I use small FM transmitter to transmit my music. It is a battery type but I've added wall wart power supply so it can run 24/7. But it hums. I've tried adding filtering caps with resistors, different switching and transformer power supply but just can't get rid of the hum and buzz. It's not extremely loud but it's annoying. Is there a way to isolate the power supply?
If you have the voltage headroom to include regulation, you may well reduce the residual hum and noise. Buzz is usually indicative of a transformer/rectifier type power supply. I have used regulated type wallwarts for radios of all kinds but even their filtering is inadequate for use with hifi devices or sensitive communications receivers.
If adding RC filters to the power supply doesn't reduce the noise significantly, you have to look at the qualities of the FM transmitter itself. Most simple and DIY hobby types are really too basic, relying entirely on the battery supply for clean power. They will have no internal filtration that prevents PSU noise from modulating the RF or preventing the RF from leaking out via the power supply wiring and then propagating via the supply and input leads as much as the aerial. That wont be so simple to eliminate without effective RF choke filters and a grounded metal enclosure as part of the filtering to the transmitter and also the source/input wiring.
Unless you have the time to research how the pro. gear is powered and filtered, it may be wiser to just use a LiPo battery with a smart charger fitted. An Ebay 5V USB type charge controller kit is often very cheap and convenient for these little transmitters too.
If adding RC filters to the power supply doesn't reduce the noise significantly, you have to look at the qualities of the FM transmitter itself. Most simple and DIY hobby types are really too basic, relying entirely on the battery supply for clean power. They will have no internal filtration that prevents PSU noise from modulating the RF or preventing the RF from leaking out via the power supply wiring and then propagating via the supply and input leads as much as the aerial. That wont be so simple to eliminate without effective RF choke filters and a grounded metal enclosure as part of the filtering to the transmitter and also the source/input wiring.
Unless you have the time to research how the pro. gear is powered and filtered, it may be wiser to just use a LiPo battery with a smart charger fitted. An Ebay 5V USB type charge controller kit is often very cheap and convenient for these little transmitters too.
Two main possible causes of hum with a transmitter are: a) - cleanliness of the DC supply, has been covered above, and b) - the modulating effect of the rectifier diodes: they act as a dirty mix between PIN diodes and varactors, and cause parasitic phase and amplitude modulation of the transmitted RF, because the ground/supply connections act as a return connection to the antenna's ground plane.
If DC-oriented measures are ineffective, try placing 10nF ceramic discs directly across each diode of the bridge. They must be as close as practicable, each mm counts.
You can also pass the supply cable through a ferrite rings (more than one turn if possible), and make a ground connection directly from the transmitter to a large metallic mass (does not necessarily need to be earthed): radiator, metallic furniture, etc
If DC-oriented measures are ineffective, try placing 10nF ceramic discs directly across each diode of the bridge. They must be as close as practicable, each mm counts.
You can also pass the supply cable through a ferrite rings (more than one turn if possible), and make a ground connection directly from the transmitter to a large metallic mass (does not necessarily need to be earthed): radiator, metallic furniture, etc
Some good ideas thanks. I did try ferrite ring but it did noting. And I'll ready have it grounded which helped a lot, that was the first thing I did when I've set it up. Also tried charger battery but that was the same how ever Lipo is a good idea as it might only charge when at 3.2V. I'll try that. Ceramic disk I've not try but will. I'll see how it goes, there are other factors that make it hum like the position of the power wire etc. Yes it's a cheap transmitter. I can get it to sound almost clear but than can't receive signal all over the house. It's a bit of a catch 22.
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