Nasty RFI problem with my monitors

[poonna]

It's my ongoing quest to solve this RF interference on my monitors (KRK VXT4). Here's the thing: they pick up radio station whenever I turn them on, and pretty loud at that (clearly audible, almost intelligible). However, when I remove the audio cables, they are dead silent -- no RF interference heard. So I think it's the cables that pick up the RF, not the speakers themselves. Grounds are lifted on my monitors (there's a switch for that at the back of my monitor) so it shouldn't be a ground loop problem. I put ferrite cores around my cables, and that help reduce the interference a bit, but still very audible.

My question is: would changing into a higher quality cable help? Currently it's Canare L-2T2S (twisted-pair microphone cable). Its features include RFI/EMI rejection, so I wonder why I still have this problem.

[Matt Syson]

Hi
Have you tried with the 'ground lift' NOT lifted?
I doubt different cables would make that much difference in your situation as you seem to have a very high field strength in the studio.
I presume you have contacted KRK and asked for help?
If it is OK with ground lift NOT lifted but it then hums, fit a 0u1 or perhaps larger capacitor from pin 1 to the shield of the cable. You may need ferrite cores as well.
I am presuming that it is not coming from the 'monitor controller' whatever that may be?
You could perhaps try then fed from another 'source' a preamp or whatever, it does not have to be a conventional desk or monitor box, just something with a line output.
If it still picks up with ANY line output source then definately yell at KRK and get them to put proper RF filtering on the input to their monitors. Your speaker drivers will not respond beyond 100KHz so a filter should get rid of it.
Matt S

[poonna]

Yes, I've tried both lifted and not lifted. Didn't notice much difference there. I haven't contacted KRK yet. Currently they are connected directly from my audio interface (TC Electronic Desktop Konnekt 6). I guess I'll try another source then, but as the cables are balanced, my other source will need to be balanced as well, right?

[Matt Syson]

Hi
Balanced would be better but unbalanced should be OK. You are trying to 'prove a point' so use as many different bits of kit as you can find. Gear SHOULD be powered up to 'terminate' the input but it is worth trying it switched off as well.
You could even try a dynamic mic. This may sound bizarre and of course you won't get much audio out but the point is that it will 'terminate' the input on the end of some cable.
If you do try a mic, try shorting the casing to some earthed metalwork (rack, PC or whatever) and see if that brings the radio up.
Is it an AM or FM station? I presume it is local?
When identified, try to find where the transmitting aerial is located. If it is on a nearby roof you may have to get into 'desperate measures'.
If you have a shorter cable available to hook between the gear and monitor, try that too.
There are other threads on 'hum' and RF pickup, have you looked at those?
There were suggestions for a guy with problems with a UA610 mic pre.
If several bits of gear and 'any' cable is bringing the problem you really do need to be getting on to KRK, at least for ideas.
Are you aware of any other gear picking up the radio? listen to putputs of mic pre's for example ON HEADPHONES.
Matt S

[poonna]

Thank you very much. Seems there are lots of ideas to try out. I'll get to them and post my results again here.

[poonna]

Here are what I have tried so far:

1. Plugging a dynamic mic to the speaker using a different cable. There's still RFI, without touching the mic to a metal case. I don't hear any such problem through headphones through the preamp on my interface.

2. Plugging different sources to the speaker. I used an unbalanced cable here because I don't have other balanced sources. I have tried a music player, a hand-held game (Nintendo DS), and headphone output from my laptop. The first two exhibited RFI. As for the headphone output from my laptop, the hum was way too loud that I wasn't sure if there was RFI or not. I don't have other speakers lying around here, or I would have tried that as well.

3. Shorter cables seem to produce lower RFI noise, but still audible nonetheless. I can't get much shorter than that anyway due to speaker placements. My current cables are 2 meters, and I believe I can't get them shorter than 1.2-1.5 meters.

4. I tried emailing KRK (service@krksys.com), but my email was immediately rejected with "no such user" error. I'm trying other contacts now.

If nothing helps, I wonder if an in-line RF filter like this one http://store.shure.com/store/shure/en_US/DisplayProductDetailsPage/productID.104211000 will help, given that I have already used ferrite cores, and there's still audible RFI.

[poonna]

One interesting observation, not sure if this helps:

The volume knob on the monitor has no effect on the RFI. It sounds equally loud no matter what position the volume knob is.

