Remove AC hum from FM transmitter power supply

[boylesg]

I discovered the hard way that powering my FM transmitter from a wall plug introduces a massive hum to the transmission that drowns out the audio signal from the microphone.

Is there any simple way to reduce or eliminate the hum?

I tried googling the problem but I did not find any sites that emphatically tell me what to do or try.

 

[betwixt]

A hum that load can only mean the output of the supply is either AC or unfiltered DC. If it is AC, you risk damaging your circuit, especially the capacitors. If it is DC, it needs more filtering and ideally a voltage regulator to keep the voltage stable.

Tell us the specification of the wall plug and the voltage you measure across it's output with your circuit disconnected.

Brian.

 

[boylesg]

A hum that load can only mean the output of the supply is either AC or unfiltered DC. If it is AC, you risk damaging your circuit, especially the capacitors. If it is DC, it needs more filtering and ideally a voltage regulator to keep the voltage stable.

Tell us the specification of the wall plug and the voltage you measure across it's output with your circuit disconnected.

Brian.

It is definitely DC output but it was a cheap plug pack from jaycar. So I guess I can't expect that they would have been particularly fussy with the filtering.

The package said it was a switch mode supply so I guess the main hum may not be the only issue.

12V/1.5A

 

[betwixt]

Before looking further, use a DVM and measure the resistance between the output wires and the AC pins. Check both wires to both pins. Some really nasty adapters do not use an isolating transformer and although there may be 12V across the output, both wires may be floating at high AC voltage. SMPS generally switch at high frequency so a steady hum probably doesn't come from the switching circuits.

Brian.

 

[boylesg]

Around 23k between the pins - measured at the end of the DC plug
Infinite between the AC pins.

 

[betwixt]

No, I meant between each of the DC output wires to each of the AC pins. I'm trying to establish whether you have an isolated adapter or one of the many cheap (and very dangerous) versions that use a capacitive dropper. A capacitive dropper type will usually show a lowish resistance between input and output sides.

Brian.

 

[Audioguru]

Please post the schematic of the FM transmitter. Maybe it has an electret mic that is powered directly from the unregulated power supply. A resistor and a capacitor will fix it.

 

[boylesg]

Please post the schematic of the FM transmitter. Maybe it has an electret mic that is powered directly from the unregulated power supply. A resistor and a capacitor will fix it.

Same schematic as before.


- - - Updated - - -

No, I meant between each of the DC output wires to each of the AC pins. I'm trying to establish whether you have an isolated adapter or one of the many cheap (and very dangerous) versions that use a capacitive dropper. A capacitive dropper type will usually show a lowish resistance between input and output sides.

Brian.

Around 38M in all cases.

 

[Easy peasy]

try 470uH ( 200mA) in the + & - leads to the FM xmtr, and put 100uF across the 22nF supply decoupler - this will reduce the 100Hz ripple on the supply markedly ...

 

[boylesg]

try 470uH ( 200mA) in the + & - leads to the FM xmtr, and put 100uF across the 22nF supply decoupler - this will reduce the 100Hz ripple on the supply markedly ...

I assume by 'across the 22nF decoupler' you mean in parallel with it?

And what exactly do you mean by 'in the + and - supply leads?

Like this with the caps in place of the two parallel inductors?

 

[FvM]

The suggestion is obviously referring to the "wall plug" DC output voltage, not the 230V input.

 

[boylesg]

The suggestion is obviously referring to the "wall plug" DC output voltage, not the 230V input.

I knew that.

But I am not entirely clear what 'in' means.

 

[Audioguru]

You have a very strange AC adapter.
The FM transmitter was designed to be powered by a battery so it has poor power supply filtering. The Capacitor filtering the battery input is only 22nF so add a 220uF capacitor parallel to it and the capacitor filtering the mic and its preamp is only 100nF so add 22uF parallel to it.

 

[boylesg]

You have a very strange AC adapter.
The FM transmitter was designed to be powered by a battery so it has poor power supply filtering. The Capacitor filtering the battery input is only 22nF so add a 220uF capacitor parallel to it and the capacitor filtering the mic and its preamp is only 100nF so add 22uF parallel to it.

How do you know what values to add Audio? Did you do some calculation or do you know from trial and error experience?

 

[Audioguru]

Since we don't have a clue about the filtering in the wall adapter then the values I selected will either cure the problem or reduce the problem.

 

[boylesg]

Since we don't have a clue about the filtering in the wall adapter then the values I selected will either cure the problem or reduce the problem.

I trust your judgement Audio, but I would dearly like to be able to look at a circuit schematic and be able to confidently make changes like that myself.
So what was your reasoning for adding those capacitors?

Also, if I was to power that circuit with 12V instead of 9V, would it muck things up?

 

[Audioguru]

So what was your reasoning for adding those capacitors?

A cheap plug pack is supposed to have a fairly large capacitor to smooth the DC and reduce the hum. But yours is a switch-mode type and maybe it does not have enough space inside for a fairly large capacitor. so add the capacitor to where the DC connects on your transmitter.
Also, the mic and its preamp has a tiny low value capacitor to prevent the preamp from amplifying the radio frequency but it needs a much higher value to filter away hum.

Also, if I was to power that circuit with 12V instead of 9V, would it muck things up?

It might need some resistor values changed to work.
But it will probably cause interference to radio stations and other communications (police, ambulance, aircraft etc) and get the RF cops chasing you. Dont doo eet.

 

[boylesg]

OK so the 'hum' is caused by inadequately smoothed DC and a 'ripple' in the voltage? I understand that! I thought the hum was something else related to the 50Hz.

Why 220uF and not 470uF or larger?

Why a separate smoothing cap at the pre-amplifier?

 

[Aussie Susan]

This is where the 'rule of thumb' part of design comes in.
Basically you are wanting to create a low-pass filter that has a cut-off below 50Hz so that it will not pass the 'hum' on to the circuit.
A standard LPF can be made from a capacitor and another impedance- in this case that other impedance can be thought of as the circuit.
The value of the capacitor will determine where the cut-off point is, but when you have a capacitor of that size, the cut-off point is so far below 50Hz that there is room to maneuver.
Therefore it is quite common to 'just stick a big capacitor' across the PSU using whatever suitably sized capacitor is at hand.
Basically as long as it is big enough to absorb the charge as the voltage rises and then release it as the voltage drops then anything will do.
There is a similar situation with bypass capacitors that can be scattered throughout the circuit. You will often find a mix of 100nF and 10nF capacitors across the supply lines. There is no "calculation" for this but these are good sized capacitors to have the high frequencies that can appear on the supply line due to circuit switching see a nice low impedance path to ground.
Of course if you are dealing with specialised situations (and high frequency circuits into the GHz can qualify) then you may need to be a bit more careful, but if you are dealing with those circuits then you generally have a great deal of experience and specialised training.
Susan

 

[Audioguru]

Why a separate smoothing cap at the pre-amplifier?

Because the preamp will amplify hum so you want to make sure the hum at the preamp is gone. Notice that the mic at the input of the preamp is powered from the preamp supply voltage.