High-pitched fizz from multiple amps... is this normal?

[jakeru19]

Hi everyone,

I have 8 tube amp heads connected to a UA OX attenuator via a KHE switcher. I recently noticed that more or less all of the amps... some of them vintage, some modern and brand new... produce a high-pitched fizzing or rattling noise. The noise occurs when I strike a note/chord, and dies out along with it... it's very faint, and is easily drowned out and inaudible through a cabinet when playing at normal volume... I only notice it when I attenuate the speaker output very low or am recording silently. It is present to some degree more or less with every amp, with multiple guitars/cables, so I don't think it's a particular bad tube or cable etc. Moreover, I can hear the noise is actually coming from within the different amp heads themselves, moving around the room to whichever amp is currently active, but I cant tell if it's coming from the tubes or the transformers... The noise increases in volume proportionally with the pre-amp gain and master volume on each amp... not a low pitched hum, I know these terms are subjective but I would describe it as a "fizz" or "buzz" almost like a very high pitched mechanical, almost metallic rattling like a string with low action buzzing against a fret. Is this a normal phenomenon? It's not audible in my recorded tracks so it's not a problem really, I just want to make sure everything's okay and I'm not about to blow something up... appreciate any insight!

 

[glpg80]

Sounds like a problem with the attenuator or the switcher (or a combination of both interacting with each other) since it occurs on every amp and when the attenuator is active and heavily loading the currently selected amp’s output transformer signal.

What if you run the same amp direct to the ox heavily attenuated without the switcher? Does it still make the same sound? If so, do you have another attenuator that you can try to isolate the problem to the ox with and without the switcher?

If it’s still apparent with a different load box then I’d say it’s due to the switcher. If it goes away with heavy attenuation without the switcher then it’s the switcher. If it goes away with a different load box with the switcher then it’s the ox. If it goes away with the switcher and a different load box but also goes away with the ox direct in to an amp without a switcher then it’s a combination of the ox not being compatible with your switcher.

My gut tells me that that impedance curve at high attenuation isn’t the same as the ox expects just plugged straight in since you have the switcher between it and the amp(s) output transformer which is normally purely inductive to the ox input.

 

[raiken]

It may be magnetostriction, which is a normal noise caused by the vibration that occurs in an output transformer due to the magnetic field produced by the signal.

Normally you don't hear it because the speaker volume masks it, but when playing into a silent load it becomes audible.

 

[psychodave]

It may be magnetostriction, which is a normal noise caused by the vibration that occurs in an output transformer due to the magnetic field produced by the signal.

Normally you don't hear it because the speaker volume masks it, but when playing into a silent load it becomes audible.

I’m thinking this post is likely correct.

OP, try attenuating one amp and going over to the output transformer and listening. Keep in mind if you get too close (like without an inch or so) you might introduce additional noise if holding a guitar.

 

[jakeru19]

I can't see the inside of of most of my amp heads, and am too lazy to take them apart, but in the one or two that I can peer inside, it seems to be coming from the smaller of the two transformers I can see. Looking at diagrams online, it seems the output transformer is usually smaller than the power transformer. So the 'magnetostriction' thing seems to be at play here. I can't explain why, but that also just jives with my gut feeling that this was some kind of organic phenomenon caused by putting a whole lot of signal through the amp... I won't worry about it then. Thanks guys!