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Condenser Mic Humm
Old 2nd March 2014
  #1
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Condenser Mic Humm

Hi all,
Complete newbie to home recording here (weeks), first post so please be gentle. Not sure whether to post here at newbies or in DAW Forum.

I have a decent laptop, running Cubase 7 essentials, to Scarlett 2i2 interface and Truth B1031A monitors. Managed to put a couple of simple midi tracks down and now experimenting with vocals. Got 2 mics 1 dynamic, 1 condenser.
Dynamic mic is fine, problem is background hum in condenser mic. There is no hum in the dynamic so it's coming up the 48v power supply to the condenser mic from the audio interface. It's either being generated by the audio interface or originating from the laptap which USB connects and powers the audio interface. I think its from the laptop as the noise changes and decreases slightly when I unplug the laptop and just run on batteries.
Is there a USB piece of gear I can place between the laptop connection and Audio interface to clean up the hum somehow?
Cheers,

Last edited by goosecat; 2nd March 2014 at 01:45 PM.. Reason: newbie
Old 2nd March 2014
  #2
Need more details:

What is the brand and model condenser mic which has the hum problem?

Have you tried a different mic cable on the condenser mic? If it's a conventional condenser connected with a known-good, balanced XLR/XLR cable there is little possibility of ground loop hum coming from your laptop. Did you try the cable you used on your dynamic mic?

Have you tried another condenser mic?

What gain setting are you using relative to the dynamic mic?
Your phantom powered condenser should be producing a much stronger signal (typically 15 to 25 dB stronger) than a typical dynamic mic and should operate with the mic gain at a much lower setting. If you are using a gain setting that is similar to, or higher than what the dynamic mic requires, you may have a defective condenser mic (or mic cable).

The interface to laptop USB cable carries only digital data and no analog audio. It does make a ground connection to the interface, but unless the mic is grounded externally somehow, there is no ground loop path through the mic possible to cause hum.

It is possible that the +48 volt phantom power supply is bad in the Scarlett 2i2, but it's a high-frequency DC/DC converter. If bad, it should not cause "hum".

Does the hum disappear instantly if you are listening to it and turn off the "48V" phantom power switch on the Scarlett 2i2?
Old 3rd March 2014
  #3
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Quote:
Originally Posted by Lotus 7 View Post
Need more details:

What is the brand and model condenser mic which has the hum problem?

Have you tried a different mic cable on the condenser mic? If it's a conventional condenser connected with a known-good, balanced XLR/XLR cable there is little possibility of ground loop hum coming from your laptop. Did you try the cable you used on your dynamic mic?

Have you tried another condenser mic?

What gain setting are you using relative to the dynamic mic?
Your phantom powered condenser should be producing a much stronger signal (typically 15 to 25 dB stronger) than a typical dynamic mic and should operate with the mic gain at a much lower setting. If you are using a gain setting that is similar to, or higher than what the dynamic mic requires, you may have a defective condenser mic (or mic cable).

The interface to laptop USB cable carries only digital data and no analog audio. It does make a ground connection to the interface, but unless the mic is grounded externally somehow, there is no ground loop path through the mic possible to cause hum.

It is possible that the +48 volt phantom power supply is bad in the Scarlett 2i2, but it's a high-frequency DC/DC converter. If bad, it should not cause "hum".

Does the hum disappear instantly if you are listening to it and turn off the "48V" phantom power switch on the Scarlett 2i2?
Thanks for the reply.
The condenser mic and cable are a brand new set from Scarlett. They do a basic first home recording set. The mic is the Scarlett Studio CM 25 attached via new supplied xlr cable. The gain setting is hugely reduced for the condenser mic, even down to zero I can still here the hum. I thought the hum might not pass through to the recording in Cubase but it does.
The laptop USB interface carries digital signal and "power" to the interface, that's why I thought it might be the laptop power source issue. The hum definitely changes when I plug in or unplug laptop from mains but still remains at a constant low hum on laptop battery power.
As soon as I push the 48v button off all sound goes off immediately.
I noticed when I push the 48v button on however the humm starts up at a higher frequency before sliding down to a constant low frequency over a period of about a second, almost like a monitor starting up. (It's a bit like a slide that I can't work out how to do in Cubase, someone should do a tutorial on that).
It's not a dirty or crackling ground loop buzz. It's a clean distinct low frequency hum.
Really I need to get another interface and condenser mic and do some trialling but I just don't have access and don't really want to buy more.
Cheers

Last edited by goosecat; 3rd March 2014 at 12:03 AM.. Reason: error
Old 3rd March 2014
  #4
Quote:
Originally Posted by goosecat View Post
...The gain setting is hugely reduced for the condenser mic, even down to zero I can still here the hum....The hum definitely changes when I plug in or unplug laptop from mains but still remains at a constant low hum on laptop battery power.

