Intro

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.

Ground Connections

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.

AC Wiring

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.

DC Leak

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.

Intro

Capacitors are electrical components used to store electric charge. They consist of two electrical conductors which are separated by an insulator known as a dielectric. Capacitors are similar to batteries in behaviour in that they store energy and then release through discharge. Capacitors however, for the most part, release their energy much quicker i.e. seconds or less.

Power Supplies

In a guitar amp power supply, capacitors are used to smooth the ripple in a rectified current i.e. AC to DC converted current which still has an alternating waveform. This is achieved by the capacitor charging when the wave peaks and discharging when the wave troughs resulting in smoothing of the wave. The ultimate goal of filter capacitors is to smooth and filter the rectified current to a more pure DC supply. If the supply is not smoothed properly due to leaky or dead capacitors then the result will be hum in the output.

Tone

Capacitors are one of the main components used to manipulate the tone of a signal. Larger value capacitors take longer to charge and as a result allow both shorter and longer frequency waves to pass whereas smaller capacitors only pass shorter frequency waves due to the shorter time available for charging.

Coupling

Coupling capacitors are used in a guitar amp circuit to pass the desired AC signal (amplified guitar signal) and block unwanted low frequency DC signal (used to power the valves). If coupling capacitors are not used then the DC leaking into the output circuit will appear as noise. In valve amps, coupling capacitors can be found between each amplification stage in the circuit. Coupling capacitors can also affect tone in that if a smaller value capacitor is used the lower frequencies will not be passed to the next stage.

Bypass

Electrolytic capacitors are used for the purposes of bypassing AC signal at a valve cathode to ground resulting in a reduction of degenerative feedback. The result is higher amplification and dynamic response. The larger the value of capacitor used for bypass will result in a greater range of frequencies having increased amplification. If the goal is to just increase the high frequencies and gain then only smaller value capacitors should be used.

Intro

Valve amplifiers use vacuum tubes for the purposes of amplifying an input signal. Predominantly pre-1947 (invention of the transistor) valve amplifiers also used vacuum tubes for the purposes of rectification of AC to DC power supply.

Feel

Making use of vacuum tubes for the purposes of rectification can itself impart a “feel” to the tone while playing. Different rectification tubes designs can impart more or less of this feel e.g A 5Y3 tube displays more whereas a 5AR4 tube does not. The reason that this phenomenon occurs is due to an effect called “sag”. Sag occurs when there is a dip in voltage as a result of higher demands for current (when the amp is working hard) and the inability of the rectifier tube being able to fulfil the requirement immediately. The sag phenomenon can also be triggered by filter capacitors of lower capacity and the internal resistance of transformers. The result of sag is that it effects the dynamics of the amp making it more responsive to guitar picking attack i.e. the amp becomes more expressive in that it compresses, sustains, notes bloom, lowers clean headroom and has delayed guitar attack.

Use of solid state rectification, on the other hand, imparts a more immediate and punchier feel to the tone.

Both types of rectification can impart a different feel to the tone while playing and are generally suited to different styles of playing therefore there is no better option. Vacuum tube rectification does however have a small drawback in comparison with solid state in that is costs more to build and can be less reliable.

Tone

Another characteristic, through the use of vacuum tubes for amplification, is the type of overdriven tones which are produced. Valve amps soft clip resulting in the addition of even harmonics at lower levels. The result of these even harmonics is the production of warm and rich tones which is more pleasing to the ears. Solid state amplifiers tend to add odd harmonics which are less pleasing.

Lower levels of negative feedback (NFB) in valve amps are used to reduce distortion, noise and sensitivity due to low levels of gain per amplification stage compared to solid state amps. The result of using lower levels of NFB is the production of greater amounts of harmonic distortion, warmer and richer tones. Because solid state amps have much greater levels of gain there is a requirment for using higher levels of NFB thus less harmonic distortion and warmer tones.