Strange problem with crossover construction... is this the drivers' fault, or...?

[fjhuerta]

Hi all!

I was building a couple of crossovers when something strange happened. When I measured impedance plots, they were both identical - except for a tiny region, from around 1000 ~ 1300 Hz. They measured significantly different - one of them was 3 or 4 ohms higher than the other. The overall shape of the curve was identical everywhere else.

I thought I had a cold solder joint, so I reheated every union. No luck.

I then disassembled both crossovers and reassembled them onto a larger board (to avoid inductor interaction). Same issue.

Disassembled everything and measured every component. They measured within the spec - except for the fact that I think I saw a cap oscillating when I measured it. I moved the probes a bit and it stabilized. I re-measured the cap at the crossover frequency and it seemed to be fine.

When I tried to reassemble the crossover - well, what do I know, broke a couple of leads. I tried soldering some wire to them, but the damage was done.

I ordered every cap and resistor again.

But I 'm now wondering - is it "normal" to have a couple of components fail every now and then? Or should I worry and start measuring the impedance of every driver... could this be the problem?

[Brian Walter]

Hi all!

I was building a couple of crossovers when something strange happened. When I measured impedance plots, they were both identical - except for a tiny region, from around 1000 ~ 1300 Hz. They measured significantly different - one of them was 3 or 4 ohms higher than the other. The overall shape of the curve was identical everywhere else.

Before disassembling everything did you try swapping or interchanging the crossover/driver combinations to see if the differences followed the crossover or the drivers? This would tell you right away if the difference was due to the drivers or the crossover. This might have saved you a lot of effort, then again maybe not.

Brian Walter

[fjhuerta]

Before disassembling everything did you try swapping or interchanging the crossover/driver combinations to see if the differences followed the crossover or the drivers? This would tell you right away if the difference was due to the drivers or the crossover. This might have saved you a lot of effort, then again maybe not.

Brian Walter

Ugh... you are absolutely correct.

Memo to myself: working at 3.00 AM is not really good if you need some common sense.

[JonP]

Another thing to think about... I don't know how your soldering habits/skills are, but if you're spending a lot of time heating those joints, or in this case, several solder/unsolder cycles, it can cause problems.

The polypropolyne caps (other film types too, but polyprop are worse) are somewhat sensitive to heat and the plastic film can melt, causing the leads to come loose, or disconnect from the many layers internal. You could end up with a pulled out lead, a cap that is no longer the rated value, or one with a bad connection.

That might have been the case with the flaky reading on that one cap you mention.

But definitely do the swap test first, to find out if it's not just a slightly different driver instead of a crossover issue...

[fjhuerta]

Another thing to think about... I don't know how your soldering habits/skills are, but if you're spending a lot of time heating those joints, or in this case, several solder/unsolder cycles, it can cause problems.

The polypropolyne caps (other film types too, but polyprop are worse) are somewhat sensitive to heat and the plastic film can melt, causing the leads to come loose, or disconnect from the many layers internal. You could end up with a pulled out lead, a cap that is no longer the rated value, or one with a bad connection.

That might have been the case with the flaky reading on that one cap you mention.

But definitely do the swap test first, to find out if it's not just a slightly different driver instead of a crossover issue...

Hopefully, the drivers are OK.

I mean.. hopefully... because I just destroyed all the caps by repeated de-soldering and re-soldering... : I wouldn't want to pay anymore for a set of drivers...

Would an impedance sweep find out any driver issues?

[Dennis H]

Would an impedance sweep find out any driver issues?

Sure, run an impedance sweep on the two drivers to make sure they are the same.

[fjhuerta]

Well, I went ahead and tested the drivers, and now I think I have even more questions...

The tweeters' curve is vastly different, but only on at or below resonance.

The woofers' curve is different by about 0.3 Ohm. I can't understand it.

So, I took each measurement and simulated it.

This is the FR plot.



It does have a slight bump in the midbass. Ouch. Hope it's not too bad.

I found out that the impedance curve does vary, just as I measured. But while my measurements were some 3 or 4 Ohm off, this one is less than 1 Ohm off. I'd assume one of the components was to blame.



The thing is, FR is quite different when I simulate using the two impedance measurements.

I'd guess my question is... is it that normal for drivers to be this different? Should I just live with the difference? (I see no other alternative). Would other drivers be more consistent? I was actually pretty surprised to see how different the tweeters' impedance is (although tweeter response doesn't vary that much).

[Bri]

What software/hardware are you using to measure the impedance? I'm in the process of building my crossovers at the moment and would like to try this out.

[fjhuerta]

What software/hardware are you using to measure the impedance? I'm in the process of building my crossovers at the moment and would like to try this out.

I have the Woofer Tester II, although I think you can do the same with LSPCad or SoundEasy and a probe.

I like the WT II, though... it's a great piece of hardware.