Interesting Audio interference

[DuckJordan]

First I'll explain my setup. I have a dell laptop with the 3.5mm Stereo out cable running to a 2 Channel RCA direct box, out from that using 2 XLR connectors to an old Mackie SR24-4-2 4 bus Mixing Console, both channels getting amazing signal. Then it goes out the mono output through an XLR to 1/4" connector into an old Squier Champ 15 amp.

Out of the speaker comes the local college radio station (this is the interference). The local radio station runs at 94.5 FM. It utterly dissapeared when some static electricity was discharged from me to the console, its back now but at a lower volume. I'm slightly confused as everything except the amplifier/speaker combo is run with balanced signal all the way across.

Not really a big problem as this is just a personal setup for listening to music but still odd none the less can anyone figure this one out. I'm betting its inside the Amp/Speaker box and the only way i can think of shielding it is create a copper mesh box around it.

 

[Stookeybrd]

I'm slightly confused as everything except the amplifier/speaker combo is run with balanced signal all the way across.

Double check, I've run systems that I was confident were balanced throughout too.

I'm betting its inside the Amp/Speaker box and the only way i can think of shielding it is create a copper mesh box around it.

You bet'cha.

 

[DuckJordan]

Double check, I've run systems that I was confident were balanced throughout too.


You bet'cha.

Well Considering I've pulled all the connectors apart and made sure the continuity between the connections was clear. I would be safe saying that one, Although I did another test on it I unplugged the amp from the signal chain and the interference was still there so. that said is there something easier to do that build a copper box to go around the amp/speaker combo?

 

[Dover]

Check the grounding on the rig. It sounds like you have a open or high resistance ground.

Dover

 

[howlingwolf487]

Make sure that the audio shield/drain wire is not connected to the shell of the connector.

Work your way back from the amplifier. Start by disconnecting the input cables; power the amp on - is the RFI there? If "yes", you've found your weak link, if "no", then reconnect the input cables but disconnect them from the output of the previous device. Cables can act like antennas if connections are floating. Work your way up the signal chain in this manner until you find the culprit.

 

[DuckJordan]

Make sure that the audio shield/drain wire is not connected to the shell of the connector.

Work your way back from the amplifier. Start by disconnecting the input cables; power the amp on - is the RFI there? If "yes", you've found your weak link, if "no", then reconnect the input cables but disconnect them from the output of the previous device. Cables can act like antennas if connections are floating. Work your way up the signal chain in this manner until you find the culprit.

I'm so happy people read past the first post, I've found where the issue is, its in the amp/speaker combo (its an old Guitar Amplifier that has a decent clean signal most of the time).

This has now come to is building a copper box around this small 1.5 foot square amplifier the only way to get rid of this interference on the amp, again this is more of a thought provoking thread not "show critical".

I want to hear the most interesting ideas on how to get rid of the interference.

And no asking the local college radio to "turn it down" is not applicable. although it only seems to occur during certain times of day, so I'm wondering if their system is putting out bleed off signals during times of lower voltage.

 

[DuckJordan]

Check the grounding on the rig. It sounds like you have a open or high resistance ground.

Dover

Interesting thought, everything in the chain is run through the same outlet, so the only chance of an open ground would be if they didn't wire the building correct (wouldn't surprise me). If it helps i did have to get a ground lift to go between the laptop and the mixer to get rid of sound ground loop issues.

 

[avkid]

If you had a ground loop, you most likely have a ground.

 

[DuckJordan]

not sure if it was a ground loop but, with a DI and ground lift running between the computer and the board got rid of some bad noise. I hope i used the correct term, I haven't been in my sound class for over a year, and i'm starting to go to the "light" side.

 

[dmoes]

Ok so the EMI/RFI problem is in the speaker amp before putting it into a copper box try this


Put a ferrite around the power cord where it enters the amp looping it a few times if possible. Also put some around the speaker leads if they are visible keeping them closest to the amp itself. basically any wire that goes into the amp part of the speaker amp combo. the wavelength of the FM broadcast band is fairly short so even a foot of wire can act as an effective antenna that contributes to the problem.



