What can I use to shield a 100 VA torroid power transformer?

[sbelyo]

So over the past two weekends I re-cased my original Dynahi.  It's always had a slight 60 Hz hum from the transformer (my best guess).  The amp boards, psu and tranny are all in one 16" x 12" x 4" case from par-metal.  This time I put the transformer in it's own aluminum project box with a cover and then put that in the case.  Then I put an aluminum barrier plate that runs the length of the case in between the amp boards and the power side.  I used unshield twisted pair wire to hookup the input and output of the amp.  The hum is still there as it was before without any of that.  

My guess is that aluminum project box is not a good shield and I should use shielded twisted pair with a drain wire for the input and the output.  Can I improve the shielding of the transformer using this http://www.digikey.com/product-search/en?mpart=11.5X12-6-1181&vendor=1067  or this http://www.ebay.com/itm/152045122881?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT  ?

 

And for hookup wire, does anyone have a suggestion for shielded twisted pair with a drain wire?  Like maybe this  http://www.markertek.com/product/bl-8451-100/belden-8451-22-awg-stranded-microphone-cable-100-foot-black

Edited June 6, 2016 by sbelyo

[Craig Sawyers]

Is it mechanical hum from the transformer, or electrical hum that you hear in the headphones?

[sbelyo]

Electrical hum that I hear in the headphones.

[dsavitsk]

Aluminum will not do a lot to shield noise. The first thing to be sure is that it is not a grounding issue. Once that is ruled out, the best remedy is space. If you can't add space, try a plastic retaining screw and plate. When that doesn't work, try reorienting the transformer - turn it upside down, turn it on its side, rotate it, etc. If moving it does not at least change the hum a little, then revisit the grounding. But if reorienting does change the hum, but does not reduce it enough, the next thing to try is mu-metal shielding. It may work - it usually doesn't. Finally, try a different transformer. See if you can get an R core or  C core or even an EI (the low radiated fields from toroids is much exacerated, and sometimes EIs are better.) Finally, contact a custom winder and tell them you need a toroid with low radiation - I have had SumR reduce the noise by 20dB with a well made transformer.

[sbelyo]

I can't add space so I'll try re-orienting and some mu-metal.  Didn't know there was plactic plate.  I'll look for one

[luvdunhill]

I suppose my questions would be:

How much current does the amp draw (measured) and how much headroom do you have from the spec'ed current?

Do you have any DC on the line or things like dimmer switches, etc. that would cause noise?

Are signal grounds isolated from earth and in general where does the E pin go (have you implemented a ground loop breaker scheme, etc.)

[sbelyo]

9 hours ago, luvdunhill said:

 

I suppose my questions would be:

 

How much current does the amp draw (measured) and how much headroom do you have from the spec'ed current?

 

Do you have any DC on the line or things like dimmer switches, etc. that would cause noise?

 

Are signal grounds isolated from earth and in general where does the E pin go (have you implemented a ground loop breaker scheme, etc.)

I will measure the current possibly this weekend.  I suspect there is a good amount of headroom.  Is it a problem if there is too much headroom?

No DC on the line.  I ran a dedicated circuit for A/V equipment

Signal grounds are isolated from ground.  The E pin is in the front left corner of the case by the power switch.  There is no ground loop breaker scheme.

[Craig Sawyers]

Well it can only be down to a hum loop somewhere, induction through stray magnetic fields from the transformer, or ripple on the DC output from the regulator.

Also, are you hearing the 60Hz fundamental, or the second harmonic at 120Hz?  What does the mains power look like - ie is it symmetric or asymmetric?

Just firing ideas - cause of hum can be a bitch to find.  But once you find the cause, it will seem to be blindingly obvious....

[dsavitsk]

25 minutes ago, Craig Sawyers said:

cause of hum can be a bitch to find.  But once you find the cause, it will seem to be blindingly obvious....

QFT

[sbelyo]

My money is on the stray magnetic field so I'm going to go in that direction.

18 hours ago, Craig Sawyers said:

Also, are you hearing the 60Hz fundamental, or the second harmonic at 120Hz?  What does the mains power look like - ie is it symmetric or asymmetric?

Just firing ideas - cause of hum can be a bitch to find.  But once you find the cause, it will seem to be blindingly obvious....

I'll try and find an example of both tones so I can make an educated guess on that.  I believe my power is asymmetric, I'm in the US and use 120V power so there is one hot leg, one neutral, and one ground to earth.  Did I answer that right?

[Craig Sawyers]

5 hours ago, sbelyo said:

 I believe my power is asymmetric, I'm in the US and use 120V power so there is one hot leg, one neutral, and one ground to earth.

