Loud background noise: cables picking up RF?

Jea48: After looking through my own copy of ARRL's Radio Frequency Interference: How to Find it and Fix it, (sec. 2-8, 22-11, and 3-18) and looking at the schematic of a couple power amps I tend to agree that it is possible to inject RFI via speaker leads. What was troubling me was the mode of getting it through the amplifier. It appears that this is possible via the feedback path all the way to just past the differential stage. Having been a Ham all my life I cannot say that I ever had any complaints from my neighbors or family when I was transmitting but it certainly appears that it is possible to feed the RF back through the amp in this manner. I would think however, that the active components would end up detecting the modulating signal and you would end up with voice at the speakers, at least in the Ham setup (unless of course transmitting CW).

09-14-08: Dawgcatching
Is there any way I can check for a ground loop w/o buying equipment? If it comes to that, so be it, but is there any way I can test it? I tried grounding my preamp, with no change.

You might try temporarily lifting all the safety equipment grounds.....

Here is some good reading material

http://www.jensen-transformers.com/an/an004.pdf

http://www.smartdev.com/pdf/ac_power.pdf

http://www.soundstage.com/weaver01.htm

http://www.audaud.com/audaud/JUL01/EQUIP/equip3JUL01.html

Describing the sound: I get a whistling sound when my CDP is engaged and I am on that input (probably some sort of DC leaking there)

From this statement it sounds like you think the whistling sound is caused by the CDP. or possibly the ics used to connect the CDP to the preamp.

By chance do you have a DVD player you could substitute in place of the CDP and connect to your preamp?

That should tell if the whistling noise is being caused by the CDP.

and a general hum when I turn the amp and preamp on together (not present when the amp only is activated).
09-14-08: Dawgcatching

You lumped this statement in with the whistling statement in the same sentence.
If the CDP is not connected to the Line inputs of the preamp is the hum still present?
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I am also getting a static background noise when I have my preamp hooked up (a Herron) but the preamp was just back at the factory and checked out fine. Perhaps it has something to do with the preamp tubes picking up something out of the air, although my backup preamp (EE Minimax, also tubed) isn't picking it up, and is much quieter.
09-14-08: Dawgcatching

Are you using the same tubes in Herron that was in the preamp when it was checked by the factory?

Static can be caused by something as simple as dirty or corroded tube pins. The static can also be caused by poor contact pressure between a tube pin/s and the socket/s of the tube socket. Tube socket/s can loose their tension memory, the ability to maintain a good pressure contact against a tubes pin/s.

Like tube pins, tube sockets can also become dirty and corroded.

And then there is the tube itself..... Did you try substituting other tubes in the preamp?

The static is pretty constant: the whistling noise and hum comes and goes, although I usually hear the Whistling around 15 minutes into listening. It tends to diminish over time, but can still be heard at high volumes.
09-14-08: Dawgcatching

Have to ask again, does all of these problems only exist if using the CDP and the ics used to connect the the CDP to the preamp?

If the CDP and ics are totally disconnected from the preamp, even to the point of unplugging the CDP from the power wall outlet, with only the preamp and the power amp on.... Does any of the problems you have described still exist? If so, please explain which ones?

Yes, speaker cables can be attennas, particularly when they are ribbons of wire with no criss-cross (rright-angle) geometry. Even short runs of parrallel wire ribbons can catch RFI.

Dave

09-14-08: Musicnoise
No Jea48 you are incorrect - his speaker wire will not act as an antenna that causes transmitted RF to be heard through his speaker - that makes absolutely no sense - if that were the case he would be hearing RF all the time, with or without his amplifier on or connected. In every system on this planet the speaker wire is past the amplifier not before it.
Musicnoise

First lets keep the subject matter in perspective.

My response was in regards to living very close to a radio transmitting tower/s.

And yes in some instances there are people who actually hear the sound of radio station broadcasts coming through their speakers.

Had you taken the time to read any threads from the archives, here on Agon, you would of found a few threads on the subject. In probably most cases the RF signal will enter audio equipment through ics, sometimes through the AC power wiring. And yes sometimes through long speaker cables.

As for the speaker cable.... I do not believe I said that Dawgcatching static was caused from his speaker cables working as an antenna. My point was in some cases long speaker cables can act as an antenna if an audio system is in the close vicinity of a radio tower's antenna.

