Hum induced in other rack gear

[beatnik]

Another thought, is there some headroom on the power transformer ? I believe they tend to increase magnetic field strength when operated close to the limit.

 

[warpie]

Do the Neve input transformers have some sort of mu metal shielding or it's just a steel enclosure ?

I really wonder the same. The older ones seemed to be mu-metal but the newer ones are painted blue, which makes me suspicious. Not to mention that they pop out like hell...

 

[beatnik]

Looking at Audiomaintenance catalog it seems Carnhill has essentially two kinds of input transformers
The Belclere style are specified to have a mu metal can but the blue Marinair style don't have any mention about shielding.
I guess with a gain of 80dB like on the 1073 an unshielded input transformer could be problematic

 

[SoundBlaster]

Be careful with steel shields over toroids. Use a non ferrous mounting bolt. Any short between the cover and the chassis (if it too is ferrous) and using a steel bolt will essentially create a shorted turn for the toroid. Once had a factory located in a country that starts with "C" try and substitute a brass bolt with a steel one.. Every unit blew fuses the second they plugged them in. Was a teachable moment.

 

[Mr. Moscode]

Sometime toroids don't cut it. I had a magnetic radiation problem at hum frequencies caused by a filament winding on a 1kw toroid that fed a DC supply. The charging current on the peaks put out 60 hz bumps all over the chassis and that was for a 2 amp winding. I took the winding off the 1kw torroid and put a smaller 50 watter in the box and that solved the problem. It wasn't a problem with the HV windings for tube power.

You can also try re-orienting the transformer for minimum hum infiltration. I did that in a preamp that has a phono stage with 80 db gain input to output which means there was closer to 100db gain at 60 hz. That was with and EI xfmer but rotating a toroid can make a big difference.

Out-boarding the xfmers is always a good idea if you can. Mu metal can help around high gain circuits. I look for hum currents on the ground track on the pcb or at the output when orienting the mu metal and the transformer. Mu metal may be more effective around the hi gain circ rather than the xfmer.

 

[beatnik]

Sometime toroids don't cut it. I had a magnetic radiation problem at hum frequencies caused by a filament winding on a 1kw toroid that fed a DC supply. The charging current on the peaks put out 60 hz bumps all over the chassis and that was for a 2 amp winding. I took the winding off the 1kw torroid and put a smaller 50 watter in the box and that solved the problem. It wasn't a problem with the HV windings for tube power.

I have heard of similar problems with toroidal power transformers but my understanding was that the noise is coupled from the rectified filament winding to the HT winding, and not to the chassis like you are saying ?

 

[MidnightArrakis]

Be careful with steel shields over toroids. Use a non ferrous mounting bolt. Any short between the cover and the chassis (if it too is ferrous) and using a steel bolt will essentially create a shorted turn for the toroid. Once had a factory located in a country that starts with "C" try and substitute a brass bolt with a steel one.. Every unit blew fuses the second they plugged them in. Was a teachable moment.

[Be careful with steel shields over toroids] -- If "steel shields over toroids" are so bad, then "WHY" would toroid transformer manufacturers even offer them?

[Use a non ferrous mounting bolt] -- In assuming that you are able to understand the drawing below, this company uses a -- NON-BOLT -- method of securing both the transformer and the shield-cover to a chassis.



[Once had a factory located in a country that starts with "C"] -- I would guess that it is the country of "Chad", right??? Either that or "Comoros", I would guess!!!



[Was a teachable moment] -- SEE??? You learned something that you will always remember.....up to the point when you are on your deathbed!!! A very valuable lesson!!! heh.....heh.....heh.....heh.....heh.....

/

 

[Audio1Man]

HELLO!
Any current carring material fastner through the Torrid will make a SHORTED TURN through the chassis.

 

[guy_4]

Hello All,

I had a similar experience with magnetic radiation affecting close outboards in a rack of a mastering studio.

A Weiss processor was creating some hum over a Chandler stereo EQ below.
I tried several " tricks ", like inserting a sheet of Mu-metal between both units, but with no success.
Finally I removed the PT of the Weiss, and replaced it with a R-Core transformer, and it was clean at the end.

Best,
Guy

 

[Ike Zimbel]

It's been a while, but I have seen instances where Mu-metal had no effect, and steel did. In one case this was shielding CRT screens that were between the large overhead monitors in an L-C-R arrangement in a new studio, around 1990-91.

 

[Audio1Man]

Hello All

Who thinks this TRANSFORMER is GOOD?

It may be OK for Japan, however not for the rest of the world. The splattering field will make HUM & BUZZ in any Audio product.

 

[SoundBlaster]

[Be careful with steel shields over toroids] -- If "steel shields over toroids" are so bad, then "WHY" would toroid transformer manufacturers even offer them?

[Use a non ferrous mounting bolt] -- In assuming that you are able to understand the drawing below, this company uses a -- NON-BOLT -- method of securing both the transformer and the shield-cover to a chassis.

View attachment 123238

[Once had a factory located in a country that starts with "C"] -- I would guess that it is the country of "Chad", right??? Either that or "Comoros", I would guess!!!

View attachment 123239

[Was a teachable moment] -- SEE??? You learned something that you will always remember.....up to the point when you are on your deathbed!!! A very valuable lesson!!! heh.....heh.....heh.....heh.....heh.....

/

HELLO!
Any current carring material fastner through the Torrid will make a SHORTED TURN through the chassis.

You Are correct sir.. Was confusing two different production issues. The toroid was supposed to use a fiberglass bolt. A speaker magnet structure was supposed to use the brass bolts. In any case. Steel in both cases created a short

 

[heatwalk]

Update from client - pulling it from the rack the resolved the issue in most of the units, and he could make it get better or worse as he moved it around the most problematic nearby unit.

Do the Neve input transformers have some sort of mu metal shielding or it's just a steel enclosure ?......

Unsure. The problematic one is a BAE so if anyone knows about them specifically.

Another thought, is there some headroom on the power transformer ? I believe they tend to increase magnetic field strength when operated close to the limit.

The heater winding is running near its specified rating, HT definitely has some headroom.

 

[JohnRoberts]

Large manufacturers who sell both hum makers and hum receivers get feedback from dealers and customers if they don't catch problems during internal pre-release testing.

JR

 

[Ike Zimbel]

I'd love to know what the most affected unit is. I bet it has either an aluminum, or worse, plastic chassis. Shielding the transformer on the unit you built is a good idea, but this unit would also benefit from getting more iron (or steel...) in its diet.