Help! i've Demons in my gear..

[xazrules]

Hi everyone in almost all my mic pres and in a guitar tube preamp there's a "whistle" at around 8khz or sometimes 6khz with associated harmonics. can you help me troubleshoot it?
this is how it looks like:

 

[RuudNL]

I think more information is needed about your configuration and how and when you get this disturbance!

 

[Bo Deadly]

Wrap a mic in some towels and then run an XLR directly into a mic input of your USB audio interface and then look at the spectrum. That is your baseline. Then systematically insert devices into the chain until you see the issue. You should be able to at least narrow down the issue a little.

 

[Tubetec]

Powering your audio interface from a laptop can be problematic .
Often on battery power the noise vanishes .
Ive also seen multiple switchmode power supplies interacting and creating noise problems in audio gear
Lighting is another likely suspect , dimmer switches are especially problematic, flourescent strip lights are another frequent offender  , Ive also heard of grow lamps causing mains noise problems , motor speed controllers are another possibillity.
Definately give us a run down of your set up as it could help us pinpoint the issue better .

 

[scott2000]

Funky outlet power strip has given me issues in the past as well....

But yeah those smps, especially warts are always suspect....

I had this one crackle driving me nuts for several months...... Finally figured out my cell phone was too close to my sound card..... Even when the screen is off that thing is doing something.... wild...

 

[xazrules]

thanks for the suggestions, yes it is driving me nuts! it is a costant whine in most of my mic pres even with no xlr cable connected.
the thing i've tried so far:

-remove ground connection of the pc. nothing
-loopback of the soundcard and there is no whine, all is clean.
- shut down everything in the house except my gear. still there
- some pres don't have this disturbance

the wifi of my router is emitting this same whine, but if i remove it the sound is still there in the mic pres.

i think that if it is a usb ground loop or the switching psu of the pc it could be present even in the loopback of my soundcard, so is it possible that it is a sound picked up from the audio and/or mains transformer?

but the worst thing is that i hear the same sound in my head now  ;D

 

[cyrano]

Which preamps filter out the noise?

Maybe we could have a look at their schematics and decide if it's likely a filter for the mic-in side, or the power supply side?

 

[Bo Deadly]

i think that if it is a usb ground loop or the switching psu of the pc it could be present even in the loopback of my soundcard

Not necessarily. If the ground of the USB interface and ground of the mic pre are connected together, that could be a source of noise. This would be especially true if your computer is grounded to earth ground. For this reason USB audio interfaces are most likely to work better when used with a laptop and best when running on battery. A specifically bad scenario for example would be if your PC has an earth plug and thus the ground of the USB is connecting to the shield of the XLR and then that is connected to the audio ground of the mic pre (because it's not output transformer isolated). Meaning the ground of the computer and audio ground of the mic pre are now connected together. That creates creates an opportunity for differences in ground potentials to develop. The most likely one being that the PC just puts out a bad ground voltage. One time I had a cheap USB audio interface connected to a small tower PC that had several volts difference from the analog ground of a device I was testing with it. I could also measure significant current between the grounds. Switching to a laptop completely resolved the issue. If you have a DMM I can tell you how to check to see if there's significant voltage between grounds. One possible fix might be to disconnect pin 1 from the chassis at the XLR output of the mic pre (AKA "ground-lift").

Grounding related noise issues are very common and require systematic study and understanding to resolve. The best method is to divide and conquer by starting with a minimal signal chain and then adding things until you see the problem. You have not been particularly explicit describing your tests but it sounds like you've narrowed it down to specific mic pres. Are the outputs transformer isolated? If no, you might never get them to work well plugged directly into a USB audio interface because the computer ground is relatively noisy and should not be directly connected to audio ground of devices. The "ground-lift" of a mic-splitter might be necessary.

 

[JohnRoberts]

thanks for the suggestions, yes it is driving me nuts! it is a costant whine in most of my mic pres even with no xlr cable connected.
the thing i've tried so far:

-remove ground connection of the pc. nothing

nothing mean no noise, or not quiet?

-loopback of the soundcard and there is no whine, all is clean.

Not sure what that is?

- shut down everything in the house except my gear. still there

even if you shut down the preamps  ;D (sorry joke)

- some pres don't have this disturbance

that is good news...

Some obvious tests are to experiment with input XLR terminations... try shorting XLR pin 2 to pin 3? Does that make a difference? Maybe try cap coupling shorted 2-3 to pin 1.

JR

the wifi of my router is emitting this same whine, but if i remove it the sound is still there in the mic pres.

i think that if it is a usb ground loop or the switching psu of the pc it could be present even in the loopback of my soundcard, so is it possible that it is a sound picked up from the audio and/or mains transformer?

but the worst thing is that i hear the same sound in my head now  ;D

 

[xazrules]

thanks guys! i'll try to mess with grounds in my pres and lifting pin1 in xlrs and then report back. what if i remove the ground pin in the usb cable to my soundcard?
and what if i change the psu of my pc to a linear one?
another hint, my guitar amp that is connected to a different wall wart don't have this problem.

my rig is comprised of motu 16a, patchbays, diy neve style pres, tab v372 and v672.

 

[xazrules]

nothing mean no noise, or not quiet?

not quiet

Not sure what that is?

connected the output of my soundcard to its input

even if you shut down the preamps  ;D (sorry joke)

still there!! if i shut down the mic pre and add a lot of gain from my daw there's the whine!

that is good news...

yes at least i can record some acoustic gtrs  ;D

JR
[/quote]

 

[xazrules]

another screenshot at 96khz magnifying the interested area, this pre is a clone of a neve 1053 (the germanium transistors one)
i've added a switch to select the mic input (transformer balanced) and line input (unbalanced pin1+3 tied togheter)

with the mic input there's no whine, if i switch to unbalanced line in here comes this beautiful sound in the screenshoot.
maybe i have a dirty earth?

 

[Bo Deadly]

with the mic input there's no whine, if i switch to unbalanced line in here comes this beautiful sound in the screenshoot.
maybe i have a dirty earth?

That narrows it down quite a bit. So try loading the line input with a 100 ohm resistor. Do you still get noise? If yes, then it's upstream like the cable shield or mic body is not actually getting grounded. If no, then try adding the XLR but with 100 load. Etc. You're getting closer.

 

[xazrules]

thank you all! i've solved the problem, it was the DI jack on the front panel badly grounded. anyway i've a very dirty ground in my house, i'm thinking that could be the antenna of the tv