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Amp picks up local radio

post #1 of 50
3/5/12 at 4:17pm
Thread Starter 
OK, so I bought a second hand Exposure 2010 power amp last saturday. Once at home I find that it picks up a local FM station plus some white noise. Only the amp is plugged into the wall outlet and a pair of speakers are hooked up to the amp. I've unplugged every other apliance on the particular circuit and it still does this. What to do? AC cable, RF filter, power conditioner, move somewhere else or simply give up on the amp and get another one?
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post #2 of 50
3/5/12 at 4:34pm
It could be picking up the signal on the inputs. Do you still have this problem with a preamp connected (but turned off)?
If so I'd suspect the amp was defective.
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post #3 of 50
3/5/12 at 4:59pm
Thread Starter 
Yup, even with the pre-amp connected, its picking up RF interference. How can I tell if the amp is defective? Can this sort of defect be repaired? The shop I bought it from claims they refurbished it and gave me a 6 month warranty.
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post #4 of 50
3/5/12 at 5:32pm
Quote:
Originally Posted by AudioVideoPhilia View Post

Yup, even with the pre-amp connected, its picking up RF interference. How can I tell if the amp is defective? Can this sort of defect be repaired? The shop I bought it from claims they refurbished it and gave me a 6 month warranty.

Then take it back and see if they can help. See if it does it in their store. If not, consider moving if you really want to keep the amp (do you have this problem with any other equipment?)

It could be a design defect, or it could be a circuit problem. Luckily you have a 6 month warranty
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post #5 of 50
3/5/12 at 5:51pm
Thread Starter 
Yeah, I guess I'll eventualy take it back to the shop but I wanted a better grasp on the situation before hand. I'm wary of that salesman. He was pushing hard to get me to buy some 150$ AC wire. I know their margins are better on accessories. How convinient it would be if what he just sold me needed a new power cord?!
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post #6 of 50
3/6/12 at 5:57am
Quote:
Originally Posted by AudioVideoPhilia View Post

Yeah, I guess I'll eventualy take it back to the shop but I wanted a better grasp on the situation before hand. I'm wary of that salesman. He was pushing hard to get me to buy some 150$ AC wire. I know their margins are better on accessories. How convinient it would be if what he just sold me needed a new power cord?!

Do you live close to transmitter? If so, I would check level of EM field in your room. It can be bad for your health.

If there no problem with EMI, your unit is either broken or poorly designed. The later is often happens with small manufacturers.
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post #7 of 50
3/6/12 at 3:51pm
Thread Starter 
I don't live close to any powerfull transmiter, I doubt thats the problem.

Last night I tried a few things to see what causes the interference. While the amp was powered on, I moved AC cords and interconnects up and down to pin point wich one was receiving the RF signal. Turns out my Audioquest interconnects going from the pre-amp to the power amp responded to movement most. The radio signal got stronger the closer they got to the back of the TV - wich is made of sheet metal and contains a tuner.

I then moved the amp across the room, away from the TV and the rest of my AV gear. Hooked up the speakers, plugged it in and pressed power. Big difference, this time the radio signal was barely audible but I still got the same level of white noise.

I asked around town in different hi-fi shops and they all have their theory. One says my interconnects aren't insulated enough. Another thinks its bad AC power and I should look at power cords and/or power conditioners. But none of them have flat out said the amp itself is broken. Then again they all want to sell me accessories.
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post #8 of 50
3/6/12 at 5:47pm
Some white noise is unavoidable. Obviously the less the better. But all amps have some.

Try replacing the audioquest cables with something else
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post #9 of 50
3/6/12 at 7:47pm
Speaker cables are better antennas than interconnect cables (they're usually unshielded), and they can feed rf interference back into the amp through the feedback shunt capacitor practically without attenuation.
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post #10 of 50
3/6/12 at 8:30pm
Quote:
Originally Posted by rntlee View Post

Speaker cables are better antennas than interconnect cables (they're usually unshielded), and they can feed rf interference back into the amp through the feedback shunt capacitor practically without attenuation.

