Some INFO ,Unavoidable Humm

[TIMBO]

I do construction for the local power company and was picking the brains of one of their techs. I'll try to explain this as he said it. When Electricity is made it is PURE and the sin wave has no lumps or bumps in it. The consumer then corrupts it by chopping the tops off the wave with their electronic gizmos (mainly motor control devices, inverter air cons, welding machines,flouros etc.) and when these tops are cut off they have nowhere to go and this produces HUM, BUT fortunately these floating tops can be absorbed by appliances that have heating elements and filiment lighting.
This is maybe why we seem to have this battle with the HUM and because of our u-bute electronic stuff upsetting the wave 

I could understand that valve amps used many years ago could have been HUM free due to an uncorrupted pure electricity supply. He mention that back to back PTs could be a way the reduce hum  Thanks

[kagliostro]

To me there is difference between "Humm" and "Noise"

Humm is given also by a pure sine wave

Noise is something that isn't a pure signal

IMHO

Kagliostro

 

[HotBluePlates]

Read up on power inlet filtering, or power conditioning devices.

"Hum," in my mind, is smooth sine-wave interference, generally at 60Hz (or the line frequency in your area, probably 50Hz).

"Power line buzz" is more like what you describe: additional frequencies above the nominal line frequency, which could extend up into the MHz range. It is generally caused by appliances, motors, or light dimmers connected to the building wiring. The extra hash on the line is a result of some kind of rectification and/or switching, and the electrical noise gets spewed back onto the building line wiring.

This is sometimes a reason why you'll notice an amp be more buzzy in certain buildings, or when your fridge compressor kicks on. Some equipment uses a line filter to help reduce that hash before the line current is applied to the power transformer. Such filters can be simple or very elaborate.

If you contemplate rigging a home-made line filter, be cautious in the type of caps you use. Line filters use class X and/or class Y caps. Class X caps are intended to cross between the live and neutral lines of your power cord, class Y caps are intended to run from live or neutral to the ground wire. Each is designed to fail safely; caps often fail by shorting, but these types fail by opening, which prevents a hazard situation.

Purpose-designed line filter often include chokes as well as caps.

[jojokeo]

I could understand that valve amps used many years ago could have been HUM free due to an uncorrupted pure electricity supply.

I strongly disagree and this isn't real world conditions. There are way too many factors to consider here. Couple that with converting AC with noisy rectification and other factors, you will still end up with tradeoffs - noise being among them. The process of rectification produces a type of DC characterized by pulsating voltages and currents (although still unidirectional). Without getting too long, half-wave & full wave rectifies half the signal pos or neg or both, and there's peak losses. Rectifier output needs further smoothing to produce steady DC from a rectified AC supply. Enter a smoothing circuit or filter. In our case it's the resevoir capacitor placed at this output. There will still remain an amount of AC ripple voltage where the voltage is not completely smoothed.
Sizing of a resevoir capacitor represents tradeoffs. For a given load, a larger capacitor will reduce ripple but will cost more and will create higher peak currents in the transformer secondary and in the supply feeding it. The peak current is set in principle by the rate of rise of the supply voltage on the rising edge of the incoming sine-wave, but in practice it is reduced by the resistance of the transformer windings.
For a given tolerable ripple the required capacitor size is proportional to the load current and inversely proportional to the supply frequency and the number of output peaks of the rectifier per input cycle. To further reduce this ripple, a capacitor-input filter and/or choke can be used.

When you have high peak voltages & currents going on at high switching rates there will be noise generated. As you can see there's a lot going on in "simply" making proper steady & constant DC power available for our uses regardless of how "pure" in the incoming sign wave might be.

[HotBluePlates]

And if you compare the pulsating d.c. output of a rectifier feeding a load resistance only to the same feeding a capacitor, you'll see the ripple in the shape of a sawtooth. The pulsating d.c. is converted to a sawtooth because of the charge-discharge cycle of the caps.

That's a reason why smooth hum is often 60Hz and related to the filament wiring (although I had one case that was a definite buzz sound; there's a hidden capacitor that caused that symptom).

[PRR]

> When Electricity is made it is PURE and the sin wave has no lumps or bumps in it

Not true; but the distortion is mild.

> consumer then corrupts it by chopping the tops off the wave with their electronic gizmos

There's really not a lot of top-cutting loads in most residences.

> when these tops are cut off they have nowhere to go and this produces HUM

No. Hum is the 120 Volts of pure 60 Hertz. Some buzz is the clipped tops, but the top-clipper closest to your amp is its OWN power supply.

> floating tops can be absorbed by appliances that have heating elements and filiment lighting.

Effect and cause. The 'tops are absorbed" by non-linear loads such as electronics. Heaters don't top-clip. Adding heaters to a top-clipped line will not make it round again.

> valve amps used many years ago could have been HUM free due to an uncorrupted pure electricity supply.

No. They hummed and buzzed.

> back to back PTs could be a way the reduce hum

No, adding transformers won't clean-up the line.

[TIMBO]

Thanks PRR, I did say that this guy worked for the local power co. and i kinda gotta believe what he says only because he does the installations of the huge transformers (110kv-33kv) that power my corner of the world BUT when it come to our little transformers and circuits and how they are affected by the electricity supply well i'll go with you PRR and to be honest i have had little problems with HUM, its the BUZZ thats giving me the sh**s. Thanks

[jojokeo]

i have had little problems with HUM, its the BUZZ thats giving me the sh**s.

and I like a buzz when getting hum

[John]

Can you play a few bars? 

[PRR]

> I did say that this guy worked for the local power co. and i kinda gotta believe what he says

There's truth in what he says. Flat-topping of the wave can be observed in many commercial (and educational) buildings. Along with that we often find huge harmonics on the 3-phase neutral. In extreme cases the neutral can overheat while the hot wires are hardly warm, something that never happens with motor and lighting loads.

But it may be unwise to extend these system issues down to amplifier hum/buzz. The only available "food" for our amps is a HIGH voltage AUDIO tone (120V 60Hz) which isn't even on the standard tuning scale. Amplifiers "must" clean-up the ugly (for audio) power source to tolerable cleanliness.

[TIMBO]

Maybe its just something that happens downunder, the other thing he said is that most of the big bands will run their own generators to help reduce noise that is in the the power supply