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Discussion Starter #1
Hi everyoone, I'm new to this forum, and I'm new to diy loudspeakers too.:jump:

I'm doing my first project, trying to keep things as cheap as possible. I need practice before going in for a more expensive and ambitious diy speaker project.

I'm making a 3way speaker system, recycling a pair of 2way loudspeakers of which I'm only reusing the boxes and the 15"woofers. Using three power amps and a digital crossover, I have determined that the whole system sounds best with crossover frequences of about 500-800Hz and 3000Hz. Using the formulas from DIYspeaker books I have come up with a set of values for the crossover design. Now, in order to get the best approximation I will have to put capacitors in paralell. Will I get any problems if I use np electrolitics in paralell with np polyester ones?:scratchhead:

I also have trouble getting inductances. :no: I can't cheaply buy .8-1mm enammelled copper wire to make them, nor ready made ones, except for some toroidal ones. Are these any good? Can I put more than one toroid in series to achieve a bigger value?
 

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Wow, I'm way impressed! I took the DIY plunge, but I stayed in the shallow end of the pool -- basically for the same reason you're posting, crossovers. Hopefully someone with better electrical know how can help you with this area.. however, here are a few websites I found related to crossovers that may help:

Link 1
Link 2
Link 3
Link 4
Link 5
Link 6
Link 7

Several of the links are related to automobile systems, but the application should be the same.

If you don't mind, I was wondering if you could answer a few questions:
  1. What kind of slope are you going to use (for example, butteworth or linkwitz-Riley)
  2. What order of slope are you going to use (6db/octave, 12 db/octave, etc)?
  3. How did you determine the box size?
  4. What is box tuned to?
I'm only just barely starting to understand how crossovers are designed, so I'm hoping to pick up a few pointers.

Good luck!

Oh, and Welcome to the Shack!

JCD
 
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Discussion Starter #3
I'm not an expert myself, that's why I'm posting in the forum.

What I'm going to say mustn't be taken for granted as I may be totally wrong.

First I would like the point out that I'm reusing an existing commercial 2way speaker. The box was, hopefully, calculated for the 15" woofer that I'm also recycling. Bass-wise, the box isn't all that bad (not good either). What I'm changing is the original piezoelectric tweeter, that sounded horrible, for a midrange and a tweeter, so I I'll end up with a 3way system.
I bought two books on speaker construction with all the formulas and knowhow for entirely designing a speaker. All I've used for this first attempt are the formulas for designing the crossover.
I chose the previously mentioned crossover frequencies by actually testing the 3way speaker with an active crossover system and a separate amplifier for each driver. My final goal is to only use two power amps, one for the woofer, and one for the mid-high range. Due to the fact that the speaker system I'm after has to run flat out and really loud (parties mainly), I have decided to use at least a 18dB rolloff for the tweeter for more protection. I should think 12dB rolloff for the midrange is sufficient. Whether I use one type of xover or another depends on the audition tests, as I will choose the one I think sounds best.
The main problem is that the impedance at the crossover frequency of the midrange is around 9ohms, and the capacitor and inductance values I've calculated aren't standard, so I have to parallel some capacitors and do something about the inductors. I'll probably have to make em myself!!

But my first question remains the same: Can mixing different types of capacitors be bad? I assume it's not the best solution, but I have the feeling no harm will come from it. I guess the benefit is that electrolitc is a cheaper solution although not as good sounding.
 

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Gotcha.. I obviously read your first post a little too quickly.

Unfortunatley, I don't have a good answer for you as I'm too much of a noob in that area myself.

I did check my copy of Dickason's LSDC related to caps.. he didn't mention anything about mixing types, so I'd assume that'd be "ok". However, he did mention that plastic film (e.g., mylar and polyprolene) caps should generally be used for the tweeter/midrange section and that nonpolar electrolytics for the rest. With the nonpolar caps, he said it's actually better to run several smaller (0.1-1uF) polyprolene types rather than use the nonpolars.

And I agree, my assumption from that section of the book is that the electrolytics aren't as good, but particularly for the tweeter/midrange section you seem to be working on.

Given all that, I wouldn't mix and just use the plastic film caps. I wouldn't think the additional cost would be that much unless you use really boutique parts. Again, I'm a total noob as well, but that was the impression I got from my book.

Regarding the box -- I'd assume it works for the woofer, however, you're adding a midrange driver to the mix, which (I'm assuming) is being used in the same box as the woofer. I'm not sure that's going to work as well.

There are other issues involved that you may or may not have to be addressed, such as the acoustic centers of the various drivers and how that affects the time delay, and the phase changes needed, etc, etc, etc...

JCD
 
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Discussion Starter #5
Yes, I understand what you say about two speakers sharing the same box. I already got round that buy building a small box inside the big one. You can't see it from the outside but it's there. The reduction in the internal volume is pretty small as it's so big ( over 100litres ), put I added a bit more damping to compensate.
I've asked around and nobody seems to think there's any problem with mixing cap types apart from the inferior quality of the electrolitic types. So I'll just go ahead and mix 'em and see what happens.
The next thing I have to solve is making my own inductors. I've been told to use Calcbob2 for calculations. It appears to work very well. Any other suggestions would be great though.
 

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I am not sure how much you have studied and whether I am teaching someone how to suck eggs but here are a few thoughts I had whilst reading this thread:

The results from using a digital crossover and three amps will audiably exceed those obtainable from passive crosovers, meaning that while the crossover point of 3Khz works with an active crossover and biamped, there is still the real possibility that a passive crossover at 3Khz will introduce phase/time delay issues and distorton.

capacitors have a thing called ESR (equevalent series resistance), when using cheaper capacitors the ESR can get quite high, thus putting them in parralel can have the effect of reducing the ESR which will reduce unwanted anomilies due to RC and RL network topologies.

Designing the crossover is the close to the hardest thing to get right in speaker design, unfortunately the crossover must take into consideration driver accoustical centres, crossover frequency and phase delay respective to driver impedance. If we mess this up we can end up with a set of speakers that have a FR graph that looks like the Rocky mountains.


Can I ask what is the difference in accoustical centres is?
 
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Hello again!! Thanks for the info.

I'm not quite sure, but I think that the acoustic center he refers to is the loudspeaker acoustical center. I might be wrong, but the acoustical center varies depending on how you set up a series of drivers in the speaker box. Driver response is not equal in all directions. Depending on the box design and driver layout, the relative angle of these with your ears varies, and so does driver response. So when designing the crossover for that speaker you have to keep these variations in mind to make an absolutely supreme and divine job. At least, that's what I think.:)

And, yes, I agree that a digital crossover with an amp per driver will sound great. I've tried it at home. But my final goal is to entertain a bunch of drunken people (I will not say names :jiggy: ), so I think a passive crossover is cheaper and I won't notice the difference! Them neither.

And I didn't think of ESR!! Yes! Putting my capacitors in parallel might be a better idea than I had thought. THANKSS!!:T
 

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Each driver has an acoustic center -- generally speaking, it's the cone of the driver. Knowing what the AC is is important when considering the time delay and/or phase needed for the crossover of the different drivers.

Since a woofer/midrange is bigger, the center is generally further back than the tweeter. In a typical flat baffle, you'd get something like this:



In order for the speaker to sound good, you'd have to have the speaker above your head unless you delayed the signal to the tweeter and or adjusted the phase of either driver.

This COMPLETELY oversimplifies the issue, but hopefully gives you an idea.

As for your original question, it sounds like you have the answer you were looking for. Let us know how it turns out!

JCD
 
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