John Reynolds
Member
A couple years ago I started working on a bass guitar preamp using a Teensy 4.0 and the Audio Shield. I was able to try lots of different topologies for gain and tone controls and used it on stage for a few live gigs(!), but there was always some annoying background noise in that setup that I was never able to get rid of.
Recently, I decided to revive this project and try to see if I could find the source of the noise and eliminate it. Here is what I tried and what I learned.
The Usual Suspects
Power and Ground were the cuplrits. There is no separation of Analog and Digital grounds on the Audio Shield. Also, VDDA (3.3V) for the codec is derived from Digital VDD with a CLC filter, which doesn't completely eliminate the digital noise on the supply rail. Fixing these issues did the trick.
Analog Power Supplies
I built two linear power supplies to power the Teensy and Audio Shield. A linear 5V supply provides power to the Teensy. A separate 3.3V linear supply provides VDDA to the codec. These both powered by a 12V wall wart, but each supply is isolated from the other via RC filters on the inputs. Separate Analog and Digital grounds are established.
On the Teensy, the USB power pad is cut and the board recieves 5V on the Vin pin and Digital ground on a Gnd pin.
On the Audio Board, the ferrite inductor is removed and VDDA and Analog ground wires are soldered across the VDDA bypass caps.
Grounding Strategy
The boards are mounted in a copper-clad box which is connected to Analog ground. Digital ground is maintained as an isolated net. The two grounds connect at the AGND pin on the codec chip.
Gain Structure
Codec input and output levels are set to maximum voltage inputs levels and the ADC Highpass filter is disabled
Next Steps
There are 5 knobs and a switch available on the front panel that can be programmed for any variable. I'm planning to investigate alternative tone control algorithms and some compressor/limiter functions. Looking forward to using this at some gigs this Summer!
Recently, I decided to revive this project and try to see if I could find the source of the noise and eliminate it. Here is what I tried and what I learned.
The Usual Suspects
Power and Ground were the cuplrits. There is no separation of Analog and Digital grounds on the Audio Shield. Also, VDDA (3.3V) for the codec is derived from Digital VDD with a CLC filter, which doesn't completely eliminate the digital noise on the supply rail. Fixing these issues did the trick.
Analog Power Supplies
I built two linear power supplies to power the Teensy and Audio Shield. A linear 5V supply provides power to the Teensy. A separate 3.3V linear supply provides VDDA to the codec. These both powered by a 12V wall wart, but each supply is isolated from the other via RC filters on the inputs. Separate Analog and Digital grounds are established.
On the Teensy, the USB power pad is cut and the board recieves 5V on the Vin pin and Digital ground on a Gnd pin.
On the Audio Board, the ferrite inductor is removed and VDDA and Analog ground wires are soldered across the VDDA bypass caps.
Grounding Strategy
The boards are mounted in a copper-clad box which is connected to Analog ground. Digital ground is maintained as an isolated net. The two grounds connect at the AGND pin on the codec chip.
Gain Structure
Codec input and output levels are set to maximum voltage inputs levels and the ADC Highpass filter is disabled
- LineInLevel(0)
- LineOutLevel(13)
- HighPassFilterDisable()
Next Steps
There are 5 knobs and a switch available on the front panel that can be programmed for any variable. I'm planning to investigate alternative tone control algorithms and some compressor/limiter functions. Looking forward to using this at some gigs this Summer!