Quick Toroid Hum Question

[Siegfried Meier]

Hey guys,

I've never seen this before, but the top panel of a 1U chassis is causing the toroid inside a pair of 312 mic pres to hum.  Usually it's the opposite, is it not??  I've never had a case where it hums when the top panel is put back on.  I've started using these Avel Lindberg trafos lately over the Amvecos we usually use, and I don't think I'm happy with these...

Any advice?
Thanks,
Sig

 

[Siegfried Meier]

One factor that often gets overlooked is the metal that the case is made of. Many rack mounting cases have aluminium front panels but mild steel sides top and bottom. The mild steel parts will 'conduct' stray magnetic fields surprisingly efficiently so I now always build my mic pres in all aluminium boxes. More expensive but a lot less grief.

Cheers

Ian

Does anyone think that if I simply ordered aluminum top panels for these from Par-Metal that my hum issues would disappear?

 

[Siegfried Meier]

Meh, I tried placing an aluminum front panel from another piece of gear on top - same hum ****.

You really do need to build external PSU's for mic preamps.  I don't think it's possible to get a clean, quiet pre with built in power trafos...

 

[desol]

Meh, I tried placing an aluminum front panel from another piece of gear on top - same hum sh*t.

You really do need to build external PSU's for mic preamps.  I don't think it's possible to get a clean, quiet pre with built in power trafos...

It's possible...you just have to be careful.

 

[Davo]

Sig,

I've had this occur with a couple of builds... get it nice and quiet, then put the top on and there's hum.  On the most recent one, I stuck a piece of mu-metal in between
the top and the trafo, and it made a difference.  On the other, I'm re-doing the psu... crapshoot but some mu-metal may work.

cheers

 

[Siegfried Meier]

It's funny, I actually had to do the opposite from what I've learned to do over the past 10 years.  I don't know if this is the 312, since I've always done them external PSU before, or if it's the Avel Lindberg.  But, I actually had to rotate it 180 degrees the other way, and keep it NOISY...then, with the top panel back on the noise cancelled.  It wasn't 100%, but the hiss of the circuit and opamps was much, much louder than the hum when everything was totally cranked wide open.

Magnetic fields are weird.

 

[alexc]

I've had this on some builds - add the lid (alu) and the psu harmonics noise gets worse.

I've usually found it to be symptomatic of wiring/placement issues.

Reworking my wiring placement, signal transformer placement and ground wiring connections
by trial and error till  I get an improvement in noise floor usually makes the sensitivity
to the alu lid disappear.

I've tried steel lids, steel shields in these situations and it hasn't helped more than a dB or 2.

Basically, when I've seen the lid sensitivity, it's a signal to improve my wiring and grounding.

Good luck with it

 

[Tekay]

Some toroid brands are better and some worse. Bad core material? I tested several brands in our DP1 and Farnell's Multicomp toroids and Carnhill's toroids are the best so far. Sometimes we need to exchange the transformer as turning the toroid doesn't help at all.
You can also try shielding sheets from Don Audio they helps alot.
If you have problem with 50/60Hz buzz, serial resistors with the secondary may help. We use 2R7/3W resistor in our DP1 to get rid of some buzzing. Check the voltage before your regulators as the serial resistors lowers the voltage.

 

[CJ]

is this mechanical or electrical humm?

if mechanical, you could locate the the xfmr outside the box somehow.
or get a box with more clearance,

any metal that gets close to a strong  mag field will start to jump around like a frog in a dynamite pond,

 

[ruffrecords]

Meh, I tried placing an aluminum front panel from another piece of gear on top - same hum sh*t.

You really do need to build external PSU's for mic preamps.  I don't think it's possible to get a clean, quiet pre with built in power trafos...

That is really odd. It might be worth trying to reduce the coupling of the transformer to the face it is mounted on my inserting a spacer of some kind.

Cheers

Ian

 

[peterc]

Is the bolt or transformer actually touching the lid?

 

[radardoug]

Surely the whole point of using a toroid is that the magnetic feild is largely confined in the toroid. Otherwise you would use a conventional transfomer.
Coming from a country using 50 Hz mains, I often see US equipment where the manufacturer has skimped on core material. These transformers will saturate on 50 Hz mains but work at 60 Hz.
So it comes down to lack of core material, or excessive load on the tranny.
If you previously had a brand that you liked, and my guess is you changed brand because of price,
now you know why the inferior tranny is cheaper!
As an aside, it is perfectly possible to put a power supply in a box with mike pre's, but it takes good design.

 

[Siegfried Meier]

Bolt is not touching the lid, it makes a different sounds when it does BTW.

Ya, it's not really the wiring as it's very solid and short/basic in this mic pre.  Just seems that the Avel Lindberg's cause different issues than the Amveco's I've used in the past - and indeed, they were chosen because of price!  The Aveco's used to be $15-$20 and now they've gone up to $60 each.  The Avel's, while they used to be made in the US, are now made in China sadly, and are around $25 each.  After rotating it as mentioned above, all is silent and sounds great at incredibly loud gain, so all is well.  I think the Avel's are fine in an external chassis, but I would be careful with them so close to mic preamps.

Thanks guys!
Sig

 

[Siegfried Meier]

Has anyone used these?  http://www.don-audio.com/trafo-shield

I'd love to hear thoughts on whether they really do work or not.

Thanks!

 

[Davo]

Yep... I've used them in a couple of builds....  preamps with internal traffo where I had a tough time getting the hum below the noise floor.  In both cases, it did
exactly what I needed and dropped the hum considerably. 

Another source of mu-metal:  http://www.lessemf.com/mag-shld.html
I'd go with .01" as the .004" is pretty thin stuff... although a lot more manageable.  I've been thinking about trying out the "giron"

cheers