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Steve.au

Speaker Impedance Out of Wack

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Steve.au   

Hi everyone, I recently picked up a pair of Mordaunt-Short ms25ti's for a decent price. There is a bit of rub on one of the woofers but nothing a recenter shouldnt fix - I think the voicecoil is ok.

 

Of greater concern is one of the speakers is reading @32ohms resistance on my multimeter whereas the other one is reading 7.7ohm. I swapped the woofer and tweater between cabinets and the impedance is the same, so I think it is something to do with the crossover. I have been looking at the crossover for a few days and testing the resistance  around components on the board but I have got to the point where Im ready to admit I know next to nothing about this stuff. So hoping to get some help! I have compared the resistance of the components to the lower impedance cabinet but not sure how to narrow it down to the fault. Any help greatly appreciated. I'm confident enough to replace any component on this thing but might need some help finding equivalents.

 

I have attached a picture of the board. 

IMG_20170911_170429.jpg

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Noum   

I'm not sure what is causing the high impedance but i would replace those Benic electrolytic caps with some film caps. Same capacitance values.

 

Someone else will help better than I.

Edited by Noum
typo

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A wild guess but the speaker reading 32ohm is the wrong one. Once more a wild guess is the woofer is open cct. or the inductor to the woofer is OC or the wiring to the woofer is OC.

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I'd suggest checking/retightening any screw terminals and reflowing all the solder joints.

 

Regards,

 

SS

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By the way the Xover is a piece of shite. Dump it. I hope you paid no more the than $20 for the speakers. I wouldn't  even use that Xover in $20 speakers. 

Edited by mwhouston

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Aha, are they polyswitches (yellowish resettable fuses) on the lower right hand corner of the crossover board? Could you do a resistance test of the larger one?

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Steve.au   
6 minutes ago, Sub Sonic said:

Aha, are they polyswitches (yellowish resettable fuses) on the lower right hand corner of the crossover board? Could you do a resistance test of the larger one?

Ahh I have been trying to work out what they were - I thought they might be ceramic capacitors but couldnt work out why they were there. The big one went into Out of Limit - the smaller one is reading 3.2ohms.

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Steve.au   
10 minutes ago, mwhouston said:

By the way the Xover is a piece of shite. Dump it. I hope you paid no more the than $20 for the speakers. I wouldn't  even use that Xover in $20 speakers. 

haha thanks for the advice! Well I dont know what Im doing so I guess for $20 so if I learn something from the process its worth it.

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Yep, keep the crossover and replace the polyswitch. You could bypass the polyswitch as a test, and replace it if you feel so inclined.

 

BTW the one with the reading of 3.2 ohms also sounds a little high, but the smaller the polyswitch, the higher the resistance it will have. It may or may not be within spec.

 

SS

Edited by Sub Sonic

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Steve.au   

I found them on Jaycar - so Im guessing the big one would be for the woofer and the smaller for the tweeter? so grab ones that match the wattage and resistance of the speakers? am I on the right track here?

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Steve.au   
28 minutes ago, Sub Sonic said:

Yep, keep the crossover and replace the polyswitch. You could bypass the polyswitch as a test, and replace it if you feel so inclined.

 

BTW the one with the reading of 3.2 ohms also sounds a little high, but the smaller the polyswitch, the higher the resistance it will have. It may or may not be within spec.

 

SS

Mate that did the trick - bypassed the big polyswitch and now the xover and speakers are reading 5.8 ohms - so now the "good" cabinet is reading a bit high at 7.8ohms! Thanks heaps for the advice - I owe you a beer.

 

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If the speakers are any good make some new or even buy some better Xovers. You will need to know the Xover points to replace them. 

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6 hours ago, Steve.au said:

I found them on Jaycar - so Im guessing the big one would be for the woofer and the smaller for the tweeter? so grab ones that match the wattage and resistance of the speakers? am I on the right track here?

 

Yes, the smaller one would be for the tweeter, and the larger one for the woofer. I'd replace all the resettable fuses, I.e. do both cabinets, and see if that evens things up.

 

Resettable fuses are rated by current, so Google the part numbers and get the correctly rated replacements. Jaycar only has a limited range from memory, but RS Components has many options. RS usually sells them in packs of 5 or 10, but postage is free.

 

Regards,

 

SS

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