This is what I call a design error. There has to be very little sensitivity to RF noise, unless you are in three hundred feet from transmitter.
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post #11 of 50
3/6/12 at 10:39pm
Quote:
Originally Posted by rntlee View Post

Speaker cables are better antennas than interconnect cables (they're usually unshielded), and they can feed rf interference back into the amp through the feedback shunt capacitor practically without attenuation.

The signal traveling through speaker wiring is high current/low impedence. Very much impervious to EM interference, in 99.99% of situations.
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post #12 of 50
3/7/12 at 5:09am
Quote:
Originally Posted by Theonetruegreg View Post

The signal traveling through speaker wiring is high current/low impedence. Very much impervious to EM interference, in 99.99% of situations.

Actually, speaker cables make great antennas, and pick up EMI quite well. You're thinking of cross-talk, which is a different matter. SS amp use massive amounts of negative feedback to achieve their low THD #'s.
As AP1 noted, poor design can allow EMI to travel back the feedback loop and enter the earlier stages of the amp unattenuated. There is speculation that EMI (which is becoming a bigger and bigger problem by the year) is large cause of the "graininess" some amps exhibit, via increasing the intermodulation.
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post #13 of 50
3/7/12 at 5:18am
Quote:
Originally Posted by ap1 View Post

Do you live close to transmitter? If so, I would check level of EM field in your room. It can be bad for your health.

If there no problem with EMI, your unit is either broken or poorly designed. The later is often happens with small manufacturers.

Guess they should stop going outdoors, getting into vehicles, running the tv, microwave, dishwasher, wash machine, dryer, furnace, air conditioner.

Sorry, but we have been surrounded by Electromagnetic waves for centuries and no one has died from them, so your point has no point.
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post #14 of 50
3/7/12 at 6:36am
Quote:
Originally Posted by gregzoll View Post

Guess they should stop going outdoors, getting into vehicles, running the tv, microwave, dishwasher, wash machine, dryer, furnace, air conditioner.

Sorry, but we have been surrounded by Electromagnetic waves for centuries and no one has died from them, so your point has no point.

There are very strict limits on EM emission for everything used in household. Though nobody knows how much health risk is from costantly connected smart phones, and wi-fi networks found at almost every home now. You need 20-50 years to get enough statistical data to analyze full effects from combination of all that energy around us. As for powerful transmitters nearby you should be as worried as if you live under high voltage power line. Negative health effects of both are vell documented.
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post #15 of 50
3/7/12 at 7:01am
If you are picking up an FM station, it's going to be somewhere in the 88-108 MHz range, which is within the band (about 25-250) that common ferrite cores operate. Try a couple of them on the power cord, close to the amplifier, with only the speakers connected (and power, of course). That will tell you if it's coming in via the AC power.
If that doesn't cure it, try one or two on each (at the same time) of the cables...speaker (L), speaker (R), and the power cord.

You can buy them at Radio Shack:
http://www.radioshack.com/product/in...ductId=2103222
http://www.radioshack.com/product/in...ductId=2996360
(Two different sized holes for the cables)

There are different "mixes" of ferrites for other frequencies, such as AM and HF radio (below 30 MHz), and UHF/microwave (above 200 MHz), which have to be ordered through a major electronic parts supplier....not usually Radio Shack, etc.

What station is it? You could be close to a low-power station, and not even know it.
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post #16 of 50
3/7/12 at 8:36am
Some interconnect cables come with ferrites already installed. I have seen a number of them from various manufacturers.

I recently bought an OPPO BDP-95 Blu-Ray player, and the HDMI cable supplied with it had ferrites built in. These will be seen as large cylindrical bulges around the cable near either end (about 1 inch in diameter).

As the gentleman said, you can buy these at Radio Shack and clamp them around the Interconnects at either end. These should be quite effective at reducing RFI. Ferrites come in several types with different permeabilities which make them most effective in certain frequency ranges. The #43 ferrite material works best from 30 to 500 Mhz, and the #77 material is best for 3 to 30 Mhz.

Unfortunately, these are not always identified that way, but the most common is #43 and it will probably be what Radio Shack sells (#273-067).

You can buy these much cheaper at Mouser Electronics; they sell essentially the exact same item as above for 61 cents each, or 10 for $4.70 (part #28A2005-OA2). (this one is for cables up to 0.14 inches diameter; for cables up to 0.26 inches diameter use part # 26A2913-OA2; other sizes are also available).

