Hum is caused by the addition of AC mains power signal to the audio signal of an amplifier. The main sources of hum include AC valve heater powering, DC power supply ripple as well as electrostatic and magnetic coupling. This article will provide some insight into how hum is caused and what can be done to reduce it.
If grounds are not firmly connected to the chassis then this can either cause intermittent or a constant source of hum. If grounds are soldered then checks should be made to make sure that the joint which is soldered is strong, has no movement and no signs of cracks in the solder. Note that any soldering to the chassis requires a lot of heat due to the chassis acting as a large heat sink.
It’s also worth noting that the grounding point should be made on the bare metal of the chassis and not any shiny coating. This means that "roughing up" the surface may be required before making a ground connection.
Grounds which are mechanically attached using nuts, bolts and lock washers should also be checked for any looseness or bad chassis contact. If there is any sign of the nut being lose or the wire spinning freely then this will give rise to hum.
Ground Placement & Grounding Scheme
Ideally, if using a ground bus scheme, there should be one ground which is located near the input stage. Full isolation of components from ground is often not possible therefore there may cause the introduction of ground loops e.g. many input jacks used for amps are metal and not isolated therefore allow more than one route to ground. It is possible to retro-isolate metal jacks by using isolation washers or replace the jacks entirely with non-metal grounded equivalents. In many instances the hum produced by not isolating jacks is negligble therefore not always required.
Ground loops in a circuit are a major contributor to hum. Ground loops can occur when more than one ground in a circuit are connected together and there is a potential difference between them. Examples of additional groundings in an amplifier include metal input and output jacks without isolation, potentiometers connected together by a ground wire, potentiometer tags soldered directly to the pot back. Hum will occur when there is a potential difference (voltage) between the grounds as well as different ground loops being in close proximity to each other and noisy current and voltages .
Hum or buzz can be caused due to sub-optimal placement of component along the ground bus. It is often good design and practice to place component ground connections close to the smoothing filter capacitor which provides them with the correct voltage. It is also good to provide a bit of distance between the power supply reservoir capacitor and other filter capacitors grounds to limit the amount of noisy current from the power supply bleeding into the rest of the circuit.
The main power supply can be considered as the Transformer (secondary HV wires including the centre tap), rectifier and reservoir capacitor. None of this circuit should be grounded as it will potentially induce noise in the circuit.
The AC 6.3v heater wires are known to induce hum in an amp circuit. This hum can be reduced by soldering the centre tap to the ground bus and if there is no centre tap then a virtual centre tap can be created by attaching one end of two 100 ohm resistors to the pilot light terminals and the other to ground bus. This can often be improved by lifting the 0v ground reference to a higher voltage and can be achieved by soldering the ground side of the centre tap to to the cathode of one of the power tubes in a cathode-biased amp.
All AC wires hosted in an amplifier chassis create electromagnetic radiation which can result in hum in the audio circuit if not addressed properly. All AC wire should be tightly twisted so that radiated fields have a better chance of cancelling each other out. The actual twisted pairs should be kept as far away as practically possible from audio cables and components so as to not pass any EMF on. It is generally good practice to place heater wires tightly into the edge of the amplifier chassis. When this is performed make sure they stay far away enough from sockets and potentiometers.
Hum can also be caused by leaky filter or coupling capacitors. When this occurs, stray DC will get into the circuit and induce hum. This is more common with old capacitors but can occur to new too. It is quite often good practice to change old capacitors as a matter of course as the cost is minimal.