Hum and noise.
Hum and noise are common problems in guitar electronics. Hum can be avoided, noise can be reduced, but never fully eliminated.
Noise is generated in every conductor of electric signals. In an amplifier these are tubes, transistors, operational amplifiers, resistors and other components. Noise is caused by very small variations in the current. These variations are caused by individual electrons that either free themselves from the conducting material or recombine with so-called holes in the conductor. This happens at random and therefor the noise does not have a specific frequency but in fact there is a bit of every frequency in the signal. This sounds as noise. Depending on the type and quality of the components used in the electronics, noise can be more or less, but it can not be fully avoided (in the best case the noise is not or hardly audible). The power supply does greatly influence the amount of noise generated in the electronics.
Hum is a low frequency (tone) that can be caused by two sources. Hum can be induced or conducted.
Inducted hum arises from the phenomena that a changing magnetic field causes a current to flow in a conductor. A bicycle dynamo uses this effect. The dynamo consists of a magnet that spins around in a coil of copper wire. Due to the changing magnetic field, caused by the spinning of the magnet, a current is induced in the coil. A lamp is connected to the coil and the current flows through the lamp. In the same way, a current can be induced in cables, elements and other parts of the guitarist’s sound system. Not so much of course that it will make a lamp burn, but sufficient to cause an audible hum in the speakers.
The magnetic fields that induce the hum in the sound system exists around powercables and devices that are powered from the mains. More power means stronger magnetic fields. So around the cables of the stage lighting you may expect a relatively strong magnetic field. In most cases the magnetic field is not very strong because the power that is used is limited. Besides that many devices are shielded to avoid these problems.
Magnetic fields from power cables induce a 50 or 60 Hz hum (depending on the mains frequency). Magnetic induction can be avoided if all parts of the sound system are shielded or separated from magnetic fields. In practise:
- Keep signal cables (microphone cables, effect-loop cables, cables from and to the sound mixer) away from powercables such as for the lighting
- Make sure all sound sources (microphones, elements) are shielded
Fortunately the infuence of magnetic fields reduces rapidly with the distance to the source. If you double the distane to the source of the magnetic field, the influence is reduced four times.
Hum can also arise from a conducted signal. In this case the hum enters the system through the power supply. If the power supply voltage of a part of the sound system (for example an amplifier) varies strong, a part of the change will turn up in the output signal. Why the power supply voltage sometimes is unstable is explained below. In a battery operated system, hum cannot aridse from the power supply since the battery supplies a perfect DC voltage that only declines very slowly with time.
If more than one pedal is powered from a single source, it is important to make sure that ground loops are avoided. A ground loop is formed when the supply current of a pedal flows through one of the signal wires, see the drawing on the right. >
Pedals A, B en C are powered from a power supply V. Pedal C uses current from the power
supply. The path of this current i is shown in the drawing. This current causes
a signal S in the signal loop. The signal S increases with an increasing cable
length between the pedals.
These problems can be avoided by using isolated power supplies for all pedals. See the drawing on the right.
Pedals can sometimes produce special effects at a certain battery voltage. If you use a power supply with a fixed output voltage, these effects can never be reproduced. The flatliner has two outputs that can simulate a discharged battery.
When a battery discharges, two things happen. The battery voltage slowly drops and the internal resistance of the battery increases. An increasing internal resistance means that the output voltage of the battery decreases with increasing load. This effect depends on how fast the battery is discharged. Both effects can be simulated on the ‘dying battery’ outputs of the Flatliner.
This section explains the features of a power supply in more detail.
Where does the hum come from when you use a low-cost mains adapter?
AC stands for Alternating Current and DC is short for Direct Current. AC is what we get from the grid, DC is what we get from a battery.
In a power supply, the mains voltage of 230VAC or 115VAC is reduced to a safe level by a transformer. This yields a voltage of around 12VAC (in most cases). This voltage is an AC voltage with a frequency of 50 or 60 Hz. If we need DC from the power supply, something needs to be done. In the worst case, the AC power will damage devices that are supposed to get a DC supply.
AC can be rectified and become DC. The alternating current that you get from an AC supply is caused by the alternating polarity of the AC voltage. Since the voltage polarity changes, the direction of the current through the load changes. Rectifying is making sure that the current always flows in the same direction.
A rectifier is made with so-called diodes. A diode is an electronic element that conducts current in one direction only. A diode is schematically presented by the following symbol:
a load is connected to this voltage through a diode, there will be a current flowing
through the load only half of the time.
This way only half the available power is used. Therfor a configuration with 4 diodes is used in practice. In the drawing below you can see that the current flows through the load in the same direction for both the positive and the negative part of the cycle of the input voltage.
If there is no load, the voltage over the capacitor will be stable. If a load is using current from the capacitor, the voltage over the capacitor drops a bit. The capacitor is charged again when the input voltage rises above the voltage over the capacitor. This results in the voltage as shown in the graph below. This is called a DC voltage with a ripple.
The magnitude of the ripple depends on the capacity of the capacitor. It is not possible to eliminate the ripple entirely by using an enormous capacitor. The current that is used by the load from the capacitor has to be added again. This happens when the input voltage becomes higher than the voltage over the capacitor. If the capacitor is very large, the voltage drop is very small (very small ripple) but there is very little time to add the current that is used, because the input voltage is higher than the capacitos voltage only a very short time. This would lead to a very high current flowing from the input voltage into the capacitor for a very short time. This can not happen since the capacity of the adapter is limited. It would lead to overload of the adapter. Poorly designed power supplies can be in almost constant overload due to this effect and they will generate enormous heat even at a very low load.
Cheap adapters usually contain a rectifier and a small capacitor. They produce a voltage with a large ripple. The ripple causes the hum. Note that the ripple has twice the mains frequency. It can help you to identify the source of the hum.
The best way to make a stable DC voltage is to use an electronic voltage regulator. Each output of a FLATLINER has its own regulator. Besides that all outputs are short circuit protected.
If you still have questions about your pedalboard power electronics, don't hesitate to ask us !