An attenuator is an electronic device that reduces the power of a signal without appreciably distorting its waveform. An attenuator is effectively the opposite of an amplifier, though the two work by different methods. While an amplifier provides gain, an attenuator provides loss, or gain less than 1.
The diagram above provides a simplified visual representation of an attenuator circuit. The circuit is made up of a signal source, input impedance (Zi), attenuator and output impedance (Zo).
For the purposes of this article only passive constant impedance attenuators ( both Zi and Zo remain the same) and their application within amplifiers will be covered.
There are common forms of passive attenuators which use simple resistor networks:
- L pad
- T pad
- Π pad
L pad are the most basic form of passive attenuators. L pad is basically a potential divider circuit which is designed to match Zi (input impedance) but not Zo (output impedance). Many guitar amplifiers and pedals use this simple design because it is cheap
and effective for attenuation. The drawback being that the speaker no longer sees the same impedance from the amplifier therefore impacts the strength of the signal and tone.
T pad are balanced passive attenuators (although can be made unbalanced). The topology has a single resistor to ground and a series of resistors on the input and output. The use of this type of topology for guitar amp attenuators is more effective because both
input and output impedances remain the same i.e. impedance presented to the amplifier and to the speaker, for the most part (see notes on reactance), remain the same as before therefore no impact on amplifier output section or speaker input. This in
effect means the tone of the amp should remain the same during attenuation.
Π pad are balanced passive attenuators (although can be made unbalanced). The topology has a single resistor in the signal line and input and output resistors to ground. Much like the T pad attenuator, the Π pad topology keeps both input and output impedences the same.
Application of Passive Attenuators in Amplifers
It is important that when using an attenuator to attenuate a guitar amplifier signal that the tone is not impacted in any way. There are two important factors to consider when designing or buying a attenuator:
Impedance Matching: It is important that the load the attenuator presents to both amplifier and speaker remain the same (or as close to) as they were before the attenuator was added into the circuit. For example, an L section attenuator is not fit for purpose due to the fact that the input impedance remains the same and the output impedance does not (e.g. 4Ω amp out/4Ω speaker in after attenuation results in the following: 4Ω amp out/2.2Ω speaker in). Any impedance presented to a speaker which is lower than the optimal impedance will result in a weak signal and poor tone i.e. muddy.
Making use of T or Π topologies will allow for both input and output impedances to match those impedances prior to adding the attenuator e.g. 4Ω amp out and 4Ω speaker in. For the most part, the tone will not be impacted by increasing attenuation using T or Π topology.
Reactance: An important factor in the circuit is the speaker and how it reacts to changes in the signal frequencies with regards to impedance. The diagram below provides a visual representation of how a speaker impedance changes with changes in frequency:
Changes in impedance occur due to 1) resonant frequencies 2) increase and decreases in impedance i.e voice coil of the speaker acts like an inductor. Because frequency and inductive reactance are directly proportional, a 4Ω voice coil only measures 4Ω at a specific resonant frequency. If the frequency values decrease, then so too does the reactance and impedance of the coil. If frequency values increase, then the impedance likewise follows.
In the diagram, the large peak corresponds to a point at which both mass (the cone, cone suspension, spider and the voice coil) of the assembly and spring resonate most freely. The movement of the coil will then induce a voltage back into the circuit and increase inductance/impedance. The diagram also shows that with frequency increase the inductance and impedance increases. This is due to the fact that the speaker coil is acting like an inductor i.e. impedes rapid change.
Speakers are denoted with a nominal impedance i.e. the minimum impedance of that given speaker cannot drop below 80% (international standard) of the value stated over a defined frequency range. So for an 8Ω speaker it will not drop below 6.4Ω , 6Ω not below 4.8Ω (shown in diagram above) and 4Ω not below 3.2Ω. In reality speaker impedances will swing from the minimum rating to very large impedances over the audible frequency range. Speakers do not present simple constant resistances; They are complex systems and present a changing impedance in that they resonate at frequencies (very high impedance) and increase impedance at higher frequencies.
Using a simple network of resistors to attenuate the signal level is successful to a point but imparts a sterile feel to the tone due to it not presenting the reactive element of a typical speaker. Being able to also match the reactive load of the amplifier and speaker will help achieve a much more accurate tone and feel. By making use of inductors in the attenuator circuit it is possible to achieve this.
The diagram below shows the introduction of inductors into the Π pad attenuator circuit at input. By placing inductors in different parts of the circuit present reactance to either the input (amp) or output (speaker). This will result in lower or higher levels of current flowing in the circuit.
Passive Attenuator Design
Simple Resistive Attenuator
Use a PI attenuator calculator to work out the values of resistors to use for the required impedance and attenuation. You will need to provide the impedance value in ohms and attenuation value in dB. For the example below the impedance is set to 4Ω and attenuation to 7.4dB - the results are 9.9514Ω for resistors to ground and 3.8353Ω for the single resistor in series to the positive terminal. The closest values to those provided is chosen - please note that by changing to closest values the impedance presented to both amp and speaker may differ slightly to that required (for 10Ω/10Ω/4Ω the impedance presented to both amp and speaker are 4.068Ω)
The following two audio clips demonstrate l pad and pi pad configurations without and with attenuation added at 7dB.
Simple Reactive Load Attenuator
Much like the basic PI attenuator use a PI attenuator calculator to work out resistor values based on impedance and attenuation requirements. The following diagram shows an air coil attenuator of 0.9mH (DC resistance of around 0.2Ω) placed in parallel with the input. The inductor will present a reactive load to the amplifier and as a result at higher frequencies the impedance will increase (much like the speaker behaviour). The input resitor to ground has been amended to 9.8Ω to accommodate the inductor 0.2Ω DC resistance