Low End Rumble in Mic

"I tested a SDC and it's quiet, no rumble.."
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Not the least of which is this.

Ok, quickie test. Get a piece of wire, while listening in phones loud enough, hold one end to the grill, the other to where ever you can bare metal on the case- or the XLR/cable shields etc. If the noise goes down, that question solved.
If not, you've at least eliminated that.

..i think you said you tried dif mic cables etc..
 
This is in the bedroom, which has no appliances at all and is the quietest room I have. It's also 40ft from my recording equipment, so I don't know how I'd actually record vocals. The hum does sound lower, I think? Can the person who graphed it before graph this clip?

View attachment Hum.mp3

---------- Update ----------

mixsit said:
..i think you said you tried dif mic cables etc..
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I tried different 3 pin cables, but I only have the one 7 pin that came with the mic.
 
MJ asked me to wrap the mic and cable end in foil. This is the result with the preamp at max gain. To me it sounds like less rumble. I'm not sure what this implies the problem is. Also, is this acceptable noise for the mic at full gain?


View attachment Hum Foil.mp3
 
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It does strike me that if this recording is with the preamp gain on full, then the hum could really be at a very low level. When you recorded the very first sample with the girl singer, how far was she from the mic? I'm wondering if the actual hum we're hearing is really very quiet, and you just have the input gain cranked right up and the singer was too far away?
 
rob aylestone said:
It does strike me that if this recording is with the preamp gain on full, then the hum could really be at a very low level. When you recorded the very first sample with the girl singer, how far was she from the mic? I'm wondering if the actual hum we're hearing is really very quiet, and you just have the input gain cranked right up and the singer was too far away?
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She was about 8" from the mic, the channel was set at -18dbfs, and the preamp at about 75% gain.

When it's wrapped in foil at full gain, the noise did seem lower. Also I recorded some clips at lower gain on the preamp, and it did seem to reduce noise a bit, but the gain had to be so low that it made the vocal unusable (output was like -30 to -35db)

I'm not sure what to make of it. The SDC mic sounded great on the first try, so I am almost certain it's not the preamp. I've never had issues with it other than being too quiet for ribbon mics, but that's more an inherent ribbon mic issue.
 
Yes there is no reference on the last two clips so it is hard to give any hard answers.

The foil has certainly changed the game! It should not have done by my lights, the mic should be properly screened anyway.

There is the probability that the feed from the mic itself to the PSU is unbalanced and it thus prey to hum fields. It would be interesting to hear from other owners of valve mics about this? I have certainly never read of it being a problem and I have been infesting audio forums for over ten years!

So, we need a noise that gives -20dBFS or so and then silence and the gains left alone. Please repeat with another mic, pfeff' two!

Dave.
 

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Thanks for the graphs, ecc.

What does it mean that the foil reduced the hum? What can we conclude from that? Again, I'm an idiot when it comes to the actual engineering side of this.
 
Geez for crying out loud. Short the grill/case to shield.
W/o the grill grounded... condensers hum
Eliminate it.
Tin foil :wtf:
 
mixsit said:
Geez for crying out loud. Short the grill/case to shield.
Click to expand...

I have no idea how to do that or what it even means.

---------- Update ----------

Something else I notice: if I wrap my hand around the grill, the hum dissipates to a more useable level. I'd say 90% gone. What does this imply?
 
I think I may have solved the problem. I noticed that my 3 prong surge protector was actually connected to a 2 prong extension cord (it had a 3 prong adapter on it). It was hidden behind my work area so I didn't realize. I removed the 2 prong extension cord and plugged the [grounded] surge protected directly into a 3 prong/grounded outlet. The hum seems significantly lower. I'll test it on a vocal tonight. Would that ungrounded extension cord cause this problem?

Also, the Focusrite 18i8 interface uses a 2 prong plug for their preamp. Is this an issue? Maybe I should look into replacing their power supply, too.
 
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No, man, the guys who manufactured your interface knew that it shouldn't make any noise, so, the 2 prong plug has nothing to do with it.

Your mic has something...my call is that is poorly grounded. Try to check the grill...maybe its not completely attached and that usually causes this humming noise.

I had the same problem and all I ever did was re-solder the 3 prong and attach the grill again. And to this day it is completely gone.
 
So this is after switching out the 2 prong extension cord. No processing other than a little bit of reverb, which to my ears is leaving slight artifacts, but since I was testing the rumble I just left it.

https://soundcloud.com/4-tracker-1/hum-fixed

BUT...most importantly...I don't hear the rumble anymore, and I think the mic sounds good now. Any thoughts? Do you guys hear any rumble or issues with the mic?

This one is louder with a little bit of peak limiting on it as well.

https://soundcloud.com/4-tracker-1/hum-fixed-2
 
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Well things are certainly s lot better but as the spectrum of the last but one voice shows (about 20secs of it) the 120Hz hum is still only some 36dB below the average modulation, not very good in my book! I don't like to see hum at all!

Changing to an earthed mains cable seems to have improved matters a lot but the need for the foil is worrying? As Mixit says, in his own elegant way, be good to check various "bits" for continuity!

I have to confess I did not think of any part of the system NOT being connected to mains supply earth. Over here there is only one, fused, earthed plug and outlet. There is equipment that does not need an earth but it is designed to be safe in that condition.
For a "quiet life" almost all audio gear needs to ultimately be earthed.