Another thing that sometimes works is to buy a replacement AC plug from the hardware store and attach a .01 microfarad 400 volt non-polarized capacitor to it, connecting the capacitor leads to the two power connections. Plug this into an AC outlet on the same circuit as the equipment, and it will eliminate RFI on that AC circuit.

Speaker cables are almost never involved in this type of problem, because any RFI they pick up is at too low a level and too high a frequency to be an issue. It needs to be picked up at a point in the amplifier circuit where it gets amplified to a great degree and rectified into an audio frequency, thus becoming audible.

I have been a ham radio operator for 50 years and have had to deal with dozens of cases where people needed relief from RFI produced by a neighbor's transmitter, so my experience at solving these problems successfully is extensive. There are several specific techniques that are used successfully, depending on the specifics of the case. I hold an FCC commercial license and taught communications electronics for 30 years. I currently am a commercial communications consultant to several major system operators for RFI and intermodulation issues at cell sites and in other communications systems.

Putting ferrites at both ends of all of your interconnects is the first step I would recommend (about 6 inches from each end).



Quote:
Originally Posted by kenglish View Post

If you are picking up an FM station, it's going to be somewhere in the 88-108 MHz range, which is within the band (about 25-250) that common ferrite cores operate. Try a couple of them on the power cord, close to the amplifier, with only the speakers connected (and power, of course). That will tell you if it's coming in via the AC power.
If that doesn't cure it, try one or two on each (at the same time) of the cables...speaker (L), speaker (R), and the power cord.

You can buy them at Radio Shack:
http://www.radioshack.com/product/in...ductId=2103222
http://www.radioshack.com/product/in...ductId=2996360
(Two different sized holes for the cables)

There are different "mixes" of ferrites for other frequencies, such as AM and HF radio (below 30 MHz), and UHF/microwave (above 200 MHz), which have to be ordered through a major electronic parts supplier....not usually Radio Shack, etc.

What station is it? You could be close to a low-power station, and not even know it.
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post #17 of 50
3/7/12 at 9:51am
Quote:
Originally Posted by rntlee View Post

Actually, speaker cables make great antennas, and pick up EMI quite well. You're thinking of cross-talk, which is a different matter. SS amp use massive amounts of negative feedback to achieve their low THD #'s.
As AP1 noted, poor design can allow EMI to travel back the feedback loop and enter the earlier stages of the amp unattenuated. There is speculation that EMI (which is becoming a bigger and bigger problem by the year) is large cause of the "graininess" some amps exhibit, via increasing the intermodulation.

Any unshielded wire can be an antenna. But when in use as a speaker cable, it doesn't transmit those signals anywhere due to the high current energy traveling through it. Even when no signal is passing through, an amplifier working properly won't be amplifying that signal as it's limited to the output chain of the circuit.
When having problems like the OP is, he could be using coat hangers as speaker wire, wouldn't matter, that wouldn't be the cause of his EM issues, something in the amplifier is the cause, he would be chasing ghosts fiddling with buying new wires in hopes that it would matter.
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post #18 of 50
3/7/12 at 9:59am
This is a theoretical possiblity which is well-known to any competent engineer who designs amplifiers.

That is why an amplifier is always designed with RF suppression capacitors in the feedback loop to prevent it from happening.

If this was not designed into the circuit, an amplifier would almost certainly oscillate at some ultrasonic or RF frequency and it would overheat badly for no apparent reason.





Quote:
Originally Posted by rntlee View Post

Speaker cables are better antennas than interconnect cables (they're usually unshielded), and they can feed rf interference back into the amp through the feedback shunt capacitor practically without attenuation.
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post #19 of 50
3/7/12 at 10:58am
Quote:


it doesn't transmit those signals anywhere due to the high current energy traveling through it.

