Crossovers & L-Pads

I'm building a 3-way speaker system. Woofer is 8 ohm, Mid is 5 ohm and tweeter is 8 ohm. By using an online calculator, the difference between 8 & 5 ohm requires difference values of L & C in the mid crossover. Now I want to control volume of the Mid and tweeter by using a L-pad, but the L-pad is 8 ohm & Mid is 5. What should I size the value of L & C for 5 ohm or 8 ohms? Or is this not going work?
 
If you use an 8 ohm variable L-pad with a 5-ohm driver, the overall character of the combined impedance will likely be more like 8 ohms. The minimum impedance will vary based on the amount of attenuation though, which makes it harder to pick a precise value for crossover calculation. For something like 0-5 dB of attenuation, the minimum value will wag around from 5 to about 7 ohms.

It's also worth noting that if you're only using an L-pad and standard crossover, the impedance seen by the crossover is going to vary with frequency anyway (since a normal cone/dome speaker does). So depending on where you are crossing, the actual values may need adjustment from those suggested by calculators/equations if you are trying to be precise. Without modeling, things aren't likely to be very precise in general though.

In practice, if you aren't doing a highly refined design you'll probably be fine. People use imprecise off-the-shelf crossovers that have the same issues, and for some that's enough.
 
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That's a good summary from mattstat.

Since this is your first post I will back him up.

The values of 8, 5 and 8 ohm for the woofer, mid and treble drivers respectfully are only nominal values and the mid driver may well have an impedance close to 8 ohm at the crossover frequency.

If you are using a simple crossover calculator and intend to use 8 ohm L-pads, simply calculate for 8 ohms for each driver.
 
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I planned on building this crossover. And would put an L-pad in tweeter line and Mid line of the driver. My speakers:

Phenolic Ring Tweeter AR-4X CTS Parts Express Part 8 ohm Part #270-252
HiVi DMB-A 2" Fabric Dome Midrange 5 ohm Part #297-716
Dayton Audio DSA315-8 12" Woofer 8 ohm Part #295-534

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I'm going to be the negative one, only to save you money and disappointment.
  1. you've chosen a 2" dome that is recommended to have no lower than 800Hz 2nd order crossover (in some references at least 1000Hz). Your 450Hz will be too low, power limiting your system
  2. The impedance of midrange and tweeter are not the most important thing for an L-Pad. Both drivers may have different impedance but have the same sensitivity (90db). Therefore you need to pad ensuring appropriate attenuation. For the 2" dome midrange this could be even more attentuation (L-padding) than the tweeter if there is bandpass gain (that is closer crossover points to woofer and tweeter and / or lower order slopes meaning more contribution from the woofer / tweeter)
I would re-think your driver choices... or at least explain why you have chosen the drivers you have.

Your dayton woofer is also 90dB sensitive, and this means your likely target system response will be ~ 86dB.

If you are dead set on going ahead, I'd strongly recommend importing all driver manufacturer FRD/ZMA curves into a crossover simulator and plugging in the values you have chosen, to see what the net system response looks like.

PS: The manufacturer curves are not ideal, but will give you a wake up call if it all looks awful (which I suspect it will)
 
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The impedance of midrange and tweeter are not the most important thing for an L-Pad. Both drivers may have different impedance but have the same sensitivity (90db). Therefore you need to pad ensuring appropriate attenuation.
This is a good point, and one that I didn't help to make clear.

There are a couple of reasons one might use an L-pad close to the nominal impedance of the driver.
1. A variable unit may be more consistent this way throughout the range of adjustment.
2. Improved likelyhood of predictability in a non-measurement design.

Another factor to consider is the reduction of variations in impedance over frequency which will vary with different resistor combinations.
 
