Polarity is correct!!!!

Discussion in 'Amps [BG]' started by Remus_Redbone, Jul 5, 2013.


  1. Remus_Redbone

    Remus_Redbone

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    I have an old Peavey 2 x 15 cabinet that I loaded with two cast frame 400 watt EV 15's that look like this;

    http://images.search.yahoo.com/imag...hmv&sigi=1175qs5fr&.crumb=eAIKtGLT/4B&fr=ipad

    I just recently sealed up the ports on this cabinet to see if it would tighten up the sound. Each speaker has it's own compartment and does not have the air pressure influence from the other speaker.

    Here's the twist; when I play an open E or A string, the speakers vibrate at the frequency being played, but one slowly moves outward from its frame and the other pulls inward deeper into its frame. Each are still vibrating at frequency.

    I've checked and re-checked the polarity by using a 9 volt battery and touching the batt. terminals to the tip and ring / ground of a speaker cable plugged in the cabinet. Both speakers always move in the same direction.

    Is the slow drift of the cones in opposite direction the result of one compartment not being sealed as good as the other? What are the other possibilities?
     
  2. T-Bird

    T-Bird

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    Hi.

    There can be something wrong with the tuning or the drivers, but more likely explanation is that the inadequacies of our eyes/brains just trick You.

    Especially with fluorecent lights, the flickering frequency and the cone movement frequency create a resultant that can be processed with our limited senses, ie. less than ~25Hz or fps.

    The max amplitude of those old EV's is not enough to see much movement accurately anyway, especialy in a sealed cab.

    Regards
    Sam
     
  3. Remus_Redbone

    Remus_Redbone

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    Since both drivers are in the same cabinet, they are generally in the same light. As for the excursion of the EV drivers, it's may not be as extreme as some newer speakers, but it's not difficult to see, and it's the primary reason these speakers still work. I've been down the road with extreme xMax speakers. They sound fantastic at first, but I haven't found one that will hold up for long. I'll admit that I've only tried a couple; it's a rather expensive experiment, but the couple I've tried started sounding like Buddy Holly's guitar speakers after about 15 - 20 gigs.
     
  4. Bass_Pounder

    Bass_Pounder

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    Are you properly modeling with cab design software and retrofitting the cab to the speakers ?

    Or are you just swapping out speakers ?

    If you are just swapping speakers, you could be damaging them due to improper cabinet volume and tuning.
     
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  6. David Jayne

    David Jayne

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    I would bet one of the two chambers is leaking. Can't think of any other explanation, other than what T'bird said. His theory can be proved/disproved easily enough by just holding a fingertip near the cone to gauge movement.
     
  7. BassmanPaul

    BassmanPaul Gold Supporting Member

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    My first thought too. So many folk seem to think that if a cabinet is cut for a 15 or even two, that ANY 15 will work well in that cabinet.

    OP, the enclosure has to support the particular driver's parameters that you are using. Anything else is a crapshoot and weird things, such as you describe, can and do happen.
     
  8. Remus_Redbone

    Remus_Redbone

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    I would guess a bit of both. I'm sort of building a retro rig consisting of a Fender Studio Bass amp section built into a head and the old Peavey 2x15 cabinet. I got the cab for almost nothing, & I had the speakers, so I loaded them up. The best I could measure the shelf porting, it was wrong for the T/S parameters of the drivers. It sounded like it, too.

    The shelf port was going to be almost impossible to modify, so I thought I would try sealing up each compartment. That more or less eliminates cabinet tuning and is much more forgiving; as long as the cabinet volume isn't too small or way too big, the speaker does its thing. Each compartment of the cabinet is 3.18 cu. ft. which is not optimum, but within the range for the T/S parameters of the drivers. The cabinet performance is greatly improved, but then I noticed the opposite drift in the drivers when allowing a long sustained note while searching for "air leaks".

    I think I discovered the issue. Even though the two drivers are identical, one has been re-coned. When I swap their position in the cabinet, each driver continues to act the same, eliminating the cabinet as the cause. Perhaps the re-coned VC winding starts on the VC former at the opposite end from the original? Not sure on that, no way to tell.

    There doesn't seem to be any phase canceling, so I'm going with it. The sealed cabinet does wonders to tighten up the low end when using the tube amp. It was sloppy when driving the cabinet with the tube amp when the cab was still ported, which is why I started this journey.....
     
  9. Downunderwonder

    Downunderwonder

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    Speed of light being what it is, it's safe to discount strobe effect as the cause. It's the means by which the odd phasing is being observed. Must be leakage or drivers are very non identical.
     
  10. Remus_Redbone

    Remus_Redbone

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    Here's a look at the beast. I had to replace a board in the grill frame that I didn't paint and does it ever show up with flash!!

    Also need to get the grunge off the back of the cab, but that pic is to show the mobility mods. Casters from hardware store and bathroom grab bar for disabled. $14 + $18.

    [​IMG]

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