GB2603116A

GB2603116A - Reactive Attenuator

Info

Publication number: GB2603116A
Application number: GB2100574.9A
Authority: GB
Inventors: Ramon Alvarez Fernandez Santiago
Original assignee: Marshall Amplification PLC
Current assignee: Marshall Amplification PLC
Priority date: 2021-01-15
Filing date: 2021-01-15
Publication date: 2022-08-03
Anticipated expiration: 2041-01-15
Also published as: WO2022153054A1; GB2603116B; GB202100574D0

Abstract

Valve amplifiers, when over-driven, produce distortion and a tonal quality that guitarists appreciate. Guitarists like to ‘push’ their amplifiers into this region of operation. However, the sound may then be too loud for normal use. The invention provides an output attenuator that lowers the sound level while retaining the desired sound quality more satisfactorily than does a resistive attenuator, while avoiding the costly inductors of a traditional ‘reactive’ attenuator (fig.4). In the circuit of the invention (fig.5), the non-ideal characteristics of an output transformer (modelled in the equivalent circuit of fig.6) can effectively replace some components in a traditional ‘reactive’ attenuator (fig.4). For example, the invention recognises that the magnetising inductance of the transformer (55, fig.6) is equivalent to the load-box inductor (32, fig.4). The attenuator circuit (fig.5) thus comprises a load capacitor 33 and load resistor 25 coupled across a winding (or part-winding, fig.7) of the output transformer 14.

Description

Reactive Attenuator

FIELD OF THE INVENTION

[0001] The present invention relates to guitar amplification and more specifically attenuation of sound from a guitar amplifier.

BACKGROUND OF THE INVENTION

[0002] When using a guitar amplifier it is normal for guitarists to 'push' the power stage into distortion to produce a distorted sound. A power stage can be 'pushed' by turning the volume control of the guitar amplifier up. In a valve amplifier, the power stage is a valve power stage.

[0003] Figure 1 is a schematic circuit diagram of a valve power stage 10 which comprises two valves 12a, 12b (also called 'electron valves', or 'tubes') arranged as a 'Class B' amplifier stage (sometimes called a 'push-pull' amplifier stage). Other arrangements of a valve power section are known, such as a 'Class A' amplifier stage. The valves 12a, 12b are coupled to an output transformer 14 at a primary winding 15. The secondary winding 16 of the output transformer 14 is coupled to a speaker 18. The purpose of the output transformer 14 is to change the voltage/current characteristics of the output power to meet the requirements of the speaker 18. It is particularly difficult to replicate the natural distortion of a valve power stage when it is 'pushed'. The main problem when doing this is that the amplifier has to run at full volume and it is too loud for normal use.

[0004] There are traditionally three ways of dealing with the high volume problem: 1) Use a low power amplifier; 2) Integrate some circuit that reduces the output power produced; or, 3) Add an external attenuator (also called load-boxes) that converts the output power into heat and only sends a fraction of the power to the speaker.

[0005] A benefit of the attenuation method is that the power stage of an amplifier can be 'pushed' while reducing the overall volume of the guitar amplifier. There are two main ways of attenuating the output signal: 1) Resistive attenuator. The power is dissipated by heat in a resistor divider and part of the signal is sent to the speaker.

2) Reactive attenuator. In order to make the amplifier respond as if it is connected to an actual speaker (i.e. have a similar frequency response), reactive elements are added to the attenuator circuit (inductors and capacitors) and then unwanted power is dissipated into a resistor attenuator as above or alternatively some additional transformer or autotransformer.

[0006] A problem with traditional external resistive attenuators connected to the amplifier is that the speaker becomes more and more isolated from the power amplifier. In a resistive attenuator this modifies the way the speaker reacts and, although volume is lowered, the desired 'tone' characteristics of a non-attenuated guitar amplifier are lost.

[0007] A reactive attenuator (also called a 'reactive load-box') tries to compensate these undesirable side effects by connecting a circuit that reacts like a speaker so the amplifier keeps operating as if a real speaker is connected to it. Reactive attenuators are typically heavy, large and expensive.

SUMMARY OF THE INVENTION

[0008] A first aspect of the invention provides a guitar amplifier system comprising: a valve power stage; an output transformer with a primary winding coupled to the valve power stage, and a secondary winding; and a reactive attenuator coupled to the secondary winding of the output transformer, wherein the reactive attenuator comprises a load comprising a load capacitor, and the load does not comprise a load inductor in parallel with the load capacitor.

