I know it wouldn't harm anything, but it wouldn't be accurate because attenuation circuits are designed with a specific impedance in mind. I just can't say for sure how innacurate it would be or if it would make any real difference in use. But...
...because you said it's for a tweeter, I'm left to wonder what scenario you have that would make you need a variable L-pad. Tweeters are normally attenuated with fixed L-pads to match the other drivers' outputs and left alone. There aren't adjustments after the initial setup. So why don't you solder in the appropriate resistors in the "L" configuration to attenuate the tweeter where it matches sensitivity best? Here is a calculator: http://www.apicsllc.com/apics/Misc/filter2.html#fourth
I often buy resistors of values for a couple of db's attenuation, like -3 db and -2 db, to experiment with for best overall speaker sound. For example, if my graph looks best with -3 db tweeter attenuation, my room acoustics or some other anomaly may make it more likely that only -2 db does the job better. I want to have the resistors on hand to try both.
I build some MTM's and I used Vifa's D26TG0506 tweeter which has a fairly high FS of 1500.... so I am having problems toning it down, it's too up in my face ..too bright...I tried to expierement with resistors to cut 3 db... but it's still bright... altough that helped a bit... so short of me having to keep opening them up all the time I thought why not a variable L-pad.
Are you jonz on the diyAudio site? If not this is virtually an identical problem, but since you're both posting from Montreal I'm guessing you're the same person.
Real quick, reverse the polarity of the tweeter at the tweeter itself. The MTM design is also known as the 3/2 alignment meaning a 3rd order crossover for the bass units and a 2nd order for the tweeter. Because of the summation of the different order crossovers, there will be a peak at the crossover frequency that can be greatly reduced simply by reversing polarity on the tweeter. Note that the crosover region may not be ruler flat without some additional component tweaking, but it should be better.
You could also look into a notch filter to flatten the impedance peak at the box resonant frequency (for the woofers) as well as a Zobel (just an RC network in parallel with the woofer). A Zobel eliminates the impedance rise in the woofer as the frequency increases; it makes the woofer near a pure resistive load rather than an inductive one. These won't help the tweeter but can really improve the low frequency performance of the speaker if you don't already have them in the crossover.
I picked up the supplies for the Zoble... as for the notch I can't find the Le of these drivers posted anywhere eventhough I contacted via e-mail Canadian Loudspeaker Corp. the makers to see if they can send me the TS. parameters and plots..it's been 5 days no answer yet.
Are you using Speaker Workshop or another program for help in crossover design? And what are the mids...can you provide links to these drivers?
I recently built a Vifa MTM and got excellent results. Very laid back tweeter response.
The quality of components you choose to use in the tweeter network is more important than the mids, midbass and sub. Using a commercially available variable L-pad assures that you will have something of lower-than-desirable quality in the signal path. Use your design tools, and ears, and by experimentation and tweaking find the combination of non-inductive 5 or 10 watt resistors that will do the job best. I suggest Mills for the tweeter network.
The advice above is good about looking again at the rest of the system. Deficiencies in the mids network will influence what you are trying to fix in the tweeter. I hope your crossover is at least 3000 Hz, or better yet 3,500 Hz with that 1500 Hz Fs tweeter.
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