Toroidal transformers and EMI & Electrostatic shielding

Swumhole

Member
2015-08-16 1:45 am
Hi all,

Bit of a newby here but have been reading solidly for a few weeks prior to taking the plunge.

I have ordered the Peter Daniel LM3875 Premium kit and after a further few days' confusion have plumped for a pair of 160va 2x22 Airlink transformers (my speakers are 6ohms).

So far so good (I think) but on ordering the Airlink transformers I am given the option of adding 'Electrostatic screen' for £7 and 'EMI/RFI shield' for £5. This bumps up the price from £52.80 per pair to £81.60 (inc vat).

Being a bit tight I'm wondering whether these extras are necessary. If I'm honest I haven't decided the housing route I'm taking i.e. a single amp case or 2 monoblocs including the PSU or 2 monoblocs with external PSUs (in which case I assume the shielding won't be required).

Does anyone think these shielding options are necessary - I had intended this project to be a bit of fun but the costs seem to be running away from me :D

Thanks very much in advance.
 

tomchr

Member
Paid Member
2009-02-11 12:58 am
Calgary
www.neurochrome.com
The EMI/RFI shield goes between the primary and secondary and, if properly grounded, can act as an RF filter preventing RF present on the mains from getting through to the secondary.

The electrostatic screen is probably what's commonly referred to as a belly band. As far as I understand it, it terminates the leakage fields. That should result in lower inductive coupling to any circuitry near the power transformer.

Whether they're worth the extra cost depends on how sensitive your amplifier is to RF on the power supply and how clean your layout is inside the amplifier enclosure.

Tom
 

infinia

Member
2005-05-15 9:51 am
SoCal
what does Peter Daniel advise since its his kit ? YOUR choices here on Power , Grounding and chassis make a difference in potential for hum. I bet he wont offer any written instructions. check the chip amp build sticky sections for much gnashing of the teeth and other stories.
me, I'd check the vendor about safety class II ratings and forget all the extras. Pure mono block Class II = lowest risk for avoiding hum.
 
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Swumhole

Member
2015-08-16 1:45 am
Thanks very much for the responses. I will take your advice and save the money on the transformer upgrades to add to the cost of a separate power supply case with 2 umbilicals, or maybe have 2 separate power supplies?

I have a week or 2 to decide as I only ordered the LM3875 kit on Friday.

Thanks again for your help.
 

tomchr

Member
Paid Member
2009-02-11 12:58 am
Calgary
www.neurochrome.com
you can allways have a separate metal chassy for the transformator and just run the DC to the amplifier..
surely requires some thick wires, and nothing beats distance.

Except that solution maximizes both loop area of any ground loop and the supply impedance.

The single supply + two mono blocks suffers from the same issue. If you want dual mono, build dual mono. This allows for the shortest speaker cables at the expense of everything else. If you want stereo, build stereo (and get longer speaker cables).

I have no issues getting below -120 dBV of residual mains hum in my power amps and 5-6 dB better than that in my preamps with everything collocated in the same chassis.

Tom
 
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tomchr

Member
Paid Member
2009-02-11 12:58 am
Calgary
www.neurochrome.com
Looks to me from reading Airlink's website its the other way around - the electrostatic screen goes between the windings with an aim of reducing common-mode noise, it comes with a drain wire for grounding.

You're right. My bad. I had the names reversed. Here's what Airlink says about it:

* Faraday shield
Electrostatic screen wound between the primary and secondary
* EMI/RFI Shield
GOSS steel band (2 layers)

I stand behind my explanation of their respective functions, however.

Tom
 

infinia

Member
2005-05-15 9:51 am
SoCal
the cost of a separate power supply case with 2 umbilicals, or maybe have 2 separate power supplies?

I have a week or 2 to decide as I only ordered the LM3875 kit on Friday.

Thanks again for your help.
Make 2 mono blocks that have their own power supplies self contained with the amp.
It's a common mistake to believe toroid's being somewhat close to the amp PCB is an issue. With good planning /layout and wiring practice it's fine. Most here don't understand the mechanisms of ground loops when you add a third wire aka safety earth into a class II system ( 99% commercial audio sources are class II that will be plugged into it... right.) use an Ipod only /batteries source and anything goes.
 
