Here's some technical stuff about guitar amp design. On this page I'd like to illustrate the reasons why no two amps sound and respond exactly the same. I've described 20 aspects of guitar amplifier design that have an affect on the overall sound and response of an amplifier. Then I've ranked the effect of each feature from 1 to 5 (a rank of 1 means it has a small but perceptible effect on tone, a rank of 5 means it has a major effect on the tone). These rankings are all my personal opinion of course but it is helpful to understanding the value of each feature. Anyway it shows how amp designers have a lot to play with.
Preamp gain stages ....
A pre amp consists of one or more gain stages that boost a guitar signal to a sufficiently high level to drive the output stage. A gain stage is usually designed round one half of a double triode valve ( e.g. one half of an ECC83 or 12AX7). Gain stages are one of the main areas where the valves soft overload and distortion characteristics are harnessed to create the basic voice of the amp. The gain of each stage can be designed to overdrive the next stage in a particular way. 5
Output configuration ....
The two common output configurations used in guitar amp design are Class A and Class A/B. This simply describes the way the output valves are biased in their normal operating state. Valves in a class A circuit draw full current regardless of the signal level being amplified. Class A/B is a more efficient way of running output tubes and uses minimal current at zero or low signal level, increasing to maximum current at high signal level. An output stage can consist of one valve (this would be called a single ended configuration and is always class A) or one or several pairs of valves in a push pull configuration. The push pull configuration can be biased as class A or class A/B The single ended and push pull configurations however introduce different distortion components when overloaded. Single ended is predominantly even order harmonics (even multiples of the fundamental frequency of the signal), whereas push pull introduces odd order harmonics. 5
Channel switching ....
Channel switching allows either the characteristics of the preamp (gain, frequency response, distortion) to be switched to an alternative configuration or a completely separate preamp stage to be switched into the circuit or substituted. This gives the amp alternate voices for rhythm and lead for example or vintage / modern. 5
Speaker ....
There are a vast range of speakers available from manufacturers, the main differences being cone size, magnet material and size, cone material, surface area and stiffness. Coil Impedance and resonant frequency. This means that the individual sounds of each model vary widely. Age / usage often loosens up a loudspeaker and changes the character of it's tone. The efficiency of the speaker has a direct effect on how loud the amp sounds. (Efficiency is a measure of how much sound energy the speaker generates per watt of input). The number of speakers loaded into a cabinet effects the overall loudness (but the loudness is not proportional to the number of speakers used because the limiting factor is the amount of power that the amp can provide). Speaker wiring configuration (series or parallel) can effect the sound, speakers wired in parallel tend to have a tighter sound, series gives a looser more open sound. Sometimes different types of speaker with complementary characteristics are loaded into the same cabinet. 5
Speaker cabinet ....
The cabinet is the final stage of shaping the sound. Its material, size, shape, all have an effect on the tone. Whether the back is open or closed, effects the efficiency of the cabinet, the way it projects sound and the response to playing. (the position of the cabinet in the room and the size and shape of the room also effects the sound ) 5
Resistors ....
Resistors are used in gain stages to bias, set the gain , attenuate signals and in conjunction capacitors to filter the signal, The value of each resistor effects the sound of the amp in some way. Designers have different preferences for carbon resistors or high stability metal oxide resistors. Carbon resistors were typically used back in the 50's , 60's and 70's and in some positions in a circuit, their non linearity at high voltage levels can be exploited to soften the tone. Metal oxide resistors are more linear less noisy and have tighter tollerances. 4
Coupling and bypass capacitors ....
Coupling capacitors isolate the DC voltage of each gain stage of the amplifier from the next stage. ( ie they only allow the signal to pass from one stage to the next). The capacitor also acts with the input resistance of the following stage to form a high pass filter and attenuate low frequencies below a certain frequency. The choice of capacitor and resistance values determines how much low frequency gets rolled off. Bypass capacitors are used in gain stages to emphasise mid or high frequencies Different types of capacitor dielectric have different characteristics and importantly some (polypropylene) perform better than the others in transferring the high frequency part of a signal without impeding it. 4
Tone stacks ....
Tone stacks are typically passive filter networks which include three variable resistors to alter the response at high, medium and low frequencies. They are positioned between gain stages in the preamp section. The frequency of the high and low frequency roll off and amount of roll off are determined by the resistance and capacitance in the network (sometimes inductance is also used) and the topology of the filter network also determines how each control interacts with the other. The position of the tone stack in the chain of gain stages determines whether the tone stack is filtering the “clean” guitar signal or the signal that has been coloured by valve distortion introduced in previous gain stages. The tone stacks can be further enhanced by allowing certain filter component values to be switched in or out. 4
Valve type ....
