Help with RFI

stereocilia · « **on:** 23 Sep 2009, 04:47 pm »

I've been thrilled with the sound of my RM10 mk ii which I purchased second-hand a few months ago. But unfortunately, I've been listening to AM radio involuntarily through the system, mostly in the right channel. At first, I thought it was just environmental acoustic noise, but now that I've identified it, it's all I hear and it's driving me nuts. I do live only 4 kilofeet from three giant towers of a 50 kiloWatt AM station.

I am using blue jeans interconnects, which I understand are well-shielded. I could try shorter speaker cables, or maybe even coaxial speaker cables, I don't know. When I switch to a different amp the problem disappears. I'll try different tubes, but any other suggestions are welcome.

JimJ · « **Reply #1 on:** 23 Sep 2009, 05:14 pm »

Ferrite toroids may help...

stereocilia · « **Reply #2 on:** 23 Sep 2009, 05:22 pm »

...but hard to say ten times fast. Thanks; I'll look into that.

stereocilia · « **Reply #3 on:** 23 Sep 2009, 11:13 pm »

Here's an update for those of you who are glued to your computers anxiously awaiting an update: I have one speaker cable much longer than the other (I wanted some slack for repositioning, and the equipment is not between the speakers). It turns out that no matter which channel I use the AM radio comes through the channel with the long speaker cable. So, I either need shielded speaker cables (any suggestions?) or I'll just move the amp for minium speaker cable length.

avahifi · « **Reply #4 on:** 23 Sep 2009, 11:39 pm »

Try twisting your speaker cables 3-4 turn per foot. You can chuck an end in an electrical drill to speed up the job.

This will provide some self shielding against RFI pickup.

Regards,

Frank Van Alstine

stereocilia · « **Reply #5 on:** 24 Sep 2009, 12:38 am »

Thank you. The Blue Jeans Cable (Belden 5T00UP) is already twisted, but I will chop off the excess which is coiled around behind the speaker. Also, I hate to move the amp between the speakers because this puts the equipment either uncomfortably near a baseboard heater, or too far out into the room.

stereocilia · « **Reply #6 on:** 24 Sep 2009, 02:56 am »

Short speaker cables worked. I wonder if the loop of wire behind the speaker became an am "loop antenna." No more creepy voices during quiet parts of the music. But, I have to wonder, how much better would my system sound with full-on shielded speaker cables, ferrite rings, shielded power cords, etc. I guess I should remember that I don't get extra points for making the noise more inaudible. (Was that a Kevin Haskins quote reference?)

Ericus Rex · « **Reply #7 on:** 25 Sep 2009, 03:17 pm »

That's weird, though, that you only got the rfi through the Music Reference and not your other amp

srb · « **Reply #8 on:** 25 Sep 2009, 03:36 pm »

I always try to avoid circular coiling with any wire. Probably one of the worst applications is an extension cord reel not entirely unreeled with a motor tool attached. This can create a back EMF which can damage motors, particularly on startup.

But you bring up a good point. Excess wire isn't good for anything and certainly provides more of an antenna for whatever lurks in the air.

I tried once to dress all my A/V cabling, but unless you can be precise in your layout and have all wiring cross each other at precisely 90 degrees, you are probably better off with a random mess!

Sterocilia, it would be an interesting test to go back to the long cable, but stretch it completely out into the room and see if it was a matter of length, or more the coiling of it.

Steve

stereocilia · « **Reply #9 on:** 25 Sep 2009, 05:21 pm »

Ericus: That is a little weird, but not too surprising given that there are multiple ways to make a circuit with the same basic function. The other amp is solid state FWIW.

Steve: That would be an interesting experiment -- I'll post the results. As it stands, if the room is quiet and my ear is a centimeter from the speaker I can hear the sports talk and understand about 15% of it. I can accept that since I could never hear it from the listening position.

I also haven't tried changing from 8 ohm to 4 ohm taps.