That is all we have in the UK, except it is 230V (well 240V in reality).

[spritzer]

The days of balanced power from the grid are long gone.  There are a few spots here that have it but it was more expensive so nothing recently built will have it. 

[sbelyo]

I'm going to rewire the signal input and output with shielded wire and measure the current on the transformer secondaries this weekend.  We'll see what happens 

[sbelyo]

On 6/7/2016 at 0:27 AM, luvdunhill said:

How much current does the amp draw (measured) and how much headroom do you have from the spec'ed current?

The amp is drawing 797 mA from each secondary. The spec'ed current is 1.67 A from each secondary so that's roughly a 50% headroom. If I reduce the size of the transformer that should reduce excess EM radiation. How much can I reduce it?

Edited June 12, 2016 by sbelyo

[luvdunhill]

Here are some links to consider - there are important things to think about, like line variance, duty, AC RMS current versus DC current, etc.

http://sound.westhost.com/xfmr.htm

(Read all three sections)

http://www.electra-print.com/custompower.php

[sbelyo]

so after reading the electraprint page I'm starting to think that when I I put the transformer in it's own aluminum project box with a cover it either made no difference, made it worse, or may be causing harm to tthe transformer.  Does that sound right?  If so maybe I sould take it out of the project box that it's in and just mount it in the amp case.

[kevin gilmore]

putting it in a metal box won't hurt the transformer. unless you use a metal post that goes thru the center of the transformer (torroids or racetrack transformers) in which case there is one very low impedance single turn short.

[sbelyo]

I only used the normal mounting hardware from avel lindberg and the top of the metal bolt does not touch the top of the metal box.  Is that ok?

[kevin gilmore]

That is ok

 

[Craig Sawyers]

For rating transformers I use a free simulation package from here http://www.duncanamps.com/psud2/ .  For a capacitor input filter (the usual sort) the transformer VA rating is always significantly higher than the power delivered to the load.  This is entirely a result of the pulse-like capacitor charging current.

Just as a for instance, Tx with 100V, 1ohm secondary, bridge rectifier, 10,000u capacitor and 700mA load.  Power to load = 0.7 x 135V = 94.5W.  VA from Tx = 1.8 x 99 = 178W.  This is typical - you need somewhere about 1.5 to 2 times the VA rating for the Tx than is delivered to the load.

The other way to do this is to use the nomograms in a classic paper by Schade (Proc IRE, July 1943, 341-361, "Analysis of Rectifier Operation, O. H. Schade).  There is an update with measurements in Linear Audio, V8, Sept 2014, 83-127 "The Otto Schade Method, A practical design method for Rectifier Circuits" by Rudolf Moers.

All of this agrees with http://sound.westhost.com/power-supplies.htm which concludes with 

Transformer
The voltage is determined by the power that you need from the amplifier. Calculate power from the formula ...
    P = Va² / R   Where P is power in Watts, Va is RMS speaker voltage, and R is speaker nominal impedance
The supply voltage (allowing only for basic losses) is calculated as follows
    VRMS = Va * 1.1   Where VRMS is the transformer secondary voltage (for each supply rail)

VA Rating - Class-AB 
The minimum VA rating suggested is equal to the amplifier power. A 50W amp therefore needs a 50VA transformer, or 100VA for stereo 50W amps. Larger transformers (up to double the amp power rating) will provide a 'stiffer' power supply, and this may be beneficial. For continuous operation at full power (never needed for hi-fi but common for guitar amps), the transformer should have a VA rating of up to 4 times the amplifier power.

It is suggested by some transformer manufacturers (and no doubt gleefully adhered to by many amplifier makers) that the VA rating needs only to be 0.7 of the maximum amplifier power. While this will work well enough in most cases, you will not have a 'stiff' power supply - a more appropriate term would be 'soggy'. The DC voltage will collapse as more current is drawn.

VA Rating - Class-A
The minimum VA rating suggested is at least 4 to 5 times the amplifier power. A 20W Class-A amp therefore needs a minimum of an 80-100VA transformer, or 160-200VA for stereo 20W Class-A amps. Transformer rating may need to be as much as 10 times output power, depending on amplifier topology and quiescent current [2]. The constructor needs to be able to work this out, or transformer failure is likely.

[luvdunhill]

Only addition to this is transformer vendors specifying a very high temperature rise or going extremely high on the flux curve are making these calculations hard. So, in addition to these guidelines, you really need to have conservative ratings - or specify them yourselves, which few do.