Another idea is put some wire mesh or metal screen over the back of the amp and over the grille cloth. be sure these are attached together electrically and to the ground. this should provide some shielding. this should work assuming that the ground is good and not acting as an antenna itself. keep in mind that grounds that may be good at DC or low frequency AC may appear open at radio frequencies. this is especially true if its a long way from the mains panel. if the ground is an issue the shield may not work well or even make it worse.

I have Ferrites all over the place in the theater particularly on the in/outs on the clear-com. we have the local tv and radio station's tower in close view and we used to get them both on the intercom Ferites made a huge difference there and I never get to hear the hockey game over the intercom during a slow show anymore.

 

[DuckJordan]

Ok so the EMI/RFI problem is in the speaker amp before putting it into a copper box try this


Put a ferrite around the power cord where it enters the amp looping it a few times if possible. Also put some around the speaker leads if they are visible keeping them closest to the amp itself. basically any wire that goes into the amp part of the speaker amp combo. the wavelength of the FM broadcast band is fairly short so even a foot of wire can act as an effective antenna that contributes to the problem.

View attachment 4128

Another idea is put some wire mesh or metal screen over the back of the amp and over the grille cloth. be sure these are attached together electrically and to the ground. this should provide some shielding. this should work assuming that the ground is good and not acting as an antenna itself. keep in mind that grounds that may be good at DC or low frequency AC may appear open at radio frequencies. this is especially true if its a long way from the mains panel. if the ground is an issue the shield may not work well or even make it worse.

I have Ferrites all over the place in the theater particularly on the in/outs on the clear-com. we have the local tv and radio station's tower in close view and we used to get them both on the intercom Ferites made a huge difference there and I never get to hear the hockey game over the intercom during a slow show anymore.

Interesting I'll have to check and see if the science dept has one that i can borrow.

 

[FMEng]

Interesting I'll have to check and see if the science dept has one that i can borrow.

Radio Shack sell a couple of toroids. They don't say what frequency range the material is tuned to, but they are worth a try.

Snap-Together Ferrite Choke Core - RadioShack.com

Snap-Together Toroid Choke Core-2-Pack - RadioShack.com

Dmoes has it exactly right, wrap the power cord and the audio cable around the toroids, close the where they enter the amp.

Generally speaking, balanced lines cable shielding are not very effective for RF interference.

 

[dbrown]

First I'll explain my setup. I have a dell laptop with the 3.5mm Stereo out cable running to a 2 Channel RCA direct box, out from that using 2 XLR connectors to an old Mackie SR24-4-2 4 bus Mixing Console, both channels getting amazing signal. Then it goes out the mono output through an XLR to 1/4" connector into an old Squier Champ 15 amp.

Out of the speaker comes the local college radio station (this is the interference). The local radio station runs at 94.5 FM. It utterly dissapeared when some static electricity was discharged from me to the console, its back now but at a lower volume. I'm slightly confused as everything except the amplifier/speaker combo is run with balanced signal all the way across.

Not really a big problem as this is just a personal setup for listening to music but still odd none the less can anyone figure this one out. I'm betting its inside the Amp/Speaker box and the only way i can think of shielding it is create a copper mesh box around it.

if your mackie board is the orginal, there was a problem with picking up radio stations

log on the mackie web site there is a fix for it, it takes putting some capicators and inductors in the xlr housing. not hard if you can solder.
the orginal pre-amps were too open on the frequency spectrum the newer series corrected for this problem

i had this happen in a church setup during a service and a soloist with a radio background..... put the fix in and no more problems

 

[DuckJordan]

if your mackie board is the orginal, there was a problem with picking up radio stations

log on the mackie web site there is a fix for it, it takes putting some capicators and inductors in the xlr housing. not hard if you can solder.
the orginal pre-amps were too open on the frequency spectrum the newer series corrected for this problem

i had this happen in a church setup during a service and a soloist with a radio background..... put the fix in and no more problems

If you read past the first post, I have mentioned, when i remove the amp from the signal chain completely (only plugged in to AC power) i still have the interference.

Please people read past the first post.

