Need some help figuring out which capacitor to use.

comfortablycurt

I've got a couple old RCA 2-way speaker cabs that I'm about to be putting new drivers in, and I'm trying to figure out which capacitor I need. The cabs are each going to have an 8 ohm 10" woofer, and an 8 ohm soft dome tweeter, wired in series with the woofer open, and a first order crossover on the tweeter.

Since the two 8 ohm drivers are going to be wired in series, that will be a 4 ohm load altogether correct?

I've got the capacitor pretty much narrowed down to two different ones...but I'm not sure what I need. Is the load on the capacitor going to be 4 ohms or 8 ohms? I want to set the crossover on the tweeters at about 3300 hertz.

From my calculations a 12 uf capacitor at 4 ohms would give me a crossover point of about 3312.

A 6 uf capacitor at 8 ohms would give me the same crossover point of 3312.

So I guess my question is...what's the impedance on the capacitor? Do I go by the seperate impedance of the tweeter(8ohms) or the total impedance of the two speakers(4ohms)?

Any help would be appreciated!
Curt

ShinAce

Seperate impedance: 8 ohm.

Face

http://www.partsexpress.com/resources/crossover-component-selection-guide.cfm

A 6db slope at 3312 will probably blow your tweeter.

jcaut

Wait a minute- Two 8 Ohm drivers wired in series is 16 Ohms. BUT, beyond that if I'm understanding correctly what you're doing, it won't work. A woofer and tweeter wired in series, with a capacitor in the line (also in series) will result in the woofer and tweeter receiving the same high-passed signal. My guess is that you have series/parallel confused. Parallel is "+" to "+", "-" to "-" each wired seperately back to the amp's "+" and "-" respectively. Series would be "+" from amp connected to "+" on woofer, "-" on woofer connected to "+" on tweeter, and "-" on tweeter connected back to "-" on amp. Which won't work.

Now if you're wiring parallel, it'll work, but it's going to be somewhat trial and error (and be aware that an error and too much volume might burn out your tweeter) and it'd just have to be up to you as far as what sounds good. Not many tweeters like a first order crossover, and not many 10" woofers sound good up high enough "wide open" to meet a typical dome tweeter. But there are exceptions. Rather than worrying about the actual crossover frequency, my suggestion would be to start with something small like a 2.2uF, 3.3uF.... maybe 4, and see what it sounds like. Go much higher, and you're going to be really working the tweeter. I'd also try reversing the polarity of the tweeter with each cap value. You might stumble onto a setup where they combine pleasingly.

comfortablycurt

The capacitor is in between the woofer and the tweeter, so only the tweeter would be getting the high pass signal. Maybe I do have series and parallel confused...I thought if I wired two 8 ohm speakers in series it would give me 4 ohms...do I need to wire them in parallel to get 4 ohms then? Would I still just be able to put a capacitor in the positive lead going to the tweeter if it was wired in parallel?

Alright...well I know the impedance to use now at least, thanks ShinAce.

Should I use a different crossover point then? It has a 2uf capacitor in it now...but according to my calculations that puts the crossover point at 9900 or so...which is well above what the woofers are going to be able to reproduce.

The tweeters I'm looking at have a rated frequency response of 2000-20,000 hz, and the woofers are rated for 36-5000 hz. I figured the crossover point would be best about halfway between the two...but if that would blow the tweeters...maybe I'll look into a different one.

Should I try to find a woofer that has a higher frequency response? It seems to me that if I leave it crossed over at 9000 on the tweeters, I'd be missing a big chunk of the frequency spectrum.

Or would I be fine just using a 2uf cap?

Thanks a lot for the help everyone.
Curt

Face

Why don't you just put together a proven design? Throwing a couple of drivers together that you have no idea what their parameters are will probably not sound good at all.

ShinAce

You want to wire in parallel. Series crossovers are no fun at all.

The woofer gets wired with jumpers straight to the terminal cup on the box. The tweeter gets wired the terminal cup with the cap cutting the positive wire(doesn't really matter in your case, but it's important once the crossover gets more complicated).

