RFI Audio Problem...

[T23]

I am having an RFI problem, I have made a custom cable to take input audio from a pre-amp and ribbon mic, I am getting audio and a static or buzzing sound, I am on 2 meters running 50 watts and the antenna is just outside the house, I have noticed that the sound isn't as bad when running lower wattage such as 12 watts. I think the RFI problem has to do with the cable I made because with just the stock mic or just the ribbion mic with no pre-amp my audio is fine.


T23

 

[KE3W]

What happens if you take the pre-amp out of the picture?

 

[Robb]

Is it a shielded cable?
If not; then there is your problem . . .

 

[T23]

Good clean audio, The irritating sound is gone when the pre-amp is not in line.

Yes its an XLR cable its shielded...

T23

 

[Robb]

Then go with the mic w/o the preamp until you can make it work.
Sounds like you don't have it wired right - to me . . .

 

[T23]

I am pretty sure it is wired correctly, I get audio even when the buzzing is about, works fine without the preamp, maby some rf chokes might help...

Even when only my headphones are connected to the pre-amp and not the radio I get the same buzz when I key up in my head phones. That tells me its RFI.


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[unit_399]

what are you using to power the preamp? If you're running it off of a dc supply, it's possible that rf is being picked up by the supply leads causing the buzzing.

- 399

 

[KE3W]

Good clean audio, The irritating sound is gone when the pre-amp is not in line.T23

That is what I thought. You have to make sure the power feeding the pre-amp is clean. Now, what are you using to feed the pre-amp? Do you just have it plugged into a 12 Volt Power Supply? Do you have it plugged into wall adapter? What is feeding the puppy?

 

[T23]

A wall brick, it supplys about 9 volts to the tube preamp.

I have tried pluging it into the power strip my radio is connected to and directly into the wall. The radio is a mobile so it has its own 13.8 volt power supply.


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[KE3W]

If you are still plugging it into the wall brick regardless of whether you are going directly to the wall AC or power strip - it probably is the wall brick. How do I know? I had the same exact problem (buzzing) with a electronic keyer feeding back into my headset. The moment I changed the power supply (9 volt) adapter (changed it for a better one) - the noise went away. So, do you have another 9 volt source you can test out?

 

[T23]

I have another multi purpose generic wall adapter, but the thing that gets me though is that even when the output gain is all the way down I still get it, and also it is not so bad with lower transmit wattages, I also get the same noise in a headset even when the pre-amp is not connected to the radio when I key up?

T23

 

[KE3W]

Whether or not you have the gain turned all the way down - whatever is "buzzing" still causes interference. If you unplug the "brick" completely (and unplug the microphone as well) and you are still getting the "buzzing" - you have something else in your shack causing the problem. Now, I am assuming you didn't have this problem until you started using your new microphone. IF after unplugging the microphone and "brick" and you still have the noise - start unplugging stuff around the room and see if the noise goes away. Now, even if it doesn't - check your other rooms because the noise could be generated along the AC line feeding your wall outlets.

 

[T23]

The new mic works fine without the pre-amp ( it could be a bit louder though ) and so does the original hand mic I have been using with this radio, but I only hear the noise when the radio is keyed whether its through just the head phones or through a monitor radio on the same frequency with the pre-amp. So I got to figure if it is the RF energy getting into my audio lines while the pre-amp is inline or is it the wall brick for the pre-amp...


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[RatsoW8]

I'm trying to figure out why you're using a mic into a pre-amp into a 2m radio. Is that correct? What type of interface are you using between the pre-amp and the radio?

I would think you need to use an interface similar to the w2ihy iBox to take the line level audio down to mic level audio. If you are running a preamp directly into the rig that's most likely where the hum is coming from..an impedance mismatch. It's also possible you could damage the 2m meter radio by doing this. if I'm off base on your setup just disregard, but this it what it sounds like to me.

You also don't make 2m audio "loud" by using power mics or other audio boosters. it doesn't work the same AM and SSB audio does.

 

[T23]

Yes it is a 2 meter radio, I know it sounds overkill for 2 meters, but I just wanted to take advantage of the FM audio quality, apparently the mic is loud enough without the pre-amp after I did some more testing, my monitor radio is a Icom IC-2A series handheld and I used some headphones with it to test the direct mic only audio and it sounds great. As for the pre-amp I keep the output gain very low so as to not damage the radio.

Hey Ratso could you elaborate on the impedance mismatch, just trying to look at this problem from all angles...

Give Robb's artical on high quality audio for your export radio a read, thats what I have based most of my audio setup on.

And the pre-amp is really just there for the "tube sound" not so much for amplification...

T23