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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
 

If the speaker keeps humming no matter where it is in the room it's not an EMF field.  Do the amp measurement.

daielf OP
9 posts
 

The DC output is something I considered and tested for when troubleshooting the phono hum. I purchased the DC blocker adapter that plugs between the IEC socket and power cable, no change. The Puritan Mains conditioner does a great job at filtering all nasty stuff coming in the power line, not that it's that bad to begin with. Measured the EMI/RFI interference out of the Puritan sockets and it's between 11-16mv or 0.011v - 0.016v seems negligible.

 

Dealer told me about a speaker he owned a few years back that kind of had similar issues with humming. Took him a while to find out the cause: oil-filled caps leaked and shorted or corroded something.

Tried measuring with a multimeter, it's super basic model, and it's measuring no DC current at the mV setting. So either my amp is 100% not leaking any DC (unlikely), or the meter is misleading, which what I think is happening.

 

daielf OP
9 posts
 

Sorry Eric, the timeline of the posts have me answering in reverse order.

If I un-short the speaker the hum comes back, whether in the listening room or elsewhere, doesn't matter. If it's EMF and a wonky something in that speaker only, then yeah your explanation fits the bill. The only thing is, that the right speaker exhibits none of this, connected to the amp or not, so that kind of excludes the amp outputting DC, no?

I unplugged every piece of gear that emits WiFi/Bluetooth (Playstation, Apple TV, TV, Router, Philips Hue/Bridge, and even disabled the WiFi on my AV receiver, but it didn't seem to do anything with the hum. The phono is canary in the coalmine so to speak, it's amplifying whatever is being emitted by the speaker. Many orders of magnitude louder than the original hum level.

erik_squires
16,056 posts
 

Leave it shorted for 30 minutes. If the problem comes back after removing the short it’s EMF from somewhere, and also damn weird!

If it vanishes and stays vanished it’s more likely to be a DC charge happening (could also be a combination of DC + wonky crossover).

Since it doesn’t matter where in the house it is, or how it’s turned around it is unliekly to be EMF though.

In any event, spend $40 on a multimeter and measure the DC on your amp outputs.

 

erik_squires
16,056 posts
 

OP:

 

I think you misunderstand me.  DC on the AC input is very different. DC on the AC line causes mechanical transformer hum in the amp. 

DC on the amplifeir's output is caused by improper biasing or leaky coupling caps.  Measure the amp's output terminals.  DC should be near zero. 

Any significant DC could charge the crossover caps and cause oscilation when the input is removed.

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