[0009] A combination of a magnetising inductance of the output transformer and the load capacitor may provide an LC resonant circuit with a resonance between 50Hz and 200Hz.

[0010] A second aspect of the invention provides a guitar amplifier system comprising: a valve power stage; an output transformer with a primary winding coupled to the valve power stage, and a secondary winding, wherein the output transformer has a magnetising inductance; and a reactive attenuator coupled to the secondary winding of the output transformer, wherein the reactive attenuator comprises a load comprising a load capacitor, and a combination of a magnetising inductance of the output transformer and the load capacitor provide an LC resonant circuit with a resonance between 50Hz and 200Hz.

[0011] The LC resonant circuit of any of the first or second aspects may have a resonance between 75Hz and 120Hz.

[0012] The secondary winding may have two or more tapping points. The guitar amplifier system may further comprise an attenuator switch which can be switched between a first state in which the load is coupled to one of the tapping points, and a second state in which the load is coupled to another one of the tapping points.

[0013] A third aspect of the invention provides a guitar amplifier system comprising: a valve power stage; an output transformer with a primary winding coupled to the valve power stage, and a secondary winding with two or more tapping points; a reactive attenuator; and an attenuator switch which can be switched between a first state in which the load is coupled to one of the tapping points, and a second state in which the load is coupled to another one of the tapping points.

[0014] The load may comprise a load resistor coupled in series with the load capacitor.

[0015] The guitar amplifier system may further comprise a speaker coupled to the secondary winding of the output transformer in parallel with the reactive attenuator.

[0016] The speaker and the reactive attenuator may be coupled to different tapping points of the secondary winding.

[0017] The secondary winding may have two or more tapping points and the speaker may be coupled to one of the tapping points of the secondary winding.

[0018] The guitar amplifier system may further comprise a speaker switch which can be switched between a first state in which it couples the speaker to a first one of the tapping points, a second state in which it couples the speaker to a second one of the tapping points, and a third state in which it couples the speaker to a third one of the tapping points.

[0019] The first, second and third tapping points may be associated with respective output powers of the speaker, and the output power of the speaker at the third tapping point may be less than 2W.

[0020] The first, second and third tapping points may have respective effective numbers of windings, and the effective number of windings of the third tapping point may be less than 10% of the effective number of windings of the first tapping point.

[0021] The effective number of windings of the third tapping point may be less than 5% of the effective number of windings of the first tapping point.

[0022] A fourth aspect of the invention provides a reactive attenuator configured to be coupled to an output transformer of a guitar amplifier, the reactive attenuator comprising: a load comprising a load capacitor, wherein the load does not comprise a load inductor in parallel with the load capacitor.

[0023] The reactive attenuator may further comprise first and second attenuator inputs; and an attenuator switch which can be switched between a first state in which the load may be coupled to the first attenuator input, and a second state in which the load may be coupled to the second attenuator input.

[0024] A fifth aspect of the invention provides a reactive attenuator configured to be coupled to an output transformer of a guitar amplifier, the reactive attenuator comprising: a load comprising a load capacitor; first and second attenuator inputs; and an attenuator switch which can be switched between a first state in which the load is coupled to the first attenuator input, and a second state in which the load is coupled to the second attenuator input.

[0025] The load may comprise a load resistor coupled in series with the load capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Embodiments of the invention will now be described with reference to the accompanying drawings, in which: [0027] Figure 1 is a schematic circuit diagram of a valve power stage coupled to a speaker.

[0028] Figure 2 is a graph showing a typical frequency response of a speaker.

[0029] Figure 3 is a schematic circuit diagram of an electronic equivalent circuit for a speaker.

[0030] Figure 4 is a schematic circuit diagram which shows an example of how the electronic equivalent circuit can be coupled to a speaker in parallel.

[0031] Figure 5 is a schematic circuit diagram of a guitar amplifier system comprising a valve power stage, an output transformer, a speaker, and a reactive attenuator.

[0032] Figure 6 is a schematic circuit diagram of an electronic equivalent circuit for a transformer.

[0033] Figure 7 is a schematic circuit diagram of a guitar amplifier system comprising a valve power stage, an output transformer, a speaker, and a load.

[0034] Figure 8 is a schematic circuit diagram of an external reactive attenuator. DETAILED DESCRIPTION OF EMBODIMENT(S) [0035] In order to maintain the functionality and the beneficial 'tone' characteristics of a reactive attenuator, while being able to reduce the negative characteristics of a reactive attenuator, it has been found that it is possible to integrate some of the additional reactive elements of a reactive attenuator into the parasitics that already exist in a valve amplifier with an output transformer. Typical inductors and/or autotransformers are large and expensive, and become larger and more expensive as the power they are designed to handle increases.

