Harmonics that modify the mains voltage waveform are above the mains frequency. Harmonic levels are also much lower than the fundamental level.
Such harmonics may couple over in to audio circuitry (eg. by AC powered heaters, or electromagnetic coupling in to sensitive circuitry), but typically won't couple via the main B+ supply due to the quite short duration rectifier current waveform, and subsequent capacitor filtering.
So an alternative approach again for cleaning up any DC offset is an isolation transformer. Any experience or points of view on this as an approach?
Sample reason for humming
I just want to share some of my recent findings regarding transformer noise:
In my self built 5.1 digital power amp I use a 200VA toroidal transformer and when connected to the rest of my set-up, there was strong humming audible. I checked the ground connections, shielding, pre-amp connection. Optimized rectifiers, capacitors, physically damped/isolated the transformer. I could reduce the noise in the speakers a bit, but not the noise emerging directly from the transformer.
Only late I realized, that the hum was not present on my lab workbench!
The reason:
In my Preamp I used a solid state switch to power up the rest of my equipment. And this solid state switch introduced a DC level to the power amp, which in turn was responsible for the hum. The offset seems to originate from variations in the used triac or thyristors and their switching behavior.
After removing the solid state switch, silence all around 😀
Conclusion:
Don’t underestimate the effects of DC offsets!
RetMarut
I just want to share some of my recent findings regarding transformer noise:
In my self built 5.1 digital power amp I use a 200VA toroidal transformer and when connected to the rest of my set-up, there was strong humming audible. I checked the ground connections, shielding, pre-amp connection. Optimized rectifiers, capacitors, physically damped/isolated the transformer. I could reduce the noise in the speakers a bit, but not the noise emerging directly from the transformer.
Only late I realized, that the hum was not present on my lab workbench!
The reason:
In my Preamp I used a solid state switch to power up the rest of my equipment. And this solid state switch introduced a DC level to the power amp, which in turn was responsible for the hum. The offset seems to originate from variations in the used triac or thyristors and their switching behavior.
After removing the solid state switch, silence all around 😀
Conclusion:
Don’t underestimate the effects of DC offsets!
RetMarut
I’m still waiting for an actual scope trace of what DC on the power line looks like. No I don’t mean the simulation from Rod Elliot either. An actual picture.
Same here. I haven't seen one yet. If anyone has DC on their power, take a scope picture. Then we can discuss the issue intelligently.
Put a AA battery in series with the main to the transformer and scope the difference? I dunno how much the AA will appreciate it though... 😛
Even if there were DC offsets on your AC mains/lines, you'd be looking for a small component superimposed onto a very large sine wave. One might be better off with a DMM. Hypothetically.
So the requested picture would look like a sine wave shifted by some tiny degree easily mistaken for the trace not being zeroed.
If one of you affluent folks with a 'scope that has FFT capability, wouldn't that show a DC Fourier component? Textbook style...
One of my scopes has knobs. No FFT.
The other might have FFT, but I learned on scopes with power transformers, so I still don't like measuring a power line with a power-line-powered 'scope. Bad experiences. Let someone else do that.
Maybe a fancy power analyzer. You could rent one for less money than you spent on certain other components.
Why am I here? I came here for a different thread...have a nice day.
So the requested picture would look like a sine wave shifted by some tiny degree easily mistaken for the trace not being zeroed.
If one of you affluent folks with a 'scope that has FFT capability, wouldn't that show a DC Fourier component? Textbook style...
One of my scopes has knobs. No FFT.
The other might have FFT, but I learned on scopes with power transformers, so I still don't like measuring a power line with a power-line-powered 'scope. Bad experiences. Let someone else do that.
Maybe a fancy power analyzer. You could rent one for less money than you spent on certain other components.
Why am I here? I came here for a different thread...have a nice day.
Well you will not get one from me.....I don't have a scope!
but a more experienced guy I know (builds commercial amplifiers) said to measure it I need a scope.....
I have an old 1KW isolation transformer I am going to try.....but errr it also hums when I turn it on....... 🙂
my assumption is that AC trace has a lift or fall below the bottom of the wave = 0 for the DC offset?
tonescout, you seemed to start this thread with the assumption that some form of DC on the incoming mains was causing noticeable hum on both your preamp and main amp.
You could be completely wrong with that assumption, and it would seem valid to try and disprove that assumption if that was the easiest path forward with your quest to fix hum issues.
It would be difficult for nearly all of us to measure a small DC component on the mains - that is not an easy measurement to make and requires all other forms of DC measurement artefact to be discounted. Also, the DC is often observed to be 'varying', depending on the mechanism that is causing the DC.
So if you have an isolation transformer, and can safely configure it to supply your preamp / amp then that would imho be the easiest action to take to make an aples:apples comparison. If the isolation transformer audibly hums, and your only 'measure' of hum is your ear and speakers, then perhaps use a long mains AC extension cord and move the isolation transformer to another room.
Good idea, I will do this today - and try the Power amp. The Pre amp now is 99% silent, which you could conclude = no mains DC offset!!
For there to be a presence of DC it means that AC has to stop oscillating. So I suppose technically for the brief instance you look at the top of a square wave you're seeing DC. If your power line has turned from sine to square I suspect your have bigger problems.
A shifted sine wave is still a wave, it has frequency and if something has frequency it can't be DC. If the wave is so distorted that the peaks are flattened yeah I could see that causing some audible noise. At this point it's distortion but not DC.
A shifted sine wave is still a wave, it has frequency and if something has frequency it can't be DC. If the wave is so distorted that the peaks are flattened yeah I could see that causing some audible noise. At this point it's distortion but not DC.
ahh - I thought you could have DC and AC together at the same time, and for these to act independently?
Well ok so my assumption was probably incorrect as there is no advantage with the isolation transformer to reduce hum on the 211 power amp. I think most of it is mechanical vibration from a less than inherently quiet transformer AND a sound board and a resonator box. The transformer sits on an aluminium plate that is bolted to a wooden box, that probably acts like a sound chamber, so I will try even more damping and see where that gets me 🙂
what do you call the signal coming out of a resistance coupled amp running in class A if you put a sine wave on the input?
The concept of an AC voltage with a DC offset is pretty well established and can be looked up in a textbook. It doesn't require any debate.
Like this:
Blue, only AC: 5*sin(2*pi*f*t)
Green, sum of DC and AC: 3 + 5*sin(2*pi*f*t), 3 = DC, 5*sin(2*pi*f*t) = AC
Case closed. 🙂
Blue, only AC: 5*sin(2*pi*f*t)
Green, sum of DC and AC: 3 + 5*sin(2*pi*f*t), 3 = DC, 5*sin(2*pi*f*t) = AC
Case closed. 🙂
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Yes there would be offset but that isn't what people are claiming. My power comes directly from another transformer on a pole. How is DC going to be imposed on it?