Please Help - Impedance confusion :S

[AndrewUK1990]

Hi everyone, this may sound simple but for a long time this simple matter has had me just a bit puzzled despite reading all sorts of threads on the matter. Some help just to clear this up for me would be very much appreciated indeed.

Basically, i am replacing the bass units in some old speakers i have. Each loudspeaker has two 6.5" bass units. They are connected to the crossover in parallel with one another. Their Impedance is 16Ohms each.

My problem is that the new drive units i have bought to replace them with are 8ohms each. So at the moment i have them wired in series, but in parallel to 2x 8ohm resistors thinking this would balance out as 8ohms overall.

But through practice, it seems the drivers are too quiet, not getting enough power in other words, is this because the power gets shared across the resistors too so the drivers only have half the power they should have?

If i wired the drivers in series to make 16ohms, they would then share the power equally as the old drivers would have done originally but obviously the overall impedance is doubled from 8ohms to 16ohms, so will this have an effect on the speaker?

The loudspeakers are 3 way design, 2 bass a mid and a tweeter. The mids are 8ohms and the tweeters are 6ohms impedance.

Please please help ive been puzzling over this for a long long time now to no avail . Thank-you in advance to anyone who can help me on this matter.

Regards,

Andrew.

[Cal Weldon]

Hello Andrew,

I am afraid there's no easy solution. When you change the impedance, you also change the XO point so you need to end up with 8 ohms. Adding parallel resistance robs the drivers of 1/2 the power and if you parallel the drivers you will need to add series resistance which again robs you of the power delivered to the drivers.

Also, you can't just use the stated nominal impedance of the drivers as your guide because the actual impedance changes with the frequency. No two drivers are the same in that regard so you will again be messing with the XO frequency.

It can be done but you will need to alter the XO itself and that can be a labourious, tedious, frustrating and time consuming venture.

Have I cheered you up yet? If not there's more. A lot more to it than I have outlined.

[AndrewUK1990]

Hmm things are not looking good lol. If i changed the drive units to some with 4ohms and wired those in series to make 8ohms at the crossover terminal, would that be ok?? :S

I am aware that resistance is not the same as impedance also, but thanks for the advice on that. I do not know the frequency response of the original drivers compared with impedance unfortunately, but if i can at least match the impedance correctly then i would assume that their behavior would be similar.

I was looking at the Scanspeak Revelators 4ohms, as they seem to be suited to bass reflex and a big cabinet size of around 60ltrs together which is ideal as the cabinets i have are 60ltrs. http://www.scan-speak.dk/datasheet/pdf/18w-4531g00.pdf

[Cal Weldon]

Originally Posted by AndrewUK1990

If i changed the drive units to some with 4ohms and wired those in series to make 8ohms at the crossover terminal, would that be ok??

That is probably the best thing to do and if you're not into redoing the XO it might work but it is hit and miss at best so even if you use good drivers the result may not be as rewarding as you hoped. Not trying to discourage you but if you owned a car with a broken windshield and tried to get another type to fit...

I think you get the picture. Not an easy solution my friend.

[AndrewUK1990]

Ok, cheers for the help. The annoying thing is i had the XO's refurbished a couple of years ago with new components, but to the original spec. For now then ill use some 4ohm drivers i have lieing about and see what it sounds like.

Just as a last hope, can you help me out, or just point me towards the basics of designing/altering a XO design? I am studying a degree in civil engineering so if it requires maths knowledge then i should 'hopefully' be able to cope lol. :P

[Cal Weldon]

You will need measuring equipment to do it properly. If that's not in the cards then just go ahead and see what happens. I've done it many times. Sometimes it works and sometimes the boxes get gutted and I start again. OTOH you may be able to take the whole package to someone to tune up for you. Or if you want real control, get yourself another amp, an active XO (something like the Behringer 2310) and run it bi-amped with the mid and tweeter using the passive XO.

[AndrewUK1990]

Ok thanks for your help, it has cleared up my confusion and confirmed what i expected may be the case. I think ill try my luck with some 4ohm drivers and see what it sounds like for now. If its ok then i think ill stick with it until i can afford something better, otherwise ill go try get them tuned at a hi fi specialist store seeing as though i cant afford to be buying my own measuring equipment.

Shame i didnt study Acoustics instead then maby i could have 'borrowed' some of that measuring equipment from uni!

Thanks again anyway, you've helped a great deal!

[Cal Weldon]

My pleasure. Good luck with it and do to tell us how it goes.

[AndrewUK1990]

Hey, i know this threads a bit old, but regarding my results for this project i thought i may as well just post them here. SO anyway, i have ordered some Scanspeak drivers taking the plunge after finding out a few more things about the old wharfedales i have. I found that the crossover freq to the mid is 800htz which is almost ideal for the scanspeak drivers as they have a very flat response up to about 900htz. I also managed to find out more details for the crossover and drive units. Judging by what i can find it seemed that the Scan-Speak 18W/4531G00 drive units would be as close as i can get to perfection working with just the info i have, they also suit a the wharfedale cabinet design and volume which sits right in their optimum range of about 55Ltrs. Obviously ill be wiring them in series to closely replicate the same overall impedence across the frequency band.

Anyway, i should receive them in the post soon, so once i get them installed and run in for a while ill post up the results! Fingers crossed :P

[Inductor]

Originally Posted by AndrewUK1990

...
Anyway, i should receive them in the post soon, so once i get them installed and run in for a while ill post up the results! Fingers crossed :P

Good. Maybe you can post a picture later (old/new). Regards.