 

[SHARYNF]

Guitar amps are notoriously susceptible to type of rf interference

Usually it comes from either the power line which is acting like an antenna and feeding the rf on the ground side. Make sure that you ahve a good ground in the outlet you are using, check to see if you have a two wire plug on your amp of three wire, and getting one of the ferrite units placed right at the point the ac cord enters the amp might help

the other one is related to the Mackie solution in theory applied to another input situation. If you are good with a meter, check to see if your input jacks (I'm guessing you are using a 1/4 inch converter on an rca cable) ground/shield side is connected to the chassis of the amp. IN a lot of cases it is not, and the Jack is isolated from the chassis on the guitar amp and adding a small .01UF cap from the Ground OF THE INPUT JACK to the chassis will clear up the problem ( it will filter anything coming in on the line/Jack If you are careful you can see if this will get rid of the problem by taking a 1/4 jack and just use a temp wire that you can touch to the Chassis. If this causes the rf to go away, then the cap will do the trick

Sharyn

 

[DuckJordan]

Guitar amps are notoriously susceptible to type of rf interference

Usually it comes from either the power line which is acting like an antenna and feeding the rf on the ground side. Make sure that you ahve a good ground in the outlet you are using, check to see if you have a two wire plug on your amp of three wire, and getting one of the ferrite units placed right at the point the ac cord enters the amp might help

the other one is related to the Mackie solution in theory applied to another input situation. If you are good with a meter, check to see if your input jacks (I'm guessing you are using a 1/4 inch converter on an rca cable) ground/shield side is connected to the chassis of the amp. IN a lot of cases it is not, and the Jack is isolated from the chassis on the guitar amp and adding a small .01UF cap from the Ground OF THE INPUT JACK to the chassis will clear up the problem ( it will filter anything coming in on the line/Jack If you are careful you can see if this will get rid of the problem by taking a 1/4 jack and just use a temp wire that you can touch to the Chassis. If this causes the rf to go away, then the cap will do the trick

Sharyn

I could see where that would makes sense. This is an old school amp though and the chassis is wooden, Also the connector going from the mixer to the guitar amp is XLR from the boards mono out, within the same cable terminating into a TRS 1/4" plug, also, ground shouldn't be the problem since i had to put a DI between the laptop and the board cause from a ground loop issue, so ground is good, although i haven't gotten a chance to get a good ferrite magnet on the AC Cable, just a heads up its 9:15 and no interference. it comes in randomly.

and from the laptop to the DI is 3.5mm on both ends.

 

[n1ist]

Does the amp have a non-polarized power plug? I have seen some strange interference issues happen when those get plugged in backwards (and some strange voltages showing up on metal parts too, due to a cap between one side of the power line and the chassis)
/mike

 

[DuckJordan]

Does the amp have a non-polarized power plug? I have seen some strange interference issues happen when those get plugged in backwards (and some strange voltages showing up on metal parts too, due to a cap between one side of the power line and the chassis)
/mike

Considering its a grounded Edison plug i cannot plug it in backwards, nice thought though although I'm wondering what amplifiers you've been working with as i have only seen them come standard with grounds on their plugs.

 

[mbenonis]

It's interesting to me that it's an *FM* station causing the interference, and not an AM'er. Typically it's AM stations that get into gear. A properly generated FM signal shouldn't be audible in your gear, though if it's not generated right there could be some FM-AM going on there. Does the school also have an AM station by chance?

I agree that Ferrites on the power cord are your best bet. Put them as close to the input to the amp as possible and see if it fixes it.

How much power is the station running (or, just give me the call letters and I'll look it up).

 

[FMEng]

It's interesting to me that it's an *FM* station causing the interference, and not an AM'er. Typically it's AM stations that get into gear. A properly generated FM signal shouldn't be audible in your gear, though if it's not generated right there could be some FM-AM going on there. Does the school also have an AM station by chance?

I agree that Ferrites on the power cord are your best bet. Put them as close to the input to the amp as possible and see if it fixes it.

How much power is the station running (or, just give me the call letters and I'll look it up).

It's not weird at all for a strong FM station to get into an audio system. The "antenna" (cables going into the amp) is far from ideal because it isn't really resonant at that frequency. Hence it creates a lot of AM from the FM carrier.