[ulysses]

What happens if you don't have anything connected at the source end of the cable? Still hum?
And how about with nothing connected at the source end of the cable, but with pins 2 and 3 shorted together (or tip and ring, depending on the connector)?
Or with pins 1, 2, and 3 (or tip, ring, and sleeve) all shorted together?
(you can briefly connect these pins together with a piece of wire or aluminum foil)
Lastly, unplug the cable and use a multimeter to check continuity from one end of the cable to the other, and from one pin to another (make sure your shield is connected all the way through, make sure there's no internal shorts from one conductor to another).

[Matt Syson]

hi
Are we right in thinking both monitors are acting the same way, both have radio on them?
Matt S

[poonna]

Originally Posted by ulysses

What happens if you don't have anything connected at the source end of the cable? Still hum?
And how about with nothing connected at the source end of the cable, but with pins 2 and 3 shorted together (or tip and ring, depending on the connector)?
Or with pins 1, 2, and 3 (or tip, ring, and sleeve) all shorted together?
(you can briefly connect these pins together with a piece of wire or aluminum foil)
Lastly, unplug the cable and use a multimeter to check continuity from one end of the cable to the other, and from one pin to another (make sure your shield is connected all the way through, make sure there's no internal shorts from one conductor to another).

All these -- unterminated cable or shorting -- still produce radio station reception.

I've just checked the cable with a multimeter and it is fine. All the pins are properly connected and there's no internal shorts.

I'm wondering... If the volume control on the monitor has no effect on the loudness of the RFI, then does that mean the RF reception goes from the cable, somehow bypassing the attenuator/volume control, directly into the power amp section?

[poonna]

Originally Posted by Matt Syson

hi
Are we right in thinking both monitors are acting the same way, both have radio on them?
Matt S

Yes, both monitors have radio on them. However, the placement of the cable has some effect on the amount of RFI. There's nowhere I can find to place the cable that will not pick up RFI though.

[Matt Syson]

Hi
Sounds like you have it BAD. You really do need to be talking to KRK about this (or dealer).
Power amps can and do demodulate radio and since adjusting the volume on the monitors themselves does not affect the level much or at all it suggests it is a 'power amp fault'. Either an actual 'fault' or possibly poor design with respect to RF rejection.
The remedy could be to put ferrite beads or an 'air wound choke' on the wires to the driver(s) but this should be done under direction of KRK.
Personally I would try slipping a choke or ferrites in there (just as a test) but you would have warranty issues, it depends how you are situated and want to play this.
Good luck
Matt S

[Time Tech]

Are both Speakers plugged into the same AC power outlet? Do you have any EMI RFI noise suppression on your AC power? I have seen this with guitar amps, unbalanced cords and moving the player around. We solved it by plugging the amp into another AC outlet. stike

[poonna]

Originally Posted by Time Tech

Are both Speakers plugged into the same AC power outlet? Do you have any EMI RFI noise suppression on your AC power? I have seen this with guitar amps, unbalanced cords and moving the player around. We solved it by plugging the amp into another AC outlet. stike

Yes, they are plugged into the same AC power outlet. The outlet has a noise suppression circuit, but I don't know how well it works. However, when I tested them, I also turned off one speaker, so it shouldn't have an effect on the other one.

Anyway, big thanks to everybody for all your help. I think I'll have to deal with KRK dealer about this issue. However, I think my monitors are out of warranty by now. When I first got them 2 years ago, I was in my old place and there was no RFI problem. It's just this new place I moved to that causes all this problem.

[Radioman]

Try to look at this

radio interference in studio monitors

[poonna]

Originally Posted by Radioman

Try to look at this

radio interference in studio monitors

Thanks! I guess that's the only way then...

[Matt Syson]

Hi
They obviously fail basic FCC regulations in the respect that 'out of band' interference compromises their operation. There is NO reason why the amplifiers inside the box need to responnd to anything above say 50KHz (you can't hear it and the drivers won't produce it) so should be 'rolled off' accordingly.
It would be interesting (to me at least) if adding a ferrite ring to the driver's wiring stops the radio which would suggest it is getting in the 'back end' or whether it is just generally 'bad'.
Gluing some cooking foil inside the box sounds like it is at least a partial solution but hey aren't you glad they are cheap speakers?
Matt S

[scharf]

This is my first post to gearslutz: I have been reading for a while.

I bought the KRK VXT4 monitors almost 3 months ago. I really like them when they are not playing several radio stations and all of the iphones in the building: seriously loud iphone 3G noise when the devices are 20 feet away!

I contacted KRK and they said this:

Brian,

Sadly this is a problem we are still working on. I will forwarded your information to our engineers for further help.

I didn't hear anything and sent another email. They said this:

They have a list of people to contact when the fix is active.

Another response from a different guy (Erik) at KRK included this:

I recommend people simply keep all offending smart phones about 6-10 feet away.

