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Discussion Starter · #1 ·
Newbie question:


I've got a setup w/ neuhaus T-2 tube amp, 2 Definitive ProMonitor 800s, connected via Monster XP speaker cables.


Regardless of what input I select on the amp (optical, CD, tuner, etc.), I often hear crackling/static from my speakers.


1) I observed that if I move/wave my phone around the room, the noise would change with it. Also, the noise gets awfully loud especially when I receive phone calls and SMS text, etc.


2) My wireless Internet network seems to have a similar effect.


Is this noise related to the EM interference from 1) and 2) ? If so, how could I alleviate this annoying issue? If not, what could be the source of the crackles and static?


Thank you very much!
 

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Try removing all the input signal cables from the amp.

Also try plugging the amps AC power straight into the wall outlet.
 

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Discussion Starter · #3 ·

Quote:
Originally Posted by Speedskater /forum/post/19542869


Try removing all the input signal cables from the amp.

Also try plugging the amps AC power straight into the wall outlet.

The noises are still present even with ALL input cables removed. Also, my amp is plugged directly into the wall outlet. Could it be the cables between the amp and the speakers?


BTW, this might be obvious, but there's no noise when the amp is off.
 

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Well the noise is sneaking into the amp on the speaker cables.

It's the amp's circuit that has the problem.

You might try a twisted pair speaker wire with lot's of twists.

You might try a ferrite on the speaker cable and maybe the AC cord.

Put the ferrites very near the amp.

But don't get your hopes to high.
 

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The impedance of the output circuit and the speaker will not allow the small signal RF radiation for as cell phone etc. to enter the signal chain at that point. The problem is the INPUT circuitry is not sufficiently shielded. This MAY or may not be solvable on a retrofit basis.


With no input cables to the amp connected and the volume control at zero(all the way down) do you still get the noise incursion?
 

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Quote:
Originally Posted by Gizmologist /forum/post/19543063


The impedance of the output circuit and the speaker will not allow the small signal RF radiation for as cell phone etc. to enter the signal chain at that point. The problem is the INPUT circuitry is not sufficiently shielded. This MAY or may not be solvable on a retrofit basis.

True, so twisting the speaker wires probably wont help.

But, the RFI can still sneak in on the speaker wire, then into the feed-back loop that is sometimes connected to the output transformer's secondary.
 

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Its not an RFI issue, that is the issue is not at high frequencies.


Many cell phone systems are based on versions of TDM (Time-division multiplexing) AT&T's GSM and T-mobile are especially good at this. I'll skip the gory details but what you are hearing is not interference from the carrier frequency but rather those types of systems break up the available transmission time into lots of time slots. In many cases the spacing of slots in time is in the audio spectrum. To put it another way the radio in your phone is pulsing on and off say 2000 times a second. The fact that its carrier frequency is 900mhz and each time it comes on it sends out a burst of high frequency information doesn't really matter as the pulsing at 2khz is the problem. Ala you don't need to approach this as a situation where you need to keep RFI out of your system but rather one were you need to keep 2khz EM waves out of your system.


This pulsing can be picked up by the speaker coil itself, inputs, cables, etc. In many speakers it doesn't need to be amplified to be audible. Not surprisingly speaker coils are relatively good at picking up EM waves in the audio band and converting them to current. The best solution is a shielded speaker, shielded cables, etc. More realistically, leave your cell phone in another room or put it on airplane mode.
 

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Since the tubes are not shielded very well in most amps, they are the first viable culprit. I have never had any type of EM noise imposed on a speaker line.


Simple test: Disconnect the speaker cable from the speaker. If the EMR is directly affecting the speaker, it will be apparent on an unconnected speaker, or headphones.


Some tube amps and receivers do use tubular metal shields but they are relatively poorly grounded to the sub chassis and as a result offer small amounts of RF shielding.
 

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The only easy thing you can do is use ferrite cores on speaker and power cables. If it won't help, then you better sell your unit. It is usually always possible eliminate EMI, but cost to do it will exceed what your amp is worth.
 

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Quote:
Originally Posted by xianthax /forum/post/19543633


Its not an RFI issue, that is the issue is not at high frequencies.


Many cell phone systems are based on versions of TDM (Time-division multiplexing) AT&T's GSM and T-mobile are especially good at this. I'll skip the gory details but what you are hearing is not interference from the carrier frequency but rather those types of systems break up the available transmission time into lots of time slots. In many cases the spacing of slots in time is in the audio spectrum. To put it another way the radio in your phone is pulsing on and off say 2000 times a second. The fact that its carrier frequency is 900mhz and each time it comes on it sends out a burst of high frequency information doesn't really matter as the pulsing at 2khz is the problem. Ala you don't need to approach this as a situation where you need to keep RFI out of your system but rather one were you need to keep 2khz EM waves out of your system.


