Pair of woofers with different Re's?!

[Sony]

I got a pair of woofers (AAD 5" aluminum) I would like to use to build a pair of 2-way monitors and I measured the DC resistance to be quite different between the two:

Re(1) = 4.6 ohm
Re(2) = 6.8 ohm

Can this sort of difference be found normal? Or will this probably mean that both the impedance curve and the nominal impedance of these drivers might be rather different?

Thanks and regards

[wintermute]

Hi Sony, what is the manufacturers specified re? small differences can be expected but a difference of around 30% (taking the higher one as the reference) seems extreme to me.

6.8 looks like a normal value for a nominally 8 ohm speaker. 4.6 is too high for a nominally 4 ohm speaker and much lower than I would expect for an 8 ohm. Were these purchased new? If so I'd be sending the 4.6 ohm one back if it is a nominally 8 ohm driver.

Tony.

[Sony]

Thanks Tony.

I haven't the original specifications, these drivers are not sold.
The dealer gave me these spare pair because the original drivers were not in perfect condition (I bought the speakers as new). The original speakers are the AAD 2002 from Phil Jones.
I agree with you, they probably have different impedances and it might be on proposite... These AAD 2002 speakers have a 2.5-way configuration with two woofers like these per speaker. I now wonder if these two spare woofers may not have different impedances since the AAD 2002 are a 2.5 and not a 2-way design. Both woofers only have low-pass filters (and zobel), one at the XO point with the tweeter and other at a much lower point (about 500Hz).
So, bellow 500Hz the overall impedance is the combined impedance of both drivers (after zobel rectification). Maybe the designer chose the bass-only woofer to be of higher impedance so that the impedance curve of the speaker wouldn't drop to very low levels bellow 500Hz.

So, maybe one woofer is really 8 ohms and the other 6 ohms. Now this takes me to another question - can I still use these woofers with different impedances in my 2-way/2-drivers per speaker project? I am assuming that:
- If I use different zobel's per side I will get closer final impedances between L and R.
- If I design the crossover assimetrically L-R with the woofer's right impedances in mind, the curve will be flat on both sides.
- Eventually one speaker will take a little more current and play slightly higher, but if I attenuate the lower impedance driver by 1dB or what it takes, both sides will be equal.

Will these assumptions be correct or should I get other drivers for my project?

Note: My project is to build a high-end purest 2-way small monitor using these spares that have really amazing bass, probably like there is nothing available on the drivers market at this 5" size.

[kbgl]

You are building 2-ways right? TM? One woofer in the left speaker, and the other in the right, correct? Not a MTM or TMM.

[wintermute]

Hi Sony, if you have access to measuring equipment you could try measuring the impedance curve of both and derive their T/S parameters. You could then try adding in 2.2 ohms of series resistance on the low impedance one, and re-measure the T/S parameters. I don't know whether the series resistance will change the params or not, having never gone into the mathematics behind deriving the params, however I do know from modelling that adding series resistance before the driver DOES change the results of the model, so I suspect it will change the T/S params.

If the two speakers have similar T/S params when the series resistance has been added to the low impedance driver, then you may then be able to simply do everything else the same (making sure the series resistance is between the driver and the crossover), and hopefully you won't even need to worry about levels... I guess there is only one way to find out

For the approach you outlined to work I would think that the drivers would need to have similar T/S params when measured without any series resistance added.

Before you expend too much effort though I would try each driver running full range in a test box (playing music you are familiar with) and see if they sound the same (when levels are adjusted appropriately). The thing that concerns me is that the one with lower impedance may in fact be faulty, perhaps some shorted windings in the voice coil, if this is the case then its motor will behave differently to the other driver, and the chances you can get a decent match between channels would be I think low.

Tony.

[Sony]

kbgl, that's right. It's a TM rear ported (compact monitor).
The woofers I'll be using are spares of a MTM 2.5 way speaker.

Tony, thanks again for such great tips.
I have no specifc measurement equipment although I have a Behringer equalizer and mic that I can use in the test you mentioned to check the response curve and levels. I have now finalized the enclosures and can use them for the test.

I am particulary concerned with the possibility you raise of having shorted windings in the voice coil. These drivers are magnetically shielded with a really heavy cover made of lead. I assume I won't be able to fix it if that's the problem.

I have spent lots of time building the enclosure and if I can't use these drivers the project will be compromissed...

[tinitus]

you can use a 4ohm and 8ohm woofer together in one channel, preferably as 2.5way

but you cannot use a 4ohm in one channel, and 8ohm in the other channel

well, you might be able to make it work for a somewhat flawed workshop speaker, but not good quality