Guitar amp designers tend to be fairly conservative and stick to a relatively small selection of valve types. The characteristic differences in sound between output valve types that were traditionally manufactured in the UK ( EL84, EL34) compared to American types (6V6 , 6L6) play a big part in defining the sound of an amp. Additionally, it is apparent when experimentally substituting valves of the same generic type from different manufacturers that they each have individual sounds of their own. In the preamp and power amp sections, similar valve types from different manufacturers exhibit different gains and overload distortion characteristics. Considering that there are several current manufacturers of valves and many more valves still available that were manufactured in factories that no longer exist, there are certainly lots of combinations. 4
HT voltage and valve bias ....
The HT (High Tension Voltage) determines the maximum wattage that can be derived from an output stage and the maximum “clean” volume that can be achieved before valve overload starts to significantly distort the sound. In the pre amp gain stages, the designer can set the HT voltage to control how much distortion each stage produces. Valves in the Preamp and the power amp stages are biased. The bias determines the operating point for the valve, the range over which it will amplify a signal without overload and how it behaves at the onset of clipping. 4
Output transformer ....
The output transformer converts a high voltage low current signal driven by the high impedance anode of an output valve into a low voltage high current signal which drives a low impedance loudspeaker. The construction of the output transformer (core material and windings) determines it's frequency response, power handling and the way it distorts when saturated. The frequency response is particularly important at the low frequency end. Transformers may be open frame, semi shrouded/drop through , or fully shrouded. The advantage of fully shrouding means the windings are completely screened from other circuitry and sources of hum interference such as the power supply transformer. A high quality transformer can make the difference between a mediocre sounding amplifier and an excellent sounding amplifier. 3
Power output ....
(wattage) largely determines the maximum volume that can be achieved by the amplifier. The sound level achieved by the amplifier that is to say, a doubling or halving of power to the speaker results in a 3dB increase or decrease of sound pressure level (3dB is a change that is just perceptible to a listener). It takes at least a 10 dB change in sound pressure level to sound double or half as loud. That corresponds to a 3 Cathode follower ....
This is a single stage in some preamp circuits that adds no gain but buffers the relatively high output impedance of a preceding gain stage from the tone stack. The result is that the tone stack tone stack gets driven by an optimum low impedance, allowing the filtering effect of the tone stack to perform with a full range. The insertion loss or the tone stack is minimised and the susceptibility of the tone stack wiring to noise pickup is minimised. 2
Negative feedback ....
Some amps use negative feedback. This means that a small part of the output signal is fed back into the input of the power amp stage and affects the signal into that stage. By varying the amount of negative feedback either using fixed components when the circuit is designed or using a knob (usually called a presence control) the character of the sound can be changed . More negative feedback gives lower output stage distortion and tighter base response. Less or no negative feedback gives a looser response and more signal distortion in the output valves. 2
Noise ....
Noise ( buzz, hum, and hiss) comes from internal sources within the amplifier and also enters the amplifier circuitry from outside (electro magnetic interference conducted through cables, power source or radiated through the atmosphere). If it is generated by or coupled into the preamp stages then it will be amplified along with the signal and become a noticeable in the output signal. The noise level may determine how much gain in the preamp is viable. Internal design usually determines whether an amp is noisy or not, but faulty components ("old" valves or leaky capacitors) can generate noise. The main design aspects are the wiring routes, the amount of filtering and decoupling on the HT supply, the amount of screening afforded by the chassis and it's covers, filtering on the ac power input, the positioning of transformers and critical gain stages, the way power is supplied to the heaters of each valve.. 2
Rectifier ....
The rectifier is part of the circuit that converts the ac voltage from the power supply transformer into a dc voltage. Traditionally valve rectifiers were used. Often semiconductor diodes are now used. A side effect of a valve rectifier is the way it drops more voltage as the current passing through it increases. This can introduce a compression effect in a Class A/B output stage when the output is overdriven. 2
Master volume ....
A master volume control enables the input to the power amplifier output stage to be controlled. This is a direct way of controlling the level of the signal that's finally presented to the loudspeaker. Master volumes were introduced during the evolution of the guitar amp to enable a player to drive the preamp into overload but keep the overall volume to a manageable level. Because the signal is being reduced before the important output stage this does mean however that one important contribution to the amps sound being compromised (the overdriven output stage and loudspeaker(s)). 1
Chassis ....
The chassis provides screening from electromagnetic interference. The position of transformers relative to each other and sensitive gain stages is important to minimise hum effects. The type of metal can also subtly effect the electro magnetic current that is conducted through it. .. 1
Power reducer ....
This is a circuit used sometimes to allow selectable HT voltages to the output valves to allow them to operate with lower power outputs whilst still operating in a region where they can handle clean and over driven signals. 1
Power input filter ....
This is used to filter conducted electromagnetic interference from the AC mains before it enters the amps power supply and potentially introduces noise to the signal. 1
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