impedance question

[rntlee]

Hi,
I have a pair of 3-way 8 ohm speakers that don't measure the same at the terminals.
I only have an ohmmeter, which I know isn't ideal, but one measures ~6 ohms on my meter and the other ~3 ohms.
3 ohms seems like a pretty big deviation so I unplugged the mid and the woofer from the x-over and checked them individually and they both measure 5.5 ohms.
Is there something in the crossover that could cause this discrepancy?
I don't want to damage my amplifier.

[speaker dave]

When you measure resistance you are measuring the DC path through the speaker. Normally that would only be through the woofer since the midrange and tweeter have capacitors in series with them that would block the DC flow. So even though your woofer and mid are both 5.5 ohms, the DCR wouldn't be from those two paralleled (2.75 ohms), just the woofers 5.5.

My first guess is that there is a series capacitor for the midrange that is shorted in the system that measures 3 ohms. Measure just across those caps. They should measure open circuit (infinite resistance).

David S.

[sakellogg]

impedance is ac resistance.
you are measuring the dc ohms with your ohmmeter.
yes things in the xover will make it read funny.
your best bet to find the impedance would be a test tone say at 2 kHz. set your volume at medium and measure the voltage at the terminals.
then put your meter in series and see how much current you are drawing.
divide the voltage by current and you will get the impedance.......@ 2 khz.
it will be different for most frequencies
or you can do all the math and figure capacitive and inductive reactants for 20-20 khz
i hate math

[system7]

Edit: Sorry, wrong guess. LOL

[rntlee]

Hi Dave,
thanks for the reply. This x-over doesn't seem to have a series mid-range cap. Here's a pic of the x-over:
Crossover
and my best guess at the circuit (sorry, I'm no engineer!):
terrible circuit drawing

sakellogg, should both speakers give the same reading even though it's only an ohmmeter?

[speaker dave]

Not so terrible! The circuit is a little unusual in that both woofers are running full bass range. This makes it seem like the lower resistance (the 3.3 ohm) system is the one that is correct and the higher resistance has an open circuit?

Can you tell if all units are playing in both systems? Can you measure the woofers and their networks individually (woofer, inductor, resistor and capacitor as one measurement, midrange, inductor, resistor and capacitor as the second measurement). This might mean a temporary cut to the board or desoldering a component.

I'm guessing that one of those strings is open circuit.

David S.

[sreten]

Hi,

SD your first synopsis was more accurate.

The woofers are not run full range, they are a series inductor with a zobel.

Adding the mid and treble units should make no difference to the DCR.

The problem with the shown diagram is the mid has no high pass,
its the same as the bass unit, series inductor with a zobel.

Could be its a 2.5 way and one "mid" or "bass" unit is open circuit / blown.

That would explain the measurements ....

Something is wrong somewhere ...

rgds, sreten.

Check the DCR of all drive units ....

[rntlee]

Thanks guys, I'll check the drivers in the other speaker.

[rntlee]

You guys nailed it, dead woofer in the other speaker.
Ughh...I was hoping it was something inexpensive.

[speedie]

you must have noticed that there was an audible difference from one speaker to the next with a faulty woofer

hope that you can get another driver for your system