Woofer:

+

---

+

-

---

-


Tweeter:
+ ----||

---

+

-

---

-

jcaut

Here's what it sounded like you wanted to do first:



This would not be the way to implement a crossover. Assuming both drivers are 8 Ohms, the amp would see a fairly high impedance (mostly the capacitor), but both drivers would be affected by the series capacitor, which would see the total impedance as about 16 ohms.

Here's a typical crossover using only a cap on the tweeter: These drivers would be considered "in parallel" though the actual impedance of the speaker system would still be about 8 Ohms as it would primarily be influenced by the woofer.



Here's a "typical" 1" dome tweeter (this one looks more like a 6 Ohm driver) , its FR, impedance curve, and how the response is affected by a 4uF capacitor (shown on the red line):



Here's a picture showing a 10" woofer that shows little breakup running unfiltered combined with the theoretical 1" dome tweeter with the 4uF cap. I reversed the polarity to get better response. See how much they still overlap? The peaks and valleys are due to the overlap in the response and whether the drivers happen to be acoustically in-,or out-of-phase at that point. The illustrated setup probably wouldn't sound too bad on-axis.

Could you mention what drivers you're using? If they're common ones like from Parts Express, you might have better luck getting suggestions on the PE board.

Jason

jcaut

comfortablycurt

Alright...I think I had my terms confused. The cabinets are wired in parallel...just got my words mixed up. Just to clarify...the input terminals are wired to the woofer, and the tweeter is wired to the woofer, with a capacitor on the positive lead in between the woofer and the tweeter. Is this parallel? This is how it's wired right now. Should I just wire each driver to the input terminals individually? I was planning on replacing the inputs with binding posts anyway.

These are the drivers I was thinking about using.

http://www.parts-express.com/pe/pshowdetl.cfm?&Partnumber=290-324&scqty=2

http://www.parts-express.com/pe/pshowdetl.cfm?&Partnumber=270-182&scqty=2

So would a 4uf cap be the way to go? Sorry I'm asking so many questions...just new to this, and want to make sure I do it right.

Thanks for the help,
Curt

ShinAce

comfortablycurt wrote: »

Alright...I think I had my terms confused. The cabinets are wired in parallel...just got my words mixed up. Just to clarify...the input terminals are wired to the woofer, and the tweeter is wired to the woofer, with a capacitor on the positive lead in between the woofer and the tweeter. Is this parallel? This is how it's wired right now. Should I just wire each driver to the input terminals individually? I was planning on replacing the inputs with binding posts anyway.

Parallel? Yes.

Rewire it? No.

4 uF? I would be talking you into a crossover for the tweeter that had 1 cap and 1 coil. But I have no intention of making you try something you don't want to. Try what you like. 4uF is in the right ballpark.

Face

comfortablycurt wrote: »

So would a 4uf cap be the way to go? Sorry I'm asking so many questions...just new to this, and want to make sure I do it right.

Nobody here can tell you for sure. I would try part's express' message board, maybe someone there is more familiar with those drivers and can tell you what you need.

jcaut

I wasn't trying to be long-winded with my reply and I hope you didn't feel like I was insulting your intelligence by trying to explain series vs. parallel. The talk of series/parallel had ME confused and I just wanted to post some pictures to make sure we're all talking about the same thing.

Those look like quality drivers. Regardless of the stated "frequency range", I question whether the woofer is smooth enough to use without a filter, but without a graph (which I can't find anywhere) or measurements, it's hard to say. In my experience, when running a woofer unfiltered, a simple crossover on the tweeter tends to sound best. In reality it all depends upon the roll-off characteristics of the woofer and whether or not the tweeter is robust enough to deal with the shallow slope. No doubt a second order would be better for the tweeter, but it might be even harder to get it to sound right.

The problem is that no one can give you any solid information unless they're at least somewhat familiar with these particular drivers being used together. My GUESS is that about a 4 uF on the tweeter would be worth a try if you're committed to using only a cap. That's a high crossover point if you're going by the formulas, but... My reasoning is that the tweeter probably has a resonance peak around 1000 Hz (or higher) and if it doesn't have ferrofluid (?) that impedance peak will be pretty high. Which makes your cap fairly ineffective down at those frequencies. You can get into a situation where the cap just produces a "sag" in the response if you go with too low a value. If the tweeter DOES have ferrofluid then that will help alot. That's not really the same thing as a recommendation. There's just not enough info to give a recommendation, and I don't think it's fair to guide you toward a more complicated crossover (even though that may be what it really needs), because without measurements you're just shooting in the dark and spending more money.

