RFI-Immune Computer Speaker Recommendations

[W2XR]

Very possibly this subject has been beaten to death in this forum previously; if so, I apologize.

I am looking for suggestions as to a make and model of amplified computer speakers that offer significant immunity to RFI in the HF part of the spectrum, when placed in the near-field of a legal limit 160 thru 40 meter plate modulated transmitter. Assume the defination of near-field to be within 50 feet of an 80 meter dipole antenna suspended 45 feet above the ground when energized by this transmitter.

One brand that I have seen on the Internet as being rated quite good at RFI immunity are the COMspkr(s) from West Mountain Radio. I have never heard of this company or this product.

As we all know, computer speakers are probably the worst offenders when it comes to RFI-prone consumer electronics, and short of gutting them and going for a complete rebuild to improve their immunity to RF, which is not really cost-effective for such low-cost electronics, I'd rather purchase something out of the box that fits the bill.

Again, the requirement is that the speakers must be amplified (internal/self-contained audio amplifier).

Thanks & 73,

Bruce

[W1RKW]

Bruce,
I have two computers in the shack. One has Harmon Kardon powered speakers and the other Cambridge Audio Works powered speakers.  No RFI issues whatsoever.

[KM1H]

The 31 mix ferrite toroids and large beads work wonders on HF, Mouser has the best prices, Amidon and Palomar the worst

Carl

[K5UJ]

Bruce, good luck. 

This is probably one of the toughest RFI problems to face.  At worst, you have:

A PC and sound card in an unshielded plastic case but maybe it is metal.
In any event, the lead from the sound card is usually unbalanced and unshielded.
Most PC speakers use a solid state audio amp in one of the speaker enclosures and it is unshielded.
Wire to the speakers is unshielded and unbalanced.

Some of this can be dealt with using ferrite cores in the right locations but the big problem is usually diode rectification in the solid state audio amp.  I've tried removing it and shielding it--no luck.  You could try bypass caps and ferrite beads but you are usually looking at a pc card with no schematic and ICs, maybe surface mount parts and.....  I didn't want to get into that with a cheap POS audio amp.   Oh yeah, another prob. is possible pickup on the DC cable to the audio amp from a wall wart. 

In my case I dealt with it by just powering down the speakers when I operate.  Actually I no longer even have a computer in the shack. 

But I did have another audio issue that comes into play--RF into my receiver speakers via the outboard audio amps to them.  It's worth noting that these are p.p. tube amps and a lot of RFI hassles seem to go away by design with them.  For example, you have Z matching transformers between the plates and the speaker leads which serve as isolation transformers.  Tubes don't seem as sensitive to RF as transistors do.  I licked the RF in the receiver audio by using a few large ferrite cores on the audio lines to the amp inputs from the receiver right at the amp chassis.   The audio cables wrapped on them as many times as possible.  Big Fair Rite cores with i.d. holes about 1.5 inches.  Don't know the material number.   And the audio amps operate with no covers and the station antennas pass right over the house; 300 watts or more on 160, 75 and 40 m.   

So if it's your shack and you have to have PC audio, I'd dump the PC speaker setup and look for a small vintage mono tube amp with a 12AX7 phono needle preamp and put that on the sound card output.  Keep the speaker lead as short as possible.  Adjust for the right levels and wrap the sound card cable a bunch of times on a material 31 core and I'll bet you'll be in pretty good shape.

Rob 

[W3RSW]

look for a small vintage mono tube amp with a 12AX7 phono needle preamp and put that on the sound card output.

Check to see if 'that' 12ax7 is surrounded with RIAA equalization circuit first.  RIAA records had heavy preemphasis and the playback preamps matching deemphasis. I guess you could say that the deemphasis would act as a kind of RF filter. 

 Some preamps had the first triode stage as a RIAA eq stage and second as just a voltage amp.   May have better results going directly to the second stage in that instance.  Some had both triodes of the 12ax7 in a RIAA feedback circuit and were followed by one or two more triode voltage amps.  Shouldn't be too hard to see where to tap in to a non-compensated stage and use light  R/C filtering of the input to squelch RF.

[W2XR]

Thanks for the replies, guys!

The issue here is not RFI within my own QTH, but rather one neighbor I have. They are very nice people, and great neighbors, but they spend a lot of time on their laptop, to which these crappy computer speakers are conected. Their irritation when I fire up the big rigs is understandable, and I'm looking for a solution that is mutually acceptable and effective.

I have tried the usual clamp-on ferrite cores, lining the inside of the speaker/amplifier housing with aluminum foil, but all of this is to no avail. Their home is about 50 feet from my antenna, and with a pair of plate modulated 4-400As or the GPT-750 on SSB at my end, you can see the problem. Aside from this computer speaker issue, I have really no other RFI issues to speak of with them or with anyone else.

Suffice it to say, it is a real PITA to be on the air and to have the phone ring............and I always know what the context of the call will be.

No, it is not practical for us to move to a QTH where our nearest neighbor is 2 miles away.

Thanks & 73,

Bruce

[WA3VJB]

Fostex 6301

Buy a pair.

Or, buy one and see if it solves the problem. If it doesn't, you will have a dandy local monitor speaker for yourself. 

I've got three or four of them and they're great.

[W2XR]

Thanks, Paul.

But do these Fostex units offer significant immunity from HF RFI?

73,

Bruce

[K1JJ]

Hi Bruce,

You already tried enclosure shielding and ferrites. Those are good tests.

Now, how about an L/C low pass filter in the speaker input leads?  

Also, a lead pipe approach - try a parallel resonant trap in series with the speaker hot lead to drop the 3.8mhz across it. If it works, then maybe a 40M trap too, etc.

Assuming it IS the speakers and not the laptop itself, then take the speakers to your workbench and couple some signal generator RF into it and see how effective you can be with filters.

T

[W1AEX]

Bruce,

I use a pair of Bose Companion II speakers and have found them to be very resistant to RFI. They are commonly used by guys running Flex Radios because of their resistance to RFI. I also picked up a set of the West Mountain Radio COMspkr units at Nearfest for 10 bucks and use them on a separate computer across the room. I have never heard them squawk when I'm operating.

My own personal philosophy on this issue is to simply say that when something that is not a radio receiver starts to act like one then it needs to be fixed or replaced. You might offer to help them write a letter to the company that makes their current speakers as it's really their issue to resolve. Back before cable, when I had issues with TVI and some phone RFI the first thing I would ask is, "Have you contacted the manufacturer?" I have not had a complaint since the early 90's but I handled those in that way and it was entirely satisfactory (for me anyway). Sounds kind of cold-hearted but are we really responsible for every piece of junk our neighbors drag home from Walmart? Just sayin'...

:O)

Rob W1AEX

[KM1H]

JW Miller used to sell LC filters for speaker leads that worked well.  If those clamp on ferrites are the RatShak type they are 43 mix which does little at 80M and lower. Even with 31 mix you have to wind as much wire as you can on them, just clamping on is useless.

[W2XR]

Many thanks to all who replied, and for their guidance.

Based upon the recommendations here, I intend to acquire a pair of the West Mountain Radio COMspkrs and give them to my neighbor, and hope for the best.

73,

Bruce