Crossover for 4ohm Tweeter and (2) 8 Ohms Woofers

[rmjeeves]

Hello All,

This is my first post. I am generally a reader and most of the times I find my answer on forums. I am truly amazed how some people know just about everything. However there is something that is killing me and I need some help.

I have now designed and constructed at least 2 sets of loudspeakers. My latest design uses 1x4 Ohm Tweeter and 2x8 Ohm woofers. Now I know that I can use these driver with different impadance. I also know that if I connect both the woofer drivers in parralel I will get 4 ohm load. Following is my question.

Question: When I calculate the crossover using one of the online crossover tools (e.g. 2-Way Crossover Designer / Calculator), what should I input as the 'Woofer Impedance'? Is it 8 ohms as the drivers are 8 ohms or should it be 4 ohms which will be as a result of the parallel wiring.

Please help me as this is driving me insane. Also if possible, please explain me the logic of the answer. FYI: I will also be using a LPad and Zobel.

Thanks so much for your help.

 

[Cal Weldon]

4 ohms. Problem is that you are not likely to be exactly 4 ohms at the XO point. Impedance is frequency dependant and you are entering nominal figures only, not precise ones so your mileage may vary.

 

[tinitus]

what Cal says, impedance varies

with two 8" woofers, 2.5way might be a good idea

handle one setion at a time
woofers first
listen, and adjust
adjust the other driver section(s) to that
finish with fine adjustment(s)
multiples

 

[speakerdoctor]

First, you need to understand it's okay to mix 4 and 8 ohm drivers in speakers.
The caveat is you need to be aware of the true impedance at the crossover points you select and also be cognizant of the potentially higher efficiency of the 4 ohm driver and pad it down accordingly to match SPL's.

Having written that, now follow post #2 & 3.

 

[tinitus]

hey, instead of L-pad on tweeter I like to use a series resistor only
works especially well with a 4ohm tweeter
but be aware that it changes xo function
to me its an advantage, for adjustments
makes series cap smaller, or lowers xo 'point'
and it sound better than having additional paralel resistor

 

[Arty]

hey, instead of L-pad on tweeter I like to use a series resistor only
works especially well with a 4ohm tweeter
but be aware that it changes xo function
to me its an advantage, for adjustments
makes series cap smaller, or lowers xo 'point'
and it sound better than having additional paralel resistor

the.. idea of the L-pad is to keep the impedance close to original, while still decreasing Spl of the tweeter.
Your methood of 4 ohm series with a 4 ohm tweeter is halfing the drive voltage, it is still an option and sometimes it is not a bad one. But be aware that tweeter impedance is -specialy at Fs naturaly- can be quite a lot higher than the 4 ohm series resistor. Why is this important? The voltage divider formed by the tweeter and the 4 ohm ressitor will not halve the voltage anymore, so the amount of padding does change, and becomes freqvency dependant. So, this solution can work, but it may do some funny things. First off it is better to compensate the drivers to have a more predictable impedance, then You can work out a crossover.
And this is where L-pad comes in. You design the crossover , accounting the effect of compensation networks, if You add a simple series resistor to pad the tweeter, then You allso change crossover point. But, if you go with a proper L-pad, the final impedance does not -or in real life, not significantly- change the impedance seen by the crossover, therefore Your crossover can function as You wanted it, the crossover point will not drift away that mutch.

This is why L-pad is better solution than simple series resistor.
And supposedly the above is a more or less good crossover design practice.
Surely one needs to mesure and calculate a lot, but the efforth is more than worth it.

For the original thread starting question:
"Question: When I calculate the crossover using one of the online crossover tools (e.g. 2-Way Crossover Designer / Calculator), what should I input as the 'Woofer Impedance'? Is it 8 ohms as the drivers are 8 ohms or should it be 4 ohms which will be as a result of the parallel wiring. "

--> You input the impdeance value You achieve after compensating the driver impedance, mesured at desired crossover point. (notice, it will be different than 4 or 8 ohms. for the above reasons. )

 

[tinitus]

Your methood of 4 ohm series with a 4 ohm tweeter......