Replacing drivers in an old cabinet can be a lot of fun, but you have to have reasonable expectations of what can be acheived with what you have to work with. I would be thinking more along the lines of "garage" or party speaker versus living room speaker. For example, I recently re-used an old cabinet by sticking a 10" paper cone woofer- something like might have been seen in a Pioneer rack system from the '80's- into a sealed box with a piezo horn tweeter. No crossover whatsoever, except for a resistor on the tweeter. I was actually stunned that they sounded as good as they did- I didn't even bother to measure them because I know they'll be ugly- But they cost me all of about $8 per speaker. They're in my garage though, not in the house.

Don't know if that helps you any. I'd suggest asking on the PE board as there's more people there that migh have experience with those drivers you have.

Jason

comfortablycurt

Actually all of those charts helped answer a lot of my questions, so I don't really feel that my intelligence was insulted at all.:D Just glad that someone else with a bit more knowledge on the subject was able to help me.

I don't have real high expectations for these speakers, they just sort of have some sentimental value, because these along with the Fisher receiver I have were my first "real" stereo. So I want to see what I can do with them. The speakers will probably soon be replaced with some Polk Monitor 5's or Monitor 7's...or something similar to those. These speakers will end up becoming garage speakers or something like that.

So I think I'm just gonna leave it wired the way it is, and try out a 4uf cap on it for now. I might end up getting a few different caps and see which one sounds the best. I don't really want to get into a more complicated crossover, because it would start costing more then...and I'm trying to keep the cost of it down.

Another question...The wiring in the cabs is kinda old and dried out, and it appears to be pretty oxidated, so I was gonna replace it with some new wire, along with some 5 way binding posts. Any specific kind of wire I should use? Would regular 18 gauge speaker wire be sufficient?

I might join the partsexpress forums and see what I can find out. I'm sure they'll be able to give more product specific advice there.

Thanks again for all the help,
Curt

jcaut

Plain old 18ga speaker cable should be fine. I have a pretty good selection of crossover parts, especially capacitors and resistors- If you want to try out several values without having to buy them, just let me know what you'd like to try: Use what you want - send the rest back. If that would help you just let me know.

(Edit) Another thing I just thought of that you might consider: If you don't already have those drivers, you might consider switching your tweeter to something like the Dayton silkie, part# 275-070. It's more expensive, but it's well-known and handles a low crossover point. It's been used in enough designs that the folks on the PE board would be able tell you how to cross it over just about anywhere you want to.
Especially if you were to pair it with, say, the Dayton classic 10" 295-315. I'm not real sure that would make a good two-way, but I'm just thinking as I type.. (Edit again) Looks like the Dayton woofer would definitely need a LP filter- at least an inductor and impedance compensation- so that might be getting complicated..

Jason

comfortablycurt

jcaut wrote: »

Plain old 18ga speaker cable should be fine. I have a pretty good selection of crossover parts, especially capacitors and resistors- If you want to try out several values without having to buy them, just let me know what you'd like to try: Use what you want - send the rest back. If that would help you just let me know.

(Edit) Another thing I just thought of that you might consider: If you don't already have those drivers, you might consider switching your tweeter to something like the Dayton silkie, part# 275-070. It's more expensive, but it's well-known and handles a low crossover point. It's been used in enough designs that the folks on the PE board would be able tell you how to cross it over just about anywhere you want to.
Especially if you were to pair it with, say, the Dayton classic 10" 295-315. I'm not real sure that would make a good two-way, but I'm just thinking as I type.. (Edit again) Looks like the Dayton woofer would definitely need a LP filter- at least an inductor and impedance compensation- so that might be getting complicated..

Jason

Yeah, that could help me out quite a bit actually. It'd be great to be able to try a few different caps out without having to buy them all. I'll let ya know if I want to take ya up on that offer.

