Noise isolation with battery speaker

I have a couple of battery- powered speakers which have USB power outlets.
I have the Zynth and another RPi-based synth.
When I try and run either synth from the USB supply and feed the audio output back into the speaker, I get a lot of background noise. I assume it is coming from the ground link, since if I take power from one speaker, and feed the audio to the other speaker all is good (i.e. it’s not a noisy USB power supply problem).

The speakers have 3.5mm stereo inputs (not balanced).

I want to be able to power a synth from the speaker and hear clean audio from the same speaker.

What’s the solution?


It seems some kind of grounding problem. The ground on the speakers (battery) is not the same than the power supply… but i don’t know how to solve it.

Some ideas, @zynthianers?

Hi Guys!

You can try this transformers on audio output from zynthian:

Lift an earth?

@gonzoB are you seeing any warning symbols in the Zynthian that might indicate low power, e.g. red lighting bold near top right of screen? This would indicate the speaker is not capable of delivering the required power to the Zynthian.

A similar issue may be that the speaker can provide sufficient power to run the Zynthian (as you have describe) but when the speaker’s amplifier is being driven then it also needs to draw power which may collapse the supply, i.e. the speaker PSU is not capable of driving Zynthian and amplifier simultaneously.

You describe “a lot of background noise”. Could you provide an audio sample of the noise which might help us understand the cause? (Different faults cause different noise.)

I don’t think it’s the power - the Zynth pulls about 600mA, and when I plug my phone into the speaker to charge it pulls 1.2A, so the capacity is sufficient. I get the same noise with a PiZero (with HiFiBerry) that pulls about 200mA.
The noise sounds like this:

The noise changes character as the Pi gets busier. There seems to be no problem with volume.


I get similar issues when using my Korg nts-1 plugged into my zynthian, going to look at getting a USB isolator

I let you if it helps me

@maniccyberdog Thanks, I couldn’t find an adapter at 5V that isolated AND was able to supply the current.
@rod_amaral I’ve ordered some audio transformers and am waiting…


Ok, I installed the audio transformers, and the problem is fixed!

Thanks all.



A new challenge with my battery-powered speaker…

When I haven’t played anything into the audio input for ~ a minute the speaker goes to sleep, and when I start playing again it misses the first note or two. Also, if I don’t play anything for ~8 mins it turns off, which is really annoying because then I have to start the synth again.

The solution is to send a continuous low volume audio signal at a frequency above hearing range, but not above what the amp is able to see. 20KHz works. But how to do that? At Zynth Club this morning I got instructions from @riban using the C* Sin sine wave generator, and a PureData solution from @ronsum. I couldn’t get the PureData one to work, but the sine wave generator worked. Thanks chaps!

The next problem was doing the same for my Pi-Zero FluidSynth engine (without Zynthian). I used the C* Sin solution on the Zynth to capture a 20KHz .wav sample, and made a .sf2 preset for FluidSynth. On start-up I configure the 20KHz preset on channel 5 (which I don’t use for anything else) and then send a noteon to trigger the 20KHz. I adjusted the noteon velocity to give just enough volume to stop the speaker going to sleep.

It all works. Thanks again.



I think we need pictures and obviously a :face_with_monocle: confirmation recording of the 20,000hz tone so we can all listen to it and make sure we don’t hear anything . . . .

Does it need to go into the Archive along with a block of silence . . … ???

Tape a bat to a microphone.

1 Like

Maybe bongs would be better i.e. occasional short burst of tone. It might reduce the processing and save your speakers (and ears) from continuous audio. Just because you can’t hear it doesn’t mean it isn’t there!

Tape a raven to a mike?

1 Like

The beeb used to ( and presumably still does ) transmit a ping like tone to keep the transmitters alive . . . .

Wots a Beeb? LV2?

Short affectionate term for the BBC…