Speaker DIY impedence ??

[luvthymunky]

Hi,

I'm looking to build a 3-way pair of floorstanders. Using parts new and old
Have butchered some Wharfedale Diamond 8.1 bookshelves for their:
5" kevlar mid/woofers (model 1333)
1" tweeters (model 0326H)
Crossover

Can't find details for individual speakers, but bookshelves range 55-22,000 Hz.

Both these measure as 4 ohm impedance with a volt meter, however the Wharfedale Diamond 8.1 states its whole self as 6 ohm imepedance. What gives?

Now I have to build crossover to low pass to a:
Vifa 8" M22WR-08-09 VIFA VIFA SPEAKER DRIVER - Digital MEGAHOUSE (bottom of page)
This woofer measures as 8 ohm (as it should).

So my question (second one) is, what sort of measures in crossover construction should be undertaken to account for this impedance difference between the 8" woofer and its smaller mid and tweeter?

Cheers, any comments welcome!

P.S. am going to keep the bookshelf crossover as the high crossover, just aiming to build a low at around 500 Hz. Was thinking a 3rd order butterworth

 

[Inductor]

Leave the xover of the Wharfedale Diamond 8.1 bookshelve as it is, and add an inductor to the Vifa 8" M22WR-08-09 to make a 1.order xover. If you change the xover you have, from the manufacturer, you will have to measure all drivers and reset your options for a new design.

 

[PeteMcK]

what Inductor said, but I'd suggest using the baffle step frequency as your crossover frequency - depends on the baffle width you're using for your mid = 115/width in metres
Also you need to use the actual impedance of the driver at this freq to calculate you inductor (Impedance varies with frequency)
Try to find the sensitivity of your mid driver, ideally the woofer should be ~ 3dB more sensitive
IIRC Digitalmegawhatsit are a bit expensive, try HAS - local to you
Hasaudio Speaker Components - Suppliers of Speaker Components, parts and Speaker Kits.

Probably a good idea to add a cap in series with your mid too, once again you'd need to know the impedance of the mid at the xover freq to calulate it accurately

 

[luvthymunky]

Inductor:
Yea the Wharfedale original crossover is huge and complicated and obviously designed to cross the mid/tweeter as well as can be done. So leaving it is probably a good option.

PeteMcK from (checks ggogle maps) Dipton:
I guess the concern is how best to fit the woofer in with the mid and tweeter. Can't find any signs of the specs for these two because they are not meant for DIY I guess. The only specs come in the form of the whole Diamond 8.1 bookshelves Wharfedale DIAMOND 8.1 Main / Stereo Speaker Review | Specifications:
Sensitivity 86 dB, range 55-20,000 Hz, imped 6 ohms.
As a consequence this project 'feels' like some control is lost, but there are budget concerns so tough!
The Vifa 8" woofer has a 88 dB sensitivity, so does this make it 2 dB more sensitive than the mid and tweeter and therefore should it be left as is regarding what you say about woofers being ideally 3dB more sensitive?

I will put a cap and inductor for a 1st order between woofer and mid.

The recommendation for the Vifa M22WR-08-09 8" crossover is 500 Hz in a bass reflex design. It's response is very flat from 80-800 Hz.
Using the 115/baffle width (assuming a rectangular box design) the xover is 115/0.27m = 425. This is close(ish)!
Impedance at 425 Hz for 8" woofer is 9 ohms, for mid.... who knows?!
Never knew about importance of baffle for xover so cheers for that!
Hasaudio is expensive but good quality from quick browsings.

 

[GeneralGato]

Tweeter Impedance

Both these measure as 4 ohm impedance with a volt meter, however the Wharfedale Diamond 8.1 states its whole self as 6 ohm imepedance. What gives?

Hi luvthymunky:

Did you meter the tweeter connected to the crossover? I read that the driver impedance have to be measure with it disconnected to get and accurate reading.

Red Spade Audio: Tweeter pad - a simple tweak that might save your speakers

I have apair of diamonds 8.4 with the same tweeter that i want to make an L-pad for but got no multimeter

 

[luvthymunky]

Hi GeneralGato:
I pulled apart the diamond 8.1s and measured the impedance of woofer and tweeter while not attached to crossover = 4 ohms each. The crossover has a single resistor attached, three capacitors and two inductors.

If an L-pad requires two resistors, these components may instead make up a crossover with a series notch filter to damp a tweeter resonance spike. Just a guess but am gonna try to map out the crossover to a circuit diagram and investigate.

You can get a cheapo but ok multimeter for like $20 easy, have a splurge and treat yourself!

 

[giralfino]

Measuring speaker impedance with a multimeter is a waste of time, because impedance changes with the frequency. You have to have some measurement device that is able to plot impedance vs frequency.

So build this (very) simple jig: Impedance jig

I had some trouble deciding how it should be built, so I asked for a photo: http://www.diyaudio.com/forums/multi-way/154751-what-should-i-do-2.html

As for the crossover components, it would be better to draw a layout, because 3 caps, 2 inductors and a resistor can be arranged is a lot of ways (for example 2nd order electrical for the woofer + 3rd order electrical for the tweeter and the single resistor is series with the tweeter for SPL lowering).

Ralf

 

[speaker dave]

Hi GeneralGato:
I pulled apart the diamond 8.1s and measured the impedance of woofer and tweeter while not attached to crossover = 4 ohms each. The crossover has a single resistor attached, three capacitors and two inductors.

You measured the DC resistance of each driver. At frequencies well below resonance that would also be the AC impedance (nearly). However, AC impedance will rise above DCR, especially at resonance or high frequencies where voice coil inductance becomes significant. The crossover network will also raise impedance, usually at the crossover frequency.

The convention for specified system impedance is to pick an average value a little above the minimum seen and well below the maximum. Rated impedance is also usually 4, 8 or 16 ohms, or sometimes 6, whichever is closer.

So a 6 ohm system rating sounds totally plausible.

David S.

 

[GeneralGato]

I talked to the Wharfedale service people here in Spain, they said the tweeter is a little under 6ohms so I will use 6 for my calculations.

I also found my speakers service manual here. The tweeter circuit diagrams are probably the same for the hole Diamond 8.x line.

I am thiking in redoing my crossovers with better components, probably Jensen caps and air core inductors with lower resistance and of course the L pad.

This is an interesting tread in a french forum.

 

[PeteMcK]

re:'they said the tweeter is a little under 6ohms so I will use 6 for my calculations' - you are confusing resistance with impedance, as Speaker Dave said. You need to use the Impedance of the tweeter at the crossover frequency in your calculations, other wise the result will be inaccurate.