Stupid question about impedance X-Overs & L-Pads

[iNeLuKi]

Sorry for this, I'm totally new to electronics (started researching a few days ago).

I'm making some basic 2way bookshelves with a Vifa P17SJ and D27SG in each.

My questions are,
Do I need to do anything about the mixed impedance of the drivers? Its an 8Ohm woofer and 6Ohm tweeter, if I leave it would it screw up my amp?

If I put a resistor in series with the tweeter to bring it up to 8Ohms do I then need to calculate the crossover values as if both drivers were 8Ohms? Which for example would make C1&2 the same and L1&2 the same (using a 2nd order X-over) How many watts would a single resistor in series need to handle for an 85wrms amp?

I dunno if this is right, but if I put a 4Ohm resistor in series with the tweeter it would bring it to 8Ohms and attenuate by 2.5dB is that right?

Be easier when I make my next ones, cause it'll be 2 x 8Ohm woofers in parallel and a 4Ohm tweeter.

Any info would be appreciated,
Thanks

[Jimmy154]

This website will answer your questions about cross-overs www.lalena.com/audio. Mainly the "crossover faq" part.

I have been told that 10 watt resistors are fine for my applications or in other words normal speaker applications such as yours. Metal oxide resistors are best or very good. Metallized Polypropylene caps are best or good too. Good place to get them is www.solen.ca.

I would look for a cross-over that's for similar drivers on the internet, might be a good starting point or maybe ask vifa for one, after all they're both vifa drivers.

[iNeLuKi]

Unless I've missed something that site doesn't answer any of my questions.

[Centauri]

As the tweeter is higher sensitivity than woofer, it could use some attenuation anyway, so use 2ohm 10W resistor in series with it and calculate crossover values as 8ohm each section as you suggest.

cheers

[Jimmy154]

That is the best website I know for x-over designing. The "crossover faq" part discussed attenuation.

Couldn't find your exact tweeter at scan-speak.com, but found D27TG-05-06 and your P17SJ-00-08 woofer. Woofer Z or nominal impedance is 8 ohm and sensitivity is 87. Tweeter Z is 6 and sensitivity is 92. Tweeter sensitivity is 5 db more than the tweeter and the Z is 2 ohm less, so add 1.5 to the 5 and you get and attenuation of 6.5 db.

Go to the L-pad calculator on the website I gave you Z=6 ohm and A=6.5 db, it gives you 3.16 ohm resistor in parellel to "+" wire and then closer to woofer 5.39 ohm resistor wired in series. Not sure why it gives you 2 values, maybe you can just use one 8.55 ohm value resistor(s) wired in series.

Although I'm not an expert in x-overs I think this is accurate. What are you going to do for the rest of the x-over? I would think designing the Lpad is the easiest part of x-over design. Did you try to e-mail vifa or look for x-overs for your drivers?

[iNeLuKi]

The crossover design isn't an issue, thats easy, the problem was equalizing the impedance of the drivers, which is all sorted now.

Thanks Centauri, I was pretty sure thats what I should be doing but didn't want to foul something up by rushing into it.

[Jimmy154]

Sorry, as usual I totally misread and/or misunderstood the question or problem.

[Centauri]

Just a note to suggest you use non-inductive wire wound resistor - Jaycar and other suppliers have them.

Cheers