unwanted noise HELP

[jcmusic]

Some one please help me, I am getting bleedover from one of the local AM Radio Staions.

My tuner is turned off, it does not even have an AM antenna hooked to it, what is causing this?

How do i fix it.

[WMcD]

I was thinking someone else might reply. But I'll give it a shot. The below is theory of why this happens, and then trouble shooting steps. The latter is just logic.

Theory 1.:

Obviously the issue is that radio frequency is getting into your set up. DrBill has pointed out that a poor connection someplace can act as a diode, such as in a crystal radio.

In the old days, very old days, the crystal was used to form a point contact diode with the use of a wire "cat's whisker". An alternative was to rest a sewing needle across two razor blades or sharpened carbon blocks. These serve as AM detectors. But I have another idea, set out below.

Troubleshooting:

One first question to ask is whether anything changed in your system shortly before the problem started. Maybe the transmitter came on the air but that seems more unlikely.

So you have to determine where the signal is getting in.

You should check all connectors to see if they're snug. This is to indicate where the "diode" could be located and eliminate it. Maybe even a cranky switch contact could do that.

Re the input connector switch. Does the signal get in as switch between inputs? If it is on only one, that tells something.

Does the volume contol effect it?

I don't see you using a turntable, tape head in, or microphone but those are high gain inputs and would make for good amplifiers.

From all the DBX equipment in your set up you have a lot of stuff in the tape loop perhaps. Don't forget that.

One final experiment would be to just run the amp without any input connections. If the signal can still be heard. I suspect the signal is getting in through the speaker wires.

I generally don't think it is getting in through the a.c. Some people have suggested removing a ground, or removing a ground, but I'm not a fan of that. People mess with this to solve ground loops. Howvever, they don't report radio problems. I can not rule it out of course.

Theory 2:

It is my belief that in rare situations the RF (radio frequency) can get into the amp through the speaker wires. Then a feedback loop does something odd to detect it. Speaker wires can be shorted with very small value caps or ferrite "beads" can be put around any feed wire.

Conclusion:

In any event, check connections, do the trouble shootng. Tell us more. The more the problem can be narrowed down, the more specific suggestions can be made.

This trouble has shown up on the Klipsch multimedia systems. I don't hang out at those forums. Maybe Amy or Trey can chime in.

Gil

[mikebse2a3]

Gil Said:

One final experiment would be to just run the amp without any input connections. If the signal can still be heard. I suspect the signal is getting in through the speaker wires.

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Gil has given you some good advise but I would suggest one thing different. Instead of leaving inputs open and testing I would suggest using an RCA type shorting plug which is simply an RCA Plug with a wire connected between the center pin and outer section of the plug.(you could also use an old Interconnect cable by cutting it off close to one end and twisting or soldering the wires together. This will keep any noise from an open input from being a problem during your trouble shooting and you could also use it on any open inputs that your not using in case the RF noise is being picked up through any of the unused open inputs in your system.

Start by using the shorting plug on the input of the amplifier (make sure the amp is off before hooking or unhooking any input connections) and if things are quite then work your way back through the system till you learn where the RF interference is entering into your system.

Another question: Is the interference in both left and right channels?

I use to own a Hafler DH110 preamp and I would get some RF interference ( My Grandfather talking on his CB Radio) in one channel. When I contacted Hafler about this they sent a fix which consisted of cutting a ground trace and installing a small capacitor to eliminate the problem.

Anyway like Gil suggested the first thing is to find where the noise is being introduced.

mike[]

[jcmusic]

OK guys, everything is off except the amp pre-amp and canhere this local talk show.

Is it possible that one of the connectors has gone bad or the shielding gone amuck?

Jay.

[hwatkins]

Well - I think the boys kind of got it right - something has turned into an AM reciever and it is picking the signal up someplace in the chain with enough umph to amplify (if even slightly). Finding this place can be a bear.

I don't have any great advice, but I had asimilar experience with an amplified subwoofer. In my case it was a new station so I first called the FCC. It turns out the folks at the new transmitter (about 1 mile from the house) were running very hot. I got a return call from the FCC that it was fixed and checked it out - no more AM.

Good luck

[JBryan]

If you've disconnected the ICs from the amp and still hear RF then make sure your speaker wire isn't coiled (makes a great antenna). If not that maybe you should try another run of speaker wire - inexpensive zip cord is fine.If its not the ICs or wire, these have actually worked in situations similar to yours on occasion... (not often but you could get lucky)...

[Erik Mandaville]

Canyon:

Looking at your system components, it seems there is the possibility that the number of interconnects hooking everything together is on the high side -- which is obviously required to hook them all up. However, depending on the type of cable being used, there is the chance the some of the cables, if they are of a coaxial geometry, might be acting as effective antennas to RFI. Since the shields in coax cables also function as the ground return path, they can sometimes actually PICKUP unwanted interference. On the other hand, cables that consist of two-conductors plus shield, do not usually have the shield grounded at both ends -- meaning that one gets the benefit of active shielding, minus the possibility of picking up RFI artifacts. If you don't know what type you have, it would be possible to figure that out.

If you ICs are long, the problem is greater. Also, I don't know the impedance values of the components you have, but, for example, tube equipment, because of it's rather high associated impedances, are much more likely to pick up RFI because of the increased sensitivity. If you are close to transmission antennas in your area, that can be problematic, too. Even the wiring in your house can be a problem.

Solving problems like this can be maddening, and we have very high RFI where we live, too. I sometimes hear a little bit in the background, particularly with all the tube amps I have running right now.

Good luck!

Erik

edit: Just remembered this. My brother an dad a very active radio operators, and both have often used split ferrite cores on cables for RFI suppression/immunity. I think Radio Shack may sell cables with ferrite. Power cords, because of being connected to the house wiring, can also sometimes benefit from the treatment. I know a circuit which is sort of a brute force AC line filter, but one can buy chassis mount IEC connectors with very similar filters already installed. They are sold by companies like Mouser, etc.

[WMcD]

Just so we know: Does the problem respond to the volume control? Is it in both channels?

Gil

[WMcD]

I see we've got another thread going. Boom3 suggests ferrite beads, which I do too.

They cost about $5 at Radio Shack and are part number 273-105. Put one around each speaker wire near the amp end to start. It couldn't hurt to experiment with other locations.

Let us all know what happens because this is becoming a common complaint.

The next solution would be to buy some 0.01 or 0.1 uF mica disk caps from RS (about $1 in a blister pack) and connect them across the amp output to the speakers.

Gil

[jcmusic]

The noise does respond to volume control, a little you have to really turn it up to hear it from a distance.

As far as both channels, yes some time both, others just one side. I am going to take down the complete system and put back just the essentials and see where it takes me. I am pretty sure it is in the pre-amp,

but I want to make sure.

[NOSValves]

Don't just shut everything off. Unhook everything from the amplifier and turn the amp on.... Do you get the noise?? Then add your preamp with nothing attached to the preamp... Do you get the noise?? Then add each component until the noise appears. This will narrow down what component or interconnect is causing the problem. With the list of gear your using the problem your mentioning doesn't surprise me all that much. That is a ton of gear.

Craig

[jcmusic]

OK guys, the noise is gone. I took it all down and went back with just the amp, pre-amp, cd, rtr, and tuner.

No more sound processing equipment, no more noise. Thanks for the advise.

Jay.