Quote:
Originally Posted by goosecat View Post
As soon as I push the 48v button off all sound goes off immediately.
I noticed when I push the 48v button on however the humm starts up at a higher frequency before sliding down to a constant low frequency over a period of about a second, almost like a monitor starting up.
Usually,when someone mentions "hum" we immediately think of line frequency related noise either at 60 or 120 Hz in North America or 50 or 100 Hz. in the rest of the world. Since your's changes in frequency it's probably mic or phantom power supply generated.

Some mics can generate low frequency oscillations when first supplied with phantom power, but they should disappear within a few seconds unless the phantom power supply is defective.

The Scarlett mic obviously should be compatible with the Scarlett 2i2. The change in the "noise" ( I won't call it 'hum") when you switch from battery to AC mains power may be due to a slight voltage change on the USB supplied power if it is a weak phantom power situation.

Without having another mic to substitute, it's difficult to isolate the problem to the mic or the interface. It is possible to test for the source of the "low frequency noise", but would require measuring the phantom power voltage under load (mic connected) and checking for any oscillation [on the mic cable] with an oscilloscope.

Since the hardware is new and is all Focusrite gear, I'd strongly recommend that you contact them and report the situation. It's possible that they may have run into it previously with this set of equipment.

I have seen and heard low and high frequency sound generated by phantom power supplies which are either overloaded or operated very near their maximum current capacity. The 2i2 should easily have enough current capacity to run one mic, especially a mic sold by Focusrite. However, the mic could be defective and be drawing more current than it should, or the phantom power supply in the 2i2 may be defective and simply can't provide the current it should.

It's conceivable that your USB port voltage is a little low (it should never drop below 4.9 volts) due to a bad regulator or that you have a high-resistance USB cable. Try a different one as short as possible if you can. Measuring the USB voltage without the interface plugged in is of no real value. It must be measured with a the full load of the 2i2. If you can, try the interface/mic combo on a different computer using a different USB cable.

I actually own a Tascam DR-100 portable recorder which produces a mid frequency oscillation (a 500 to 1kHz tone) if two moderate current mics (3 mA) are connected, but works fine if only one is used. It's apparently just a weak phantom power supply in that model recorder. To use it with most mics I have to use an external battery powered phantom power supply and leave the internal one off.
Old 3rd March 2014
  #5
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Quote:
Originally Posted by Lotus 7 View Post
Usually,when someone mentions "hum" we immediately think of line frequency related noise either at 60 or 120 Hz in North America or 50 or 100 Hz. in the rest of the world. Since your's changes in frequency it's probably mic or phantom power supply generated.

Some mics can generate low frequency oscillations when first supplied with phantom power, but they should disappear within a few seconds unless the phantom power supply is defective.

The Scarlett mic obviously should be compatible with the Scarlett 2i2. The change in the "noise" ( I won't call it 'hum") when you switch from battery to AC mains power may be due to a slight voltage change on the USB supplied power if it is a weak phantom power situation.

Without having another mic to substitute, it's difficult to isolate the problem to the mic or the interface. It is possible to test for the source of the "low frequency noise", but would require measuring the phantom power voltage under load (mic connected) and checking for any oscillation [on the mic cable] with an oscilloscope.

Since the hardware is new and is all Focusrite gear, I'd strongly recommend that you contact them and report the situation. It's possible that they may have run into it previously with this set of equipment.

I have seen and heard low and high frequency sound generated by phantom power supplies which are either overloaded or operated very near their maximum current capacity. The 2i2 should easily have enough current capacity to run one mic, especially a mic sold by Focusrite. However, the mic could be defective and be drawing more current than it should, or the phantom power supply in the 2i2 may be defective and simply can't provide the current it should.

It's conceivable that your USB port voltage is a little low (it should never drop below 4.9 volts) due to a bad regulator or that you have a high-resistance USB cable. Try a different one as short as possible if you can. Measuring the USB voltage withot the interface plugged in is of no real value. It must be measured with a the full load of the 2i2. If you can try the interface/mic combo on a different computer using a different USB cable.

