buzzing toroid

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ed
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#1 buzzing toroid

Post by ed »

oh waily

I was listening to the 'probably the best hifi in the world' this afternoon and suddenly had the urge to mess with the crossover on the MA monitors. So I got the phillips screwdriver out and went to undo the rear panel from the left hand speaker, which is in front of the F4...

The transformer was buzzing, quite audibly whilst standing in front of the amp.....ooerr

So I immediately ordered a Sjostrom DC blocker board in my panicked state.

The F4 has never made any kind of noise in all the years I've had it(excepting moosic noise), so I'm a bit alarmed. We bought a new fridge 2 weeks ago, which is the only thing that's changed in the house, so I'm wondering if that might have affected the mains in some way.....

I haven't measured the mains yet, but I understand the method is just to measure the dc between neutral and earth, can anybody confirm this?

and has anybody got any experience of the Sjostrom dc blocker boards?
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#2 Re: buzzing toroid

Post by Nick »

but I understand the method is just to measure the dc between neutral and earth
There shouldn’t be any voltage between neutral and earth unless you have a earth fault current somewhere or broken wiring.

Either way the transformers wont see it as it knows nothing about earth. Whats called DC on the mains is normally an asymmetric waveform, so there is more area under the +vc or -ve cycles. As to if your meter will show that, its down to the meter I guess.
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#3 Re: buzzing toroid

Post by jack »

Depending on your earthing system, the protective earth (PE) and neutral (N) may be connected together at your premises or at the substation. There are other earthing arrangements too...

There should only be minimal voltage between PE and N.

The issue of "DC on the mains" is, as Nick says, normally due to to asymmetry or harmonics in the L-N sine wave. There can be a number of causes of this, but the usual one is something local with a poor power factor or asymmetric loading. This used to be a problem with old PC power supplies, hairdryers and fridges etc., but newer equipment complying with modern regs has removed a lot of this.

There will always be some humming in a transformer due to magnetostriction - there is very little you can do about this as it's a physical feature of all transformer core materials. If the transformer is tightly coupled to the chassis, thus the chassis is acting as a sounding board, consider using mechanical isolation mounts for the transformer. Some transformers produce less noise than others - Plitron (and probably others) have designs for low noise.

Other exacerbating factors are high mains voltage or when under high output current stress.

Elliot Sound Products (ESP) have a good section on this.
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#4 Re: buzzing toroid

Post by ed »

thanks chaps

all good stuff....I am aware of offset, the intriguing bit is how it got there and how to get rid of it.

There is loads of stuff on the interweb about 1/2 wave rectifiers from the days of old(old TVs etc) and badly designed or cheap smps and fridges etc etc so the culprits could be all over the place, and might not even be at chez moi.

More intriguingly is the detection..I have found 3 references to the voltage between neutral and earth and I admit it didn't ring true, which was why I questioned it...but but but...

It seems there are loads of theories out there, and some may be misinformation.
There will always be some humming in a transformer due to magnetostriction - there is very little you can do about this as it's a physical feature of all transformer core materials. If the transformer is tightly coupled to the chassis, thus the chassis is acting as a sounding board, consider using mechanical isolation mounts for the transformer. Some transformers produce less noise than others - Plitron (and probably others) have designs for low noise.
All very good, but this particular amp has been 100% silent for 13 years and suddenly it's not silent, so I think it's a bit deeper than just design.

I will install a blocker and see if it makes a difference.....My Fluke may measure the pos and neg of the wave but I'm not sure and I'm not going to risk the scope so I'll wait and see if a blocker has any affect.

thanks again
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#5 Re: buzzing toroid

Post by Ali Tait »

Daft question, have the transformer mountings come loose?
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#6 Re: buzzing toroid

Post by ed »

Ali Tait wrote: Wed Sep 09, 2020 7:06 am Daft question, have the transformer mountings come loose?
not a daft question at all.....but sadly not the answer...transformer is well tight.
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#7 Re: buzzing toroid

Post by ed »

I've been thinking a bit about this, probably more than I should have.
I may need to buy a tinfoil hat soon.

Before I ditched the homeplugs I was keeping a beady eye on them, principally because of the drop-outs and disconnects I was experiencing. Things had been fine for 15 years and then problems started. It was mentioned that they are known to wear out but I wasn't sure because I had upgraded them periodically as I wanted to add to the network. It's very clever marketing because if you started with 100mb units and you wanted to add an extra unit you found that there were no longer 100mb units on the market. It was necessary to replace the whole lot as they weren't backward compatible. So I've upgraded 3 times, the last time about 4 years ago when I bought 500mb units, too recent to be worn out I thought.

anyway, back on topic....watching the homeplug units, the quality light varied between green(good) to yellow(slight degradation) and red(poor quality reception). There would be green light all round for maybe 4 or 5 days and then suddenly lots of red lights.....so what happened to the mains to make the homeplugs have difficulty talking to each other?

May these 2 things be connected? the buzzing transformer and the failing home plugs...

did I mention I might be thinking too much!!
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#8 Re: buzzing toroid

Post by jack »

IME tin (aluminium) foil doesn't work for most ETs...

Use mu-metal/Ultraperm - blocks 99% of all known brain scans...

On a slightly more serious note, if you are having a lot of issues, see if you can borrow a mains power quality analyser (PQA) - Fluke make one, but it was about 9,000 quid last time I looked. I've rented them in the past when we've had odd behaviour of equipment in the office and they've been great. One thing I learnt early on was that UPS systems generally produce rubbish sine waves full of harmonics when operating at less than about 20% load...

It's obviously not practical to hire one of these things for the 24-48 hours they need to record over, but having a grumble to your electricity supplier may possibly help... I have done this before at home and Seeboard attached a 3-phase Dranetz PQA for a week FOC before concluding that, yes, the utility feed we were getting was rubbish and they ended up replacing the small substation down the road from us.

Note: The only reason I knew about this was that I'd hired one for a month for our office in London and out of general interest, took it home for the weekend to see what our very rural supply was like (bearing in mind we are at the end of a very long cable and out in the sticks a bit). The results from the PQA was fairly grim and I used that as a lever with the utility provider.
Last edited by jack on Wed Sep 09, 2020 10:32 am, edited 3 times in total.
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#9 Re: buzzing toroid

Post by Ali Tait »

ed wrote: Wed Sep 09, 2020 8:23 am
Ali Tait wrote: Wed Sep 09, 2020 7:06 am Daft question, have the transformer mountings come loose?
not a daft question at all.....but sadly not the answer...transformer is well tight.
Daft as in you'd almost certainly checked it already. :D
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#10 Re: buzzing toroid

Post by pre65 »

Did you try unplugging your new electrical domestic appliance for a short while ?
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#11 Re: buzzing toroid

Post by Nick »

and I'm not going to risk the scope
Use the scope on the secondary of a small toroid. That way you can safely look at the mains waveform.
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#12 Re: buzzing toroid

Post by Mike H »

I remember Dave Brooks (Danbury Electronics) saying once about how sometimes a toroid transformer can get a loose lamination, as the core is a long strip wound into a roll, and sometimes the outer end comes loose and vibrates. Probably due to repeated expansion and contraction. Not much you can do with it I'm afraid as it's sealed in the former, which is moulded plastic or resin. :(
 
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#13 Re: buzzing toroid

Post by ed »

I'm still waiting for the dc blocker board to arrive...
In the meantime the toroid is still buzzing...
so
I put a 10uf cap in series with a 100k resistor across the mains
it consistently shows between 25 and 45 mvDC across the cap

this shows the same in the music room and the kitchen which are different rings.

I have no real idea what this means....does it sound excessive to anybody?
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