Noise in tube mic pre gets worse when chassis is closed

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

hop.sing

Well-known member
Joined
Jun 21, 2005
Messages
160
Hi

I built the Lorenz V241 https://groupdiy.com/index.php?topic=67386.0 into this enclosure:
nb5ci0.jpg

With the Back off the thing looks like this: (This is not the pre, but the original Voltmeter):
5angqg.jpg

Tubes are not visible here, they are on the top.
The enclosure is a bit to small to ensure a good distance from power transformer to input circuit, but it is such a beautiful case with a power transformer and good tube sockets for free, so I had to use it.
Everything works beautifully and the sound is great, but I have this strange noise issue.
Noise gets worse when the chassis is closed.
When the back of the chassis is taken off and the electronics have no shield except the cover over the input circuitry  you see in the picture on the right.
I have this Noise spectrum with shorted input and 55dB gain on a RME FF400 with +19dB full scale.
20zzdw3.jpg

Respectable and no problem even with quite sources.
But when I close the enclosure and everything is shielded the noise gets much worse:
oj3703.jpg


How is this possible? The enclosure is metal, not aluminum, does it maybe reflect the power transformer noise back into the sensitive input circuitry? Or is there some ground loop going on here because the backcover takes up a lot of space and redirects ground currents, even though circuit ground is connected to the chassis only at one point. 
I searched, but did not find anything that could help in my case...
Thanks.

 
I have seen this before (in solid state designs). The metal chassis distorts the internal magnetic field causing more hum in one or more sensitive circuits.  In my case I was able to knock it down using a simple metal shield separating the transformer from the rest of the guts.

Maybe get some mu metal and play around with placement.

JR
 
Thanks for the replys.
I just wonder: are 150Hz and harmonics above part of the electromagnetic radiation spectrum of a power transformer?
I always thought transformers mainly radiate 50Hz.
 
CJ said:
3rd harmonic?

the lid completes the flux loop, like adding I bars to an E lamination.
:)
A bit over my head, what might be a possible remedy, more and better shielding inside the chassis? Transformer orientation? (I already tried that with little success)
Building a new backlid from a different material, probably possible but would be ugly, the case as is is so beautiful.
 
hop.sing said:
:)
A bit over my head, what might be a possible remedy, more and better shielding inside the chassis? Transformer orientation? (I already tried that with little success)
Building a new backlid from a different material, probably possible but would be ugly, the case as is is so beautiful.

Yes, the metal lid "shorts" the magnetic field of the power transformer. Aluminum would be better than steel.  I had the same issue with a 1HU 19" case made of steel.

Something like this could be a solution.

https://www.don-audio.com/trafo-shield_3
 
Back
Top