I actually own a Tascam DR-100 portable recorder which produces a mid frequency oscillation (a 500 to 1kHz tone) if two moderate current mics (3 mA) are connected, but works fine if only one is used. It's apparently just a weak phantom power supply in that model recorder. To use it with most mics I have to use an external battery powered phantom power supply and leave the internal one off.
Thanks again.
Yeah I think it's time to contact the supplier. I just know how long a process that's more then likely going to be. I even grabbed the laptop, interface, mic and cans and took it to another room entirely with no AC power to see if that made a difference and got the exact same result. It can only be Laptop, mic or interface.
Going to music shop later and gonna ask if I can borrow a mic (they do lessons as well out the back so they should have some used ones around).
That will hopefully narrow it down some.
Then it's time to contact Focusrite.
Thanks for the help.
Old 3rd March 2014
  #6
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Try a new short USB cable.
Old 3rd March 2014
  #7
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Quote:
Originally Posted by ghellquist View Post
Try a new short USB cable.
The current short USB cable is also brand new and came with the audio interface and mic so it should be good. I'll try it though.
Old 5th March 2014
  #8
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FYI.
Found a great tech at a big music store (who I was gonna plug for helping me but won't cause he gave me some info Focusrite might not like to hear). Went through my "noise" issues with him and he diagnosed it over the phone. Very confident it was neither laptop, mic, cabling or groundloop but the interface itself and the generated 48v power. He even offered for me to bring in my laptop, mic and cans etc exactly as is and switch in a brand new 2i2 to compare for the noise. Sure enough his 2i2 had no issues, mine faulty. He's a supplier and supporter of Focusrite but reckons his last order of 2i2 units some months ago from Aussie distributor was a bad batch. Had numerous warranty claims he doesn't normally have. Has ordered a new batch (one of which we tried and had no problems) and hoping it was a one off bad batch for some reason. Reckons mine came from the same batch.
Couldn't warranty it for me as I didn't purchase from him but brought it from Eastern states supplier (Ebay).Guess I just got unlucky.
Here's hoping the support/warranty here in OZ is OK.
Thanks all

Last edited by goosecat; 5th March 2014 at 03:02 PM.. Reason: mistake
Old 5th March 2014
  #9
Yep, from your description, the phantom power source in the interface was way up on the list of probable causes.

Unfortunately, in USB powered interfaces the amount of total power that is available to the complete interface is strictly limited to 2.5 watts. That must power the A/D and D/A converters. the USB communications chip, the mic pres, the line-driver output amplifiers, the headphone amplifiers and the phantom power supply. Usually some corners are cut and the most likely areas to have possible problems are the headphone amps, and the phantom power supplies.

A mains-powered 2-channel interface will commonly use 3 to 5 times more power than a USB powered interface because the designers are not constrained by the USB port's limited power.

The same issues can happen in small, battery-powered recorders because of the limited power available from the battery, as in the case of the Tascam DR-100.
Old 6th March 2014
  #10
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Sounds like you found the culprit.

The reason I suggested a new USB cable is because it looked like power problems. It is not uncommon for laptops to be slightly low on the 5V and then more again is eaten by bad USB cables which are abundant.

Suggested solutions or to be tested if you cannot exchange the interface:
- external powered USB HUB (higher voltage at the sending end).
- extra high quality very short USB cable (less voltage loss in the cable).
- external phantom power supply between the mic and the USB interface (less power draw in the USB interface and not running the phantom power circuits) .

// Gunnar
Old 8th March 2014
  #11
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Quote:
Originally Posted by ghellquist View Post
Sounds like you found the culprit.

The reason I suggested a new USB cable is because it looked like power problems. It is not uncommon for laptops to be slightly low on the 5V and then more again is eaten by bad USB cables which are abundant.

Suggested solutions or to be tested if you cannot exchange the interface:
- external powered USB HUB (higher voltage at the sending end).
- extra high quality very short USB cable (less voltage loss in the cable).
- external phantom power supply between the mic and the USB interface (less power draw in the USB interface and not running the phantom power circuits) .

// Gunnar
Thanks,
Might look into the external powered USB Hub as I'm just starting out and could see a use for that in the future.
Focusrite actually responded to an e-mail from me with all the details really quick and said they would repair any fault. Sent it back to them yesterday. Will see how it goes.
Old 8th March 2014
  #12
Sounds like they have a known issue with the phantom power supply. I haven't seen a schematic of the 2i2 PP supply, but typically these interfaces us a small high-frequency oscillator and an AC-coupled voltage multiplier to avoid having to use a transformer and to keep the filter caps as small as possible. There are many reasons why a such a circuit can fail. They may have simply received a batch of bad coupling caps or filter caps. Having audible noise when using only one mic means something is seriously wrong. The 2i2 has (2) mic pres and the PP should be able to supply enough current for each.

