DC Power Filtering

REDUCING NOISE ON DC POWER FOR FX PEDALS

When looking at a guitar pedal schematic, you're really looking at two circuits that overlay each other, sharing resources, and working together.  There's one for the AC signal and one for DC power.  

We use capacitors to block DC, and let AC signals pass - this is called coupling, and you will see it everywhere on circuits used for audio.  Capacitors are also used to store power and create filters.

What is noise on a power supply?

Noise on a power supply can come from various sources, and essentially it's an unwanted signal on the DC supply.  It can be present in your home wiring, radiate from other electrical devices, or be left over ripple from the conversion from AC to DC in an external pedal power supply. 

DC has no frequency, as it's a flat line (0hz), so it's blocked by capacitors.  Adding signal (noise) to DC is like having another source of AC entering the circuit, that can move past the capacitors blocking DC, and join your guitar signal.  We don't want this to happen - especially for high gain circuits. 

How can noise be reduced?

The most common way used in guitar effects is by decoupling the power supply and adding a low-pass filter.  This, used in conjunction with a high-quality external power supply, will go a long way to reducing noise.

Decoupling is very similar to coupling, but now the capacitor is used to ground AC signals, letting DC pass by the capacitor.

If noise appears on the DC line, hopefully, most of it will be taken care of by the decoupling capacitor.  Remember AC signals see capacitors as shorts - DC sees them as a blockage or break in the circuit.

Adding a 100ohm resistor with the capacitor forms a low pass filter with a cut-off of around 16hz - this helps filter out additional noise in the audible range.  Keep in mind that the filter cut-off frequency is not a hard wall.  It’s really a gentle slope, so this doesn’t eliminate all noise,  but it can definitely make a big difference.  

Sometimes a 100n ceramic capacitor is added, which helps with high-frequency filtering - this is specific to ceramic caps by the way, don't add a polyester cap.  Quite often 100n capacitors are also placed as close as possible to V+ and V- of ICs to help reduce noise.  

There's a good chance you will also see something like this on schematics - it's the same thing as above, but with a voltage divider to provide a bias voltage at half the supply.  Bias voltage is often required for ICs, but I will not go down that rabbit hole here.  

Quite often, a smaller capacitor is used, as much of the work has been done with the one that comes before it.  The 100k resistor also forms a low pass filter with the 47u capacitor, with a cut-off frequency of 0, so there's not much point of having anything bigger.   Even if the voltage divider used 10k resistors and the capacitor was 10u, the cut-off frequency would be a very low 1.6hz

Battery Power 

Now, if you really want to avoid any power-related issues - use batteries.  A battery is about the purest supply of DC you will find.  I often test with my pedals with a standard DC power supply and a battery just to see what's happening with noise.

They are terrible for the environment, so most people do not use them for this reason.  Some manufacturers don't even have it as an option - for my builds I only ever add battery snaps to vintage builds that are positive ground.   

There are rechargeable battery packs on the market specifically made for pedal supplies these days.   I have not tried one, but they are probably worth a look.