That doesn't make any sense. Speaker circuits are less susceptible to induced noise due to the very low impedance connected to them.
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post #20 of 50
3/7/12 at 1:26pm
Thread Starter 
Quote:
Originally Posted by kenglish View Post

If you are picking up an FM station, it's going to be somewhere in the 88-108 MHz range, which is within the band (about 25-250) that common ferrite cores operate. Try a couple of them on the power cord, close to the amplifier, with only the speakers connected (and power, of course). That will tell you if it's coming in via the AC power.
If that doesn't cure it, try one or two on each (at the same time) of the cables...speaker (L), speaker (R), and the power cord.

You can buy them at Radio Shack:
http://www.radioshack.com/product/in...ductId=2103222
http://www.radioshack.com/product/in...ductId=2996360
(Two different sized holes for the cables)

There are different "mixes" of ferrites for other frequencies, such as AM and HF radio (below 30 MHz), and UHF/microwave (above 200 MHz), which have to be ordered through a major electronic parts supplier....not usually Radio Shack, etc.

What station is it? You could be close to a low-power station, and not even know it.

The station in question is 95.1 and their transmitter is over 4 miles away.

Thanks for all the info. I'll fool around with it over the week-end and let you know how it went.
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post #21 of 50
3/7/12 at 2:02pm
Quote:
Originally Posted by Theonetruegreg View Post

Any unshielded wire can be an antenna. But when in use as a speaker cable, it doesn't transmit those signals anywhere due to the high current energy traveling through it. Even when no signal is passing through, an amplifier working properly won't be amplifying that signal as it's limited to the output chain of the circuit.
When having problems like the OP is, he could be using coat hangers as speaker wire, wouldn't matter, that wouldn't be the cause of his EM issues, something in the amplifier is the cause, he would be chasing ghosts fiddling with buying new wires in hopes that it would matter.


*sigh*
I copied this quote from here...it may shed some light.

Quote:


1) Never use parallel wire (zip cord) for loudspeaker wiring. Always use twisted pair
cable for loudspeaker wiring. Why? It's quite common for audio output stages to
lack the low-pass filters needed to reject RF, so any RF picked up by the loudspeaker wiring will be coupled back to a driver stage via the feedback loop, where it
will then be detected and amplified. And, of course, it is well known that a twisted
pair provides good rejection of magnetic and electric fields, but we tend to forget
that this still applies at radio frequencies.
2) Avoid the use of shielded cables with a drain wire if the system will be exposed to
strong RF below 10 MHz (AM broadcast, ham transmitters, light dimmers). Any cable with a drain wire will have significantly greater SCIN below about 10 MHz than
a cable that uses a good braid shield. On the other hand, foil-shielded cables provide better shielding above 20 MHz. The best cable, if you can find it, is one that
has both a foil and braid shield. Second best are those with very dense braid
shields.
Both of these cable mechanisms are quite powerful -- simply switching from zip
cord to twisted pair for loudspeaker wiring, or from foil/drain cable to a good braid
cable, can easily reduce RFI by 20-30 dB if that is how the RF is getting in.
3) We know that pin 1 problems are a common cause of RFI, so fixing them is always
worth the trouble. The best fix is one that disconnects the cable shield from the
connector pin that goes to the circuit board, and connects the cable shield instead
to the equipment's shielding enclosure. This is particularly important with VHF and
UHF interference sources. Don't forget that pin 1 problems can occur with unbalanced inputs and outputs too - if the equipment has RCA and/or ¼ connectors
mounted to a circuit board, you can bet your lunch that it's got a pin 1 problem!
Vol 31 #2 and #3 of the SynAudCon Newsletter has a detailed discussion of Pin 1.
Unfortunately, lots of equipment with pin 1 problems is designed in a manner that
makes it difficult to fix them. That's where ferrite chokes save our bacon!
4) There's a tendency on the part of many audio equipment manufacturers to design
excessive bandwidth into their products. This happens for two reasons. First, it
costs a few extra dimes to include the components needed to limit the bandwidth. Understanding How Ferrites Can Prevent and Eliminate RF Interference to Audio Systems Page 12
Second, some (many?) misguided souls think they can improve audio sound quality
by extending the bandwidth into the MHz range, which in turn can improve the
phase response at very high audio frequencies. While flat phase response is a wonderful thing, extending the bandwidth of an audio system's inputs and outputs to
100 kHz should achieve that objective with even the simplest of filters, and extending it beyond about 200 kHz almost guarantees interference from nearby AM
broadcast stations. The use of more sophisticated filter topologies allow sharper
cutoffs with minimal phase shift in the passband.
Using braid-shielded audio cable greatly reduces the likelihood that equipment like
this will see enough RF for audible detection to occur, but if additional help is
needed, a good input transformer with a Faraday shield (Jensen and Lundahl are the
good brands) acts as an effective low pass filter to block the RF.
5) RF interference often enters equipment and systems by more than one path. You
may eliminate or reduce the interference coupled into one path, but not achieve the
full elimination. Always suspect more than one path, especially with interference
that is especially strong or persistent. Continue implementing all of the right techniques throughout the system, even when the first things you do don't seem to be
accomplishing much. One dominant path may be swamping the weaker ones
you are fixing. Eventually you'll find the dominant one.
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post #22 of 50
3/7/12 at 2:28pm
Quote:
Originally Posted by AudioVideoPhilia View Post