I haven't gotten any of the drivers yet, so I'm still open to suggestions. I just checked out those Daytons you recommended...and I think that might be what I end up going with. That's the best one I've seen for what my needs are. It says it's recommended for crossover points down to 1800 hz, and a bunch of the people in the review section reported crossing them over at 2000-2500 hz. So I think that's what I need. I think I'll have to widen my cutout hole by an eight of an inch or so...but that's not a big deal.

Alright...I've got another question now. This time it's about "VAS". How do you determine the ideal VAS for an application? The cabinets I have are sealed, and have an internal volume of approximately 1.31 cubic feet. So would the ideal woofer for these cabinets have roughly the same VAS? Or am I understanding it wrong? The Goldwood woofers I'd picked out have a VAS rating of 1.12 which is the closest to the cabinets volume I could find. Those Dayton woofers have a VAS of 3.93, so would they not work for me?

jcaut

Picking the proper box for a woofer is actually one of the easier parts. You'll need three important parameters for your woofer: Qts, Vas, and Fs. Generally, a woofer for a sealed box will have a Qts above .4, up to about 1.2,and a low Fs. Good woofers for ported enclosures will have a Qts in the range of about .25 to .4 Sealed boxes are more forgiving and some woofers can work well in either box type.

Since you're looking at PartsExpress, the easiest thing to do would be to check out their woofer selection guide, here:

http://www.parts-express.com/resources/woofer-selection-guide.cfm

If you know the T/S parameters of the woofer you want to use and want to play with box modeling yourself, one of the easiest programs is here:

http://www.linearteam.dk/default.aspx?pageid=winisd

comfortablycurt

Well, I used the Woofer Selector that PE has, and they pointed me in the direction of the Dayton RS270S. At 85 bucks I think that might be a little out of my price range though...so I might have to shop around for that a bit more.

I think I have a much better understanding of what I'm doing now though. I'll have to look around and see what I can find. I'm trying to keep the cost of the woofers somewhere around 30 dollars each...up to like 40-45 at the most. I'm sure I'll manage to come up with something.

I'll have to check out the program you posted...that'd probably help a lot.

Just wait til I start building my first pair of speakers from scratch. I'll be bombarding you with questions then...lol

Thanks a lot for the help though. I've got a lot of my questions answered now.

Curt

jcaut

You can always glue in a block of styrofoam to make an enclosure "smaller", if you need to.

That Dayton woofer would work pretty well in your enclosure and could even be made to work ported if you wish. The Goldwood might be fine as well. The real problem is the issue of a two-way with a 10" woofer. It's not ideal -and that Dayton is known for its bass, not its midrange quality, but from the posted FR I think its workable. You would have to cross low and you'd probably end up with a more complicated crossover than you envisioned. Even if you didn't end up with a real pretty response curve, how it sounds is what's important. I know you won't get away with a simple crossover with the metal coned RS drivers as they tend to have really nasty breakup peaks, often requiring notch filters in addition to high order slopes. A pro sound 10" might be a good candidate for running unfiltered... Alpha 10A, maybe? .... just thinking as I type again, sorry.

I've been interested in building speakers for years, but the availability of free modeling and measurement software and the wealth of information available on the internet has made DIY speakers so much easier and more rewarding than it used to be. It's addictive. I don't have a lot of money to sink into it, so almost all of my projects end up being "budget" projects. Years ago when the only thing you could really do was trust manufacturers stated specs and use the enclosure calculators and crossover formulas, the chances of making anything that actually sounded good were pretty slim: It mostly came down to luck and a lot of by-ear tweaking, unless you built a proven design. Not to discount the "by-ear tweaking" part- because that's still necessary- but nowadays I can just about put a set of drivers in a cabinet, run FR and impedance sweeps and then use that data to model a crossover that won't be far off. It amazes me really, and I guess the sheer "neatness" of it is what makes the hobby addictive for me. I certainly don't NEED any more speakers. I don't think the argument that it's economical even holds up.

If you get further into it and want to look at some more software, here are some suggestions:

Speaker workshop is a free "do it all" speaker building program. It's not the user-friendliest program ever, but it's not too hard. It'll do box modeling, but it's real strength is the crossover modeling function. You have to have some other hardware and a PC with a compatible sound card to take measurements, but the program itself is free. I've looked at some of the other programs, but I keep coming back to this one.