I do not know of the man or his works but that microphone and its supply do not seem to be made to professional standards.

Note on sound clips if I may? To get then into the analyser it is much easier for me to have an MP3 attachment (.wav would be better but not allowed) ten seconds at the best bit rate (but mono is fine). Stuff sent on Clouds etc I have to first record and then edit and export. Plus, a "whooo" at neg 20ish and then silence is best.

Dave.
 

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You touch the metalwork with your hand and the HUM changes. Not a rumble, it's a HUM. When your body touches the metalwork you extend the grounding area to include your body. If you have a fault and the mic case was live, then that is what the ground pin is for, to provide a low resistance path to ground to save your life, worst case. If it changes the hum when you touch the case that's a good indication grounding is poor, somewhere. Your body enlarges the path for hum and noise to enter the system, and can help or hinder, depends on if you are touching the grounded strings of maybe a guitar. This is where shocks can become a proper problem. Much of the equipment we use separates the electrical safety ground from the electronic ground used for circuit continuity. Linking the two types of ground is considered bad. So linking xlr pin 1 to a metal case can produce hum, or sometimes solve it. If it solves or reduces it then something is still wrong, it shouldn't hum. Your mic hums. None of your connection systems so far have cured it, just reduced it to something you can live with. If I was you, I'd be sticking a voltmeter on the AC range and measuring between so etching really grounded and the case of your mic. Even a volt or two shouldn't be there. If you have tens or worse, hundreds of volts, you really want to know. An old and perhaps stupid thing I did (and sometimes do) is to hold onto something grounded, and then very gently touch the skin on the back of my other hand to the problem device. A tingle is bad. A spark that can be repeated, so as to rule out static, is not good, and a burn very bad news. This is a last resort stupid thing to do. However, a shock through the lips when you touch the mic is even nastier. This really sounds like a poorly designed or constructed power supply. And the hum is probably from the transformer inside the psu. It has two windings in most cases, a low voltage one and a high one. Maybe over a hundred volts or more.
 
ecc83 said:
Note on sound clips if I may? To get then into the analyser it is much easier for me to have an MP3 attachment (.wav would be better but not allowed) ten seconds at the best bit rate (but mono is fine). Stuff sent on Clouds etc I have to first record and then edit and export. Plus, a "whooo" at neg 20ish and then silence is best.
Dave.
Click to expand...

Okay tomorrow I will provide an mp3 upload of what you need to analyze it. Thanks!
 
rob aylestone said:
You touch the metalwork with your hand and the HUM changes. Not a rumble, it's a HUM. When your body touches the metalwork you extend the grounding area to include your body. If you have a fault and the mic case was live, then that is what the ground pin is for, to provide a low resistance path to ground to save your life, worst case. If it changes the hum when you touch the case that's a good indication grounding is poor, somewhere. Your body enlarges the path for hum and noise to enter the system, and can help or hinder, depends on if you are touching the grounded strings of maybe a guitar. This is where shocks can become a proper problem. Much of the equipment we use separates the electrical safety ground from the electronic ground used for circuit continuity. Linking the two types of ground is considered bad. So linking xlr pin 1 to a metal case can produce hum, or sometimes solve it. If it solves or reduces it then something is still wrong, it shouldn't hum. Your mic hums. None of your connection systems so far have cured it, just reduced it to something you can live with. If I was you, I'd be sticking a voltmeter on the AC range and measuring between so etching really grounded and the case of your mic. Even a volt or two shouldn't be there. If you have tens or worse, hundreds of volts, you really want to know. An old and perhaps stupid thing I did (and sometimes do) is to hold onto something grounded, and then very gently touch the skin on the back of my other hand to the problem device. A tingle is bad. A spark that can be repeated, so as to rule out static, is not good, and a burn very bad news. This is a last resort stupid thing to do. However, a shock through the lips when you touch the mic is even nastier. This really sounds like a poorly designed or constructed power supply. And the hum is probably from the transformer inside the psu. It has two windings in most cases, a low voltage one and a high one. Maybe over a hundred volts or more.
Click to expand...

I do have a voltmeter. Are you saying to switch it to AC, touch it to the grill, and see if there are any volts?
 
"I do have a voltmeter. Are you saying to switch it to AC, touch it to the grill, and see if there are any volts? " Yes but you need to also connect to a KNOWN good earth, the chassis of a desktop PC should be such (assuming a 3 core supply to an IEC connector! Cannot get used to a mains supply system where you can CHOOSE to endanger yourself! "We" gave up 2 pin plugs decades ago) .

I have been looking up few valved cap mic speccs and reviews. The noise performance of a modest SDC would be around 20dB, better ones some 12dB. The Rode (FET) NT-1a is around 5dB iirc and so, as you would expext, their valve mic is a really good (for valves) 10dB.

These figures equate to signal to noise values of around -80 to -90dB (reff 1Pa I guess! Mic speccs are notoriously sparse!) and are "weighted" which means the hum contribution is improved but even so I cannot find a review of any valve cap mic that cites hum as a problem.

Fork! "We" can build valve guitar amplifiers with "Hi Fi" noise specifications so no excuses!

Dave.
 
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