The IEC standard for phantom power (IEC 61938) recommends that a 48 Volt PP source (the standard also covers other possible voltages) should be able to provide up to 10mA of current for each microphone. However, a manufacture does not have to meet that recommendation to claim they have PP on their mic inputs, and many phantom powered mic inputs cannot provide the full 10mA. They can "get away" with this because today, most mics only require 2 to 5 mA.

The published specifications for the Saffire 2i2 only mention that "it has a "48 Volt Phantom Power switch" Focusrite, makes no mention of how much current their phantom power supply can provide. I suspect that most small USB-powered interfaces can't come near providing the full 10mA on each mic input.
Old 18th March 2014
  #13
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Just bought the Scarlett Studio yesterday. Same issue. All the gear works great with an sm58 but can't get rid of the hum on the cm25. Running on i3 Toshiba laptop plugged in to mains. Will attempt other USB ports and computers to see if they fair better. Thank you for posting your issue and let me know how it goes. Cheers! - Noah
Old 18th March 2014
  #14
Sounds like it's a production defect on a batch of 2i2's and/or Studios.
Probably best to contact Focusrite, send it back right away and get a new one in exchange. There is no excuse or cure for a "whimpy" phantom power supply.
Old 18th March 2014
  #15
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I have just bought a used Scarlett CM25 (electret) mic.
Used it on my mixing desk...(don't have the interface)
None of my other mics have a hum problem, but this mic does.
Not a cable or grounding problem.
Still have to delve deeper into this, but it seems to me that this mic is not balanced properly (maybe parts tolerance).
Leo..
Attached Thumbnails
Condenser Mic Humm-cm25.jpg  
Old 18th March 2014
  #16
Leo,

Does the mic truly have "hum" (noise that is being picked up at the local mains frequency at 50 or 60 Hz), or does it exhibit the sound the O.P of this thread called "hum" which was, in fact, a phantom power related variable frequency noise that was created in his Scarllett 2i2 interface and was corrected by replacing the 2i2 interface. See post No.8

I fear this thread is covering two distinctly different issues, because of the title and the O.P's. use of 'hum" for his phantom power noise issue.

The O.P.'s issue was corrected by exchanging his 2i2 and didn't seem to be a mic imbalance issue.

I would not be surprised if the Scarlett mic is unbalanced, but that apparently was not the problem the O.P. was experiencing.
Old 18th March 2014
  #17
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Hi Lotus 7.
Should have explained a bit better.
This CM25 mic, connected to a cheap Tapco mixer, has background supply hum, rattle, noises etc.
Probably coming from the phantom supply.
I think this cheap mixer is very sensitive to un-equal loading of XLR pin 2 and 3.
It is noise free with e.g. a Rode Nt-1.
Gremlins I have to work out some day when I have time.
Leo..
Old 26th March 2014
  #18
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Just a quick update on the original OP faulty 2i2 audio interface.
Focusrite were very quick to respond and warranty it for me. Mailed it to their warranty tech shop in Eastern Australia and they returned it 2 weeks later.
Plugged it in and problem has been fixed. No word on what exactly caused the issue within the unit but the service was pretty good.
It did cost me $20.00 in postage which you would think on admitting fault they would cover but they wouldn't go that far.
Generally good warranty backup though.
To clarify my problem was a definite 48v phantom power issue within the audio interface.
Cheers.
Old 18th April 2014
  #19
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I've had the Scarlett Studio package 2i2 with CM25 mic with the same hum problem.
Old 22nd May 2014
  #20
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I returned the 2i2 unit to the store I bought it from. They sent it to Focusrite for repair. Focusrite returned it to the store after 2 weeks saying no problems found. The store contacted me to let me know but said that they can hear the hum and have sent me a new 2i2 unit to me, which works perfectly without any hum. The store intends to pursue it with Focusrite.
Old 22nd August 2017
  #21
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I had the same issue with an MXL 991 yesterday when it was plugged into the Scarlet 2i2. I swapped it with an SM81 and it went back to working fine. I believe you may have a defective microphone component as I did.

Last edited by Azotosome; 22nd August 2017 at 10:14 PM.. Reason: bad grammer
Old 1st October 2017
  #22
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I'm using one of the cheap bm800 and I had this issue that it creates a low hum and was quite noticeable on my youtube videos. I tried lots of fixes including replacing the cable. After I replaced the cable it worked better but then came back. Giving the connector a good push into the mic seemed to fix it completely now. You can measure it on audacity by pressing record and seeing the idle wave nice and small, preciously it was almost 1/2 cm. Happy now I don't need to buy a new one!
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