L-PAD vs POT

[dumptruck]

mm, but then you can never have full power to your driver, right? Surely R2 is 200+ ohm with 8-ohm R1 instead so that you can ditch the extra series resistor?

 

[cyberey66]

mm, but then you can never have full power to your driver, right? Surely R2 is 200+ ohm with 8-ohm R1 instead so that you can ditch the extra series resistor?

That is true, you will always dump power and can never have full power delivered to the speaker with that circuit. Power is dissipated through the series resistance and the parallel resistance for small values of R2. The only way around this is to get rid of the series resistance and use large values for R2 as you said.

So you could have a high impedance value for R2 and the ideal impedance of the speaker for R1, but the input impedance would not be constant and rise slightly as you move the wiper "down" in the schematic. If the impedance of the speaker is much less than R2 (say for 200+ ohm values of R2), you see in effect the impedance of the speaker when the wiper is "high up" on R2 (the low impedance of the speaker dominates when in parallel with high values of R2). When the wiper is all the way up, in effect we have nearly 100% power to the speaker. But as the wiper moves down, this impedance is added to the increasing impedance from R1. So you would have the impedance of the speaker + the increasing impedance of R2, leading to a rise in input impedance.

This would occur until the wiper is near the bottom and the resistance from the wiper through R2 is comparable to the resistance of the speaker. At the very bottom, 100% attenuation, we only see R1 which is the desired impedance.

They may be doing some trick like this and choosing values that give a "good enough" impedance. The impedance vs frequency of the speaker is not constant anyways, so they can live with having a variable impedance. I really need to pickup an L-pad now and play with it.