0
\$\begingroup\$

I'm working on my portable music production setup and I'm having noise issues due to connecting all my gear to the same power source.

I have three devices (a sampler, a keyboard/synthesizer and a multi-effect pedal) all connected to a USB power bank which supports Quickcharge.

I'm using a small device to trigger the powerbank to output 9V instead 5V and then a splitter to power three devices.

The issue is that each device produces a certain amount of electrical noise (mainly due to the 8-segment LED display each has) and injects it into all the other devices.

When I power my three devices separately, everything is nice and quiet but when I power all of them using a single power source such as my USB power bank then the noise becomes almost unbearable.

I've done some reading and it seems that there are two possible solutions to this issue:

  1. Using an isolated DC-DC converter to convert the DC power 1:1 (9V in, 9V out)
  2. Using an optocoupler

Those need to be applied to each "power line" of course, so for three devices I'd need three of those.

What would be the correct and most money efficient way to tackle this issue?

\$\endgroup\$
5
  • \$\begingroup\$ You probably have power supply switching noise circulating in your ground wires. If you don't see it, post some diagrams and someone will point them out. \$\endgroup\$
    – Mattman944
    Commented Dec 2, 2023 at 15:48
  • \$\begingroup\$ Optocouplers in the power lines won't do you any good. Optocouplers don't transfer power. Optocouplers aren't any great shakes with audio signals, either. \$\endgroup\$
    – JRE
    Commented Dec 2, 2023 at 16:10
  • \$\begingroup\$ My bet for the source of the noise would be the boost converter in the 5V to 9V converter, followed by the 5V boost converter in the power bank. \$\endgroup\$
    – JRE
    Commented Dec 2, 2023 at 16:56
  • \$\begingroup\$ @JRE I thought that as well, but when I use it to power only one device I don't have any noise issue. \$\endgroup\$
    – Luca
    Commented Dec 2, 2023 at 18:19
  • \$\begingroup\$ More devices = more load on the converters = more noise from the converter. \$\endgroup\$
    – JRE
    Commented Dec 2, 2023 at 18:30

2 Answers 2

Reset to default
0
\$\begingroup\$

I've drawn two devices connected together by an audio cable, and plugged into the same power source (battery on the left).

enter image description here

The supply current of Device1 (blue arrows) comes from the battery, flows through the device, then returns to the battery via ground. However in doing so it will split between the ground wires in the power cables, and the shield in the audio cable.

Supply current for Device2 does the same.

These currents create voltage drop in the "ground" wires and shields.

Therefore, the internal "0V reference" of each device, which is presumably its internal ground, is different. If you're using unbalanced connections (the usual RCA/Jacks) each device takes as input the voltage difference between the "hot" (tip) and the "ground" (sleeve)... and since "ground" is at a different voltage at both ends of the audio cable, that voltage drop is added to your signal.

Pro audio gear uses balanced connections (XLR, or repurposed TRS jacks) with "hot" and "cold" wires, and the inputs take the difference between "hot" and "cold" wire. Since these two wires are separate from the shield, any current in the shield doesn't upset the voltage on the signal wires, which means no noise (usually).

In your case the audio cable shield is not doing anything useful, it's just in parallel with the ground wires in the supply cables. This is sometimes called a "ground loop" but in this case the term is incorrect: the noise isn't created by the loop. Rather it is "common impedance coupling" because the same conductor (cable shield) is used for two different things (supply current and audio signal reference).

Unfortunately with unbalanced connections the only solution is to have no current flowing in the cable shield, which mean:

  • Use devices that draw constant current, which turns the voltage drop in to DC which is not audible. This is not practical.

  • Isolate the audio with transformers, differential receivers, etc

  • Or isolate the power supplies by using one battery per device. Or isolated DC-DC converters, but that may also make more HF noise.

Honestly, a battery holder for 6xAA (total 9V) plus some NiMH cells will probably cost less than the other options...

\$\endgroup\$
5
  • \$\begingroup\$ Right on the money. Thank you so much, that makes perfect sense and it's exactly what's happening in my setup. Adding all those batteries might end up making the setup pretty bulky, but I'll definitely think about it... some of the devices might actually work with even less than 9V, so 4xAA batteries might be enough, I bet that they all have internal regulators to get the 9V input down to 5V, so I might get away with 6V input. I'll also look for isolated DC-DC converters and see how much they cost and how much circuitry they require and decide accordingly. Thanks again! \$\endgroup\$
    – Luca
    Commented Dec 2, 2023 at 18:23
  • \$\begingroup\$ You're welcome ;) So yes indeed you are hearing supply current variations from the LEDs turning on and off in the multiplexed 7 segment displays! Note if your stuff uses a regulator like 7805 to drop 9V to 5V it will require at least 7V on the input, unless you replace the 7805 with a more modern low dropout version. But AA rechargeables will only give you 1.5V per cell when fresh off the charger, it quickly drops to 1.2V per cell... \$\endgroup\$
    – bobflux
    Commented Dec 2, 2023 at 18:44
  • \$\begingroup\$ AA rechargeables, if my memory serves right, provide 1.2V, so by having 4 of them I'd get 4.8V... which might be enough if I bypass the internal 7805, but I always try to keep my gear stock and with minimal to no modification. Still, definitely an option to keep in mind. I wish balanced audio connections would be more common on audio gear. \$\endgroup\$
    – Luca
    Commented Dec 2, 2023 at 20:30
  • \$\begingroup\$ Just wanted to post an update: I've rigged a horrible battery box, one for each device and the noise is completely gone, as expected. I guess this will be the way forward unless I go and use isolated DC-DC converter, because audio transformers will end up being way too expensive. I have to see how much those DC-DC converters cost and how big they are to make an educated guess on how to proceed. \$\endgroup\$
    – Luca
    Commented Dec 3, 2023 at 17:46
  • \$\begingroup\$ Nice! Audio transformers also limit bandwidth (especially in the bass) or if they don't they're very expensive... \$\endgroup\$
    – bobflux
    Commented Dec 3, 2023 at 17:51
0
\$\begingroup\$

I'm trying to do exactly the same thing (mobile set of midi instruments with an audio link and powered by an unique powerbank) and I got exactly the same issue (noise in the output of the device that is taking the sound from the other one).

The fact is that I'm using for now unbalanced TRS between the two devices, but on the one that receives the sound, I got the option to enter by a microphone XLR input. From all I've read in your posts and after some research it seems to me I could fix the issue by using a passive direct box, that convert the unbalanced TRS to the balanced XLR and that is popular to get rid of electrical interferences. Perhaps you've the same opportunity.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.