desirable and vintage, try listening to Starlicks Brian May lesson where he demos his amps and the need for a treble booster. What you are hearing up close and personal is what your signal chain sounds like at the speaker without taking 20 steps back and getting the room and distance involved. The ghosting is a need for more filtering if you don’t mind the extra tightness. Also make sure you are not using a Mic input of your DAW, that will distort. Transformers and tubes singing is 100% normal, you just don’t hear it when a speaker is going too. That is not related to your other concerns.
     
    Last edited: Sep 7, 2019
  7. Husky

    Husky Gold Supporting Member

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    Yes switches down is resistive. Mostly that will roll off high end though so it might give him a false sense it improved. I’d say pick an amp and increase filter cap value and maybe get rid of some bass if you don’t get along with this.
     
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  8. Dan40

    Dan40 Supporting Member

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    Does the noise occur only with the volume on 10 or does it also happen at lower amp volumes when using a reactive load?
     
  9. Husky

    Husky Gold Supporting Member

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    That musical vibration in the transformer and tubes is going to get louder the louder the signal is but that part of his question is 100% normal and happens with speakers too, it’s just way drowned out.
     
  10. Bluesful

    Bluesful Supporting Member

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    Tried that.

    My understanding is that with all of the switches set down is the resistive load. Even at those settings I get the 'ghosting', 'harmonic' or whatever it is.

    I tried a Treble Booster and I still get the noise.

    I always go Suhr RL > Hi Z 1/4" input of my interface.

    It only seems to be at the higher amp MV settings, which would suggest the filtering in the amp I suppose.

    With this particular amp though I love getting it above the point on the MV that induces this noise. The feel and tone up there is fantastic, it's just the 'extra' noise that comes along with it that is annoying me.

    It also seems to worse with certain notes - the C# on the B string at the 14th fret is the worst culprit.

    The weird thing is that I only get this with a couple of amps. I don't get it with my HW AC30 or '67 Vibrolux Reverb (which doesn't have the original filter caps).
     
  11. Husky

    Husky Gold Supporting Member

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    The “noise” I assume you mean when there is no speaker and you can hear buzzy signal around transformers, tubes or the reactive load is 100% normal. It’s always there with the amp. There is a lot of bass in the preamp of the amp or the guitar which will not be a very pleasing tone in the first place. The ghosting is a matter of filtering and if you were my customer it would be so easy to show you on a scope and then I think it would all make sense to you.

     
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  12. Bluesful

    Bluesful Supporting Member

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    I should clarify, I meant the ghosting not the transformers etc.....

    So in essence, we're saying that to avoid the 'ghosting' noise, I'd need to swap out the filter caps in any amp that exhibits this, for caps of a higher value?

    Do power tubes that are biased too hot also experience the same things?
     
  13. Husky

    Husky Gold Supporting Member

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    If the power tubes are running hot the current will be higher and you will get more ghosting yes, in theory. If in the normal ranges really probably not too big of a contributor. You could swap them out if the amp is over 15 years old. Otherwise that might not be enough, I like big filter caps because it’s punchy with gain but it’s also stiff so there is a compromise.

     
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  14. Bluesful

    Bluesful Supporting Member

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    So can anyone explain the technical reason why certain notes seem to produce a more prominent occurrence of this noise than other notes?
     
  15. HotBluePlates

    HotBluePlates Member

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    Intermodulation Distortion.

    Heterodyning is when two frequencies are mixed in a non-linear circuit (like a distorting tube), and creating 2 new frequencies that are the sum & difference of the original two frequencies. That means there are 4 frequencies at the output, one of which is higher than either of the originals, and the other lower than either of the originals.

    You can hear the effect in pedals with a lot of distortion when you do an oblique bend, or similar pedals steel type stuff.

    In this case, one of the frequencies is power supply hum and it is heterodyning with your guitar notes to produce that hash riding on top of your sound. Some people will just call it "intermodulation distortion" which is what it is, but the core process is the same as heterodyning. @Husky keeps recommending that your amp's filtering may not be sufficient for the amount of distortion you're generating, as the power supply hum intermod stuff is getting loud enough to be objectionable. The goal is to knock down one of the intermod source frequencies to reduce the hash until it's not so much of a problem.

    ___________________________

    Why certain notes? Ripple hum in the power supply is 100 or 120Hz (depending on if your wall voltage is 50Hz or 60Hz). The difference frequency from heterodyning/intermodulation probably won't be a problem, cause your guitar only goes down to ~80Hz and you're probably cutting a lot of bass anyway. Whenever you notice your guitar sounds "really fat" after adding a fuzz, etc, that thick bass is probably coming from the difference frequencies due to intermodulation/heterodyning.

    If the sum frequency of 100/120Hz and whatever note you're playing is still in an audible range, and not being heavily cut by the speaker (or other EQ), it's gonna be audible. Most musical notes in the equal temperament scale aren't going to be the specific sum frequencies you get when intermod happens, and they may beat against the notes/harmonics you're already getting/playing. So the whole thing turns into a discordant mess.
     
    Last edited: Sep 8, 2019
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  16. Bluesful

    Bluesful Supporting Member

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    Which would be by increasing the value of the filter caps?
     
  17. HotBluePlates

    HotBluePlates Member

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    Right. Higher filter cap values, less power supply ripple, so smaller 100Hz/120Hz intermod source frequency, so smaller-sized resulting intermodulation & hash. "You hear less trash in the distortion."

    Downsides are bigger filter caps make the power supply voltage higher (by some amount, depending on the existing cap values), and the amp feels stiffer because there's less voltage sag (which can happen depending on the transformer & filter caps, even if your amp has a solid-state rectifier).
     
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  18. Bluesful

    Bluesful Supporting Member

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    Do you feel that having another amplification device (like the Power Station) in line after the source amplifier can exacerbate the noise?

    Almost like there's the noise inherent in the source amplifier and the secondary amplifier (Power Station) is further amplifying it?
     
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  19. HotBluePlates

    HotBluePlates Member

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    I would guess the fact you'll turn up more than you otherwise might (more distortion, more intermodulation) and that you're hearing everything in fine detail (amps aren't blaring loud, where your ears naturally starts desensitizing to some of the high frequency hash) is part of it.

    Beyond that, Husky seems to know a lot more about it than me, so I defer to his advice. I haven't used a Power Station, and don't want to assume I know exactly what may or may not happen with that product.
     
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  20. Bluesful

    Bluesful Supporting Member

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    I think there's absolutely no doubt about that one.

    And the fact that when I'm at home I'm really close to the speaker cabs.
     
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