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Swumhole

Member
2015-08-16 1:45 am
Thanks Infinia, that would be the most ergonomic way of doing this but that brings me back to my initial query of whether paying out for the Electrostatic and EMI/RFI shielding is worthwhile. If I have a separate PSU with umbilicals I guess the shielding is less critical?

Maybe I should bite the bullet and pay for the shielding in the assumption that it can't harm and it will give me further options in the future (I can foresee that DIY audio will not just be a passing phase!!)

Thanks again
 

infinia

Member
2005-05-15 9:51 am
SoCal
Thanks Infinia, that would be the most ergonomic way of doing this but that brings me back to my initial query of whether paying out for the Electrostatic and EMI/RFI shielding is worthwhile. If I have a separate PSU with umbilicals I guess the shielding is less critical?

Maybe I should bite the bullet and pay for the shielding in the assumption that it can't harm and it will give me further options in the future (I can foresee that DIY audio will not just be a passing phase!!)

Thanks again
Hi SwumH
The double steel band option is for stray magnetic field control, usually not needed if keeping sensitive stuff a few inches away. Besides toroid's have inherently tight magnetic fields so that's good. More important is keeping all wiring twisted E.g low loop areas and canceling any currents that are induced.
The electrostatic shield option is for common mode noise reduction E.g outside HF noise to inside reduction, mostly bad stuff higher up than audio. This is up to you to decide, but it has more potential for making any ground induced loop hum worse depending on the final grounding plan.
I prefer the 2 identical mono block approach ( BTW you can put mono blocks in the same chassis)) . besides that umbilical cords = more custom design work E.g non standard DC connectors / unwieldy cables and two different chassis designs.

Id come up with a chassis layout plan 1st, with cad drawing or paper dolls before ordering more stuff. ordering w/o one is likely to paint yourself into a corner so to speak. ( mechanical ) is Peter D. forte' so look at his stuff for good ideas and ask him about wiring XFMR. IMO some of his electrical ideas are voodoo LOL
 
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Swumhole

Member
2015-08-16 1:45 am
Thanks again Infinia (and apologies to John Blackburn earlier as I misunderstood his post!).

I am going to go down the route you both suggested and mount the transformer with the amp and have 2 monoblocks.

Thanks for the advice, as a newbie it's very much appreciated :)
 
Hi Swumhole.

Do you own a multimeter?

You will need one for measuring your resistors to make sure you get them in the right places when you put the boards together, testing the psus, and to measure your existing pre outs for DC so you know what's what.

Have a browse of the links below (especially Decibel Dungeon) for some well written tutorials
 

Swumhole

Member
2015-08-16 1:45 am
Thanks John.

Yes, I have a few multimeters. Used to have an oscilloscope once too but flogged it on ebay because I didn't think I was likely to use it again!

I was prepared for testing the PSU outputs, resistors and DC offset of the completed gainclone but checking the Preamp DC offset is a new one on me!

Will continue reading, thanks for the heads-up.
 
DC offset on the completed amp is measured with the inputs shorted so it could pass the test with flying colours, until the shorting plugs are removed and a source connected. Any offset there now will be amplified by the same gain ratio as your signal and passed to your speaker drivers.

It is briefly mentioned on page 2 of the build guide http://www.audiosector.com/nigc_kit-users_guide.pdf

An electrolytic is recommended in the guide, others would say it must be a film capacitor (and others would say nothing short of angel hair and unicorn p#ss will be good enough for the precious signal)

I've used these in a few builds CCTV Camera Systems and DVRs from Cricklewood Electronics

John
 
No worries.

Have a read on dim bulb testers, there is a piece on Decibel Dungeon about making one in the building a power supply section if I remember rightly.

It puts an old style light bulb in series before your transformer to limit the available current on first start up and for test purposes. The real beauty of it is that it gives an instant visual indication of a fault.

When you switch on, the bulb will light very briefly as the psu capacitors charge and then dim down or extinguish completely. If you have a short (for example) the bulb will continue to glow brightly showing you have excess current flowing in less time than it would take to switch on your meter.

It saved me from letting the magic smoke out of my last build. The first channel powered up fine but on the second one the bulb didn't extinguish. The whole procedure only took a few seconds from switching on, spotting something was amiss and having the power switched off again before any damage was done.

Have you had any thoughts on enclosures yet?

John
 
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