Low impedance signal sourcing to reduce noise - Page 1
Forum section: electronics / projects

author: sahko123
author_rank: Frequent Contributor
timestamp: January 03, 2021, 03:20:58 pm
content: I have a simple question I have a signal source (audio DAC) which outputs a signal to some speakers. When there is a heavy load on my computer my subwoofer makes a low hum noise. Would it potentially reduce this hum and some noise if to the signal source i add a voltage follower with a lower output impedance? My thought process is that it is easier to adversely affect a high impedance signal than a low(er) impedance source. Has anyone done something similar to this or knows that this would be dumb or a decent solution?

author: TimFox
author_rank: Super Contributor
timestamp: January 03, 2021, 03:29:43 pm
content: With a normal audio DAC, the output (source) impedance from the DAC should already be low. Do you have a specification for it?

author: sahko123
author_rank: Frequent Contributor
timestamp: January 03, 2021, 03:39:01 pm
content: Its an external dac (umc404hd) and i dont know what the dac chip it has is or what it has after that.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 03, 2021, 03:50:55 pm
content: Hum with heavy load on the system that shares the same supply more point's towards ripple from the supply of maybe magnetic coupling from some inductors on the input filter that than see more current. So an extra buffer amplifier would not likely not help, as the hum would likely be already in the signal. It would be more like supply filtering / regulation or maybe shielding around the supply.

author: sahko123
author_rank: Frequent Contributor
timestamp: January 03, 2021, 03:59:45 pm
content: Right ill look into the power supply because i initially though that the hum was picked up through the cabling

author: David Hess
author_rank: Super Contributor
timestamp: January 03, 2021, 08:29:26 pm
content: The umc404hd looks nice but has a fatal design flaw; without isolation of the USB interface, a ground loop is created between the PC and any audio equipment and that is very likely what you are hearing. To solve this I recommend placing a USB isolator between them. Note that this means that the umc404hd will have to be powered by its +5 volt DC input.If you do not want to add a USB isolator, then you might get away with plugging all of the audio equipment and the PC into the same power strip.USB was never intended to be used with interconnected audio equipment.

author: bson
author_rank: Unknown
timestamp: January 03, 2021, 09:38:56 pm
content: Another potential source of ground loops is if you DC couple chassis to ground. This then goes to the USB shield, which goes to the PC chassis and makes that part of the current return path. Try AC coupling it and use a largeish bleed resistor, like 50k (to prevent the chassis from collecting charge).

author: gf
author_rank: Super Contributor
timestamp: January 04, 2021, 05:40:55 pm
content: Quote from: sahko123 on January 03, 2021, 03:20:58 pmAre the speakersdirectlyconnected to the umc404hd (without any amplifier in between)?Or do you mean active speakers (with buit-in amplifier)?If there is an amplifier in the play as well (either dedicated or built into the speaker cabinet):Is the audio connection between umc404hd and the amplifier balanced (XLR) or unbalanced?Does the amplifier have a power plug with protective earth?Does the PC have a power plug with protective earth?Does the umc404hd's power supply have a power plug with protective earth?(or is it USB-powered?)Depending on the answers, one (of several possible) ground loops could be for instance:power socket's protective earth -> PC's GND -> USB cable -> umc404hd's GND-> unbalanced audio cable -> amplifier GND -> power socket's protective earthEDIT:If you disconnect the speakers and connect a headphone to the umc404hd, does it hum as well?

author: TimFox
author_rank: Super Contributor
timestamp: January 04, 2021, 07:06:44 pm
content: Quote from: David Hess on January 03, 2021, 08:29:26 pmTo watch YouTube, etc. on my TV, I put an USB-stick DAC on a laptop, and connected the output (3.5 mm) to the analog input of a normal sound bar without noticeable hum.The laptop, TV, and sound bar all run off the same outlet strip, with an HDMI cable from laptop to (muted) TV. Also, the cable modem connects to the laptop with an ethernet cable, and the modem is powered from the same outlet strip.

author: gf
author_rank: Super Contributor
timestamp: January 04, 2021, 08:36:43 pm
content: Quote from: David Hess on January 03, 2021, 08:29:26 pmIMO this is not a big issue per se, ifonly balancedconnections (usually XLR) to other audio devices are used. This requires of course that the other audio devices do support balanced inputs/output as well, which is usually granted for (semi-)professional audio equipment.I've beeen usingthis mixerin a similar configuration and I don't remember noticeable hum. PC was directly connected to the mixer via USB (no USB isolator), and the mixer's audio output was connected to active speakers. But the audio connection was balanced, of course, via XLR.

author: gf
author_rank: Super Contributor
timestamp: January 04, 2021, 10:15:09 pm
content: After some search I foundthese slidesagain, which provide a good overview of the potential issues and how to address them. I also remember another paper covering these aspects, but I did not find it again.