Hum and Noise in Effects

(and how to squelch it)

Questions about hum and noise in FX designs come up repeatedly in the AMZ e-mail. These problems are of three basic types: powerline hum, high frequency hiss and radio frequency interference.

The problem cannot be fully evaluated until the circuit is mounted in its enclosure and sealed up. PC boards and wiring that are strewn across a workbench are especially susceptible to induced noises and will not be indicative of the performance of the finished unit. You cannot adequately evaluate the extent of your hum or noise problem while the circuit is on a breadboard or open perfboard.

Hum is leakage from the AC power source that has made its way into the circuit and subsequently amplified. It may be 60 cycles per second, which is the fundamental frequency of the power grid, or it may be a multiple of the base frequency. A 120 Hertz frequency is produced by a full-wave rectified power supply and 180 Hz., 240 Hz. and higher harmonics are also produced but at a lower level. The multiples of the base frequency are difficult to remove and the best plan is to eliminate the source of the interference.

A common way that hum enters a circuit is through the power supply, or DC input to the circuit box. Poorly regulated AC adapters are often to blame because of inadequate filtering. Putting some series resistance before the pc board is a way of reducing powerline induced hum. A value of 100 ohms for the resistor is usually adequate though it may be increased if required.

If this is not sufficient, or if there is no filter capacitor on the pc board (as with the Fuzzface), adding some additional capacitance from the voltage supply to ground on the pc board can prove helpful (this is the 220uF capacitor in the drawing above).

Why is my 9v adapter putting out 12v+?
This is usually detected when one is measuring the voltage of the adapter without a load on it. An unregulated AC to DC adapter will have a voltage and a current rating. When the circuit being powered is drawing the rated current from the adapter, the voltage will sag from the 12v or 13v unloaded down to the rated 9v. If the current requirements are less than the full rating of the adapter, the voltage will be proportionally higher since the load is lighter and there is less sag. The solution to this potential problem is to use a voltage regulated adapter which will supply a steady voltage at any current draw from nothing up to its maximum rating. They are more complex than the unregulated versions and more expensive.

Radio frequency signals can also be picked up by the power supply lines and instead of the resistor shown in the previous example, a pair of ferrite beads used on the wire connecting the power input to the pcb. The beads will increase the impedance of the wire to radio frquency signals while offering little resistance to the dc power. The Lab Notebook article about the Radio Shack choke demonstrates an easy and inexpensive inductor for powerline filtering that is more effective than ferrite beads.

Hum may also enter a circuit via routes other than the power input. Light dimmer switches, fluorescent lamp fixtures and computer monitors radiate substantial amounts of hum and noise that may be picked up and amplified by the circuit. Move your pedals as far away from these sources as is practical, but even that may not be enough. While the hum from these sources may be induced into the power supply line, the filtering of the power circuit previously shown will eliminate that source of the noise.

An alternate point of entry for the hum is the input connection from the jack or footswitch to the circuit board and any hum or interference that finds its way into the input will be further amplified by the circuit. The wire connecting the input to the pcb should be shielded cable with the shield grounded at the jack end only. RG-174 wire is small diameter and flexible enough for this purpose.

Additionally, the box that holds the circuit should be metal. The circuit ground and the power supply grounds should be connected to a common point on the box. A convenient place that is most often used is a ground lug on the input or output jacks. This effectively places the box at ground potential and it acts as a shield against induced hum and radio frequencies (RF).

The cord connecting the guitar to the effects box should be top quality since it can act as an antenna which is how some effects boxes can receive and amplify powerful nearby radio stations. A flexible cable with a good shield connection is required. Braided wire is best but some types of foil shields are acceptable. In some locations, radio signals are so close and powerful that they still manage to find their way into an effects circuit, especially if it is a high gain design such as a fuzz. The way to eliminate this radio pickup is to add a couple of ferrite beads to the signal input wire between the jack and the pc board and a small capacitor to ground on the signal input to the circuit.

A 100 to 1k resistor may be substituted for the ferrites. This usually has no audible effect on the tone except with certain low impedance circuits such as the Fuzzface and even then it will be hard to detect.

Select the smallest value resistance that eliminates the interference. The capacitance value may be increased for additional filtering. Start with the values shown above and increase the capacitance (or resistance) until the problem is eliminated. By applying the simple techniques shown above, most of the noise and hum problems in your effects devices can be eliminated.

GROUNDING: There have been entire books written on the subject of grounding and it is a hotly debated subject among homebrew amp enthusiasts. Fortunately, it is somewhat simpler for fx builders. The box should ideally be made of a conductive metal though I have made and used numerous pedals that had plastic casings. The input and output jacks will be automatically be grounded to the metal case when mounted in the holes in the box (though there are isolating jacks with plastic bodies that do not do this).

A well designed pc board usually has a single ground point that is connected by a wire to one of the jacks. I usually use the output since the input jack may already have battery wires going to it. A single wire to one jack is all that is needed. If the box is plastic, a wire should be added from the ground lug of the output jack to the ground lug of the input jack as well. Off-board controls are usually not too critical about grounding and I usually jump their ground points together and take a single wire from there to the output jack ground lug. That should do it!

AMZ-FX Home Page Lab Notebook Main Page Guitar Effects Blog