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Focal Kanta No. 2 Speakers hum with speaker with speaker cable disconnected

Hi folks,

I'm a new member, and I couldn't find a solution for the specific issue I'm having on google. I'm thinking maybe someone here has any ideas on this weird issue.

As the title states I've stated noticing a hum emanating from the speaker's mid range woofer, specifically the left speaker. I only noticed it after upgrading my phono stage, which had a hum issue from the get go (not a ground loop or line hum evidently). Took about 2 months for my dealer and Pass Labs to send me a replacement unit. Long story short, same hum is still there, so it obviously wasn't the phono, though rotating the phono 90 degrees relative to the speaker reduced the hum. Did a bit of cleaning last week and noticed that the left speaker was humming at a very low volume, only noticeable when your ear is close to the woofer. The preamp, phono, power amp, and turntable are all turned off, yet I'm getting a hum that's unexplainable.

I disconnected almost all of my components' power and interconnect cables (Pre/Pro, Phono, DAC/Streamer, Turntable power supply) one by one hoping that any source of interference/hum could be identified. I also disconnected a separate power strip that powers the Router, TV, Philips Lights, and Apple TV with no change in the hum. For reference, my audio gear is fed by a Puritan Audio PSM 1512 mains purifier, pretty clean power.

For some reason I decided to disconnect the speaker cables to switch them around and there I noticed that the speaker is still humming very faintly with no speaker cable connected. My thinking at that stage that it might be some sort of wireless interference, so I unplugged anything that has bluetooth/wireless functionality. All of the audio components were disconnected from power as well. I noticed then that it's the exact same hum that the phono has been plagued wit all this time, but amplified at a much higher level. Maybe the phono cart is picking up this minute hum and sending it to the phono. I left the speaker for an hour to see if it discharged any of the crossover components that might be causing this, nope, still humming. 6 month old speakers so I'm thinking it's unlikely a bad cap, although possible.

I'm really out of ideas on how to sort this out. I did experiment with grounding wire paths, and ground lift adapters/DC blocking adapters when I thought the issue was with the phono stage, was not successful. Anyway any thoughts or ideas would be greatly appreciated.

 

 

 

daielf
erik_squires
16,056 posts
 

PS: It’s most likely an inductor, or coil, is acting as the antenna. They couple to magnetic fields better in some alignments than others. That’s why crossover designers will often turn the coils on the board in different directions. If you think of the coild’s direction as a pencil going through the center, there’s up, horizontal and vertical directions on a board. Here’s what I mean:

https://images.audiojudgement.com/2017/09/IMG_3455-min-1100x825.jpg

 

So you can see the desinger used all 3 directions to minimize coupling. If you have a strong EMF field, turning the crossover around (well, your speaker) should change how well it is coupling to the EMF. Of course, the drivers themselves are coils... < sigh>

Image from audiojudgement

erik_squires
16,056 posts
 

PPS - I'm assuming this issue is happening to a well made speaker.  If you swap the speakers, the new speaker in that place should develop the same problem, if not, it's possible you have something funky in the speaker that makes it susceptible.

I once had a Focal speaker with a broken inductor wire, for instance.  It was nearly impossible to see and only impedance testing revealed what was going on.

erik_squires
16,056 posts
 

PPS - Of course, remove the shorting wire when you do these tests. :) 

daielf OP
9 posts
 

Fair enough... It might be too early to start celebrating. Though I did move the Left speaker to a different spot in the house, far away from the listening room and the hum was still there but disappeared when shorted. No high-voltage or transmission lines within a few miles of the house. No radio towers/cell towers close by either, and I believe they operate at a much higher frequency range (MHz/GHz) than the speaker can pickup. If that was the case, wouldn't both speakers be affected equally? Right speaker hasn't been moved at all and doesn't hum, whether connected to amplifier or not. No large electrical equipment.

I'd like to point out that the hum is not heard at all unless your press your ear to the woofer/tweeter, even then it's very faint. If it didn't cause any interference with the phono, I wouldn't have noticed or cared about it. Unfortunately the quieter/more sensitive the phono is, the more these issues are noticeable/annoying.

erik_squires
16,056 posts
 

Maybe I misunderstood.

If you unshort the speaker, does the humming come back? If so it’s an external EMF field, if not, you are right, there’s some weird ringing in the circuit. If that’s the case the best thing to do is have it examined by a pro. If you feel like digging in though you could purchase a DATS V2 from Parts Express and compare the impedance traces of both speakers. My guess is one will be wildly different.

And no, this isn't a leaky cap.  It's the opposite.  It's a cap that can't discharge.  In an amp power supply you want your caps to HOLD charge.  In a speaker you most certainly do not.  Any charges should be dissipated immediately after the signal passes. 

Power supply reserve caps are buffers.  Speaker caps are filters.  Very different expected behavior.  The only way I could see this happening is if a cap had a poor connection somewhere and therefore gets charged, but not released. 

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