There Have Always Been Common And Exotic Attenuators...

I have used many different kinds of attenuation types over a long career. Some I'm sure many of you have never heard of. All used for analog signal processing. All requiring high-definition. And all considered precision. For what they were.

Potentiometers: There are many different types of 'potentiometers' using different materials and geometries in order to achieve the same effect. To alter the flow of signal (current). Mosty... They use a configuration known as The 'Voltage Divider' where the potentiometer represents a resister and a variable tap along its resistance forming, in essence, two resisters (in series) R1 and R2 with a 'tap' at the junction (wiper). Thus the tap represents the ratio of R1 to (R1 plus R2) with respect to the input signal allowing attenuating it as necessary.

That said... Potentiometers do not provide accurate or stable control over analog signals. Additionally they are for the most part noisy. Furthermore, as the example shows (do the math), source and load impedance introduces non-linearity into the control which can seriously effect frequency response in AC coupled circuits.

Step Attenuators: Step attenuators operate on essentially the same principal as potentiometers. however instead of a purely linear adjustment a set of discrete steps are used to provide the attenuation. Interestingly... This method was most commonly implemented as a rotary switch with a few dozen discrete attenuation points. The major distinction, and attraction, was that each step represented a very specific step in terms of 'db' (decibels), a logarithmic method for scaling analog signals. Furthermore the design ensures that the input-impedance and output-impedance remain constant for any position (step) making exact gain adjustments possible. This would have been an ideal match for connecting and controlling various audio tube equipment however the cost of providing reliable controls (hardware) was reserved to fine 'instrumentation' equipment.

Multiplying D.A.C.: Uh!

Differential Mixers: This class is my all time favorite. Basically... A differential transistor pair is used in a 'Mixer' or 'Modulator' configuration. In other words the differential pair is driven by a variable (modulated) 'current-source' which sets the gain for the stage. Hence Modulation and additionally Mixing. When the modulator input is replaced by a steady control voltage a very usable audio attenuation range can be achieved. With incredible fidelity (transparency). This is the ultimate 'Voltage Controlled Attenuator' (VCA).

Advantages range from high impedance input and low impedance output which remain constant at any attenuation level to 'pure' DC coupled. Meaning... No phase incoherency at low frequencies.

Variable Reluctance: This one is not usually suitable for high bandwidth attenuation. I mention it only because of its extreme long-life and high-reliability. As such it might be suitable as a candidate for a control method as opposed to an actual attenuation device. Variable-Reluctance takes advantage of the magnetic principal that perpendicular lines of flux do not couple (influence). Practically this means that a transformer whose secondary can be rotated about the primary will exhibit a ratio of coupling proportional to the angle of rotation where 0 degrees rotation will represent 100% (theoretically) coupling and 90 degrees will represent 0% (ideally). Because of the magnetic coupling the responsiveness is controlled by the excitation frequency and how quickly the demodulation circuitry can stabilize. The lower the response required the less critical the control circuitry needs to be. It's the same circuitry but lower the modulation frequency the less attention has to be paid to demodulator phase errors due to lag (delay) in analog processing.

Mixed Signal Devices: Uh!

Conclusion...

Uh!