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In Reply to: RE: How do I test my IIIA to make sure they are OK? posted by neolith on December 14, 2014 at 17:08:20
The sub is fed by a separate pre-out off the pre-amp, so the speakers are getting a full range signal.
I checked the impedance and they both came in about 4.2 ohms.
I will try the change in polarity on one and see what the results are.
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Did you adjust phase on the subs? What is the XO freq? How far is the sub/s from the listening seat and how far is the middle of the bass panel? If you don't have fine adjustment for phase on the sub you can adjust the relative distance to reach in phase performance on the pass band or the XO freq, which one you choose depends on how much sub output you need - the more overlap you have with the speaker the more you would want the passband of the sub to be in phase.
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I took the sub out of the loop - and am trying to figure out of the Magnepans are operating as they should on their own.
With the sub powered I think the system sounds great. I have the sub crossed at about 60hz and if you look at the pic I posted you can see it behind the right speaker. Its a sealed 12" Atlantic Technology 422SB
Looking at the manual it is a 3rd order XO so should be at -135 deg @60 hz. and that is going to get more negative as you place the sub further away from the speakers, meaning that you will probably generate a bit of a suckout at least in one frequency. The half wave (180 deg) is about 11 ft. to get to -180 deg, would take 45/180 X 11 or about 2.8 ft of distance. Which is what you have with the realtive distance of speaker to ear and ear to the sub when you pull the speaker out. That suckout around 60 hz would be mirrored in the other speaker. since it is farther away and thus has its cancellation at the speaker itself at a lower freq, it may have an effect on the reflected bass from the left side.
If you flip the phase switch it will leave the at +135 and it will get less positive as the speakers move towards you, so the freq at which you will have cancellation would be further up and at a lower amplitude of sub output so you would have a smaller dip but at a more important freq around 90-100 hz.
If you brought the subs a bit closer than the speakers then the suckout would become smaller and at a higher freq. You can try that out by moving the sub along the sidewall towards your listening position in order to retain similar boundary support.
Another option is to lower the XO down to 40 hz where the freq would not happen to have that accidental set of numbers to cause dips in your bass.
Another option is to use the high pass of the sub to feed the speakers and put the XO wherever you like since then the speakers and subs would have a complementary phase and one of the inverse or straight phase positions will get it right.
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