KDus · « **Reply #10 on:** 29 Sep 2009, 11:21 pm »

The RM10 feedback comes from the secondary of the output Xmer. That would give RF a path to the gain stage. The length of the speaker cable must have been just right to pickup enough voltage to make it past the RF filter on the front end.
As a broadcast engineer, I've used 3-4 turns through a torroid to get rid of AM on telephones and long unbalanced audio runs.
If you can find a big torroid, a few turns through it would help. Otherwise, you'll need a bunch of turns around a large bolt or ferrous object.

stereocilia · « **Reply #11 on:** 30 Sep 2009, 05:02 pm »

Thanks! I'll work on this when I'm not so overwhelmed with other goings on and let everybody know the results.

stereocilia · « **Reply #12 on:** 12 Oct 2009, 09:27 pm »

As luck would have it, the little 160 mA fuse blew a few weeks ago in one channel, so I am awaiting replacement tubes and fuses before I start trying to figure out how not to have radio signals, long-ish speaker cables, and a tube amp all in the same system. I may have to put my long interconnects back in and stay with the short speaker cables.

Niteshade · « **Reply #13 on:** 12 Oct 2009, 09:55 pm »

You have to find out where it's coming in to prepare a good solution.

1. The power cord
2. Ground wire if present(yeah- it can happen!)
3. Speaker cables
4. Through the source (preamp or device connected to preamp)

I'd believe it would come through the input before anything else since high impedance circuits can soak up RFI . That means through the preamp or whatever is connected to the preamp.

stereocilia · « **Reply #14 on:** 13 Oct 2009, 03:47 am »

Quote from: Niteshade on 12 Oct 2009, 09:55 pm

You have to find out where it's coming in to prepare a good solution.

1. The power cord
2. Ground wire if present(yeah- it can happen!)
3. Speaker cables
4. Through the source (preamp or device connected to preamp)

I'd believe it would come through the input before anything else since high impedance circuits can soak up RFI . That means through the preamp or whatever is connected to the preamp.

I think it's the speaker cables; here's why: I was using asymmetric lengths, and when I swap the left and right channels at the speaker terminals of the amp then the same speaker alwas has the a.m. radio signal in it regardless of which channel is driving it. The same is true if I swap the L and R inputs (after turning the amp off, of course). Also, shorter speaker cables seem to cure problem. Finally, my other amp doesn't amplify the noise even with the long cable. Seems like the only thing it could be is long speaker cable + amp = a.m. radio, unless I'm not thinking it through completely.

I picked up some clip-on ferrite thingies, and I'll try them on the speaker cables to see if that helps when my replacement fuses and tubes arrive. Thanks for putting your mind on this, I appreciate it.

Quiet Earth · « **Reply #15 on:** 13 Oct 2009, 02:13 pm »

You should try a zobel network on the binding posts of your speaker. A very cheap and easy experiment.

Start with a .1uF film cap connected in series with an 8 ohm, 2 watt resistor. Put the cap end on the positive terminal, and the resistor end on the negative terminal of the speaker. Buy cheap parts to play around with before you splurge on premium parts.

stereocilia · « **Reply #16 on:** 14 Oct 2009, 03:41 am »

Cool. I'm not far from a store which is sure to have those in stock. Now, I have to go look up what a zobel network does, exactly. I'm guessing it provides a parallel path through which noise prefers to go instead of through the speakers. But, before I get to build any Zobels, I'm still waiting for a fuse and some tubes.

Niteshade · « **Reply #17 on:** 14 Oct 2009, 12:04 pm »

Huh...the speaker cables! That's interesting. RF does have a mind of it's own.

It can be a royal pain.

A small cap like a .1uf across the speaker terminals might help. It should not effect the sound.

stereocilia · « **Reply #18 on:** 28 Oct 2009, 12:32 am »

I hooked up the long speaker cable to see if the problem still exists with the new tubes. It does, but it's much less, and I doubt that I would hear it from the listening position anymore. Weird, but good. Moving and coiling/uncoiling the cable makes litte difference as far as I can tell.

So, since I could trim the speaker cable to a little less than 2/3 the original length and still have enough, I'll see how that works. (I hate to cut a long speaker cable just in case I need it someday). If the a.m. radio still bugs me then I can go back to the setup with long interconnects and short speaker cables and amp between the speakers. That works okay.

Thanks everyone. I will try the Zobel thing, too, if I need to. Or, maybe even if I don't.

stewie · « **Reply #19 on:** 28 Oct 2009, 04:12 am »

Maybe someone has mentioned this, but you might also try reversing your interconnects, one at a time and see what happens. I changed ics a few weeks back, got a terrible hum and a.m. radio problem, reversed the ics (in this case, when against the manufacturer's suggested directionality) and the problem disappeared.