Hello all, this is to share an ongoing project where we mount Cycfi Nu (V2) pickups on a double bass and bring them into the Bela through a CTAC Face, do a little processing and induce feedback by sending the processed signal through a power amp and into a 150w low end speaker cone embedded in the back of the instrument, see here. As the project is predicated on audio feedback any noise in the system will tend to build up so we really want all the stages of the system to be as quiet as possible. Having encountered a few issues we have a good result (low noise) and want to share our process if it should help someone else with a similar setup. We are also happy to hear thoughts on how to do things better.
Powering the pickups
These components are housed onboard the double bass and our original ambition was to have a single brick supplying all three components (four of the pickups but you know what I mean).
First we tried feeding the Nu pickups off the Bela 5V supply, that introduces tons of (digital) noise. It was a seductive prospect in terms of simplicity of the system but we doubt this will ever give a good result even if filtering is attempted (it was so dirty that we didn’t even try).
Feeding the pickups off the same brick as the Bela and amp, with a regulator and filtering at 5V came up with much better results than the 5V from the Bela but still too dirty to be acceptable. We think that it’s because the components are sharing ground and sharing noise through there, or the pickups are just very, very sensitive to the power they are getting.
Final solution to supplying the pickups: Feeding them off a 9V battery.
This is the cleanest we have gotten those pickups sounding (and they are very nice). having to depend on a battery is a little (conceptually) annoying, but practically it’s not really an issue as the Nus draw very little current and the battery is connected to our main power switch so they are not drawing when the system is off.
Getting a good signal from the pickups into the Bela
With the Nu signal straight from the onboard buffer we found we had to add a lot of gain digitally to get it to the level we wanted out of the Bela, this digital gaining turned out to be a source of consistent dirt in our signal. The solution was to create a preamp circuit 5x gaining the signal from the pickups before taking them into the CTAC. The circuit is fed off the same 9V battery as the pickup and draws very little (perhaps as much as one pickup). This is now very clean and sounds nice.
Powering the Bela/CTAC and power amp
Again, it was our ambition to have a single brick powering everything. We had some hiccups in the beginning as we did not fully appreciate how much current the Bela and CTAC want, so feeding everything (minus pickups) with 1.5 Amps the Bela kept cutting out in the boot sequence. Now we are using a 24V 5A power supply from MeanWell and we are supplying the power amp directly from it with some filtering capacitors and from the same power supply we are doing a step down from 24V to 5V with a MeanWell DC/DC Converter for the Bela/CTAC and all is well.
Power amps off Amazon
Buyer beware. Same specs, even same seller but you never know what you get. Some are very dirty and then (in our case, four orders later!) you get a clean one. We also tried a car amp but it needed ridiculous amounts of current and so didn't make sense for the power supply we wanted to use. An alternative could be this 160 watt amp: http://www.smsl-audio.com/productshow.asp?id=99 . While costing at least ten times that of the cheaper amps we are using, smaller versions from the same brand have proven to be very quiet.
It took some mucking about to figure this out but the system is now clean sounding and we are happy with it, if anyone is interested in seeing documentation of further development stay tuned at: