Low End Rumble in Mic

[4tracker]

Thanks for the help, ecc.

This is the Nady 1050 mic that is modded by Jolly. I can look up the specs for the Nady tomorrow, or if you're bored feel free, but it is 2am here and I need to sleep soon. I've been working on figuring this out all day. It could still be something with my setup, so tomorrow I am going to try to go through everything piece by piece, and then I'll upload another sound file for analysis after that.

 

[rob aylestone]

To be safe, find something in your recording area that is connected to ground. Here in the UK, radiator pipes are commonly reasonable grounds, and while the 3 pin outlet is the obvious ground source, there are problems if you get it wrong. If you have a piece of metal clad audio equipment with a 3 conductor power cable, then that will usually be connected to the house ground. This is where you measure from, and touch the other end to the metalwork of the microphone. In a building with many heavy users of power, there may well be a volt or so on the ground of the power socket compared to the 'real' building ground, but in a home far less likely. This is where the old advice to only feed equipment from one phase and one location comes from - making sure ground remains at ground potential. All joking aside, you should only do this if you understand what you are doing.

Playing live, it's common to get a tingle when your lip touches the microphone. In most circumstances it's leakage current that has voltage but hardly any current capability - but it shows things are not quite right. In my won case, getting a tingle almost guarantees that when the PA guys plug into the DI socket on my bass amp they get a hum. If there is no hum, touching the mic is tingle free. Real faults at mains supply voltage, with plenty of current available are VERY dangerous. Up your arm, and across your heart and out of your lips. Plenty of people have died, and there are even some youtube videos of it happening. It's real.

Mains power supplies that have audio level signals passing through them are always prone to safety issues - especially if dropped or damaged or worst still, modified poorly.

I doubt your mods involved changes within the power supply - but a missed offcut of solder, or cable screen can easily float about in the PSU and eventually land somewhere dangerous! Stupid things like misplaced washers, or pop rivet shafts can hide in the innards!

 

[ecc83]

All good stuff Rob.
I have looked up the Nady 1050 and it has the quite respectable but not startling noise specc of 18dB and a S/N figure of -74dB ref 1Pa.

"A" weighted figures to be sure but I cannot see noise or indeed hum being a problem with the mic as sold?

Dave.

 

[4tracker]

Ecc, I am in an apartment complex and my neighbors are being noisy, but I think I got 8 seconds or so of silence. I recorded at -20db on the channel and exported one copy at that level and one at 0db (raised the channel fader to 0, then exported). Which is better for your analysis?

Here are the two files:

View attachment Rumble Hum Test -20db.mp3
View attachment Rumble Hum Test Unity DB.mp3

There are some cars driving by and such and you can hear them at times and maybe even distant voices. I wonder if this distorts the test results. i had the preamp set at around 80-90% gain. Let me know if I should redo this with any of the settings changed. Thanks buddy. I'll also try to do another mic, but these neighbors are all getting louder since it's Friday night. I might have to wait until the middle of the night.

 

[ecc83]

Yes, these clips are more useful but I still do not have a calibrating "blip" from your system!

You can see that in my setup the analyser under reads -1dBFS by about 5 dB? This would put the 120Hz peak at about -80dBFS for the lower level clip. BUT! I cannot be sure because I don't know where -18dBFS is (for e.g.) on your recording when processed thru Right Mark Anny.

However, if we add an A weighting to the clip (anyone know of any weighting software BTW?) that would push the hum down about another 15dB but such a visible 120Hz spike worries me, I suspect poor filtering or/and regulation on the HT or/and heater supplies?

Dave.

 

[4tracker]

How do I give you the calibrating blip? I can do it now maybe it's pretty quiet here.

Attached is another file I just created. This is my preamp with the tube mic plugged into the power supply and preamp but without the mic turned on. Preamp is at max gain.

View attachment High Pitch.mp3

You can hear at the 15 second mark I unplug the mic cable from the power supply, and things get quieter. Then I plug it back in and we hear the noise again. Does this imply the power supply is creating that hum? Maybe this is normal for a power supply to create that noise (?), but I am just trying to isolate the issue.

What is your opinion of the sound file before this? Do you think it's acceptable and normal noise level or no?

 

[jimmys69]

How do I give you the calibrating blip? I can do it now maybe it's pretty quiet here.

Attached is another file I just created. This is my preamp with the tube mic plugged into the power supply and preamp but without the mic turned on. Preamp is at max gain.

View attachment 94009

You can hear at the 15 second mark I unplug the mic cable from the power supply, and things get quieter. Then I plug it back in and we hear the noise again. Does this imply the power supply is creating that hum? Maybe this is normal for a power supply to create that noise (?), but I am just trying to isolate the issue.

What is your opinion of the sound file before this? Do you think it's acceptable and normal noise level or no?

I personally do not hear any abnormal 'bad' things here.

What would be an issue is if any noise was heard during a recording.

I am not even sure what else to say other than continue recording. Use the appropriate gain level from your preamp and make some music.

 

[ecc83]

"How do I give you the calibrating blip?" Just make a noise, "Mary had...." at about -20dBFS (tell me where) then leave the controls alone and let it run for few more seconds.

But, as Jimmy says, those noise figure now seem easily good enough. If you have one, a bit of acoustic guitar at a foot (300mm) would be a pretty stringent test. If the hum is not audible at sensible and original sound levels, you have cracked it!

I really hate to see hum in a system! I think it should be a thing of the past now that chip regulators are so cheap and that 120Hz is 24dB above the surrounding noise floor (there is an approx' 6dB per octave rise in the noise floor ref 1kHz. Not sure what that is but I often see it in peoples clips.)

I tend to agree with James! Time to move on?

Dave.