L-PAD vs POT
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spkrdtr
spkrfixer
IMO, they are one and the same.
L-Pads are also referred to as the simple two-resistor circuit used to attenuate. That's because the series/parallel configuration forms a letter L.
However, there is a difference between a rheostat like those used on early AR speakers and pots used as L-Pad in speaker attenuation circuits.
jwalker
Well-Known Member
L-pads are a form of lossy matching used for attenuation of typically tweeters. The idea is that the load impedance is stable regardless of the attenuation setting and will not cause undesirable shifts in the crossover corner frequency due to changing impedance.
True POTS in series or in parallel can be used to attenuate drivers too since it is just a variable resistor. The downside is that the attenuation comes at the expense of impedance mismatch. In the series case, the attenuation is basically the product of a voltage divider function.
Interesting is that a resistance such as a pot or resistor in series with a tweeter can cause a rising response vs frequency due to the inductance of the tweeter from the voice coil. With a true ribbon, this obviously wouldn't happen.
cyberey66
AK n00b
I didn't want to start a new thread for this simple question. But, is an L-pad physically the same as a pot and simply just the name given to a particular wiring configuration of a pot?
From the drawings I've seen, electrically an 8 ohm l-pad is the same as an 8 ohm pot.
For a an 8-ohm L-pad and an 8-ohm speaker, looking into the L-pad:
Fully open: we have 8 || 8 = 4 ohms
Mid way: we have 4 + 8 || 4 = 6.66 ohms
Fully attenuated: we have 8 ohms
If we connect the input to legs 1 and 3 of a pot and the speaker to legs 2 and 3 of the pot, we get exactly the same impedance looking into the pot as we do the L-pad. I'm a bit confused, is L-pad just a name for wiring a pot in this fashion? The circuit diagrams for and L-pad and pot are drawn differently, but electrically they are equivalent.
These are the diagrams I have seen for an L-pad:
http://www.colomar.com/Shavano/lpad3.gif
From the drawings I've seen, electrically an 8 ohm l-pad is the same as an 8 ohm pot.
For a an 8-ohm L-pad and an 8-ohm speaker, looking into the L-pad:
Fully open: we have 8 || 8 = 4 ohms
Mid way: we have 4 + 8 || 4 = 6.66 ohms
Fully attenuated: we have 8 ohms
If we connect the input to legs 1 and 3 of a pot and the speaker to legs 2 and 3 of the pot, we get exactly the same impedance looking into the pot as we do the L-pad. I'm a bit confused, is L-pad just a name for wiring a pot in this fashion? The circuit diagrams for and L-pad and pot are drawn differently, but electrically they are equivalent.
These are the diagrams I have seen for an L-pad:
http://www.colomar.com/Shavano/lpad3.gif
mhardy6647
Lunatic Member
No, not at all. A pot (potentiometer) is a single, variable resistive element. An L-pad is two resistive elements; one in parallel with the load and the other in series with it. An L-pad need not be variable. Two fixed resistors are often used to "pad down" (attenuate), e.g., the output of a tweeter without altering its frequency response.
Also, the power handling of a "pot" is typically limited - high power applications typically use a rheostat.
so... let's see if we can hotlink these images...
Here's a simple potentiometer:
http://www.beavisaudio.com/techpages/SchematicToReality/fig4.2.gif
Here's an L-pad:
http://www.colomar.com/Shavano/lpad3.gif
... and here's the equivalent circuit of a (fixed) L-pad:
http://amps.zugster.net/images/articles/attenuation/lpad.gif
Also, the power handling of a "pot" is typically limited - high power applications typically use a rheostat.
so... let's see if we can hotlink these images...
Here's a simple potentiometer:
http://www.beavisaudio.com/techpages/SchematicToReality/fig4.2.gif
Here's an L-pad:
http://www.colomar.com/Shavano/lpad3.gif
... and here's the equivalent circuit of a (fixed) L-pad:
http://amps.zugster.net/images/articles/attenuation/lpad.gif
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cyberey66
AK n00b
That's why I'm confused, I guess an L-pad is in fact just a name given to a circuit that uses two resistors to attenuate the output of a tweeter.
Electrically, you could do this with a single POT, but they aren't suitable for high power applications?
mhardy6647
Lunatic Member
No, you can not... at least, not if you wish to maintain constant impedance.
We're not in DC land anymore, Toto.
http://ccs.exl.info/calc_cr.html#lpad
cyberey66
AK n00b
I am confused about that. The impedance seems to change, unless I am missing something fundamental. I quickly worked out the impedance (say at a single frequency) for a speaker (drawn as a box) using both an 8 ohm L-pad and an 8 ohm POT. The impedance varies from 4 ohm to 8 ohm in both cases
Hmm, maybe I made the incorrect assumption that the two resistors are equal to each other? If the first resistor is 4 ohms and the second is 8 ohms, the change in impedance would be smaller. In the same analysis, I would get 4 ohms at no attenuation, 4.6 ohms at mid way, and 4 ohms again at full attenuation.