[0036] Figure 2 is a graph showing a typical frequency response 20 of a speaker, illustrating how its impedance (Z) changes with respect to frequency (f). A low frequency impedance bump 22 is caused by the mechanical resonance and mass of the speaker acting like a simple mass-spring system. A high frequency impedance rise 24 is caused by the compliance of the speaker cone and other factors.

[0037] Typical frequency ranges of the bump 22 can be 50-200Hz, 75-120Hz, 85-110Hz, or 95-105Hz.

[0038] The frequency response 20 can result from using a speaker 18 as shown in Figure 1. However, a similar frequency response 20 can also be replicated or approximated with electronic components as shown in Figure 3.

[0039] Figure 3 is a schematic circuit diagram of an electronic equivalent circuit 30 for a notional speaker which can reproduce the frequency response 20. Load inductors 31, 32, and load capacitor 33 represent the 'reactive' elements of the notional speaker and load resistors 35 and 36 represent the nominal impedance and damping factors. These components can act together in place of a speaker, as a load for the valve power stage of the amplifier. Example values of these components can be derived by characterizing a frequency response of a speaker, for example: load inductor 31 can be 1mH; load inductor 32 can be 50mH; load capacitor 33 can be 100uF; resistor 35 can be 16 Ohms; and resistor 36 can be 75 Ohms.

[0040] The load capacitor 33 and the load inductor 32 are connected in parallel with each other to form an LC resonant circuit which produces the low frequency bump 22 of Figure 2. The frequency of this low frequency bump 22 is 71Hz if calculated with the example component values. Load resistor 36 is connected in parallel to the load capacitor 33 and the load inductor 32 and represents the damping factor of the low impedance bump 22. Alternatively, the LC resonant circuit may have a resonance between: 50Hz and 200Hz; 75 and 120Hz; 85 and 110Hz; or, 95 and 105Hz.

[0041] The load inductor 31 in series with the load resistor 35 form a low pass filter which increases the impedance at high frequencies, i.e. it generates the impedance rise 24 of Figure 2. This filter will have a critical frequency (double the impedance) at 14.5 KHz if calculated with the example component values.

[0042] Figure 4 is a schematic circuit diagram which shows an example of how the electronic equivalent circuit 30 can be coupled to a speaker 18 in parallel. The secondary winding 16 of the output transformer 14 inputs into both the speaker 18 and the electronic equivalent circuit 30. If the speaker 18 and the electronic equivalent circuit 30 have identical frequency responses, then power output from the output transformer 14 will be evenly shared between the speaker 18 and the electronic equivalent circuit 30 at each frequency. In an example with a 100W guitar amplifier, the speaker 18 will receive 50W, and the electronic equivalent circuit 30 will receive the other 50W. This will result in about a 10dB reduction in a user's perceived volume as the 50W is converted to sound pressure energy via the speaker 18. At the electronic equivalent circuit 30, the 50W will be converted to mainly heat energy via load resistor 35. The speaker 18 may be internal to the guitar amplifier (as a 'combo') or external (in a 'speaker cabinet').

[0043] The secondary winding 16 of the output transformer 14 can have multiple tapping points 42. A tapping point changes the voltage ratio of a transformer by reducing the effective number of coil windings on the secondary winding 16. The relationship between voltage and coils windings for a transformer is: Voltage out Turns on secondary coil Voltage in Turns on primary coil [0044] In an ideal transformer the power into the transformer is equivalent to the power out of the transformer. If the speaker 18 is connected to one of the multiple taps 42 with half as many turns as the electronic equivalent circuit 30, then the speaker 18 will only receive a fraction of power and the electronic equivalent circuit 30 will receive the remaining power. It is possible for the speaker 18 to be fully disconnected, such that the electronic equivalent circuit 30 received all 100W, such that the valve power stage 10 is under full load but no sound is produced from the speaker 18.

[0045] Figure 5 is a schematic circuit diagram of a guitar amplifier system according to a first embodiment of the present invention, comprising a valve power stage 10, an output transformer 14, a speaker 18, and a reactive attenuator 50 coupled to the speaker 18 in parallel. Surprisingly, the functionality of the circuit of Figure 5 circuit is similar to the circuit of Figure 4.

