Amplifier transformer hum

[bumpy]

In a very quiet environment and listening near field its often a little invasive hearing mechanical hum coming from the (mains) transformers, particularly in valve amps.

My reading tells me it's probably a small amount of DC on the mains.

I cant find any obvious culprits in the house and believe you can buy kit that you plug into the mains, then plug your gear into that.

Any contributions?

[scotty38]

If it is DC you have an issue with then have a look here:

https://sjostromaudio.net/cart/gb/ and here:

https://www.atlhifi.com/

[Ali Tait]

If you are handy with a soldering iron, a DC blocking IEC socket can be fitted to the amp. Failing that you can buy a filter like this-

https://www.mcru.co.uk/product/mcru-in-line-dc-blocker/

[bumpy]

https://sjostromaudio.net/cart/gb/

DCT02 DC FILTER FOR TOROIDAL TRANSFORMERS, BUILT UNIT

Does this mean that this unit works best with toroidal transformers or that toroidals are notoriously more noisy?

[bumpy]

If you are handy with a soldering iron, a DC blocking IEC socket can be fitted to the amp. Failing that you can buy a filter like this-

https://www.mcru.co.uk/product/mcru-in-line-dc-blocker/

Am happy to wield a soldering iron, and a pre-built PCB looks good. This is very much to prove this is the source of the hum, so will invest in a better bit of kit later if it works :)

[Ali Tait]

https://sjostromaudio.net/cart/gb/

DCT02 DC FILTER FOR TOROIDAL TRANSFORMERS, BUILT UNIT

Does this mean that this unit works best with toroidal transformers or that toroidals are notoriously more noisy?

Toroids are less tolerant of DC than IE types.

[enrae8]

I have a similar problem - but maybe more severe,the mains seems to be very noisy particularly when the boiler is on, I can't use the Phono when it is on !!
I have a valve amp and Phono-I have found SS phonos are also affected but not to the extent of the valve one
I am trying to get my engineer to look at the phono to make sure its ok - just wondering if the DC blockers would help in the meantime
Would this do https://www.atlhifi.com/shop/fully-assembled-devices/high-power-dc-blocker-trap-filter-c20-ac-inlet-2-ac-outlets/
Would I be able to plug in the Amp and Phono?
Cheers
Sorry to gatecrash the thread

[scotty38]

Just need to make sure it's DC that's your issue otherwise it probably won't do all that you need.

[AD Audio]

In a very quiet environment and listening near field its often a little invasive hearing mechanical hum coming from the (mains) transformers, particularly in valve amps.

My reading tells me it's probably a small amount of DC on the mains.

I cant find any obvious culprits in the house and believe you can buy kit that you plug into the mains, then plug your gear into that.

Any contributions?

Hi,
I think it's a little more complex than the above replies imply. Many valve IE transformers hum, just ask any Radford STA 25 owner! In some cases this may be because of DC on the mains but IME it's down to transformer design /build. EG my Leben CS 1000p with Hashimoto transformers does not hum at all whereas the Radford transformers I've used in a Radford STA 100 monoblock build hum like a New Orleans gospel choir, as do the Edcor EI transformers I used in the past in my pre builds and Cannonball power amps. A couple of things I've learned are:

1. DC blockers have no effect on the hum in these transformers and I have heard they kill the sound on valve amps but havn't expereinced that myself. But for the cost it's worth a try.

2. Toroidal Transformers
Again, whether they hum or not seems to be down to transformer design. Decent ones don't hum whilst some cheaper ones do and DC blockers don't tend to help these either. But DC blockers do seem to work well on large solid state amps with big toroidal transformers without any impact on SQ.

@enrae8 You say the 'mains is noisy' but are you referring to transformer buzz or something else?

David

[enrae8]

Hi,

@enrae8 You say the 'mains is noisy' but are you referring to transformer buzz or something else?

