SOLVED Unicab short weird sounds when powered

[Nico-L]

Hello,

I am building an ampless setup based on the unicab pcb. I would like to be able to mix a backing track or a metronome so I put the paramix pcb from guitarPCB. The output of the paramix is sent to a headphone amp.

I added a LiPo battery, a 5V charger and a 5V to 9V stepup to power all the pcbs to be able to use the gig without a power source.

The Unicab is not linked to a foot switch, the input jack and the output are directly connected to the pcb. A toggle switch allows to cut the 9V power. When the power is switched on, a brief piercing sound with an increasing frequency is sent to the headphone (I recorded it). Then everything works as expected.

As everything works OK, I am wondering what could be the origin of the sound. I tracked the origin of sound and it seems to come from the unicab. The sound exists even with the unicab pcb directly to the headphone amp. The sound is also produced with a regular 9V power source.

I am wondering what could produce such increasing frequency sound? Any suggestion would be welcome. Could it come from the power source?

Thanks,
Nicolas

 

[temol]

Can you lower the value of R101 and R102 to 10k and C101 to 10u? I'm not sure if you can eliminate the oscillation completly but there's a chance you can make it much shorter. It's possible that it takes a while to charge C101. You can hear the oscillation while Vref increases to 1/2 of the Vcc.
If you have the oscilloscope then you can connect the probe to C101 and observe what's going on there.

Here's the example. Yellow trace signal, blue trace - Vref. Oscillation stops when Vref reaches 1/2 of the Vcc

 

[Nico-L]

Hello @temol,
Thank you for your insight. I'll try your modification, it may take a while since I probaly don't have the 10 µF cap in my (small) stock. I'll report when it's done. I have not oscillocope, I'll try to get one to probe the circuit.

So if I understand well, this is not noise catched while the filtering is not efficient enough but rather self oscillation?

Thanks

 

[zgrav]

you are hearing the noise produced until the capacitor charges when the circuit is turned on. this is not an issue in a regular effects pedal that is already-powered and the footswitch just brings it in or out of the audio path. if you are powering the unicab on and off to save the battery life and the sound bothers you the simplest solution would be to take off your headphones or turn down the volume before powering the unicab.

 

[Nico-L]

OK, if this is the way the circuit is expected to work, I am a bit reluctant to modify the resistance and capa values. I may as well "live with it" and turn down the volume indeed.

Thanks.