The station in question is 95.1 and their transmitter is over 4 miles away.

Hi AudioVideoPhilia,

4 miles isn't very far away when you consider that CBF-FM is transmitting at 100,000 watts. There may not be a lot you can do. Check that the case is grounded. Shielding may be all you can do, if you can find where the station is leaking in.
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post #23 of 50
3/7/12 at 4:11pm
Thread Starter 
Quote:
Originally Posted by MarkHotchkiss View Post

Hi AudioVideoPhilia,

4 miles isn't very far away when you consider that CBF-FM is transmitting at 100,000 watts. There may not be a lot you can do. Check that the case is grounded. Shielding may be all you can do, if you can find where the station is leaking in.

When you say "where the station is leaking in" are you refering to the room or the amp itself? Thanks for your input.
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post #24 of 50
3/7/12 at 5:41pm
Quote:
Originally Posted by AudioVideoPhilia View Post

When you say "where the station is leaking in" are you referring to the room or the amp itself? Thanks for your input.

By where, I mean the amp and any electrical connections to the amp. I would not rule out any cables. Start with very short speaker and power cables, and grow from there. Is it a grounded power cable? If not try grounding the chassis of the amp to shield in innards. If so, check that the ground is good.

I don't think there is much you can do about the room . . .
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post #25 of 50
3/7/12 at 5:56pm
Thread Starter 
Good to hear. I was not looking forward to tin foil wallpaper!
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post #26 of 50
3/7/12 at 7:08pm
Sorry, but we have been surrounded by Electromagnetic waves for centuries and no one has died from them, so your point has no point.

As for powerful transmitters nearby you should be as worried as if you live under high voltage power line. Negative health effects of both are vell documented.

Where I grew up was on the top of a hill. Through the valley ran power lines from the Bowen power plant in North Georgia to a substation. All manner of people on the streets near these power lines died of cancer, including my mother and my next door neighbor. Not proof positive, but I am convinced.

When I bought a power conditioner the radio noise from my turntable virtually vanished.
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post #27 of 50
3/7/12 at 11:09pm
.
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post #28 of 50
3/7/12 at 11:19pm
Quote:
Originally Posted by rntlee View Post


*sigh*
I copied this quote from here...it may shed some light.

Excellent work on your ability to copy and paste from another source.
Reading is an important life skill.

Did this paper also go on to say that the speaker wires that are cryogenically treated and blessed by saints are even better at resisting interferrence?

I have never seen any speaker wire in a properly designed and working system introduce audible noise, have you?
Have you happened to open any hi-end speaker's cabinet and peak around inside? Did you see twisted pairs in there? You didn't. It must be that wood is an amazing shield!
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post #29 of 50
3/8/12 at 5:52am
Dude, your 'high current' theory is bunk...lay off the arrogance, you're just embarrassing yourself.
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post #30 of 50
3/8/12 at 8:32am
Quote:
Originally Posted by SAM64 View Post

Dude, your 'high current' theory is bunk...lay off the arrogance, you're just embarrassing yourself.

read back...high current/low impedence....is not bunk, is fact.
Posting crap about using twisted pair wiring and swapping out unneeded items has the OP chasing ghosts, and will not help him at all.
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