David
Hi - I have a bit of transformer hum - but I can live with that
The mains problem - loud pops, I guess when switches relays are activating in the boiler also loud buzzes maybe from the pump, pops also occur when other things are used, electric carving knife, blender etc..I've tried extension lead from upstairs etc..The Kitchen diner has its own board as it was refurbed a couple of years ago - the boiler is ancient, we should be getting a new one and having it on its own feed should have happened last year

[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[emoji3] 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.

[theabbot]

Barry and David, I totally agree with you thought regarding testing mains etc, you should see what happens if you get it wrong when you have +50 and -50 volts dc buzzbars in close proximity and the fuses are rated at 250 amps each, you don't get up if you get that wrong. I spent more years playing with that than I care to remember. Just because someone asks for advice it doesn't mean they have no idea what they are doing, guess i'll have to search on Google and hope the information is correct.
Thanks
Mike

[Barry]

Hi Mike,

Neither of us were criticising you for asking how one measures the DC offset on the mains, rather we simply pointing out that making measurements on the mains is always potentially dangerous. In the past we have had some Members suggest 'improvements' to the mains side of their equipment which varied between the unbelievably stupid and dangerous, to those which on the face of it seemed harmless enough, but under certain external fault conditions could be dangerous and a fire hazard. So that is why we always suggest that such measurements be only undertaken by a fully qualified electrician.

You also have to realise that we could have a casual reader pop in and look at our threads and think "I'll try that", but being a novice could well end up harming themselves and/or their equipment. The excuse that "I read about on the Art of Sound forum, and they said it was OK to do it" would not impress insurers, and could make the Forum subject of litigation.

[AD Audio]

Hi Mike,

Neither of us were criticising you for asking how one measures the DC offset on the mains, rather we simply pointing out that making measurements on the mains is always potentially dangerous. In the past we have had some Members suggest 'improvements' to the mains side of their equipment which varied between the unbelievably stupid and dangerous, to those which on the face of it seemed harmless enough, but under certain external fault conditions could be dangerous and a fire hazard. So that is why we always suggest that such measurements be only undertaken by a fully qualified electrician.

You also have to realise that we could have a casual reader pop in and look at our threads and think "I'll try that", but being a novice could well end up harming themselves and/or their equipment. The excuse that "I read about on the Art of Sound forum, and they said it was OK to do it" would not impress insurers, and could make the Forum subject of litigation.+1

Sent from my ONEPLUS A5000 using Tapatalk

[anthonyTD]

From experience, the main reason Mains I and E transformers hum is down to loose windings, and gaps between the potting agent used to hold everything in place, now this can happen over time, as the windings expand and contract with heat, they can become loose, the interwinding material thins out, and also the potting agent can dry out, leaving gaps between the windings, allowing them to vibrate, Toroid transformers also suffer from similar problems, however, due to the nature of their construction, and depending on their size, and VA Rating, it can sometimes be more difficult to make them quiet, either way, in my opinion hum in transformers is mainly down to their construction, how they have beern used, [over run, over heated etc] and time.
Also; it is very important that mains transformers are used on the correct input voltage, lots of equipment these days have 230v primaries, and that sometimes mean that they don't even need 230v on them to be correct, if you look at our mains voltage, it can vary quite a bit from one part of the country to another, especialy at diffrent times of the day, mine is usualy between 238v, and 243v most days, but i have had reports of some places having in excess of 255v, if you run a mains transformer that has a 225v to 230v primaries on these higher voltages, then they can become saturated, hence the hum.
There has also been a lot of equipment sold over the years from that well know internet site that have 220v primaries, and from measurements i have taken over the years on numorous equipment with 220v primaries some need less than 210v for the secondary voltages to be correct!
Hope this helps.
A...

[Wakefield Turntables]

Might have a play around on my P10 tonight and set voltage to 230V and see if my old Sondex hums less.

[Maggusch]

Hi,

I bought a Radford Renaissance ST 25 and the main transformator is humming quite loud. Is living with it or replacing the only 2 possible options. Radford uses this stail of EI Main Transformers for decades. Are they reallymanufactured with bigger tollerances? Was this the reason to change on toroids in some versions?