Lots of tools, mostly Excel spreadsheet-based HERE. Unibox is a more accurate enclosure calculator then WinISD, but it's a little harder to use. PCD is nice. Often you can find the .frd and .zma files for drivers to plug into these calculators (or into speaker workshop). That's not as accurate as measuring them yourself, but it'll get you started.

comfortablycurt

Another idea I've been pondering is turning these cabs into 3 ways.

[IMG][/img]

Think I could squeeze a tweeter in there if I used the tweeter mounting spot for a midrange? The way the tweeter mounts is kind of weird...that black "donut" is what the tweeters attached to, from the backside. The whole "donut" comes out and leaves a hole that's roughly 5.5 inches across. I was contemplating putting a midrange in that spot, and trying to put either one or two tweeters in the empty spot between the woofer and mid.

Sound like a good idea? Thing's would start getting more expensive going that route...but that's something I might do a little farther down the road.

I think I'll probably stick with the Goldwood woofers for now, and I may end up switching to the Daytons if I decide to make them 3-ways.

AudioGenics

try finding active crossovers and utilize separate amplifiers to provide greater control of the transitional ranges. really opens the possibilities.

comfortablycurt

joseph.v.chen wrote: »

try finding active crossovers and utilize separate amplifiers to provide greater control of the transitional ranges. really opens the possibilities.

I think that might extend the cost a bit above my projected budget for this project...lol That'd be nice...but when I go for a bi-amped setup, it's going to be with speakers much better than these ones.

jcaut

What I'd probably do with them is cut out the front baffle, leaving enough material on the edges to glue in a new baffle, then I'd turn them into this. A well-respected design, not too much money, and all the hard part already done. If you're handy at all with woodworking it would be pretty easy.

I think that'd be a good alternative to all the compromises you'd have to make to use them "as-is". My opinion, of course.

Off topic, is that your Monte Carlo SS on your photobucket account? I always liked those cars. I'd probably like that one with the blower even more...

comfortablycurt

jcaut wrote: »

What I'd probably do with them is cut out the front baffle, leaving enough material on the edges to glue in a new baffle, then I'd turn them into this. A well-respected design, not too much money, and all the hard part already done. If you're handy at all with woodworking it would be pretty easy.

I think that'd be a good alternative to all the compromises you'd have to make to use them "as-is". My opinion, of course.

Off topic, is that your Monte Carlo SS on your photobucket account? I always liked those cars. I'd probably like that one with the blower even more...

Yeah...it's not "my" car so to speak...it's me and my dads car though. We've had it for going on 10 years now...it's our baby. There's not to much thats stock about it anymore though. 350 chevy bored .030 over with a B&M supercharger and a 150 wet shot of nitrous, through a 6 speed Richmond manual. The rear end and suspension/brakes are next on the list. Can only do so much at a time though.

I hadn't even thought about putting a new baffle in it. If I wanted to go the 3-way route that probably would be the easiest way to do it...My dad used to be a professional woodworker...and has a full wood shop, so I've got plenty of experience in that department. What would be best to use for a baffle? 3/4 inch to one inch MDF or something like that? I wouldn't mind going that route actually...The baffles on their now aren't really the greatest looking...basically bare MDF from the looks of it. A nice veneered MDF baffle would improve the looks quite a bit...and give me a lot more room to experiment.

comfortablycurt

I think I'd be able to squeeze in some 8" woofers if I wanted to go the 3 driver route...it'd place them pretty close together though. Probably within 3/4 of an inch or so of the top and bottom of the tweeter. Would that be too close together?

Now you've got me thinking about a bunch of different possibilities...lol...

I was also thinking about going with a new baffle...and making them 3 ways, with 8" woofers, along with a mid and tweet stacked on top in the normal configuration. How expensive do you think it would be for me to design a 3 way crossover for them? Would one of the preconstructed crossovers from parts express work or are they not recommended?

comfortablycurt

If I was going to go with the 3 way setup...these are the drivers I was looking at.

woofers-http://www.parts-express.com/pe/pshowdetl.cfm?&Partnumber=295-366&scqty=2

midrange-http://www.parts-express.com/pe/pshowdetl.cfm?&Partnumber=295-358&scqty=2 that's actually a full range driver...is there any reason that wouldn't work as a midrange?

tweeter-http://www.parts-express.com/pe/pshowdetl.cfm?&Partnumber=275-070&scqty=2 The Dayton Silkies that you recommended me a few posts back.