[0046] The reactive attenuator 50 comprises a load 51 comprising a load resistor 35 and a load capacitor 33 coupled in series with each other. Unlike the circuit of Figure 4, the load 51 does not comprise a load inductor 32 in parallel with the load capacitor 33. The reactive attenuator also does not have a load resistor 36 in parallel with the load capacitor 33, or a load resistor equivalent to the load resistor 35.

[0047] The circuit of Figure 5 is able to perform in a substantially identical way to the circuit of Figure 4 because the non-ideal characteristics of the output transformer 14 can effectively substitute for the inductive components 31, 32, and the load resistor 36. This will now be explained.

[0048] Figure 6 is a schematic circuit diagram of a non-ideal model of a transformer. Transformers in general work by converting electrical energy into a magnetic field, then converting the magnetic field back into electric energy. This process is not ideal and there are some losses exemplified in Figure 6.

Figure 6 shows a notional transformer separated out into an ideal transformer 54, that is, a transformer which is 100% efficient (i.e. no losses as a result of the power conversion), and the losses of the notional transformer (sometimes called parasitic components). The parasitic components are: a magnetising inductance 55; a leakage inductance 56; and an iron core loss resistance 57.

[0049] By considering the output transformer 14 as its non-ideal model of Figure 6, it was realised that by substituting the non-ideal model for the output transformer 14 of Figure 5, the result would be the circuit of Figure 4. The magnetising inductance 55 corresponds to the load inductor 32, the leakage inductance 56 corresponds to the load inductor 31, and the iron core resistance 57 corresponds to the resistor 36.

[0050] To summarise, the combination of the magnetising inductance 55 of the output transformer 14 and the load capacitor 33 provide the LC resonant circuit which results in the low frequency bump 22 of Figure 2. Similarly, the leakage inductance 56 of the output transformer 14 in combination with the load resistor provides the low pass filter which results in the high frequency impedance rise 24 of Figure 2.

[0051] A benefit of this 'inductorless' architecture is that output taps on the secondary winding 16 of the output transformer 14 can be used to accomplish an adjustable output volume without having to add any additional external components.

[0052] Figure 7 is a schematic circuit diagram of a guitar amplifier system according to a second embodiment of the present invention. The system of Figure 7 has some of the same components as the system of Figure 5, and these components will not be described again.

[0053] The output transformer 14 of Figure 7 has five tapping points 62-66 on its secondary winding. The speaker 18 can be coupled to any one of the tapping points 62-66 by a speaker switch 60. The load 51 can be coupled to either one of the tapping points 63, 64 by an attenuator switch 61.

[0054] The effective number of windings of the five tapping points 62-66 is about 100%, 75%, 50%, 10% and 5% of the total number of windings of the secondary winding. The effective number of windings is defined as the number of windings between the common potential (typically ground) and the respective tapping point on the secondary winding.

[0055] The speaker switch 60 can be switched between a first state in which the speaker 18 is coupled to the first tapping point 62, a second state in which the speaker 18 is coupled to the second tapping point 63, a third state in which the speaker 18 is coupled to the third tapping point 64, a fourth state in which the speaker 18 is coupled to the fourth tapping point 65, and a fifth state in which the speaker 18 is coupled to the fifth tapping point 66.

[0056] The attenuator switch 61 can be switched between a first state in which the load 51 is coupled to the one tapping point (in this case the second tapping point 63) and a second state in which the load 51 is coupled to another tapping point (in this case the third tapping point 64). The attenuator switch 61 can be switched such that the load is not connected to any tapping points, in this case the load 51 is isolated from the circuit.

[0057] The tapping points selected by the switches 60, 61 can be the same or different.

[0058] A selector 68 controls both switches 60 and 61 to vary the power delivered to the speaker 18 and the load 51, while keeping the total load seen by the valve power stage 10 substantially unchanged. The selector 68 may be a mechanical switch or an electronic control interface.

[0059] In an example where the load 51 is bypassed (i.e. not coupled to the secondary winding 16 such that there is no attenuation), and the speaker switch 60 couples the speaker 18 to a tapping point, then the speaker 18 will receive 100% of the power from the valve power stage 10. The speaker 18 may be a 16 Ohm, 8 Ohm, or 4 Ohm speaker (although other sized speakers are available and may be used). If the speaker 18 is a 16 Ohm speaker then the speaker switch 60 couples the speaker 18 to the first tapping point 62. If the speaker 18 is an 8 Ohm speaker then the speaker switch 60 couples the speaker 18 to the second tapping point 63.

