5V speaker amplifier - Page 1
Forum section: electronics / beginners

author: msknight
author_rank: Regular Contributor
timestamp: January 13, 2022, 03:35:54 pm
content: OK - I admit defeat. I'm just no good.I have a TV that I wanted to use the headphone output to drive a pair of speakers. Obviously, the output from the TV was low and needed to be boosted (the speakers are a pair of Logitec PC speakers which are now totally passive) ... so I needed an amplifier.The intention was to drive an amplifier from a 5V USB supply so I could save myself a wall socket. What could be easier?I first started with building my own with some LM386N chips and putting it in a case for an old amp (which used to be 9V input and would have needed a separate wall socket and transformer)http://msknight.com/technilife/?p=1298http://msknight.com/technilife/?p=1327Technically, it worked, but it was picking up noise from the power input. So I consigned the effort to the bin and decided that I obviously wasn't good enough and started to look up 5V amplifiers that other people had build.Like a fool, I thought that headphone amplifiers with variable voltage input would deliver more output the higher the input voltage. Well... it didn't work out that way either.So now I'm stuck and need some help please.Target is an amp that works from 5v and delivers enough punch to drive some desktop loudspeakers.Help?!?!

author: msknight
author_rank: Regular Contributor
timestamp: January 13, 2022, 03:42:39 pm
content: I also suppose that a DC-DC 5V decoupled um... something... might work by cleaning up the power input to my original design, but I took a look at the thousands of various things (even after narrowing them down to through-hole varieties) and had a headache and needed to lie down.

author: Ian.M
author_rank: Super Contributor
timestamp: January 18, 2022, 10:04:39 am
content: Your problem is: if you use the PC, or TV's USB 5V, or even a grounded 5V wallwart, and your amplifier circuit grounds the audio cable shield, you have just made a large area ground loop, via the audio cable shield, the USB power ground, and whatever noisy parts of the motherboard/main board are between the USB PMIC or port itself, and the Audio chipset. You need to break that ground loop without breaking the bank.Its *MUCH* easier to float the signal ground rather than trying to do isolated power. A simple LoFi approach would be to use audio coupling transformers - 600 ohm 1:1 would be good. You need one for each channel. However the frequency response will suck, so you may wish to use AC coupled differential amplifiers constructed with a decent dual OPAMP and eight matched precision resistors. Even 0.1% resistors will give you around 55 dB of common mode rejection. Hand select for matched ratio pairs, and you can push that to around 70 dB. It will probably need RF filtering in front of it so RF EMI breakthrough doesn't get demodulated in the OPAMPs' input stages.

author: magic
author_rank: Super Contributor
timestamp: January 18, 2022, 10:16:21 am
content: Sorry to be a disappointment, but...been there, thought about that, got a wall wartYes, the problem is ground. The ground you get from USB is not the same ground you get from audio out.

author: mikerj
author_rank: Super Contributor
timestamp: January 18, 2022, 10:21:24 am
content: As Ian says your noise problem is almost certainly a ground loop, effectively your signal ground is passing non-signal currents that get amplified by your LM386 and changing the amplifier device will not fix this. The issue may well be exacerbated by your breadboard construction as well, they can have significant impedance across the contacts.That said the LM386, whilst simple and cheap, is not a very good device by modern standards. With a single ended output and limited output swing it will only deliver a fraction of 1 watt with a 5v supply rail, which is likely lower than the TVs own amplifier. Modern class D amplifiers such as the PAM8403 or PAM8406 can deliver a few watts into a 4 ohm speaker at 5v. These are cheap and readily available as pre0built modules for low cost.