After learning more about this problem, I have continued this question here.
I am trying to remove noise from an electric-guitar circuit that I have created called the ArduGuitar. My circuit comprises 2 opto-isolated parts: the audio circuit between the guitar pickups and the output jack, and the control circuit consisting of an Arduino Micro and a Roving Networks RN42 bluetooth module powered by a 9V battery.
A full description is available on the ArduGuitar webpage but in brief, I wrote an Android App that sends controls over bluetooth to the Arduino via the RN42 bluetooth module. The Arduino interprets the commands and adjusts PWM signals on its output pins to control Vactrol (LED-LDR) LEDs which change the resistances attached to the pickup outputs. Thus, I simulate potentiometers and switches, just as per an "normal" electric guitar circuit, while keeping the analog audio fully opto-isolated from the digital control circuit.
Globally, this works very well and I am happy with the result which I would say scores 90% of perfect.
However, despite the use of all shielded cables and a fully shielded RN42 bluetooth module, significant noise still gets into the audio and comes out the amp. This noise is of 2 kinds:
- whistling: this is present as soon as the bluetooth is switched on. I have done extensive testing of the use of capacitors across VCC/GND to try to filter this but it all fails. This noise is not very strong, but significant;
- clicking: at each data transmission, a click comes out over the audio. It is not very loud, but significant nevertheless.
I use all shielded cables and wiring internally on the audio circuit.
The noise level changes depending on where I position the Bluetooth module, and strangely, the best result seems to be when the module is inside the guitar electronics bay (which is unshielded). The worst result is when the bluetooth module is close to the guitar output cable that connects to the amp.
I have been working on the problem for several months and am now at a loss as to how to proceed…
I wonder how this problem is handled in wireless guitar systems?
Would anyone have any ideas as to how to reduce or eliminate this noise?
Best Answer
There are two ways to solve these types of problems. Either at the source and or hide the effect. Fortunately the whistling and clicking appears directly related to the RN42 and not the PWM's.
Note the HiZ inputs of the pickup coils are going to pick up everything. From Florescent Lights to the RN42 aliased noises. Typically a band pass filter is used at the point of observation or A2D. Where here your pickups are likely directly tied to the AMPs input. And a filter here is undesirable.
The RN42 is creating noise that is bleeding out onto the power supply and radiating to Pickup's. And shielding the pickup's is impractical. Rather one needs to filter out the noise from its source. My own experience with the RN42 has a lot of clicking in the audio connected circuits.
Recently Elektor's July & August 2013 had an article for Android Elektorcardi♥scope where it used an RN42 to send the result to the Android. Where it was littered with Filters on the power supplies to all the IC's. Below is the filter used on the RN42.
simulate this circuit – Schematic created using CircuitLab
Where the 30R@100MHz was used on other supply inputs with different capacitance. At DC it typically is under .2 Ohms, but with spikes and noises that will rise dramatically and react with the capacitance to block the noises.