*Edit* Thanks for your answers, at least I know that a POT and L-Pad are not the same. L-pad = speaker attenuation with two resistors. I was confused by the drawings I have seen of a variable L-pads by incorrectly assuming the resistors drawn where the same.
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gkimeng
Forum Mod, ClassicSpeakerPages.net
The windings in the L-pad are equivalent. The double-ended wiper in your sketch is what causes them to balance each other out as the wiper moves across the resistance. The pot's single-ended wiper means that the impedance changes as the wiper moves.
WRT to rheostat vs. pot vs. L-pad. "Rheostat" is the generic name for any variable resistor. L-pads and Pots are both types of rheostats. Labeling gets a bit confused by the fact that pots are often generically labeled as rheostats by the electrical industry (power, motors, heaters, etc.), while the electronic industry pretty much always calls them potentiometers.
WRT to rheostat vs. pot vs. L-pad. "Rheostat" is the generic name for any variable resistor. L-pads and Pots are both types of rheostats. Labeling gets a bit confused by the fact that pots are often generically labeled as rheostats by the electrical industry (power, motors, heaters, etc.), while the electronic industry pretty much always calls them potentiometers.
mhardy6647
Lunatic Member
Run some numbers in a calculator and see how it works.
cyberey66
AK n00b
That was the source of my confusion, looking at the diagram provided for the L-pad below. If the two resistors are the same, then it is electrically equivalent to a POT.
I plotted the input impedance of this circuit as a function of 'wiper' position, or knob position, 0 = wiper at top, 1 = wiper at bottom in the diagram. (assuming linear for now) I did this for R1=R2=8 ohms and R1= 4 ohms, R2 = 8 ohms. The input impedance clearly changes. So my diagram drawn a few posts above must be incorrect. I'll figure it out. I might just buy an adjustable l-pad and take it apart to see what it physically is. Figuring out what combination of resistors to use for a non-adjustable L-pad is trivial, I just want to figure out how the adjustable ones work.
cyberey66
AK n00b
Do it again for R1=R2=1000 ohms (or 200 or a billion). Ta-daaa
edit, er... wait, you'd never want R1 over 8 ohms.. shoot. now I don't know how L-pads work either, or rather what is in them.
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mhardy6647
Lunatic Member
Here's the equation for an L-pad:
http://www.diyaudioandvideo.com/Calculator/LPad/Help.aspx
where:
Z = Total Driver Impedance in Ohms
A = Required Attenuation in -db
A variable L-pad contains two variable resistors rigged in such a way as to provide variable attenutation while maintaining fixed impedance (e.g., 8 ohms); according to the equations above.
Here's a pretty good explanation presented for an 8-ohm L-pad:
http://www.bcae1.com/lpad.htm
http://www.diyaudioandvideo.com/Calculator/LPad/Help.aspx
where:
Z = Total Driver Impedance in Ohms
A = Required Attenuation in -db
A variable L-pad contains two variable resistors rigged in such a way as to provide variable attenutation while maintaining fixed impedance (e.g., 8 ohms); according to the equations above.
Here's a pretty good explanation presented for an 8-ohm L-pad:
http://www.bcae1.com/lpad.htm
cyberey66
AK n00b
Here's the equation for an L-pad:
http://www.diyaudioandvideo.com/Calculator/LPad/Help.aspx
where:
Z = Total Driver Impedance in Ohms
A = Required Attenuation in -db
A variable L-pad contains two variable resistors rigged in such a way as to provide variable attenutation while maintaining fixed impedance (e.g., 8 ohms); according to the equations above.
Here's a pretty good explanation presented for an 8-ohm L-pad:
http://www.bcae1.com/lpad.htm
Thanks for the links!
The mistake I made was to assume both resistors on both sides of the wiper in the diagram were the same. They actually have to be such that R1 varies directly as the dial position and R2 varies as (1-A)/(A+1) for a dial position A, which varies from 0 to 1.
A decent approximation is two Rheostats ganged together, the first has a resistance of 1/2 the load and the second has a resistance of the load. This would provide an input impedance approximately equal to 1/2 the load impedance. Put a series resistor of 1/2 the load resistance and you are good to go. I bet this is what is inside some L-pads. I'll buy one to rip apart and check
If this is the case, I'll update the Wikipedia article to be more clear.Here is the circuit I drew to come up with my conclusion:
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