Amplifier transformer hum

[nutteronthebus]

I use a BT MCU5A mains unit ( I worked for BT for 37 years ) and we had a few recovered spares . I live on a farm with O/H power lines for the supply and they do remove DC and spikes on the mains . They do crop up on ebay from time to time and may be worth a punt

Dave

[Barry]

DC on the mains should not be a problem with toroidal transformers having a rating of < 300VA owing to the high resistance of their primary winding. Above 500VA, the DC resistance is so low that even a small DC offset of 100mV on the mains will cause mechanical noise due to saturation.

Because toroidal transformers have no air gap, the saturation is hard limiting. When saturation is reached, the inductance falls, the primary current rises and the primary impedance falls.

In general most transformers are on the verge of saturation. This is deliberate; if the transformer uses more primary turns to avoid saturation it will have poor regulation because the primary resistance will be too high.


I have a Radford STA15 Mk. III valve amplifier which uses transformers using EI style laminations. They do not hum.

[AD Audio]

DC on the mains should not be a problem with toroidal transformers having a rating of < 300VA owing to the high resistance of their primary winding. Above 500VA, the DC resistance is so low that even a small DC offset of 100mV on the mains will cause mechanical noise due to saturation.

Because toroidal transformers have no air gap, the saturation is hard limiting. When saturation is reached, the inductance falls, the primary current rises and the primary impedance falls.

Unfortunately this is simply not the case. I use toroids in DACs constantly and in this application they are all smaller than 300va. AS stated, cheap ones hum, better quality ones don't.

I have a Radford STA15 Mk. III valve amplifier which uses transformers using EI style laminations. They do not hum.

This latter point is better expressed as 'mine doesn't' as my STA15 does hum, although not to the extent that most STA25s do. I put down the gentler hum to the valve rectification which generally gives the transformer an easier time than solid state.

[AD Audio]

DC on the mains should not be a problem with toroidal transformers having a rating of < 300VA owing to the high resistance of their primary winding. Above 500VA, the DC resistance is so low that even a small DC offset of 100mV on the mains will cause mechanical noise due to saturation.

Because toroidal transformers have no air gap, the saturation is hard limiting. When saturation is reached, the inductance falls, the primary current rises and the primary impedance falls.

Unfortunately this is simply not the case. I use toroids in DACs constantly and in this application they are all smaller than 300va. As stated, cheap ones hum, better quality ones don't.

'I have a Radford STA15 Mk. III valve amplifier which uses transformers using EI style laminations. They do not hum.'

This latter point is better expressed as 'mine doesn't' as my STA15 does hum, although not to the extent that most STA25s do. I put down the gentler hum to the valve rectification which generally gives the transformer an easier time than solid state.

[Barry]

It might be due to the low DC offset on my mains supply: 18mV, or it might be I am fortunate (lucky) with my sample of the STA15.

With 'cheap' toroids it might be that the two windings are poorly wound together, such that there is unwanted leakage reactance.

[AD Audio]

It might be due to the low DC offset on my mains supply: 18mV, or it might be I am fortunate (lucky) with my sample of the STA15.

With 'cheap' toroids it might be that the two windings are poorly wound together, such that there is unwanted leakage reactance.

I think you're right re cheap toroids. I'll check the offset of my mains too. Good tip but the Hashimotos don't hum at all......

Sent from my ONEPLUS A5000 using Tapatalk

[theabbot]

How do you check for DC on the mains, do you need special equipment?
Thanks
Mike

[struth]

Smps can produce dc on mains too afaik

[AD Audio]

I don't think anyone should be checking their mains for DC offset unless they really know what they're doing: in which case they don't need to ask There was something really strange going on with my mains yesterday. Three different amps-SE, push pull valve and solid state push pull-all sounded dreadful. I also tried two different DACs with no improvement. A total lack of soundstage and dynamics all round. Today everything is back to normal. A shame I didn't take any measurements as I'm sure something was way off.............

[Barry]

I would concur with David that one should never 'mess around' with mains supply unless you know exactly what you are doing.

For reasons of safety, all measurements concerning the UK mains supply should only be performed by a fully qualified electrician.




Also I agree with Grant: switched mode power supplies can introduce DC onto the mains, as can some older hairdryers with a reduced heat setting.