[0060] In an example, both switches 61, 60 couple the load 51 and speaker 18 respectively to the third tapping point 64. In this example, the speaker 18 is an 8 Ohm speaker. The speaker 18 will receive approximately 50% of the power and the load 51 will receive approximately 50% of the power. In this switching arrangement the impedance is well matched. If the attenuator switch 61 then coupled the load 51 to the second tapping point 63, the load 51 and the speaker would share power unequally such that the speaker 18 would receive less than 50% of the power (this would also be an impedance mismatch).

[0061] In an example where the speaker switch couples the speaker 18 (e.g. an 8 Ohm speaker) to the fourth or fifth tapping points 65, 66, then volume is greatly reduced and the speaker may only be provided with 1W or 0.25W of power (i.e. 1% or 0.25% of the total power). In this case the power to the speaker 18 is so low in comparison to the power to the load 51 that in practice the loading of the speaker 18 can be considered negligible (there therefore impedance matched). The load 51 is therefore coupled via the attenuator switch 61 to the second tapping point 63 in order to provide the maximum power to the load 51. Alternatively, with this configuration, the speaker 18 may not need to be connected at all -this may be desirable for silent, DI, recording.

[0062]Table 1 below gives five pairs of settings for the tapping points of the speaker and the attenuator, which may be used for an 8 Ohm speaker.

Table 1

Speaker Power level Attenuator tapping point Speaker tapping point 100% OFF 63 50% 64 64 1% 63 65 0.25% 63 66 0 (MUTE) 63 OFF [0063] The architecture of the guitar amplifier system of Figure 7 can be adapted to accommodate any number of secondary transformer tapping points, which could result in a larger or smaller number of switching options. In its most simple example, switch 61 may be a binary on/off, i.e. either coupled to output transformer 14 or not coupled at all. In another example there may only be three tapping points, with three switching states for speaker switch 60 and two switching states for attenuator switch 61. The effective number of windings associated with each tapping point can be varied depending on the values of the load 51, the possible tapping points coupled to the load 51, the desired output volume, and/or the desired output power of the speaker.

[0064] In the embodiments of Figures 5 and 7, the attenuator 50 is integrated into a guitar amplifier system. However, it will be appreciated that the attenuator may be provided as an external unit, which is not integrated into a guitar amplifier system. Figure 8 is a schematic circuit diagram of such as an external unit, according to a third embodiment of the present invention. The reactive attenuator 70 of Figure 8 is provided as a stand-alone part and can be coupled to a guitar amplifier, and optionally also coupled to a speaker. The attenuator 70 of Figure 8 has some of the same components as the system of Figure 7, and these components will not be described again.

[0065] The reactive attenuator 70 is configured to be coupled to the output transformer of a guitar amplifier via inputs 71-77. The reactive attenuator 70 also has an output 80 which can be optionally be coupled to a speaker.

[0066] The attenuator switch 61 can be switched between a first state in which the load 51 is coupled to a first attenuator input 76, and a second state in which the load 51 is coupled to a second attenuator input 77. The first attenuator input 76 and second attenuator input 77 can be coupled to different tapping points of an output transformer via external speaker cables (not shown).

[0067] The functionality of the reactive attenuator 70 is the same as the functionality described above with reference to Figure 7. In addition, all of the alternatives of Figure 7 can also apply to the reactive attenuator 70 of Figure 8. Specifically, the reactive attenuator 70 can have any number of inputs to correspond with more or fewer options at the selector 68.

[0068] Alternatively the inputs 71-75, which can be used to couple to the speaker 18 via output 80, may be separate from the external reactive attenuator 70. In this example, there may be a system which detects the external load on the valve power stage 10 caused by the external reactive attenuator 70 and automatically selects a tapping point on the output transformer 14 to couple to the speaker 18.

[0069] The skilled person would understand that the iron core loss resistance 57 may be any type of core loss resistance depending on the construction of the output transformer. The switches 60 and 61 may each be a set of switches, relay banks, or any other architecture of switch. Relay banks 60 and 61 may be a single relay bank or many individual relays or any architecture in-between. The selector 68 may be distinct from the switches 60 or 61, or the switches 60 or 61 may not be controlled by a selector 68 and instead controlled manually.

[0070] The load 51 may consist of only a load capacitor 33 and not a load resistor 35. This example may be used with a transformer which has a large internal resistance, for example from a parasitic copper/winding resistance of the transformer. The copper/winding resistance can correspond to the load resistor 35.

