Mismatched mids (same drivers different DCR) - how to fix?

jeffrey88

Member
2012-05-07 7:40 pm
Hey DIYers,

I need a little help. I am currently working on a pair of Wharfedale W90 speakers and, in investigating, I found the midrange drivers are mismatched.

They appear to be identical in terms of overall construction, so I am satisfied that the cone, magnet and basket are identical, but one pair of midrange drivers measures at 11.6 ohms DCR and the other pair measures at 6.2 ohms DCR.

As you can likely guess, this is creating balance problems between the two speakers.

First of all I have made sure both crossovers are identical and are now wired up the same way.

The crossover is a very simple one. See a picture here:
post-100237-0-18568800-1314646954.jpg


My question to you is, how should I go about remedying this? I'm thinking my options are the following:

A) Modify the crossover in one. I think this means I need to halve the capacitance on the 11.6 ohm DCR drivers to have a similar crossover point to the speakers with the 6.2 ohm DCR mids.
B) Unwind the coils on the 11.6 ohm drivers until they measure 6.2 ohms to match the other drivers.
C) Purchase original mids that have a DCR closer to the 6.2 ohm pair. I found a pair for sale that measure 5.3 ohms DCR.

What do you recommend? I know very little about drivers and crossovers, only enough to be dangerous, so I'm not sure what the implications are of each option I have. I have limited measuring tools - I have a UMIK-1 USB microphone I can use to measure frequency response, but that's about it (and also a DMM, obviously).

Also in case you are wondering why I am leaning towards making the 11.6 ohm DCR driver match the 6.2 ohm drivers instead of the other way around, it's because the speaker with the 6.2 ohm DCR mids sounds correct to my ears, and the speaker with the 11.6 ohm mids does not.

Any ideas?
 
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eriksquires

Member
2013-05-10 4:11 pm
The 11.6 is probably bad in more ways than one!

Without doing a thorough analysis, I'd get the new pair.

My guess is that what's really going on is in the motor, unwinding the coil is probably not the fix. Either the coil started to melt, or there is mechanical problems in the assembly. The way of trying to unwind a coil to make it match leads only to madness.

Best,


Erik
 

jeffrey88

Member
2012-05-07 7:40 pm
The 11.6 is probably bad in more ways than one!

Without doing a thorough analysis, I'd get the new pair.

My guess is that what's really going on is in the motor, unwinding the coil is probably not the fix. Either the coil started to melt, or there is mechanical problems in the assembly. The way of trying to unwind a coil to make it match leads only to madness.

Best,


Erik

Thanks for chiming in Eric. Oddly, there are no signs of damage, and both mids (there are two per speaker) measure identically at 11.6 ohms. Also, Wharfedale did make the same drivers with different coils, however if that's the case here I'm not entirely sure.

Is it possible for a driver to measure a higher DCR but have mechanical problems?
 

eriksquires

Member
2013-05-10 4:11 pm
I would buy the two different coils. I was going to ask if that was in fact the case.

Try this. Hook up both to an amp with a balance control. Ajust volume so they are equal and listen. If they both sound good, without any rubbing or distortion, I' d say the real problem is that you have two different speakers.

If you measure double the DCR in what is allegedly the same speaker, it's a lot more than a slight manufacturing variance! That just doesn't add up, and you should not attempt to correct it.

Like I said, you are missing more analysis. What would make more sense is to do a complete driver analysis, including an impedance plot, T/S and frequency to tell what's going on.

Still, if they both sound good, I'd say you have two different drivers, and your best bet is to get a matching pair.

Best,

Erik
 

jeffrey88

Member
2012-05-07 7:40 pm
I would buy the two different coils. I was going to ask if that was in fact the case.

Try this. Hook up both to an amp with a balance control. Ajust volume so they are equal and listen. If they both sound good, without any rubbing or distortion, I' d say the real problem is that you have two different speakers.

If you measure double the DCR in what is allegedly the same speaker, it's a lot more than a slight manufacturing variance! That just doesn't add up, and you should not attempt to correct it.

Like I said, you are missing more analysis. What would make more sense is to do a complete driver analysis, including an impedance plot, T/S and frequency to tell what's going on.

Still, if they both sound good, I'd say you have two different drivers, and your best bet is to get a matching pair.

Best,

Erik

Thanks Erik, I really appreciate your insight.

I think that they are simply different drivers. My thinking with adjusting the coil was that would be the only difference, but you are right that to know the full story I need to get more analysis done. Unfortunately I'm not equipped to obtain that information, so it sounds like my best option is to simply buy matching mids. I'll try hooking them up and will take a frequency response measurement to see if they have similar response.

Also, let's say they are the same but simply different coils, would I only need to adjust the 24uF capacitor to change the x over point? To halve the capacitance I only need to disconnect two wires, it would be very easy to do. But the coil, is that part of the mid crossover? I thought that was for the woofers only. Sorry, I am such a greenhorn with crossovers!

The reason I want to try that is so I can at least have something to listen to while I source mids. If I can get a reasonable match between the two by adjusting the crossover in one, that would work for the time being.

Jeffrey
 

eriksquires

Member
2013-05-10 4:11 pm
The crossover and the level would need to be matched, that's assuming it's that easy. :)

So, yes, if you absolutely want to save money, go ahead. I'd test them first though. Run them full range from your amp, and adjust the volume.

The problem is going to be adjusting the cap exactly right. :) Having a different DCR also implies different inductance, and a different impedance shape. That's going to be a lot of fun for you..... :D :D :D :D

Seriously, get matching drivers and be done with it.

Best,


Erik
 

jeffrey88

Member
2012-05-07 7:40 pm
The crossover and the level would need to be matched, that's assuming it's that easy. :)

So, yes, if you absolutely want to save money, go ahead. I'd test them first though. Run them full range from your amp, and adjust the volume.

The problem is going to be adjusting the cap exactly right. :) Having a different DCR also implies different inductance, and a different impedance shape. That's going to be a lot of fun for you..... :D :D :D :D

Seriously, get matching drivers and be done with it.

Best,


Erik

Oh of course, I am buying the mids, no question about that. No way am I going to leave them like this and adjust one crossover, that's silly. But for the next couple of weeks while I get the mids, I'm just wondering if I'd get sorta in the ballpark of the other crossover if I simply changed the 24uF to 12uF with the 11.6 ohm drivers? I read somewhere that doubling the impedance means you need to halve the capacitance in order to achieve the same xo point. It's a bandaid solution, just so I can listen to them while the mids arrived.
 

dogshome

Member
2010-07-17 8:15 pm
You could fit a 6 and a 12 in each cabinet. That would make both speakers technically wrong, but equal. At the moment, one of the pair is wronger than the other.

I suppose the pair of 12s are correct an the pair of 6s is wrong and meant for a simple 3 way
 

afa

Member
2008-07-31 12:57 pm
11.6 ohms DCR usually means its a 16 ohm driver, 6.2 ohms DCR means its probably a 8 ohm driver, no shorted turns or damage, just different voice coils, how to fix? maybe stick a 12 ohm (10 watt)resistor across the 11.6 ohm driver to bring it down to around 6 ohms and then see how it sounds.
cheers, Arthur