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testing a crossover

hi guys and girls i have a
problem with my 3 way  wilmslow-audio mirrage speakers , I am hoping you can help me with the problem,.
Lately my power amp went pop and i sent it away for repair,when i
received it back {repaired} it went pop again,on sending it back to
the repair technician ,he has come back to me ,and asked me to take
some ohms readings on the speakers, on testing the speakers , i run
these speakers using 2 power amps{Arcam Alpha 10s,using one for low
pass{bass} and the other for MID/HIGH,this is the amp which keeps
blowing,on the high /mid,on testing the terminal the bass{low pass}
read {left speaker]7.9 ohms ,right speaker bass 6.8ohms, But on the
left and right speakers hi/mid ,both read 0.0 ohms,{which we all know
is wrong}
BUT the strange thing is,when the blown amp was away,i only used one
power amp to run both speakers in bi wire config,and they worked
ok,all being a bit lack of bass and tightness,but still sound great.
So i stripped both speakers and checked the drivers,all 6 drivers
where good no shorts reading approx correct ohms for age,{Speakers
disconnected from x over}.can any one help with this,i would be so much in debt.
kind regards Mark
likklegerry

Showing 3 responses by millercarbon

millercarbon's avatar
millercarbon
12,531 posts
 
I have a love/hate relationship with puzzles like this. On the one hand I love em because it is nice to think they can be solved if only you know enough. On the other hand I hate em because you never can be sure you are getting the whole story. Like, one train leaves at 30mph, another at 40, how many painters does it take to earn $40?  

In this case I can't help wondering, how on Earth did two speakers go bad at the same time? In other words maybe zero ohms is a red herring. No trains left the station. Three buckets of paint, and the waitress earned a $40 tip?
millercarbon's avatar
millercarbon
12,531 posts
 
The formula for parallel resistance is 1/Rt= 1/R1+1/R2+1/R3....   
https://www.electronics-notes.com/articles/basic_concepts/resistance/resistors-in-series-parallel.ph...

Since he measured almost 8 ohms then the total resistance will be 1/8 ohm + 0 ohm or 0.125ohm.  

I think we all can agree that is a pretty low resistance and would suck the bass and dynamics out of most amps. 

What I don't understand is where is the short? And how does the driver manage to still get enough signal to make sound even though there is a dead short?
millercarbon's avatar
millercarbon
12,531 posts
 
You have a dead short. They worked when connected normally because then the amp sees the sum total resistance including the 6.8-7.9 ohms. But connected bi-wire one amp sees only a dead short and so pop, there she goes.

The lack of bass and definition is because the dead short results in a total impedance much lower than 6.8. It is simply running out of gas and so what seems strange to you is perfectly normal and expected given the situation.

Now it seems something happened to create the dead short. This is what caused the pop and the first amp failure. It seems the drivers are putting out sound or you would have noticed that, and they measure fine. So probably the short is somewhere between the terminals and the drivers. That leaves wires and crossover.

Get in there and see. (I hope it is not one of these potted crossover situations....)
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