boylesg
Advanced Member level 4
- Joined
- Jul 15, 2012
- Messages
- 1,023
- Helped
- 5
- Reputation
- 10
- Reaction score
- 6
- Trophy points
- 1,318
- Location
- Epping, Victoria, Australia
- Activity points
- 11,697
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.
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.
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.
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 ...
The suggestion is obviously referring to the "wall plug" DC output voltage, not the 230V input.
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.
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.
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.So what was your reasoning for adding those capacitors?
It might need some resistor values changed to work.Also, if I was to power that circuit with 12V instead of 9V, would it muck things up?
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.Why a separate smoothing cap at the pre-amplifier?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?