Confusion on variable L-pad vs. potentiometer

[Michael_in_Ohio]

I've been looking at schematics of crossovers. Some of them have a knob for adjusting the attenuation going to the tweeter.

So here is where I am confused. On some schematics, it shows two potentiometers that are ganged together, and their wipers are connected together. But on some schematics it shows just one potentiometer.

Here's my question: are they equivalent? I studied them, and they seem to be equivalent to me. The drawing below describes what I am talking about.

Here's another question: when you purchase an "L-pad attenuator" (such as one of these sold by Parts Express), is it simply a standard potentiometer as shown in B? Or does it contain two resistive elements and two wipers as shown in A?

 

[Looneybomber]

I’m not a potentiometer expert so there could be subtle differences between the actual components used in each diagram, but they function the same. For example, an iron core inductor has a different symbol than an air core inductor, but they both function the same.

The Potentiometer: Pinout, Wiring, and How It Works

This is a potentiometer tutorial where you'll learn how it works and with examples of how potentiometers can be wired for various circuits.

www.build-electronic-circuits.com

In university I saw the simpler diagram on the right, but that doesn’t always indicate what is commonly used in the real world. For example, we used 1.5 ohm in university, but outside academia, I see 1R5 more commonly used.

 

[Michael_in_Ohio]

Thanks!

I just did a bunch of Googling on the subject. It's really hard to find info on L-Pads. But I discovered that I made a bad assumption in my drawing. In my drawing I am assuming both resistors in the L-Pad (Drawing A) are 8 Ω. For a real L-Pad, the resistances are different: the series resistor is usually 8 Ω or whatever, and the parallel resistor is much higher, anywhere from 5X to 50X higher.

If the two resistors in the L-Pad both had the same resistance, then it would indeed be the same as a potentiometer, electrically speaking. But they're actually different.

 

[Wolf]

What you are seeing in your graphics are a T-pad, consisting of 2 variable series resistances and a fixed shunt resistance between them, and an L-pad, variable series and shunt resistor, with added 4 ohm resistor. They do the same thing; attenuate. However, you may have more minute adjustment and control with a Tpad over an Lpad.

Edit: I read it wrong. The left one has a shunt resistor in series with shunt part of Lpad, and then the series variable resistor. In effect the difference between the 2 is the series R is in a different location, fore or aft the shunt resistor. They both still attenuate, but differently.

 

[DVDdoug]

An L-pad is supposed to keep a constant load on the crossover. If the resistance/impedance changes the crossover frequency changes (a little).

 

[Wolf]

This is actually not as true as much it gets repeated. That is the goal of the Lpad to keep the impedance constant, yes, but the resultant shift in Fc is not as many think.

If you are using a 1st order electrical xover, then the frequency of xover (Fc) will shift due to the change in load impedance. Double the impedance, and the caps will halve and coils double in value to maintain the initial Fc.

If using anything higher than a 1st order electrical, as most speakers are, then only the electrical Q of the xover will change. This does affect the the angle and effectiveness of the resulting curves, but not the slope of the rolloff, which can in turn move the F3 of each driver ever so slightly. I mean between 50-100 Hz shift for a standard 2way design, which isn't much. In effect, the xover point between drivers really itself does not change more than a few cycles.