A Question About HUM

[GLM]

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

Into a line-output.

 

[Khron]

Into a line-output.

One of them, or both, or..?

 

[GLM]

One of them, or both, or..?

One channel at a time. Results the same on both channels.

 

[Khron]

One channel at a time. Results the same on both channels.

So basically you COULD have only listened to one-side headphone at a time, right? Or since each mono output is transformer-balanced, your left & right headphones were opposite-polarity, so especially the low-frequencies the hum is at, got cancelled out.

Tests are good, as long as you're aware WHAT you're testing and HOW... Point being, the headphone test is first of all flawed, and second, completely irrelevant.

So since the thing hums when connected to whatever else in your setup EXCEPT your headphones, kinda narrows it down to the unit itself, imho. Regardless of what those technicians have claimed (since neither you, nor we, know how and what they tested)...

 

[scott2000]

The hum is present when isolated from all other gear

Why are you picking on the WARM unit and not "all other gear" ?

 

[transient]

What is your mains voltage? It could be that it is too low, and causing the preamp's power supply to lose regulation. That would explain why it works fine when professionally tested. Some devices are better about dealing with low mains voltage than others. Be very careful about sticking the probes of your meter into a live mains socket - maybe find a local electrician to check!

 

[GLM]

So basically you COULD have only listened to one-side headphone at a time, right? Or since each mono output is transformer-balanced, your left & right headphones were opposite-polarity, so especially the low-frequencies the hum is at, got cancelled out.

Tests are good, as long as you're aware WHAT you're testing and HOW... Point being, the headphone test is first of all flawed, and second, completely irrelevant.

So since the thing hums when connected to whatever else in your setup EXCEPT your headphones, kinda narrows it down to the unit itself, imho. Regardless of what those technicians have claimed (since neither you, nor we, know how and what they tested)...

Thank you, I appreciate the straight-talk.

This all goes far beyond my knowledge, so I do respect that I could be going about this the entire wrong way.

I think I am going to take a step back and think about this more here.

Also, it seem I may have not been clear earlier, but the hum is present with the headphones.

In other words, I haven't been able to find an environment where I have not had hum present. With the exception of the 'clean power' environments of the tech.

I have sent it back under warranty for full bench testing by Warm Audio themselves. They passed it through 2 of their techs.

I also had a local independent tech test it.

Thanks again for the insight.

 

[GLM]

What is your mains voltage? It could be that it is too low, and causing the preamp's power supply to lose regulation. That would explain why it works fine when professionally tested. Some devices are better about dealing with low mains voltage than others. Be very careful about sticking the probes of your meter into a live mains socket - maybe find a local electrician to check!

This is a very good idea actually. The last property I was in had issues with other domestic equipment that were caused by this.

My next step is really going to have to be to get an electrician in. I wont be going near that myself. Anything but electricity and plumbing!

Thank you.

 

[Cqwet Dbdfte]

It is supposed to be 220VAC?
Low mains voltage can indeed set DC levels below regulator minimums.
Ask me how I know.
There are many things that can be the culprit, divide and conquer. Even cables and connectors.
Use shielded AC wires if you can, they are not expensive, but only help certain problems.
Use a sensitive DMM to measure AC noise, may need 5-1/2 digit or so.
The AC mains wiring is a loop, or home-runs? In metallic tubing? (EMT)
If you have low voltage a variac can be useful in troubleshooting.

 

[Bonnie1]

Hi GLM,
When you're testing with nothing connected to it, how is the input terminated?

 

[GLM]

Why are you picking on the WARM unit and not "all other gear" ?

Not sure I understand the question? The issue is only present in the WA. Its not to say its not a system-wide problem, but its the only link in the chain where the problem is apparent and having immediate affect.

 

[GLM]

Hi GLM,
When you're testing with nothing connected to it, how is the input terminated?

Hi Bonnie,

Excuse my pigeon understanding of these terms, but I have nothing connected to the inputs, but I switch between MIC and INSTRUMENT or even insert etc etc.

 

[GLM]

Just a general note of thanks for all the ideas and suggestions coming in.

Im totally out of my depths, but I at least have some directions to go.

I was still wondering does anyone have a basic thought on the device I linked to in my original post?

 

[Khron]

Use a sensitive DMM to measure AC noise, may need 5-1/2 digit or so.

How's that DMM gonna tell the difference between the 50Hz mains, and whatever "noise" one is trying to detect?

 

[Cqwet Dbdfte]

How's that DMM gonna tell the difference between the 50Hz mains, and whatever "noise" one is trying to detect?

Battery operated should avoid mains problems, but need very good sensitivity.

 

[Khron]

Battery operated should avoid mains problems, but need very good sensitivity.

That still doesn't answer my question 50Hz mains is AC, and so is the noise. How will the "one-dimensional" numeric indication correlate with ONLY the noise, as opposed to the 50Hz PLUS the noise?

I was still wondering does anyone have a basic thought on the device I linked to in my original post?

That isolation transformer? What should that achieve, if "low mains" turns out to be your actual issue?

 

[Cqwet Dbdfte]

That still doesn't answer my question 50Hz mains is AC, and so is the noise. How will the "one-dimensional" numeric indication correlate with ONLY the noise, as opposed to the 50Hz PLUS the noise?



That isolation transformer? What should that achieve, if "low mains" turns out to be your actual issue?

If the mains noise 50Hz or 100Hz is low enough to be buried in the noise, can it be heard? With a shorted input there should be only a very low level of noise, anything in excess may be mains noise.
An isolation transformer wouldn't reveal a low voltage problem, it may be useful for other issues. Even those have isolation specs.

 

[GLM]

That still doesn't answer my question 50Hz mains is AC, and so is the noise. How will the "one-dimensional" numeric indication correlate with ONLY the noise, as opposed to the 50Hz PLUS the noise?



That isolation transformer? What should that achieve, if "low mains" turns out to be your actual issue?

I have zero idea what it might achieve! Reminder that I am a dummy, but someone elsewhere had casually suggested that it might help.

I was and remain interested in hearing advice and guidance from people who know more about this stuff.

 

[Khron]

If the mains noise 50Hz or 100Hz is low enough to be buried in the noise, can it be heard? With a shorted input there should be only a very low level of noise, anything in excess may be mains noise.

Oh, you meant on the audio output! For whatever reason, my mind was stuck on the mains AC, and was wondering how that would be measured with a DMM Nevermind then...

 

[Cqwet Dbdfte]

I have zero idea what it might achieve! Reminder that I am a dummy, but someone elsewhere had casually suggested that it might help.

I was and remain interested in hearing advice and guidance from people who know more about this stuff.

Do you have a DMM? Did you measure the AC voltage at the socket with all the equipment turned on?