DIY audio player with Loud noise - Page 1
Forum section: electronics / repair

author: Prince Gautam
author_rank: Newbie
timestamp: March 15, 2020, 01:52:15 pm
content: Hello all,I recently picked up a DIY project where I want to used my old home theater speakers to make audio system which I can use with my computer.For this to complete I got1-stereo amplifier ckt using IC cd44401-smps 12v 5amps and supply cord1-stereo cable (to connect my audio system to my PC)And a pair of old 7w RMS 6ohms speakersNow, I have connected everything in correct sequence which I know bcoz its giving out sound which I am playing but with lot of high volume noise.the problem is when I power the entire thing with 12v 5amps SMPS this noise is generated. But, when I connect my variable bench power supply by setting it at 12v the sound is comparatively very clear (there is still some humming/cracking noise at very low volume).I think the problem is with the high frequency of the SMPS bcoz I tried changing 3-4 SMPS from different vendors or it might be the earthing which this SMPS don't have.I am not sure of the cause.Is there some way that could solve this problem by adding some filter or component to the circuit.Thank you for your time.

author: greenpossum
author_rank: Frequent Contributor
timestamp: March 15, 2020, 02:20:51 pm
content: Can you make a recording of the noise and attach to your post? Also what happens if the input is something totally isolated from the mains, like say a phone?

author: Haenk
author_rank: Super Contributor
timestamp: March 15, 2020, 02:27:55 pm
content: Use a linear power supply instead - these behave much friendlier (noise-wise). FIltering is also much easier (try a large cap, maybe that's enough).

author: Prince Gautam
author_rank: Newbie
timestamp: March 15, 2020, 04:18:33 pm
content: Listen to audio in low volumeI have attached 3 audio files,please checkAudio1-when no Input is connected.Audio2-when mobile phone is connected as input source and song is playing.Audio3-when song is not playing but phone is connected.There is slight change in input noise when audio jack is connected and not connected which can be heard in audio3

author: DaJMasta
author_rank: Super Contributor
timestamp: March 15, 2020, 06:50:25 pm
content: It sounds like an air compressor!There may be some circuit design/layout things you can work out, but it does sound like power supply is a first good step. What voltage do your parts actually need to run? If you can run from, say, 10V instead, you can just put a linear regulator to step the 12V from the SMPS to a lower, regulated voltage. Choosing a regulator with good PSRR rating and some extra filter caps on the input/output to it could go a long way towards cleaning up the power rail.Do you have an oscilloscope? An easy test would be to look at the output of the player and the power input and trying to see if the noise you're seeing on the input is indeed making it to the output. Also make sure all your project's grounds are connected together to a point (star grounding), and that the point is grounded to what the power supply uses.

author: Prince Gautam
author_rank: Newbie
timestamp: March 15, 2020, 07:27:04 pm
content: As per the data sheet for IC CD4440 the voltage is all good. As I have previously mentioned I have tested everything on my bench power supply at 12v DC.Yes i have a oscilloscope and I will check on it as per your suggestions and reply on it.I think the problem is with earthing.

author: DaJMasta
author_rank: Super Contributor
timestamp: March 15, 2020, 10:15:19 pm
content: I didn't mean the voltage was a problem, I meant that if you put a linear regulator between the SMPS and the audio player you should be able to filter out a lot of that power line noise. If that means SMPS at 12V and regulator lower (there needs to be some drop out voltage), then that works fine. If it means an input at like 13.5-14V from the SMPS and a regulator down to 12V that works fine too. Maybe if your SMPS has an output adjustment potentiometer you could turn it up to use a regulator in between.Earthing can often be problematic in audio stuff, but unless your SMPS is ground referenced, there wouldn't be a difference between the bench PSU and the SMPS for that. It does sort of sound like really loud ground hum, though - make sure all of your circuit grounds are actually attached to each other, and if possible, attach them to each other at the same point.

author: EHT
author_rank: Frequent Contributor
timestamp: March 15, 2020, 10:20:56 pm
content: Run it off a linear bench PSU or battery to confirm that it is the SMPS. If so, easiest to just replace it with a linear one.