"Okay clients, before we enter the "C" room I am going to need to confiscate all of your mobile electronic devices, otherwise the near-fields may start speaking in tongues."

BTW - Erik offered that he might have other suggestions, but I am still waiting for his solution.

Maybe if I wrap myself in aluminum foil ...

[Matt Syson]

Hi
To get this problem sorted should take a couple of months MAX, to do tests, redesign some circuitry and new PCB layouts with maybe a second 'iteration' to get it nailed completely.
OK if KRK is one guy working in a shed it may take a week or two longer.
Why no fix yet?
Matt S

[Radardoug]

I have looked at some KRK speakers, not too impressive in the electronic department. The fact that the volume control has no effect indicates that the cable is merely acting as an antenna conducting r.f. into the cabinet, and the amplifier is doing all the detecting and amplifying. None of their amplifiers are sheilded. Possible fixes include sheilding the cabinet using copper or aluminium foil, and then filtering on the input connector to short the r.f. to ground. have you tried two things: 1/ a 10k:10k transformer such as a Jensen just before the monitor, with caps to ground on the input XLR? Also wrap the cable through an interference blocking toroid or similar core before the caps. Use foil sheild Belden rather than wire sheild, better r.f. rejection.

[h4nc0]

To OP,

Have you solved your problem? I have exactly same problem with my new monitors. Not KRK, but Geithain RL922K. No radio interference when there's no audio input cable connected. As soon as I plug a balanced XLR cable into the monitor, I can hear radio station faintly from the tweeters. I've tried different cables and results are the same.

Any idea?

Thanks

[poonna]

Originally Posted by h4nc0

To OP,

Have you solved your problem? I have exactly same problem with my new monitors. Not KRK, but Geithain RL922K. No radio interference when there's no audio input cable connected. As soon as I plug a balanced XLR cable into the monitor, I can hear radio station faintly from the tweeters. I've tried different cables and results are the same.

Any idea?

Thanks

I haven't solved the problem yet. The last thing I haven't got around to do is open it up and cover the inside with foil.

[h4nc0]

Originally Posted by poonna

I haven't solved the problem yet. The last thing I haven't got around to do is open it up and cover the inside with foil.

But since the audio cable is acting as antenna in *our* cases, I wonder how that will help. Not only that, I don't really want to put foil in my speakers as they costed a fortune. My dealer is sending me another pair of cables that are wired differently and if that doesn't solve the issue, I think I am returning these speakers. :(

[Matt Syson]

Hi H4nco
When you have a cable plugged in and get the noise, is there anything on the other end of the cable (mixer, monitor controller or whatever) or just an 'open' cable?
If it is simply an open cable then it is acting as an aerial as mentioned previously.
Are you close to the received radio station's transmitting aerial?
I would have a go with ferrite rings or RF filters on the cables as close to the monitor amp/speaker as possible.
More experimentation needed I am afraid.
Matt S

[brianroth]

Gentlemen,

I also am curious if this is a "classic Pin 1" problem....

Best,

Bri

[poonna]

I've used ferrite rings with my KRK. Well, I think they help, but only a little bit. So I think, for my KRK, it's not just the cables that act as antenna, but the shielding inside the cabinet is poor as well.

[Dan Kennedy]

Have you put ferrites on the AC inlet cable as well? It's another antenna path
into the box!

[poonna]

Originally Posted by Dan Kennedy

Have you put ferrites on the AC inlet cable as well? It's another antenna path
into the box!

Just tried, but didn't help...

[h4nc0]

Originally Posted by Matt Syson

Hi H4nco
When you have a cable plugged in and get the noise, is there anything on the other end of the cable (mixer, monitor controller or whatever) or just an 'open' cable?
If it is simply an open cable then it is acting as an aerial as mentioned previously.
Are you close to the received radio station's transmitting aerial?
I would have a go with ferrite rings or RF filters on the cables as close to the monitor amp/speaker as possible.
More experimentation needed I am afraid.
Matt S

Happens when the cable is "open" or connected. Doesn't make any difference. Just got a new pair of cables from the dealer and it made absolutely no difference. I have no idea where the radio station's transmitter is located. I've talked to the dealer and he suggested ferrite rings, putting transformers, etc. but I feel pretty bad because these speakers are not cheap exactly.

[Matt Syson]

Hi
Is it only 1 radio station (difficult to tell I know!)?
Sounds like these should be going back to the dealer / manufacturer however, contact the manufacturer first and get their take on this. At least they should send you some ferrite rings / filters or whatever. If no joy, send them back.
Matt S