I'm starting to think that I want to make this a little bit more complicated than just dropping new drivers in...You had to mention the idea of getting a new baffle didn't you! j/k

strider

jcaut wrote: »

If you get further into it and want to look at some more software, here are some suggestions:

Speaker workshop is a free "do it all" speaker building program. It's not the user-friendliest program ever, but it's not too hard. It'll do box modeling, but it's real strength is the crossover modeling function. You have to have some other hardware and a PC with a compatible sound card to take measurements, but the program itself is free. I've looked at some of the other programs, but I keep coming back to this one.

Lots of tools, mostly Excel spreadsheet-based HERE. Unibox is a more accurate enclosure calculator then WinISD, but it's a little harder to use. PCD is nice. Often you can find the .frd and .zma files for drivers to plug into these calculators (or into speaker workshop). That's not as accurate as measuring them yourself, but it'll get you started.

Cool links. Thanks for pointing them out.

Ben

jcaut

I would try to talk you out of the three-way idea unless you can find an existing design that would work in the cabinet. The crossover design gets substantially more complicated and there are more phase-related issues, etc..

The stock, pre-assembled crossovers don't usually work well. It's the same as using the crossover formulas: The values are set up as if the response of the speaker was a flat line and the impedance was a constant value. A pre-built crossover can sometimes be a good starting point (then you might be able to add impedance compensation, or piggyback capacitors to alter the values, etc.) but you'd still need some measurements to do it right and if you've got that capability you might as well start from scratch.

Regarding the spacing of drivers: Normally it's best to keep drivers spaced as close together as possible. It's less of a problem when the crossover frequency is low (longer wavelenghts) but at higher frequencies it becomes rather critical. So in a 3-way, woofer to midrange distance can be greater than mid to tweeter. Also regarding the 4" RS100: It would probably work as a mid, but there are probably better choices. It would need its own enclosure (or sub-enclosure) since it's open-backed.

Regarding your enclosure and a new front baffle: 3/4" MDF would be fine. It looks like the orignial grill was inset, so if you were going to try and veneer it, it would need to fit reasonably flush, right? If I were doing it (I'm not the greatest cabinet builder, so I have to work within my means) I would cut 3/4" MDF or whatever it took to make it stick out a little past the original cabinet, then route a roundover around the edges and spray it with bedliner. Then a grille could be attached to the front.

Nice car! You mean the stock 10-bolt is still alive in there?

Face

jcaut wrote: »

Nice car! You mean the stock 10-bolt is still alive in there?

That was my first thought too.


As for designing a three way, you should check out Madisound.com. If you purchase the drivers from them, you can pay an additional $30(IIRC), and they will design a crossover for you.

jcaut

Face wrote: »

As for designing a three way, you should check out Madisound.com. If you purchase the drivers from them, you can pay an additional $30(IIRC), and they will design a crossover for you.

Good point.

Here's something I just saw on the PE board that's relevant. This guy knows ALOT more about this stuff than I do..
http://techtalk.parts-express.com/showthread.php?p=1544865#post1544865

comfortablycurt

jcaut wrote: »

Nice car! You mean the stock 10-bolt is still alive in there?

Unfortunately. It's hanging in...I don't think it likes having 500+horsepower being thrown at it though. You definitely can't bang the gears in it right now. We're still trying to decide between a 12 bolt or a Ford 9"...kinda leaning towards the 9" though.

Face wrote: »

As for designing a three way, you should check out Madisound.com. If you purchase the drivers from them, you can pay an additional $30(IIRC), and they will design a crossover for you.

That's definitely something I'll keep in mind. I was looking at their site a while back...it'd probably be worth it to go through them if I was going to try the 3-way setup. I'd be able to make it myself I'd imagine if I had the design for it.

So I guess I've gotta figure out if I wanna go with a 2.5 way or a 3 way now...

What are the general opinions on the SEAS Prestige series drivers? If I was gonna go the 3 way route I was kinda looking into those ones.