RF chokes for audio cables?

[Greg_E]

I’m finally getting back to working with the stuff I bought to use with my hearing protectors in the shop, and I’m getting a high frequency whine when I turn my CCrane FM transmitter on. Pretty obvious that I need an RF choke on the audio lines and was wondering if anyone had suggestions on components and design. I had an old rectangular choke from a flat FFC cable that I wound the audio cable around, this helped but still not enough.

So in the end I should probably put a choke in left and right and ground audio lines, and maybe on the power connection for both the TX and the audio processor. For the power runs I can probably just find some wrap around chokes.

[MICRO1700]

Hi Greg:

I have a C Crane transmitter. If I don’t
apply modulation, and turn the radio
volume up high, I hear a high pitched
complex audio whine about 40 DB down.

I have heard that some C Cranes do this. I
believe it is just a by product of the
multiplexing process.

But what is happening with you may be
a whole different thing.

I guess we need to hear from some other
people that are using C Crane FM transmitters.

By the way, I love mine. I have had it for
6 years and it just keeps going and going
with no problems at all. I also really like
the LED that helps you check modulation,
and the internal timer is great, too. Also,
when you are running audio through it,
you don’t hear that whine, (At least I don’t)
so it is not a problem.

Best Wishes
Bruce, MICRO1690/1700 (And NANO 90.9)

P.S. But again, I may be talking about
something that is completely different
from what you are experiencing there.

[radio8z]

The whine may not be coming in the audio cables. Unplug them at the transmitter and check if it gets quiet. If so, reconnect them to the transmitter and unplug them at the audio source. If it gets quiet then the audio source is causing the whine. The noise may not be RF in which case ferrites won’t help. There is certainly no harm in trying them though. I believe Radio Shack has the type that clamp around the cable.

You mentioned a “processor”. This may be the source of the noise especially if digital circuitry or software is used.

Neil

[radio8z]

I have another thought about this which I will pass along. Are there any switching power supplies in use in or near the system? These produce both radiated and conducted high frequency interference. The ferrites may help with the radiated noise but if the noise is due to high frequency ripple you may need to replace the switcher supplies. Switching power supplies are usually in the cord as opposed to being in the “wall warts” but not always.

Neil

[kk7cw]

The whine you are hearing way down in the base band audio is very likely the stereo pilot in the FM modulator. It is actually at 38 KHz and usually shows up in a receiver as a whining noise. Program audio is supposed to cover it up. This situation also means you have a reasonably wide band FM receiver that can hear base band audio above the 15 KHz maximum audio allowed on FM transmission in the U.S. And finally, the preemphasis circuit in the transmitter could be making the stereo tone louder. These transmitters are usually modulated and controlled by cheap FM exciter chips.

[Author]

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