SOS - audio hum on AV receiver

[bmdesigns]

hi there -

i am building some bits and pieces for my video filming work including a 2.4ghz video/audio (stereo) transmitter/receiver which works fine so far.

however, when all plugged up and working, i always get a hum on the sound from the receiver. when i unplug the video cable on the transmitter (i.e. to no longer transmit a video signal but still the audio), the hum goes completely. obviously i would like to keep the video signal but lose the hum.

am i missing something here or should i be adding another component that will sort this?

many thanks in advance,
nick.

 

[Nigel Goodwin]

It seems a fairly bizarre thing to try and build?, and almost certainly illegal, such commercial devices can be licence free - but have to tested and approved, your's will be illegal because it's not passed the required tests.

But apart from that, how are you transmitting the stereo audio?, as removing the video stops the 50Hz hum, it's probably the video signal coming through on the audio (or the audio is muted in the absence of video?).

 

[bmdesigns]

i'm sorry for not being clearer in my earlier post.

my knowledge in electronics is limited so i will try to explain where i am at:

the transmitter and receiver parts are pulled from a consumer version very similar to the one seen on the Rapid Electronics website (part no. 43-0308).

http://www.rapidelectronics.co.uk

the transmitter side is left pretty much untouched (although will eventually will be coupled with a 7" tft monitor). the receiver side is connected with a dual L386-amp circuit that allows me to monitor the incoming audio on a pair of headphones. this amp circuit works fine and i have made sure to keep the input grounds seperate from the output grounds to avoid oscillation.

in both the transmitter and receiver circuits, the video and audio channels share a common ground with the power supplies (both regulated 12V dc through mains adaptors). this is how they were in the original consumer version so i have kept it that way.

like i said, the hissy-hum dissappears when i pull out the video cable in the transmitter and varies depending on the image make-up (bright versus dark areas). also, if i were to pull the video cable out of the receiver, an extremely loud buzz is heard - i'm not sure if this is because i need termination...?

like i said, i don't know much but i am trying to learn and any help that you might be able to offer me would be most appreciated.