Courtesy of hificompass... if you want to cross the 2" dome anywhere near resonance (~ 600Hz) - you're going to need to take that into account, as the peak impedance will throw off your on-line calculator 2nd order electrical crossover. the on-line calculators assume a flat impedance... as you can see, the 2" dome doesn't. Neither will your tweeter nor woofer. Hence why a simulation programme is normally recommended. By loading FRD/ZMA curves, all this is accounted for, that no on-line calculator can match
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By loading FRD/ZMA curves, all this is accounted for, that no on-line calculator can match
One thing that I have noticed is that full simulation software will always come-up with a circuit even if you
present it with a totally unrealistic crossover frequency. IE a very complex circuit, but not realistic because of
the not realistic input of crossover frequency. This dome mid. as mentioned, should surely cross @ 1K / 12dB.
The (adjustable) Lpad 'requirement' can certainly be solved without using one. :)
 
I'm building a 3-way speaker system. Woofer is 8 ohm, Mid is 5 ohm and tweeter is 8 ohm. By using an online calculator, the difference between 8 & 5 ohm requires difference values of L & C in the mid crossover. Now I want to control volume of the Mid and tweeter by using a L-pad, but the L-pad is 8 ohm & Mid is 5. What should I size the value of L & C for 5 ohm or 8 ohms? Or is this not going work?
Did you already buy the speakers? If not look at using only Dayton drivers reason being you can download their FRD & ZMA files and then use xsim a free simulator to design your crossover you can even get correct cabinet volumes and port size from parts express website. Using a online calculator is inaccurate to say the least it wont even get you in the ball park. The crossover is not that simple the online calculator can't tell you that you need a 2nd order on the woofer and a 3rd order on the mids & highs don't go this route you wont be happy with the end product. You can go on parts express website right now and pick some Dayton drivers download their FRD & ZMA files download xsim and start designing a crossover. If you can't measure the drivers then going with the Dayton is the second most accurate option.
 
Thank you everyone for the advice. I can see this is way over my head. It seemed cut and dried with the crossover calculator. I have some re thinking and do some more reading. I already have the speakers, so have to find a way to cross them over so they sound half way good. My cabinets are old Pioneer CS-77a's a friend gave me. The surrounds were all rotted and missing. I'm not sure what to do now. Yes, I planed on using 2 manufactured variable L-Pads 1 in tweeter & 1 in the Mid line.
 
A variable L-pad control is placed between the crossover and the driver and is supposed to present a constant impedance (say 8 ohm) to the crossover.

If that is the case then the variable L-pad control should not affect the crossover frequency.

I'd value an expert opinion on the above.
 
Thank you everyone for the advice. I can see this is way over my head. It seemed cut and dried with the crossover calculator. I have some re thinking and do some more reading. I already have the speakers, so have to find a way to cross them over so they sound half way good. My cabinets are old Pioneer CS-77a's a friend gave me. The surrounds were all rotted and missing. I'm not sure what to do now. Yes, I planed on using 2 manufactured variable L-Pads 1 in tweeter & 1 in the Mid line.
If it were me, I'd save the midrange for another project. I'm not sure about the tweeter quality you have. Without distortion and off-axis measurements on the tweeter, it's hard to know.

You could search for projects using the Dayton woofer and buy matching drivers and crossover parts.

If you are dead set on your own design - I'd go for a midrange driver that can cross around 200Hz - anything with 3mm of xmax would be fine / plenty. Something like a 5" driver would work fine. SB Acoustics and Dayton have some options here. I'd go for a midrange with no breakup (that is - avoid drivers with a big peak up in frequency) as that will require more complexity in the crossover to "notch out". In terms of tweeter - there are many 1" soft dome options. Again SB Acoustics has some excellent value for money drivers. Anything in the SB26 range would be good and can cross around 2KHz.

The above (do it yourself) will require loading FRD/ZMA files into a crossover simulation program (e.g. VituixCAD or XSim). I can help / guide you if you want to do the legwork (i.e. source or create via tracing the files).
 
My cabinets are old Pioneer CS-77a's a friend gave me ... I'm not sure what to do now.

Nice boxes!

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The crossover experts on here have the facilities to make loudspeakers sit up and beg!

However, you can start by designing a very basic crossover - one which hopefully won't stress the individual drivers.

Three-way speakers are harder to design than two-way speakers and that's why Dave Bullet raises the possibility of leaving the mid driver aside for now.

It may be possible to design a simple two-way crossover of appropriate crossover frequency and slope to suit the woofer and tweeter drivers you have already purchased.

Using a variable L-pad on the tweeter would go a long way to simplifying the design process as far as level matching is concerned.