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L-pad vs. pots for speaker level control

michiganpat

michiganpat

Super Member
Hey everyone, I've got all the pieces for a custom speaker project based on some $10 AR2ax cabinets I picked up. one question I have concerns the speaker level pots, which I have thoroughly cleaned.

the two potentiometers that are used for mid and high range attenuation are a 0-16 ohm pot, with 2 legs and a common ground, so one leg is X ohms, the other 16-X. measuring it all the way one way or the other, it's acutally ~.3-15.7 ohm according to my multimeter. The way they are originally wired in, one leg is in series, the other in parallel to the driver. in it's "high" position, with the parallel leg at 15.7 ohm, the series leg at .3, it should provide ~0.7db of attenuation. turning all the way down essentially will turn off the driver as the parallel leg goes to 0 if I understand it correctly.

with an L pad circuit, which is the main "influence" of performance, the series leg or the parallel leg? playing with an online L-pad calculator, it looks like as the series resistor approaches the driver's impedence, attenuation approaches infinity....consequently, as the parallel leg approaches 0, the attenuation approaches infinity. what if you have what I have, a nominal 4 ohm impedence driver, where say, the series leg is 4 ohms, but the parallel leg is ~11.5 ohm? will it have infinite attenuation, or ~2.6db (depending on which value I look at)?
 
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The purpose of an L-pad is to maintain a constant impedance as seen by the filter, such that the crossover frequency does not change with attenuation. We originally built them with reciprocally connected ganged wirewound pots.

Using a single pot as in AR, Altec, and others, the frequency does shift, typically in an undesirable fashion. I'd dump the pots in favor of L-pads, for this reason.

HOWEVER, restorationists, in the interest of vintage "correctness" often want the system to behave as original, for better or worse, and there are several approaches to accomplishing that. I documented the situation with AR4x and AR3a in separate threads posted in this forum:

http://www.audiokarma.org/forums/showthread.php?t=123113

http://www.audiokarma.org/forums/showthread.php?t=215006
 
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that's kind of what I was thinking/afraid of, that the impedance of the circuit would change since the circuit resistance would change (increase as the series resistance increased to 16 ohm), causing a shift in the crossover frequency upward, and creating a hole in the sound. I was thinking maybe I could put a 5.3 ohm resistor in parallel with the series leg to at least minimize the shift (would put the max resistance of the series pot at ~3.9 ohm)

all the drivers I chose are going to be 4 ohm.

the woofers and tweeters I picked up (10" DVC eminence and JAMO/phillips 1" dome) are both within .5db of each other as far as their published sensitivity for 1w/m, and the mid's (a 4" dayton aluminum cone/alcino magnet) published sensitivity is down about 3db, so for now I think I'll run without any attenuation and see how I like it. if I feel I need it, maybe I'll sell the pots for the restoration crowd and pick up some L-pads, or if my parallel resistor idea would work, just do that.
 
bump for the day crowd....

Zilch, what resistor is the main influence of the speaker attenuation, is it the parallel one? (as resistance goes to 0, more current until essentially all flows through it?) would my idea of running a parallel resistor in the series leg to get as close as possible to a 4 ohm load with the series leg "all on" essentially make my pots into L-pads (other than the fact that the parallel leg will be 16-0 ohm instead of the typical 300+-0 ohm range)?
 
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Calculate the fixed-resistor pair required for your desired attenuation, and vary the series-connected one about the requisite nominal.

[OR, buy 4-Ohm L-pads.... ;) ]
 
do you have a source for 4 ohm L pads? all I can find are 8 ohm, or would putting an 8 ohm resistor parallel to the L pad series leg essentially do the same thing?
 
Parts Express used to stock them, but no more, apparently. What does the car audio gang use?

Run the calcs (or measure), and I believe you'll find the schemes you are proposing are both non-linear in attenuation and wrong with respect to maintaining constant impedance.

Option: use a three-way toggle to switch between three fixed-resistor L-pads bracketing your desired attenuation.... :yes:
 
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