This pulsing can be picked up by the speaker coil itself, inputs, cables, etc. In many speakers it doesn't need to be amplified to be audible. Not surprisingly speaker coils are relatively good at picking up EM waves in the audio band and converting them to current. The best solution is a shielded speaker, shielded cables, etc. More realistically, leave your cell phone in another room or put it on airplane mode.

Yeah, I think I have this problem at work with my really cheap computer speakers. My office is in a building on the same block as AT&T's central station and huge cellular tower/transmitter. I also have an AT&T cell phone in my pocket most of the time.


My cheap computer speakers act as radio receivers for that EM pulsing even with the volume at 0. If the power to the speakers is on, they squeal and wail like crazy intermittently, always at the same frequency. It's so bad, I leave them off except when listening to voice mail (my office's IT network is set up so we can listen to our voice mail through Outlook).
 

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Discussion Starter · #11 ·
Wow, this is more complicated than I thought.


I moved my wifi router further away from the amp, and that practically eliminated its impact.


As for cell phone, the noise still occurs, and yes it seems to be more amp related.


Thank you for all of the suggestions posted! I am going to need to think about whether I should do a major repositioning of my AV equipment, keep my phone on airplane mode when the amp is on, or some other arrangement.


BTW, I did a rough, little experiment: The noise always happens when I turn my phone on/off. While doing that, I waved a steel cooking pot around the tube amp. When the pot is positioned at certain points above/near the amp, the noise gets noticeably quieter. Any ideas on what this might mean? Perhaps I can scrape together some sort of "faraday cage" to alleviate the interference?????


Thanks again for educating a rookie, this forum has been really helpful!
 

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Quote:
Originally Posted by avamk /forum/post/19544343


Wow, this is more complicated than I thought.


I moved my wifi router further away from the amp, and that practically eliminated its impact.


As for cell phone, the noise still occurs, and yes it seems to be more amp related.


Thank you for all of the suggestions posted! I am going to need to think about whether I should do a major repositioning of my AV equipment, keep my phone on airplane mode when the amp is on, or some other arrangement.


BTW, I did a rough, little experiment: The noise always happens when I turn my phone on/off. While doing that, I waved a steel cooking pot around the tube amp. When the pot is positioned at certain points above/near the amp, the noise gets noticeably quieter. Any ideas on what this might mean? Perhaps I can scrape together some sort of "faraday cage" to alleviate the interference?????


Thanks again for educating a rookie, this forum has been really helpful!

It looks like exposed front end tubes pick up interference. You need to find a way to shield them or the whole part of your amp, where tubes are exposed.
 

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Try ferrite on the speaker leads as close to the amp as you can get.
 

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The noise is getting into the INPUT stages, not the speaker cables.


Does the amp have metal shields on the tubes? THAT is the first line of defense. If the input cables are disconnected and the gain control is at zero, then the only point of incursion will be in after the first stage.


If there tubes have metal shields, check with a good ohmmeter to see if there is continuity between the shields and the chassis and between the shields. Some tube sockets have simple mechanical pressure clamps to secure the shields and some are simply friction fit over the tubes.


In order to be truly effective the shields must be grounded to the chassis.
 

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Quote:
Originally Posted by Gizmologist /forum/post/19544730


The noise is getting into the INPUT stages, not the speaker cables.


Does the amp have metal shields on the tubes? THAT is the first line of defense. If the input cables are disconnected and the gain control is at zero, then the only point of incursion will be in after the first stage.

This amp does not have any shield over tubes. See here http://www.neuhauslabs.com/amplifiers/
 

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If you could find these shield now, they would ruin the sexy appearance of the amp. The preamp tubes are the only ones that would likely need shielding.


Is the amp in a location where it is easily visible?


One possible solution would be to use a large diameter braided tube (think the braided shield in a cable but much larger) could be slipped over each tube and have a common drain wire soldered to each sleeve then grounded to the chassis.


The big issue is that the shield must have equal spacing for the full diameter of the tube. (fit snugly)


What are the tube types in the first stages? Something along the lines of 12AU/X7?
 

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You could put a metal box around the amp - attach it to the chassis, and a good ground (aka grounded faraday cage). If the problem has always been there, you could return the amp - it might not be suitable for your environment. If the problem is new you might have a failing tube. Talk to the people who made your amp, maybe they have some experience and can suggest a fix.
 

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Quote:
Originally Posted by Gizmologist /forum/post/19545824


found some. this is the style I am referring to for the 4 preamp tubes.

This will likely require replacement of tube sockets (2 - 3 billable hours of work) and violate warranty. I would return or sell it instead.
 
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