Noob XLR Grounding Question

[schmidlin]

Hi All,

I finished a spiffy tube compressor that has some hum.  I have XLR pins 1 (I & O) going to chassis ground (at the bus bar) and heard this is not good: causes ground loops, which makes sense.

So, what is the standard way of treating these grounds?  I have balanced in and out.

Thanks in advance.

 

[lassoharp]

I generally ground the XLRs right to the jack tab(chassis) - so closest to point of entry.  More related to electrostatic noise issues than 60/120 hum though.  I've never had it create hum on a 3 wire system from input vs downstream connection.

First guess on hum would be filament grounding/CT/balance esp on Federal type plan. 

Tried stage isolating yet?

 

[abbey road d enfer]

Hi All,

I finished a spiffy tube compressor that has some hum.  I have XLR pins 1 (I & O) going to chassis ground (at the bus bar) and heard this is not good: causes ground loops, which makes sense.

So, what is the standard way of treating these grounds?  I have balanced in and out.

Thanks in advance.

what makes you think a ground loop is the problem?
Does the unit hums even with nothing connected at the input? If the answer is no, then the culprit is not a ground loop.
Hard-wiring all pin1's to the chassis is the AES recommended practice, but also they recommend using cables where the male plug has its pin 1 floating (which makes them incompatible with microphone use).
Generally, I connect pin 1 to chassis via a 0.1uF cap (as close as possible) and a 100R to audio ground (0v). Audio ground is hard-wired to chassis in one place.
But I really think the problem lies somewhere else.

 

[Nora]

60Hz or 120Hz hum?

If there's no 60Hz present, it means the power supply isn't filtered properly.

If it's 60Hz, it could be ps transformer radiating inside the chassis.
Or bad (AC!) filament layout.
There could still be a ground loop, like multiple connections from case to audio ground.

I always ground at pin 1 in/out and I've never had any noise/ground issues.

 

[Butterylicious]

Try floating pin 1 at the output only. 

 

[schmidlin]

This is why I come here.  Thanks, gang.

My first thought was heaters, but got to talking with my audio engineer and...

The primary culprit is 60 hz, with a 290hz (?!) following.

Right now I have AC heaters with (2) 100R to ground.  Should I go DC or something else?  I have a toroidal PT.  Gain doesn't effect hum level: it's fixed.  Small, but fixed.

 

[Butterylicious]

To rule out a filament hum problem, disconnect the AC filaments and heat the tubes with a lantern battery.  Batteries don't hum.  AC filaments run close and parallel to signal paths can cause 60Hz hum.  AC filaments can be acceptable, the trick it to keep them up and completely away from audio paths. This box was supposed to have a DC supply (note the unused turrets and chassis holes) but I never bothered because it's hum free with AC filaments.  With tube circuits, layout is king.

 

[lassoharp]

This is why I come here.  Thanks, gang.

My first thought was heaters, but got to talking with my audio engineer and...

The primary culprit is 60 hz, with a 290hz (?!) following.

Right now I have AC heaters with (2) 100R to ground.  Should I go DC or something else?  I have a toroidal PT.  Gain doesn't effect hum level: it's fixed.  Small, but fixed.

The last heater related problem I had wound up requiring a hum balance pot + an elevated CT 'float'(like Pultec EQP1 PS) to give lowest hum.  The PT had a CT but evidently the balance between the windings was far enough off to require the pot.

Always chance that the PT field is getting into the input  or output transformer if on the same chassis. That's easy enough to catch since the hum will appear immediately with AC power up vs after warm up.

Have you tried to isolate it to a stage via grid and plate grounding?

I'd exhaust all else then try and test with DC if possible before major surgery.  I've started making a habit of keeping my AC filament wiring polarity DC correct on bigger projects in case it comes down to that.

 

[Gareth Connor]

For in-depth info on grounding & the related implications of EMC, study the work published on the website of Tony Waldron.