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DC=>DC converter to power audio converter

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MightyJohn

New Member
Hi everyone,

First of all, I didn't find any presentation part to do a little introduction, so I'll do a little one here.
I'm working as tech support (development of patchs mainly) for a software publishing company, but I have an embeded tech diploma and training (mostly automotive application, but it isn't relevant here, just wanted to introduce myself a little).

Here is the deal, I have make a custom 5.1 amplifier for an entertainment system with 3 TPA3116A2 2x50w chips (24v alimentation).

The bases of the amplifier :
- The signal is send by the ps4 (or the TV, but I'll stick with what I have actually plugged) via an optical cable.
- Then the signal is converted by an audio converter which output in RCA (6 output, 1 by channel).
- Then the 6 RCA output of the optical converter converge in 3 stereo jacks, which are plugged in the amplifier.
- Finally, the amplifier send the sound to the speakers at every side of the room.

Some complementary information :
- There is a 24V (DC) power supply (chinese one, but it seem to work well, just loud, I'll upgrade the ventilation system) plugged to the house and is send to some relays
- The relays then close the 24V circuit when activated (3 switch, 3 amplifier, pretty basic, works well), the amplifiers are now powered.

Then the "cool" stuff, the 5V part (where I have my issue) :
- I use a buck converter to reduce the voltage from 24V to 5V to power the master switch and the relays

There is my issue : I tried to use the 5V part to power the optical converter (which input optical cable and output in RCA), but when I do that, there is an awfull white noise exiting the speakers (not that loud, the music is still a lot louder, but it's not great)

I diagnosed a little bit, try to unplug the audio input of the amplifier, no more noise, so i deduced that the noise come from the converter (I trust the output of the ps4 ^^)

I then tried to plug the optical converter to an usb port of the ps4, the noise is gone, used my raspberry power supply, no more noise ...

The thing is, I want to be able to use the 24V power supply to power the converter, to keep things clean, not use 2 outlet for 1 amplifier ...

Is there anybody that have any idea of what could be the issue and how to solve it ?

Thanks !
 

JonSea

Well-Known Member
Most Helpful Member
Make the DC-DC converter supply around 7 volts, and follow it by a 5 volt linear regulator (7805 for example) with plenty of filter capacitance.
 

audioguru

Well-Known Member
Most Helpful Member
The buck converter works by oscillating and switching on and off all the time producing noise. If the current for the 5V is low enough then replace the buck converter with a 5V linear voltage regulator.
 

simonbramble

Active Member
Make the DC-DC converter supply around 7 volts, and follow it by a 5 volt linear regulator (7805 for example)
I would use something a bit better than a 7805. The PSRR (power supply rejection ratio) or ripple rejection of this component is poor. (This is ability of the LDO to remove noise at its input and stop it appearing at its output). Also it is only characterised at 120Hz. The dc/dc converter will be oscillating at a frequency well above 120Hz, so the ripple rejection will be extremely poor at 300kHz and above, so will be of no benefit at all. The LT3042/45 or other more expensive LDOs have a higer PSRR so their ability to filter out noise coming from the dc/dc converter will be superior.

I just don't want to see you putting in a cheap LDO and seeing it does not work when it is the LDO that is the problem, not the concept of putting an LDO after a dc/dc converter.

You could always put in a 7805 to bring the voltage down to a voltage that is acceptable to the LT3042, then add the LT3042. Then you get the rejection of the 7805 plus that of the LT3042. This should work really well
 
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