Hello, i use patchbox with Modep on a Raspberry Pi3 and a Novation Launchkey midi keyboard.
I also have dac audio card connected.
It works nice … but i found that available raspberry power supply units are generally to noisy so, there is a hum sound in the audio. I tested so far some power supply units but even if the noise is sometime lower in some unit among them never disappear completely.
I solved the problem as a workaround using a 10000mAh power bank insted of a Line power supply.
Since i do not want to random buy and test power supplies hoping that they work … i wonder if some of you have such a power supply suitable for audio use and can give me an hint.
In our experience with Pisound, the Raspberry Pi power supplies proved to be silent enough to not be a problem at all, hence we don’t have much feedback on other brands of supplies. We’ve seen Apple USB chargers to be inducing noise, so better to avoid those.
Something else to consider is getting a ground loop isolator or DI box with ground lift.
I use raspberry pi, USB and battery powered devices and they are essential to remove the power hum from your signal chain. You can get cheap 3.5mm in line boxes about the size of a cigar from eBay or Amazon. Be aware that there is a certain ratio of defective ones, so buy from.a good source or get a couple in one go. Retrokits have them to use with their RK line.
DI boxes are bigger but also worth an investment, they can be total lifesavers especially when playing smaller venues. Plus have other features that make them useful in a studio. Radial are the best, but almost any stereo DI with ground lift should do the trick.
Hi tnx for suggestion. I already have a Proel DB2A stereo DI box. It is very useful on stage expecially when there are long cables to the main mixer. However in my experience the DI boxs are usefull only when the HUM noise is captured by some ground loops. Actually when some devices are connected by wires is possibile to have some ground paths that form a large single coil. This coil act as an antenna and the 50hz or 60hz electric field carried by the power lines around superimpose a few millivolt signal to the mix. In this situation breaking the loop is beneficial … even if for long path and hi power amplification it is better optical connection.
DI are not useful when the “source signal” already have noise. Cheap power supply are nowadays switching power supply (no trasformer) and the principle of functioning is based on high speed commutation of main power line. In this devices is mandatory to have a very good filtering output stage to avoid injecting of noise. May be you sometime put your hand on a power cable that have some big cylinder on the cable. That is a ferrite choke the stop (or reduce) hi enegry pulses.
if the signal coming out from device itself start with a noise in the mix … the DI box can do nothing to reduce it, because DI action is on the ground loops only.
Tecnically speaking if you measure with an Oscilloscope a power supply DC 5v output you will find an AC noise signal which amplitude range from a few millivolt to some hundreads of millivolt.
The less is the better. A noise is present every time in an output signal and cannot be avoided beacuse there is a phisical phoenomenom that is thermal noise caming from the electrons movement, but under a certain value the human ear is unable to detect it.
In syntesis we can categorize from noise point of view a power supply simply asking for Specs aboud Peak to Peak AC noise value over the DC output.
Hope this will help other to understand better the whole.
