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Ghost Notes

FractalAudio

Administrator
Fractal Audio Systems
Moderator
A phenomenon present in some vintage amps is an artifact known as "ghost notes".

Ghost notes are the result of intermodulation distortion between the note being played and ripple on the power supply. The ripple is at 120 Hz because the AC voltage is full-wave rectified. So there are frequency components of 120 Hz and its harmonics in the power supply.

These frequency components mix with the note being played and create new tones that are not harmonically related to the note being played. Since it is intermodulation distortion, tones are created at the sum and difference frequencies. For example, if you play a D at the seventh fret on the G string this is 294 Hz. The intermodulation will create new tones at 294 - 120 = 174 Hz and 294 + 120 = 414 Hz. The harmonics of the note being played also factor in. The aforementioned D will also produce tones at multiples of 294 Hz and these mix with the 120 Hz and its harmonics.

The G string above the 5th fret is most prone to this because of the harmonic spectrum of those notes.

The amount of ripple on the supply is a function of the supply impedance. More capacitance and less resistance will reduce the ripple. Conversely less capacitance and/or more resistance will increase the ripple. You can adjust these values in the Axe-Fx using the Supply Sag and B+ Time Constant parameters. Supply Sag adjusts the virtual resistance of the power supply. B+ Time Constant adjusts the resulting time constant of the supply resistance and capacitance, i.e. as you increase the sag the time constant stays constant (capacitance decreases). To counter this increase B+ Time Constant.

Old 100W Plexis exhibit this the most of any amp I've seen due to the high resistance of the power supply transformer. Our reference 100W Plexi has so much power supply resistance that the power supply sags up to 120V! This along with only 50 uF of power supply capacitance leads to prominent ghost notes.
 

SeeD

Power User
Cool info on ghost notes

But.. is this a thing us guitar players will go for.. i mean,if i select this 100 w plexi,i know i will get alot of ghost notes (when i play a solo..or let a note sing out)
I will not be chosing this model in the future.

But some will mabey like this ghost note thing.. hehe
 

VegaBaby

Fractal Fanatic
Cool info on ghost notes

But.. is this a thing us guitar players will go for.. i mean,if i select this 100 w plexi,i know i will get alot of ghost notes (when i play a solo..or let a note sing out)
I will not be chosing this model in the future.

But some will mabey like this ghost note thing.. hehe
I think there might have been some guitarists who used vintage Plexis in the past, even recorded an album or two with them, so I guess the answer could be "yes". advantage of the Axe is that you can even dial it back as Cliff pointed out...
 

Rex

Legend!
Cool info on ghost notes

But.. is this a thing us guitar players will go for..
Too much ghosting will be unpleasant. But just a touch of it can be a different story.

Intermodulation distortion is a fact of life when you have gainy tones. And when you play anything more complex than power chords, intermod will produce notes with no harmonic relationship to the notes your playing. But that thickness is part of the charm of high-gain chording.
 

SeeD

Power User
I think there might have been some guitarists who used vintage Plexis in the past, even recorded an album or two with them, so I guess the answer could be "yes". advantage of the Axe is that you can even dial it back as Cliff pointed out...

hehe,i know. But when i roll back the volume on the guitar,i i play some clean notes (just the guitar,no bass and drums) and then i hear..dissnotes..ghost notes. I will pick another amp next time.
Yes i can dail it out.. but i like to keep it simple,in the amp block.
But for the tweeker.. this is a cool thing ! Dont get me wrong :)
The axe fx is the coolest thing i own
 

Ed DeGenaro

Experienced
Aussie power frequency is 50 Hz, rather than the 60 Hz we have over here. That means your intermod-causing frequencies will be multiples of 100 Hz instead of 120 Hz. Otherwise, same game.
Inter modulation distortion is the subtraction then multiplication of its overtones.
 

Ed DeGenaro

Experienced
Intermodulation is subtractive/additive.
By way of how overtones work it's additive.
However the biggest thing on it is that it creates an "under tone" and it's harmonic series...
Run a sine wave generator if a 100 and 150 hz tine into an analyzer and see what you get.
 

Rex

Legend!
I'm not sure I follow you. If I just sum the two tones, I'll get a spike at 100 Hz and another at 150. If I overdrive the analyzer's input, I'll get harmonics of both tones, as well as sum and difference frequencies of the fundamentals and their harmonics.
 

Ed DeGenaro

Experienced
I'm not sure I follow you. If I just sum the two tones, I'll get a spike at 100 Hz and another at 150. If I overdrive the analyzer's input, I'll get harmonics of both tones, as well as sum and difference frequencies of the fundamentals and their harmonics.
And one at 50 and it's over tones... And if you replace the sine the inherent overtones of the particular instrument.
And the overdriven the signal is a given since that is what happens with any gear that has non linearities it models them.
And if we then look at Class A it gets even funnier...
And just so we're clear I'm not denying that they also add the 50 above but the amplitude is low enough that that isnt what you hear as ghosting. It's the frequency where below the 1st harmonic/fundamental that subharmonic that sticks out. That's the whole concept of pedals that give you a sub harmonic by way of IMD....
Btw bend 15th fret E string about 800 hz and it yowls like a donkey in heat.
 
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Rex

Legend!
And one at 50 and it's over tones...
Which by coincidence, are the same frequencies you get when adding and subtracting the two original frequencies and their overtones. :)

Though I'll accept that the intermod products also intermodulate with the original tones and each other, resulting in a thick soup of goodness or badness, as the case may be.
 

Ed DeGenaro

Experienced
Which by coincidence, are the same frequencies you get when adding and subtracting the two original frequencies and their overtones. :)

Though I'll accept that the intermod products also intermodulate with the original tones and each other, resulting in a thick soup of goodness or badness, as the case may be.
Right, the one below is the audible ghost is the point.
 

Dendrite

Inspired
...yowls like a donkey in heat.
Donkey in heat? I see you made it to hear me solo at my last gig! Key of E in 4/4, though the band was convinced it should be in A# at 6/8... :)

Interesting stuff really. I'd always wondered what that sound was. It always comes back to physics!
 
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