PDA

View Full Version : Buzz/hum in transit audio



KB1JCY
10-18-2007, 01:52 AM
I have an Icom IC-718 which I have connected a Heil PR-781 mic to (I also have the Heil CC--1XLR Icom cable). When keying up, there's a buzz or hum in my audio before I speak. It also happens when I stop speaking before I let off the PTT. I'm not sure what's causing this or if it's something for me to be concerned with. What could be causing the buzz/hum in my audio?

AG3Y
10-18-2007, 02:14 AM
The first place I look for when I know that there is buzz or hum on a microphone is connection to the shield of the audio cable, most likely near the radio end. This allows the entire length of the cable to act as a pickup wire and all sorts of hum, noise and buzz are possible to enter at that point. Obviously, you should check the radio with a different microphone, and check all the connections between the microphone and the radio, making sure that there are no incorrect connections, or broken wires, especially to the ground or shield.

The hum is probably there all the time, but when you speak, the gain is varying, depending on settings of mic level, ALC action ( fast slow, mild or heavy , etc. ) which causes the hum to be much more noticable during pauses in the speech or at the beginning and end of your transmissions.

90% or better chance that it is the mic wiring, most likely the shield or grounded side!

good luck, 73, Jim

W5HTW
10-18-2007, 03:18 AM
First of all, turn OFF the rig's compressor.

Secondly, set any audio gain control for the microphone to a very low setting.

Third, set the microphone gain on the 718 for clean modulation.

It sounds to me like you have "everything to the right" and are amplifiying every trace of noise you can find.

Once you have done that, see if the noise is still present. If it is, you may have a grounding problem. But for the moment it just sounds like you are running everything, or at least the compressor, wide open. This is what compressors "do." They bring up tiny, weak audio.

In most cases, to be honest, any kind of amplified microphone is a mistake, but if it is kept at a very low setting it can be used without overdriving the audio stages of the transmitter.

One way to start is to switch back to the original hand microphone that came with the 718. Set the microphone gain no higher than perhaps 30. With the compression OFF.

edit: I don't know how the gain is marked on the 718. If it is a 1 to 10 setting, pick nothing higher than 5. If it is 0 to 100, pick nothing higher than 35.

I also do not know if the Heil you are using if amplified or not. If it is, on a volume setting of 1 to 10, pick 2. If you then need more gain, do it with the 718's microphone gain control.

Finally, the compression level should be as low as you can use it. With normal speech, the power RF output shoudl average about 30 percent of peak, that is, 30 watts. If you are averaging more than that, you are probably either distorting.

End edit.


At this point you should be able to get proper operation of the 718's audio chain. Now that you have done that, switch to the Heil, and set the microphone gain on it, to maintain the same settings you had with the hand microphone.

Avoid using compression for most operations. It can be handy when chasing DX in crowded conditions, but it is extremely easy to over-use. Many people use it to try to drive the RF output to the rated 100 watts, unaware the output IS at 100 watts peak power, without comrpession.


Good luck and enjoy!



Ed

AG3Y
10-18-2007, 05:12 PM
Ed, one difference between Icom and other radios is that in Icom rigs, the first stage of preamplification is located in the microphone itself. This is because Icom has used Electret elements for years, and in order to operate properly, the element has to be biased and amplified very close to the element itself. There are two things to watch for in Icom rigs. The level into the mic jack is very close to "line level" as opposed to "mic level", and the bias voltage feeding the preamplifier in the microphone rides on the audio line!

Therefore, an amplified microphone will work on any Icom radio I am familiar with, but you do have to make sure that if you are not using a microphone specifically designed to go with an Icom radio, that you do isolate the bias voltage from the audio output of the mic.

Hope this helps. 73, Jim

KB1JCY
10-18-2007, 06:49 PM
Quote[/b] (AG3Y @ Oct. 18 2007,05:12)]Therefore, an amplified microphone will work on any Icom radio I am familiar with, but you do have to make sure that if you are not using a microphone specifically designed to go with an Icom radio, that you do isolate the bias voltage from the audio output of the mic.

Hope this helps. 73, Jim
Are you sure about that, Jim? I thought you had to use mics specifically designed for Icom radios for that reason.

/me confuzzled by that statement...

AG3Y
10-18-2007, 07:13 PM
If the microphone IS designed to be used with an Icom radio, you do not have to do anything but plug it in. It will take the power from the "phantom power" bias supply normally furnished by the radio.

If the microphone IS a self-powered amplified mic, you should isolate the microphone from the "phantom power" bias supply furnished by the radio, as the voltages might be of different amplitude, and even polarity ! This could damage the radio if the isolating capacitor is not used on an externally powered amplified microphone. Is that clear? I hope so.

Actually, I don't know if actual damage will occur, as I have never read a report of someone who was so unfortunate, but certainly improper operation could result, and the audio would NOT sound right, by any means!

73, Jim

KB1JCY
10-18-2007, 10:55 PM
I just had a QSO with a station that was unable to hear a hum/buzz in my audio. I did double check my mic connections (those were fine). The only thing I did was turn off my VOX and use my foot switch.

AG3Y
10-18-2007, 11:32 PM
Check your power output by activating your "transmit" key, and not saying anything into the microphone. You should see very little, if any deflection of the indicator in the "Power" position ( or "Forward" , or however they designate it on your radio ) until you speak into the mic. If there is any deflection whatsoever, it means SOMETHING is modulating the radio, a fan, AC unit, 60 hz magnetic induction from a nearby transformer, etc. etc.

That will tell you a lot more about the quality of your audio, than all the "audio audio AUDIIIIIOOOOOO ! " tests that you hear so often on the bands !

73, Jim

ad: SOTA-1