Weird Noise in Only Some Situations!

[Hexo]

... the weird thing is the sound cards don't make that noise when they're connected to my iMac while nothing is connected to their inputs. So it looks like they behave differently in almost the same circumstances!

Not weird at all.

With nothing connected to the inputs, the connectors act as aerials, picking up whatever RF might be in the local environment. Subtle differences in the circuit board layout and grounding will give different channels different sensitivities to different RF interference.

It would appear that there is more RF at your friend's place than yours...

But the bottom line is that inputs aren't intended to be left disconnected. If you want to measure preamp noise performance you should use a 'dummy load' in a screened case to provide a suitable source impedance for the preamp.

For a mic input, solder a 1% 150 Ohm metal oxide resistor between pins 2/3 of an XLR plug. Some manufacturers cheat their specs by shorting pins 2/3 directly without the resistor, which improves the EIN figure by a few decibels.

I totally agree and I will test them as suggested as soon as I can. But please consider that we've already tested them with different microphones connected and not only the noise is not gone, it's increased relative to gain boost. One more thing: he brought his laptop to my room and we re-ran all tests. The noise is there in his laptop, and absent in my iMac in all circumstances!

[resistorman]

I totally agree and I will test them as suggested as soon as I can. But please consider that we've already tested them with different microphones connected and not only the noise is not gone, it's increased relative to gain boost. One more thing: he brought his laptop to my room and we re-ran all tests. The noise is there in his laptop, and absent in my iMac in all circumstances!

Well, this is different than what you've been saying

[Drew Stephenson]

Does his laptop have a mechanical disk or an SSD?
If the former it might be picking up some physical vibration.

[Wonks]

Microphones pick up noise from the room. It's what they do. You're in it and your computer's in it and there's probably wind, traffic and street noise at low level. Unless you are in a soundproof and acoustically well-treated room and your computer(s) are in a separate room, there will be a lot of ambient noise you aren't aware of being picked up on a microphone that's way above the electronic noise level.
In real life your brain tunes a lot of that noise out, but when you hear it on a recording, it doesn't. It's weird the way it happens, but it's a fact.

Average room noise is between 30 to 50dB(A), and 30db is a very quiet room. A loud singer (not shouting) is around 80dB(A), maybe 90dB (A) when close-up. So you may only have between 30dB and 60dB between your vocal and the background noise in a standard untreated and non-soundproof room.

Which is why you need to judge the equipment noise purely based on using terminated cables (as has been said, shorted to ground cables will be slightly quieter).

Test like that to work out the interface preamp noise. You can't do it any other way. If that still measures high, then you have a problem with the set-up.

Otherwise any noise above that is noise is almost certainly going to be noise picked up from the room. Capacitor mics do make a little bit of electronic noise, but that is still way down compared to any ambient noise. Dynamic mics shouldn't have any self-noise.

[Hugh Robjohns]

Please consider that we've already tested them with different microphones connected and not only the noise is not gone, it's increased relative to gain boost.

I don't think you mentioned that. Nevertheless, the situation remains the same. RFI is getting into the interface somehow, and there is more RF interference at your friend's house than yours.

The noise test with the dummy plug might give some further clues as to how the interference is getting in. But it's not unknown to get in through some microphones and/or cables. And interface grounding can also play a significant role.

One more thing: he brought his laptop to my room and we re-ran all tests. The noise is there in his laptop, and absent in my iMac in all circumstances!

More helpful news.

So, it might be that the laptop or its power supply are generating the interference. Or it may be that the laptop is not providing a good ground for the interface. Or it might be that the laptop is injecting noise via the interface connection. Sometimes different USB ports can have more/less noise than others.

This kind of problem can be tricky to resolve but careful, logical testing usually gets there.

[Hugh Robjohns]

Dynamic mics shouldn't have any self-noise.

Of course they do. They generate thermal noise due to the resistive element of their output impedance. In most modern mics this is nominally 150 Ohms and results in a self-noise level around -131dBu (at 20C and measured between 20Hz and 20kHz).

A capacitor mic has a similar output impedance so generates a similar thermal noise, but the active circuitry in the impedance converter inevitably adds a few dB more noise.