A Question About HUM

[GLM]

Greetings.

I asked this over on another audio forum and they kindly pointed me in this direction.

Hopefully this is the correct section for my particular query.

I am running a small home studio.

Mostly everything works fine, no issues.

However, I run a Warm Audio WA273EQ pre-amplifier on my main mix bus. The Carnhill transformers just love to let you know you have bad power.
I have a persistant 100hz hum. This only occurs with this pre-amp. The hum isnt always consistent in volume.
I have had the hum present at the different locations (bad luck with bad power/old buildings/circuits).
But I have also had the unit inspected and tested. No problems. And 2 x technicians have reported no hum in their 'clean' workbenches.

I have done the typical troubleshooting, including turning off all other power and plugging the WA273 in directly to the main socket and monitoring with headphones ... and still .. hum .. It has a ground lift, but this does not change anything.

I also have tried an IEC hum remover, which seems to have a slight improvement.

I live in Ireland in a semi-rural area. There are 2 small farms either side of me. I am pretty sure this is a general mains issue.
Longer term plan is to get a full re-wire and inspection, but not anytime soon unfortunately.

Can I simply purchase one of these and run my studio from this single socket? Is this going to give me any relief from the 100hz hum?

https://www.screwfix.ie/p/carroll-me...30v-blue/320hv

Or something like this? I know there are large/expensive PSUs you can get, with battery back up etc.
Would the above product be worth trying?

I am relatively competent with technology/gear/DIY, but power issues go beyond me.

I would really, really appreciate any clear guidance on this.

Thanks

 

[radardoug]

You dont give us any detail on what is feeding into this pre, Is it a mixer? If so, why?

 

[GLM]

You dont give us any detail on what is feeding into this pre, Is it a mixer? If so, why?

I use it both as a mic pre/DI and as an insert on my mix bus, but as noted in my post, even with the box is totally isolated, it still produces the 100hz him.

 

[dmp]

If you orient it in different orientations (rotate 90 deg in all three axis) does the hum change?

 

[radardoug]

What mix bus? If you dont tell us about your gear, how are we supposed to be able to solve your problem? We are not psychic.

 

[GLM]

If you orient it in different orientations (rotate 90 deg in all three axis) does the hum change?

Curious suggestion! I will try this out ASAP.

 

[GLM]

What mix bus? If you dont tell us about your gear, how are we supposed to be able to solve your problem? We are not psychic.

Im sorry, I dont understand the hostility? I mentioned above that the problem exists regardless of my practical use of the pre-amp. The hum is present when isolated from all other gear, and powered independently and without any local interference.

Hence, I assume this is a problem coming from my mains power. And thus, my question about using the power isolation unit as linked.

 

[radardoug]

You came on a forum to ask for help with your problem. But you dont give us enough information.
Apparently you took this peice of gear to a tech, and they said it was fine. So if it is fine, it is how you are connecting it to other gear.
But you wont tell us what other gear and how it is connected. So we cant help you. Do you think we magically know what gear you have?
I would love to be that clever!

 

[Walrus]

My understanding is that the unit outputing a hum, using headphones, when it is just plugged into the mains and NOT connected to anything else.

You say it is 100 Hz, have you actually measured this?
I would normally expect 50 Hz for radiated pickup.

 

[soapfoot]

Curious suggestion! I will try this out ASAP.

I was going to suggest the same thing—this will help rule out (or target) radiated interference as a potential cause

 

[GLM]

My understanding is that the unit outputing a hum, using headphones, when it is just plugged into the mains and NOT connected to anything else.

You say it is 100 Hz, have you actually measured this?
I would normally expect 50 Hz for radiated pickup.

Correct. Fully isolated. Removed from rack. No connections. Nothing else powered on in the the studio.

The spike it very much at 100 hz when viewing on a visualizer/EQ.
I can replicate today today and screenshot.


I’m going to do as kindly suggested by others and move/rotate the device today and see if any change.

Thanks for trying to work this out, appreciate everyone’s time.

 

[GLM]

You came on a forum to ask for help with your problem. But you dont give us enough information.
Apparently you took this peice of gear to a tech, and they said it was fine. So if it is fine, it is how you are connecting it to other gear.
But you wont tell us what other gear and how it is connected. So we cant help you. Do you think we magically know what gear you have?
I would love to be that clever!

 

[GLM]

Ahead of trying the suggestion above,
does anyone have any thoughts on the power isolator device?
Assume 2 things;

- I’m a power dummy
- The problem is with my mains power(reminder that I live semi-rural, farms left and right) and not the problem is not the pre-amp itself.

Thank you thank you.

 

[ruffrecords]

The Audio WA273EQ pre-amplifier seems to be a Neve 1073 clone. I hope when you tested it with headphones that you used 600 ohm phones because the 1073 is not rated to drive a load less than 600 ohms.

I think the problem is unlikely to be your mains supply but it might be a poor mains earth. Does the hum change at all if you touch the metalwork?

When you run it alone with headphones. does the hum vary with the gain setting?

The reason changing the orientation of the unit has been suggested it to ascertain if an external magnetic filed is the problem.

Cheers

Ian

 

[GLM]

The Audio WA273EQ pre-amplifier seems to be a Neve 1073 clone. I hope when you tested it with headphones that you used 600 ohm phones because the 1073 is not rated to drive a load less than 600 ohms.

I think the problem is unlikely to be your mains supply but it might be a poor mains earth. Does the hum change at all if you touch the metalwork?

When you run it alone with headphones. does the hum vary with the gain setting?

The reason changing the orientation of the unit has been suggested it to ascertain if an external magnetic filed is the problem.

Cheers

Ian

Cheers Ian,

Appreciate you expanding on the orientation test!

I’ll report back soon.

 

[GLM]

Dear all,

Some more information ..

I took a screen shot of the problem frequencies. I have boosted the gain by 24db for the sake of clarity. Obviously, a lot of scenarios make this relatively acceptable with noise floor etc, but it is more often a problem than not.

You can see the 100hz spike and then a 200hz harmonic.



In terms of adjusting the gain, the hum doesn't really change, but if it's helpful to know, when the gain is pushed and some hiss/noise is introduced, it seems to overwhelm the hum. The hum gets quieter. (IE it's not just that the hiss gets louder relative to the hum, but that the hum does reduce in volume slightly. Maybe this is expected behaviour, as is moves through the circuits).

Adjusting the output on the preamp of course has an affect on the hum.

No change if the EQ is engaged or disengaged. Ground lift does nothing at all.

I have an Audiolab DC Block for my HI FI that I have tested her. This also has no effect.

I have tried the 'orientation' test too. No change at all when the unit is rotated. No change if I touch metal parts or switches.

I hope this offers some help!

 

[Khron]

You can see the 100hz spike and then a 200hz harmonic.

That pretty much points towards some issue AFTER the bridge rectifier, since that's where the 50Hz mains gets rectified into 100Hz (plus harmonics) "one-sided" pulses, which at least "should" get smoothed out by the bulk capacitors in the PSU. But i could be wrong

How old is th unit, by the way?

 

[GLM]

That pretty much points towards some issue AFTER the bridge rectifier, since that's where the 50Hz mains gets rectified into 100Hz (plus harmonics) "one-sided" pulses, which at least "should" get smoothed out by the bulk capacitors in the PSU. But i could be wrong

And if you were saying this to a 5 year old ..?

 

[GLM]

Unit is maybe 5 years old.

It is indeed a clone of a NEVE 1073.

https://warmaudio.com/mic-pre-wa73-eq

 

[Khron]

And where were the headphones connected, during your unit-only tests?