I never suggested a 4 ohm resistor

on my own build I'm using a 1ohm resistor

I know L-pad is convenient because you can change tweeter SPL without affecting xo function
but doing your own thing gives you the freedom to do it the way you think is best
and admittedly, I hate L pads
they never seem to work 100%
but when you get the optimal interaction of a series resistor and the xo, it simply sounds better than L pad
but ofcourse you need to know how it works

if you look at famous speaker builders you will find that they do it too more and more often
some will place it between amp and xo
look at Troels Gravesen
I think 'Elsinore' by Joe Rasmussen uses series resistor only too
I have used and adjusted L pads extensively myself
and wouldn't dream of using it again, unless I have to
though I happen to like it to interact with xo

but as things stands, this is really a minor issue 😉

 

[Arty]

"hey, instead of L-pad on tweeter I like to use a series resistor only" ->
"Your methood of 4 ohm series " ->
" never suggested a 4 ohm resistor"

Actualy i do not get it. Do not get it wrong, it must be my lack of english knowledge. Actualy i do not see where did i write You suggested a 4 ohm resistor in series.
Al i did say was in a nutshell that Your methood is adding a series ressitor instead of an L-pad.
And You did confirm You prefer a series resistor.

so why so offensive?

 

[tinitus]

so why so offensive?

oh, I didn't know I was

but you did write it like it was me suggesting a 4ohm resitor
which I didn't
probably the only reason for my respond
english problem, it seems

otherwise I would just have said hey, you are correct too, I think we both are 😉

 

[Jay]

I have now designed and constructed at least 2 sets of loudspeakers. My latest design uses 1x4 Ohm Tweeter and 2x8 Ohm woofers. Now I know that I can use these driver with different impadance. I also know that if I connect both the woofer drivers in parralel I will get 4 ohm load. Following is my question.

Question: When I calculate the crossover using one of the online crossover tools (e.g. 2-Way Crossover Designer / Calculator), what should I input as the 'Woofer Impedance'? Is it 8 ohms as the drivers are 8 ohms or should it be 4 ohms which will be as a result of the parallel wiring.

So you have built 2 sets of loudspeakers. Now let me tell you that you cannot build a good speaker only using online crossover calculator. Even if you have to use simple tools like that, you need to have basic knowledge.

If you want to understand more (it is more complicated than just theoretical fRLC formula), download a SpeakerWorkshop, and download a SpeakerWorkshop's project (*.swd) from RJB site. You can open this SWD file using SpeakerWorkshop. There you can find crossover for specific driver combination and the resulting chart. You can change the crossover value and recalculate to see changes to the charts.

 

[rmjeeves]

Thanks all for sharing your knowledge. So based on what I read following are my options.

1: Use 4 Ohm but it may be different at the xo point
2. Consider 2.5 way
4. Use a series resistor (I am not sure I fully understood this option)
3. Use the impdeance value achieved after compensating the driver impedance, mesured at desired crossover point

Now here is the followup question.

I am planning to use Scan Speak 15W/8434G00 MidW. It has a Re of 5.7 ohm. So if I calculate and use a Zobel network, it would flatten impedance around 6 ohm at the XO point. So per the original question in terms of what Ohms to consider while calculating the crossover components, wouldn't it make sense to use 6 ohm for woofer as Zobel should flatten the woofer impadance. Or should I be going with 4 ohms as Cal suggested and expect some imperfection.

Obviously I am still learning and what I have built so far is not perfect. So trying to seek help. Thanks again.

 

[Arty]

oh, I didn't know I was

but you did write it like it was me suggesting a 4ohm resitor
which I didn't
probably the only reason for my respond
english problem, it seems

otherwise I would just have said hey, you are correct too, I think we both are 😉

in this case my deepest apology , i should perfect my english as i suspected.
And thank you for understanding.


For rmjeeves,
the plan You describe is quite good, but before doing anything, have a look at:
Passive Crossover Network Design
Baffle Step Compensation
Voltage Dividers and Attenuators
Power Handling Vs. Efficiency

this should be a fairly long stuff to read. The articles are excelent, I em more than sure You will be able to deal with the problems of makeing a better than off the shelf xover afterwards.

The proper methood would be to calculate the zobel for the midwoofer, and mesure the impedance at xover point You wish to use, and input thato to the calculations.
Same goes for the tweeter, after You get rid of the impedance peak they have at Fs, You should allso mesure the impedance of the tweeter at crossover point, and use that in Your calculations.
Afterwards -as the articles do explain in an excelent way- You can account losses, and make the needed L-pad to suit Your needs.
Or You can go with the series resistor approach, but i would say L-pad is less of a pain to make.

 

[tinitus]

 

[rmjeeves]

Thanks so much. You guys are awesome. Really appreciate the insights.

 

[dewardh]

Or, as an alternative, use a miniDSP and never worry about driver impednce again.

 

[Cal Weldon]

if you look at famous speaker builders you will find that they do it too

Yes, my favourite builder, Altec Lansing, used to use the variable L-pads as simple series resistors (only two wires connected) and it drove me batty.