As I said in my previous post I do not believe Dawgcatching's problem is necessary caused by the communication towers.

I suggest you reread my last post.....

I also provided you a link to read.

Here is a quote from the link,
Quote:

AUDIO AMPLIFIERS
It's quite likely that the audio amplifier is performing RF detection. You need to place ferrites on the speaker cables as close to the audio output (right at the PC board if possible) with as many windings as possible. If it is audio detection, it won't matter at all how many ferrites you install on the power cord or cable TV or antenna cable although this will cut down on the amount of
RF getting into the TV via those conductors. Direct pick-up onto speaker wires causes most problems. Don't go into the television if it is not your own.

I know first hand on this one. Several years ago I bought a house that was two houses down from a Ham radio operator.

Well it appears the guy had not been using his rig for a while and all was fine. Until one day he decided to start chatting again.

I had noticed in the basement on the incoming phone line this large filter bank, but never really gave it any thought.

I quickly discovered what the filter was suppose to do, but didn't.

If we were using the phone and the Ham keyed his mike we could hear him talking. The speech was garbled but you could tell it was a man.... and it turned out it was the Ham.

Moving on.... I had just recently, at that time, purchased a new 31" tube TV. The 31" tubes hadn't been out that long.

The TV also had what they called synthesized rear speaker home theater sound system. For its day it didn't sound too bad.

But, that was before the Ham started chatting again. When ever he would key his mike and talk there would be video noise on the screen of the TV and the garbled sound of his voice through the speakers of the TV and the rear speakers of the family room. The cable length on the rear pair of speakers was about 30'.

Here is one you won't believe. The TV could be off and if the Ham was chatting you could still hear him from the TV speakers..... Faint but loud enough you could hear him. Especially late at night!

I tried ferrite chokes, I tried filter capacitors, shielded speaker wire, you name it, I tried it.

I finally gave up and disconnected the rear speakers and speaker cable wires from the TV.... No more Ham operator garbled voice over my TV. And most of the video noise on the TV screen disappeared.

The speaker cable was working as an antenna, or what ever else you want to call it.

Is there any way I can check for a ground loop w/o buying equipment? If it comes to that, so be it, but is there any way I can test it? I tried grounding my preamp, with no change. Unfortunately, I cannot run a balanced system, as my preamp and CDP (a modified Sony 9000es) won't allow, but if necessary, I can always look at getting balanced components in the future. And, as an answer to another question, I am using the same outlet for my components. Should I be using 2 outlets, or 2 circuits? Perhaps I should think about installing a dedicated audio circuit? There isn't anything else on my current circuit, save for a lightbulb, that I am aware of.

Again, thanks for the help. I will try the shielded IC's and get back to you with the results when I am back from vacation.

As far as the frequency scope goes, I will pick one up when I get back. The background static noise gets more audible as the volume increases: if I want to run my volume control to high listening levels (but not deafening) I can hear the static in the other room. The Hum level, however, is constant no matter the volume.

Thanks for the advice. Describing the sound: I get a whistling sound when my CDP is engaged and I am on that input (probably some sort of DC leaking there) and a general hum when I turn the amp and preamp on together (not present when the amp only is activated). I am also getting a static background noise when I have my preamp hooked up (a Herron) but the preamp was just back at the factory and checked out fine. Perhaps it has something to do with the preamp tubes picking up something out of the air, although my backup preamp (EE Minimax, also tubed) isn't picking it up, and is much quieter.

The static is pretty constant: the whistling noise and hum comes and goes, although I usually hear the Whistling around 15 minutes into listening. It tends to diminish over time, but can still be heard at high volumes.

I should have posted this earlier: I am headed out on vacation on Monday, so I won't get time to start troubleshooting the system until I get back.

No Jea48 you are incorrect - his speaker wire will not act as an antenna that causes transmitted RF to be heard through his speaker - that makes absolutely no sense - if that were the case he would be hearing RF all the time, with or without his amplifier on or connected. In every system on this planet the speaker wire is past the amplifier not before it.

You are not hearing transmitted RF through your speakers - you cannot hear RF and your speakers cannot reproduce it - outside the frequency range for both.

His audio system can work like an antenna and receiver. And long speaker cables can work as an antenna and the transmitted RF will be heard through the speakers.
Example
You might want to check out the archives here on Agon.

