Subject: Is no load on amps OK? To: Dear John K Browne:

A laptop PCMCIA modem (by Megahertz) is picking up very loud interference of what sounds like a talk radio station. I tried the Radio Shack modular plug-in with pigtail device and it was useless, no surprise considering it's marked 3-30 Mhz. Yours is specifed at 0.5 -150 Mhz which covers both AM and FM, great! The part I don't understand is that yours is 3-5 times more effective: since they are of widely different frequency ranges, what does the 3-5 mean?
My other question is about listening through headphones with disconnected speaker wires.
I'll keep that in mind for future troubleshooting. Can I assume most audio receivers/amplifiers are designed to withstand power-on without a load?
Thank you.

Dear Sir:

Thanks for the inquiry. You seem to be quite a clever fellow.
Most Stereo amplifiers have a headphone only mode of operation. There is either a button or a switch to turn off the speakers whilst keeping the headphones active.

While RF is often picked up by the speaker leads it is usually detected by an earlier (more sensitive) audio stage not the audio power amp stage so the above test helps detect that type of problem.

I assumed that most people would know that the term "listen with the headphones" meant to listen with the headphones as described in their stereo owners manuals. However you may have a point and I may need to further explain that statement on the web page(s).

In the case of testing computer sound cards for RF pick-up, the headphones are the load, because they plug directly into the stereo speaker output jack.

As far as using the term "3-5 times more effective" I have qualified that in most places but I guess I need to check over the pages once more.

There is quite an overlap in the frequency ranges covered by our filters and the Radio shack version.

Recent measurements show that:
At 850 Khz. our filter attenuates RF by a factor of 6 times more than Radio Shack's filter. Their filter is identical to K-COM'S filter by the way.
At 1400 Khz. we get 7 times more filtering.
At 1.8 Mhz it is 6 times better. The latest version of our filter measures 8 times better (ie. there is 8 times less RF than with the Radio Shack (K-COM) filter).
At 3.5 Mhz it is 6 times.
At 7 Mhz our filters attenuate RF more than Radio shack's by a factor of 3 times
At 14, 28 and 50 Mhz our filter works about the same as Radio Shack's.
At 108 and 144 Mhz our filter attenuate Rf approx. 3 times more than the Radio Shack filter.

Does the laptop interference disappear when you disconnect the telephone line from the modem?????????????
I suspect the interference is an AM station since a majority of the talk shows are AM.
So! What do you think sir???? Sincerely, John Kirk Browne

Click on the picture for RFI filter construction.


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