[0071] Advantages of the embodiments of the invention described above, compared with conventional designs are: * there are fewer reactive elements (which can be bulky and expensive) * the amplifier still drives the speaker directly without any attenuator 'in between' the amplifier and the speaker * by changing which tapping point the attenuator is connected to and which tapping point the speaker is connected to, it is possible to modify the speaker volume and simultaneously keep a constant load to the amplifier (all loads being reactive).

* the attenuator can be used as an external attenuator.

[0072] The words "coupled" or "connected", as generally used herein, refer to two or more elements that may be either directly coupled/connected, or coupled/connected by way of one or more intermediate elements.

[0073] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

CLAIMS1 A guitar amplifier system comprising: a valve power stage; an output transformer with a primary winding coupled to the valve power stage, and a secondary winding; and a reactive attenuator coupled to the secondary winding of the output transformer, wherein the reactive attenuator comprises a load comprising a load capacitor, and the load does not comprise a load inductor in parallel with the load capacitor.
2. A guitar amplifier system according to claim 1, wherein a combination of a magnetising inductance of the output transformer and the load capacitor provide an LC resonant circuit with a resonance between 50Hz and 200Hz.
3. A guitar amplifier system comprising: a valve power stage; an output transformer with a primary winding coupled to the valve power stage, and a secondary winding, wherein the output transformer has a magnetising inductance; and a reactive attenuator coupled to the secondary winding of the output transformer, wherein the reactive attenuator comprises a load comprising a load capacitor, and a combination of a magnetising inductance of the output transformer and the load capacitor provide an LC resonant circuit with a resonance between 50Hz and 200Hz.
4. A guitar amplifier system according to claim 2 or 3, wherein the LC resonant circuit has a resonance between 75Hz and 120Hz.
5. A guitar amplifier system according to any preceding claim, wherein the secondary winding has two or more tapping points, further comprising an attenuator switch which can be switched between a first state in which the load is coupled to one of the tapping points, and a second state in which the load is coupled to another one of the tapping points.
6. A guitar amplifier system comprising: a valve power stage; an output transformer with a primary winding coupled to the valve power stage, and a secondary winding with two or more tapping points; a reactive attenuator; and an attenuator switch which can be switched between a first state in which the load is coupled to one of the tapping points, and a second state in which the load is coupled to another one of the tapping points.
7. A guitar amplifier system according to any preceding claim, wherein the load comprises a load resistor coupled in series with the load capacitor.
8. A guitar amplifier system according to any preceding claim, further comprising a speaker coupled to the secondary winding of the output transformer in parallel with the reactive attenuator.
9. A guitar amplifier system according to claim 5 or 6, and claim 8, wherein the speaker and the reactive attenuator are coupled to different tapping points of the secondary winding.
10. A guitar amplifier system according to 8, wherein the secondary winding has two or more tapping points and the speaker is coupled to one of the tapping points of the secondary winding.
11. A guitar amplifier system according to claim 10, further comprising a speaker switch which can be switched between a first state in which it couples the speaker to a first one of the tapping points, a second state in which it couples the speaker to a second one of the tapping points, and a third state in which it couples the speaker to a third one of the tapping points.
12. A guitar amplifier system according to claim 11, wherein the first, second and third tapping points are associated with respective output powers of the speaker, and the output power of the speaker at the third tapping point is less than 2W.
13. A guitar amplifier system according to claim 11 or 12, wherein the first, second and third tapping points have respective effective numbers of windings, and the effective number of windings of the third tapping point is less than 100/o of the effective number of windings of the first tapping point.
14. A guitar amplifier system according to claim 13, wherein the effective number of windings of the third tapping point is less than 5% of the effective number of windings of the first tapping point.
15. A reactive attenuator configured to be coupled to an output transformer of a guitar amplifier, the reactive attenuator comprising: a load comprising a load capacitor, wherein the load does not comprise a load inductor in parallel with the load capacitor.
16. A reactive attenuator according to claim 15, further comprising first and second attenuator inputs; and an attenuator switch which can be switched between a first state in which the load is coupled to the first attenuator input, and a second state in which the load is coupled to the second attenuator input.
17. A reactive attenuator configured to be coupled to an output transformer of a guitar amplifier, the reactive attenuator comprising: a load comprising a load capacitor; first and second attenuator inputs; and an attenuator switch which can be switched between a first state in which the load is coupled to the first attenuator input, and a second state in which the load is coupled to the second attenuator input.
18. A reactive attenuator according to any of claims 15 to 17, wherein the load comprises a load resistor coupled in series with the load capacitor.