I do think in the case of Dawgcatching if the sound he is hearing from his speakers is static, just my 2 cents, I don not think it is caused from the communication tower antennas. But no guarantees....

Static can also be caused from a HV cold solder joint or point on a circuit board with in the preamp or the power Amp. Could be a bad connection on a tube socket for example.

Unless Dawgcatching joins in the discussion it will be very hard to help diagnose the problem.

You've gotten lots of great advice here. Please follow up to let us know what you figure out.

If the ICs are a big help, then you're in one of those situations where almost every improvement you make will be noticeable. If that's the case, then I'd recommend going fully balanced if your pre-amp doesn't already allow it. Think of RFI with every IC and cord selection and consider putting a conditioner in the line. These improvements will be small relative to fixing your initial problem; however, when your bathed in RFI then plugging every hole is important.

Dave

"I get the noise as soon as my preamp and amp are engaged, whether or not I have a source hooked up. Can I buy a cheap AV-style IC from Best Buy and and see if the cables are the problem, or are those low-end IC's they sell really shielded? Are the power cords also a likely culprit?"

In the previous post I did not address this part of your post. The power cords are not the problem. The wire feeding your outlets will still be there and that is not shielded. RFI filters immediately inside your preamp and amp should take care of that anyway unless it is of extremely large magnitude. In which case you may need an external RFI filter. Again - using an oscilloscope to troubleshoot will make life much easier. As to ic's picking it up - maybe. Any shielded IC should work fine. What is important is that the shield is complete and terminated correctly. Make a pair of IC's yourself, terminate the cable so that only one end of the shield will be connected to ground when it is hooked up - make that end the source end in each case. Radio shack best buy or whatever - no need to go to an expensive cable. To minimize any problems as to coverage of the shield (if you are using a mesh vs foil shield)- avoid sharp kinks in the cable. As a quick check- try adjusting the position of the cables that you do have hooked up now. The effectiveness of an antenna for a given signal depends on its orientation - if you start moving the ic's while listening - as if adjusting an antenna - and you hear a change - then yes, your ic's are definitely picking up the interference.

Is everything plugged into the same electrical outlet? Also look at this - Jensen Transformer - it might be a ground loop and this will eliminate that. Of course it is best not to have anything at all in the signal path but you already have a major problem.

Fully agree with dcstep's recommendations.You have to go for fully balanced system,using shielded and sielded IC..

You might also shield the unused RCA inputs usings specially made plugs{Cardas,Acoustic revive and the likes]

Lastly if you use any type of ionizers make sure they do not fire directly to your preamp input.
good Luck,and dcstep is right on the money

When is the noise present? Always ? With or without a signal present ? For all sources ? etc. Does the volume of the noise change with a change in the position of the volume control? The first thing I would do is find out what the noise looks like and what frequencies are present - audible and inaudible. Scope the various points in the signal chain to see where it is present and to see what it looks like. If you don't have a scope - buy one - Kenwood sells fairly inexpensive scopes new and you can probably pick up an old tecktronix cheap. Its not like you'll never have another use for it. You are not hearing transmitted RF through your speakers - you cannot hear RF and your speakers cannot reproduce it - outside the frequency range for both. By seeing what it looks like and where it is you can do more than guess at it.

You don't have to recognize the sounds for it to be RFI.

True. There are many forms of RFI. But with that said do you think if his problem is RFI, is it being caused from the communication towers?

An arcing from a loose electrical power connection can also create RFI. And in many cases it can be heard in the form of a static sound..

Of course, a whistle can be tube related also.

Also agree.

But, what about the static sound heard 15' away?

You don't have to recognize the sounds for it to be RFI. A whistle, as described, can be an RFI issue. Usually it's just low level hash, but it can present as actual radio signals.

Of course, a whistle can be tube related also. I'd check all the tubes. Does the whistle come and go?

Dave

I have a significant background noise problem in my system. At normal listening levels, I am getting static that is audible from 15 feet away, and also a whistling sound coming through the speakers from time to time.

I don't think I have read where transmissions from radio towers caused static.

Usually what is heard through the speakers is audible sounds. Sometimes the actual radio station can be heard. Sometimes the sound is garbled.

Can you be more specific what you hear.

Yes, you should use shielded ICs and go to balanced mode if you pre-amp allows. RFI and EMI can get into your system through IC, PCs, chassis, etc. Start with the ICs; however, you may have to move toward a balanced system.

Dave