I constructed this rf amplifier:

http://ba1404.blogspot.com/2010/06/1-watt-universal-rf-amplifier.html


I am using this amplifier for a video transmitter with audio. The circuit works really good, and the video is very good, but I have an irritating background hum in the audio. Any RF pros here? I'd like to dramatically quiet the hum, or eliminate it. I tried different rf coils(also compressed/expanded coils), added a trimmer in place of C1, and I also made sure of the calculation for my frequency. my RFC is an epoxy coated inductor of the desired value, and the 2 other coils were made from 20ga wire on a 7/16" form...to the desired turns/values. I also made a custom steel enclosure for the entire circuit, and use a coax connector at the input/output. Can I add a high value resistor somewhere? Or an additional pico capacitor? All help appreciated. Thanks

 

What are you using for a power supply?
You may have have a 60 cycle power supply interference problem.

 

The typical source of hum is from the power supply. Please answer some questions.

What are you powering this with?
If you remove the video and audio from the carrier do you still hear the hum?
Where does the amp get its signal from?

 

Does the hum vary in any way according to the picture content? This may point to non-linear "vision-on-sound" effects in the transmitter, perhaps because it is being driven too hard. Note that this can also happen at the receiver. Is there any chance that the received signal is too strong?

Another thing to check is the quality of the signal from the modulator feeding the transmitter. Are you sure that this is clean in the first place?

 

doug08,

What you describe sounds an awful lot like "60 cycle hum". 60 cycle hum is caused by eddy currents in larger coils and transformers. Try Googling for "60 cycle hum" and you should get plenty of information to help solve your problem.

williamj

 

definitely not a power supply issue.

Thanks

 

definitely not a power supply issue.

Thanks

Yes, that eliminates the supply but you didn't answer the other questions that I asked. They were asked because that's a class C amplifier and I wanted to determine if the audio or video was AM. You can't feed AM into a class C amp.

 

Are you keeping all your component and coax leads connected to the PCB as short as technically possible ?

Darn nice link to the circuit. btw -- Welcome to AAC !!!

 

I have a lot of experience with electronics. When I build something I do it extremely well/professional or I don't do it at all. Check out the pics below of this amplifier. I made the board/housing/all of it. I cut out the coil from the board to try different values. The other side of the board you cannot see has another coil on it. I can make the audio hum almost completely gone, but I have to sacrifice video quality to do it. I think I will try adding a trimmer cap in certain places to better fine tune the unit. Check out other things I made on Youtube under the user ID "whereisdbn".

 

Is this being connected to an antenna? It's not related to the question but re-read post 4.

 

Improved video translates into more audio hum. The antenna which connects to this amplifier(which was not shown)....is a dipole which I made out of standard coax, and some wire for the radiators. I used a dipole calculator for a 1/2 wave 67.45 mhz antenna. I think I will add a 4:1 balun to my antenna tomorrow then see what effect it has on the hum/video, and also see if the rf field is detected further away with my rf counter/meter.

 

Are you located in the USA?

 

No problem transmitting where I am located.

 

Ha, I guess you saw where I was going with this. Anyway, have you tried a small pad on the input?

 

I made a new antenna today. I constructed a folded dipole with a coax balun, and fed it with coax. I tried the transmitter with the amplifier and had huge success. The hum is now able to be adjusted out....the old dipole w/o the balun was the problem.

Thanks