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. Husky

    Husky Gold Supporting Member

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    I hate getting a direct shot of speaker. I much prefer off axis to a nice 4x12. Unfortunately those room tones get lost in a mix, I learned the hard way many years ago. You hear all the ugliness though close mic or getting right in the beam zone. But honestly I didn’t hear all that much that would stop me from playing. Add some reverb or maybe different IRs
     
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  2. Bluesful

    Bluesful Supporting Member

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    Definitely won't stop me playing.

    It's more prominent via the Power Station and my cabs compared to the RL > DAW and IRs.
     
  3. Bluesful

    Bluesful Supporting Member

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    FWIW - I had a listen to this video:



    The ghosting as highlighted in that video isn't the same as what I'm hearing. The ghosting in the video sounds lower than the note, whereas what I hear with my amps is higher than the note.

    Not sure why that is?
     
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  4. HotBluePlates

    HotBluePlates Member

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    I think someone talked about that in this thread... :huh

    It's worth pointing out no one has scoped your setup, checked it out on a spectrum analyzer, and knows for certain that intermodulation with power supply hum is the cause of your issue. It would be nice to rule out some sort of oscillation happening when you hook up the reactive load. However, that is probably unlikely, and it's way easier for a tech to tack on some extra filtering capacitance to see if it eliminates the hash.

    Separately, the video is also of a pedal emulating amplifier behavior. So... maybe not the ideal place to glean tech detail of how things work.
     
    Last edited: Sep 11, 2019 at 8:34 PM
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  5. HotBluePlates

    HotBluePlates Member

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    That was too dismissive of me to say. I wasn't able to watch the video earlier, and now that I have, Simon gives quite a great explanation. I also liked the demo of the Twin Reverb.

    What he didn't say:
    • In the process of rectifying the wall a.c., most guitar amps have a ripple frequency double the wall frequency (where I got 100/120Hz before rather than the 50/60Hz he mentions).
    • The filter caps are like a reservoir, and they charge up 100 or 120 times per second.
    • When the filter caps are fully-charged (or close to it), the residual ripple voltage is small.
    • When you play LOUD the output section sucks a ton of power from the power supply, and this tends to drain those reservoir filter caps.
    • While the amp is LOUD ripple voltage becomes very much larger than it was at idle. There's a bigger difference between the voltage at the (pretty drained) filter caps and the incoming voltage applied by the power transformer & rectifier.
    • There was always a hum signal being mixed with your guitar output; now there's a very much bigger hum signal mixing with the desired guitar signal.
    • The intermodulation distortion due to power supply hum ("Ghosting") adds signals 100Hz (in Oz) above and below the actual notes you're playing.
    • Which of the two will be more audible (or if both are equally audible) depends on a lot of other stuff in your amp's circuit.

    ______________________

    Oh yeah... Heterodyning/intermodulation is happening to some degree in all your amp's stages, if they're distorting. It's most obvious when you play 2 notes on the guitar and compare them to a starkly-clean amp (where there are no added harmonics due to distortion, and no intermodulation due to a distorting/non-linear amplifier).

    You might know about harmonic distortion, adding whole-number multiples of the original frequency. A 110Hz note (5th string A) gets traces of 220Hz, 330Hz, 440Hz, etc.

    Add an E on the 4th string (rounded to 165Hz for easy math). That E has traces of 330Hz, 495Hz, 660Hz, etc.

    But intermod of A and E (110Hz & 165Hz) also produces 165Hz - 110Hz = 55Hz (and octave below A) and 110Hz + 165Hz = 275Hz (slightly out-of-tune C# and 2nd fret B-string).

    And there's some degree of intermod between the A fundamental (110Hz) and all the harmonics of E, as well as the E fundamental and all the harmonics of A. And the harmonics of both A & E are interacting to create their own intermodulation products.
    ______________________

    Now throw a bunch of 100Hz hum in this soup... 110Hz A - 100Hz hum = 10Hz (can't hear it), as well as 110Hz + 100Hz = 210Hz (very out-of-tune G# beating against the octave harmonic of our A).

    And 165Hz E - 100Hz hum = 65Hz (probably rolled off, but audible), and 165Hz E + 100Hz = 265Hz (close to a C natural, but out-of-tune).

    And on & on... High levels of distortion bring on high levels of intermodulation, and higher levels of hum mean wild out-of-tune distortion products become louder.
     
    Last edited: Sep 11, 2019 at 9:03 PM
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  6. Bluesful

    Bluesful Supporting Member

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    Thanks for the explanation mate.

    When you said:

    "The intermodulation distortion due to power supply hum ("Ghosting") adds signals 100Hz (in Oz) above and below the actual notes you're playing.The intermodulation distortion due to power supply hum ("Ghosting") adds signals 100Hz (in Oz) above and below the actual notes you're playing."

    it's the frequencies above that are most prominent to my ear in my situation.

    I'm enjoying reading all of this stuff.
     
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  7. HotBluePlates

    HotBluePlates Member

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    I understand. But I'm telling you the lower ones are there, too.

    Know how to play a pedal steel bend? Say, fret & hold a 12th fret B while bending the 11th fret F# up to G#. You're bending up to a partial E major chord. If you release that bent G string, it drops down to a partial B major chord.

    Play your amp, kick on a heavy fuzz sound, and play that pedal steel lick. You may notice a thick deep note when you bend the F# up to G#. But you will absolutely notice the subharmonic when you release the G# back down to F# while holding that B.

    That's the same difference-tone from intermodulation distortion (ghosting) as what I described. The sum-tones get lost most of the time in the rest of the higher frequency mess from distortion harmonics.
     
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  8. Bluesful

    Bluesful Supporting Member

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    Apologies mate, I'm not doubting you at all. I believe you.

    My ears are just cr@p ;).

    I'll give that a go.
     
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