torroïdal trafo buzz

[billfitzmaurice]

There are DC components/offsets on a typical household AC line, typically cause by things like lighting dimmers and ballasts that inject a bunch of crap into the line. The resulting asymmetrical waveforms have a DC component. Stick a scope on a wall outlet and see how not sinusoidal the signal is.

Anyway, in a non-air gapped toroidal transformer, it doesn't take much DC current to saturate the core because of the high magnetic efficiency. Once the core is magnetically saturated, it's susceptible to magnetostriction. The magnetic field actually deforms the iron molecules in the core, which causes the core to vibrate. There are two cycles of this per cycle of AC, which is why transformers buzz at 120Hz (or 100Hz in places where line frequency is 50Hz). It takes very little DC (a few milliamps or less) to produce this effect in toroids.
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That I can see, but the physical vibration of the windings shouldn't show up as hum in the output of the amp. I've never heard toroids I've used sing, but I usually overdo their capacity by at least fifty percent, if that makes a difference. I also use potted toroids in critical applications, like pre's.

 

[Mark Reccord]

That I can see, but the physical vibration of the windings shouldn't show up as hum in the output of the amp. I've never heard toroids I've used sing, but I usually overdo their capacity by at least fifty percent, if that makes a difference. I also use potted toroids in critical applications, like pre's.

No, you're absolutely right that it shouldn't show up in the amp's output, I just got off on a transformer tangent...
All of our transformers are potted, which really damps core/winding vibration, though I can get them buzzing like a mother if I drive them hard enough... I like destructive testing...


What the OP is describing sounds like a bad ground somewhere in the amp, not a transformer issue.

 

[Matthijs]

What the OP is describing sounds like a bad ground somewhere in the amp, not a transformer issue.

So how can you explain these two things:
The noise coming through the amp is not a 50 cycle hum but sounds similar to the transformer buzz.
The noise through the amp (not from the trafo itself) diminished a great deal when I turned the transformer half a turn.

 

[bongomania]

Though I'm not fit to clean some of these guys' soldering-iron tips, I'll take a stab at those questions:

Grounding problems do not always result in a 50 (or 60) cycle hum. They can cause transistors and other components to behave erratically, they can allow noise carried on the ground line to be injected into the audio line, and they can be both mechanical and electronic at the same time- mechanical vibration can cause intermittent or rapidly-repeating make/break contacts. So one hypothesis might be that the vibration of the xfmr is causing a rapidfire make/break contact at a ground point, resulting in buzzing through the audio line that mimics the mechanical buzzing of the xfmr. It is possible that turning the xfmr a half turn caused its vibration to have a lesser impact on the affected ground point.

Anyway, even if I'm way off, it's a fun challenge to think about.

 

[Espidog]

There are DC components/offsets on a typical household AC line, typically cause by things like lighting dimmers and ballasts that inject a bunch of crap into the line. The resulting asymmetrical waveforms have a DC component. Stick a scope on a wall outlet and see how not sinusoidal the signal is.

Anyway, in a non-air gapped toroidal transformer, it doesn't take much DC current to saturate the core because of the high magnetic efficiency. Once the core is magnetically saturated, it's susceptible to magnetostriction. The magnetic field actually deforms the iron molecules in the core, which causes the core to vibrate. There are two cycles of this per cycle of AC, which is why transformers buzz at 120Hz (or 100Hz in places where line frequency is 50Hz). It takes very little DC (a few milliamps or less) to produce this effect in toroids.

Lots of manufacturers take steps to block DC current from passing through the primary winding of a power transformer. Including big respected ones like Bryston. You can build an effective DC blocker with a diode bridge and a couple of capacitors placed on the neutral side of the transformer’s primary. The caps pass the AC and the diodes block any DC on the line as long as the voltage is less than the forward drop of the diodes.
This is one of the few things in the audiophile world that isn’t voodoo or snake oil.

Mark, it looks like you and I have very different understandings of what qualifies as DC. Yes, we all know how noisy the AC mains is: a jagged beast indeed, rich with spikes and ripples acquired, as you say, from all manner of sources - but I can't agree with the idea that this noise, being made up of components nearly all of which have a higher frequency than the 50 or 60Hz mains itself, could be classed as a "DC component".

Or am I missing the point here? Are you, in fact, suggesting that the mean average effect of numerous sources of AC noise, each with its own characteristic frequency and waveform, is such that an overall DC offset is created? Interesting....

 

[Mark Reccord]

So how can you explain these two things:
The noise coming through the amp is not a 50 cycle hum but sounds similar to the transformer buzz.
The noise through the amp (not from the trafo itself) diminished a great deal when I turned the transformer half a turn.

1) Most ground issues in an amp will produce noise at twice line frequency, because the ripple on the DC supply is at that frequency.

2) Really? That's pretty interesting. So you loosened the bolt and turned the transformer half a turn and the noise diminished? Maybe it is a transformer issue after all. I can't really make a judgement on it without seeing it of course. Sounds a bit strange though.

 

[Mark Reccord]

Mark, it looks like you and I have very different understandings of what qualifies as DC. Yes, we all know how noisy the AC mains is: a jagged beast indeed, rich with spikes and ripples acquired, as you say, from all manner of sources - but I can't agree with the idea that this noise, being made up of components nearly all of which have a higher frequency than the 50 or 60Hz mains itself, could be classed as a "DC component".

Or am I missing the point here? Are you, in fact, suggesting that the mean average effect of numerous sources of AC noise, each with its own characteristic frequency and waveform, is such that an overall DC offset is created? Interesting....

Yep. It's based in Fourier analysis. If the AC waveform is asymmetrical (i.e. more area under the curve on one side of the zero crossing) it will indeed have a DC component. It's not obvious looking at the waveform on a scope, but it's there.