Building Speakers: 8ohm Load with Built-in Crossover Issue

[darkeagle]

Hello,

I am in the process of building a set of speakers (not my first pair), however I have run into a little bit of an issue. The tweeter I am using has a built in crossover, which cuts out the lower-end frequencies. If I am to wire both the tweeter and the woofer in series, the woofer will also lack the low-end frequencies. Both loads are 8ohm. If I am to wire them I am parallel, they will work fine, but I will end up with a 4-ohm overall load, and my amp can only handle 8ohm. I understand I can wire a 4ohm resistor into the circuit, but I can't seem to find one. Based on what I have, are there any other ways to wire the speakers so as to keep at least an 8ohm load, and not have to buy any other parts? Any suggestions are greatly appreciated!

-bryan

 

[Cliff_J]

Series is a great way to get bad sound.

Use a passive crossover network. Done properly, both speakers will end up presenting a nominal 8 ohm load across the frequency band. I'll explain how they work, and I think that will answer any fears you have about running these two in parallel. Hint: This isn't a pair of resistors on a battery, its an AC signal and we're using reactive devices.

For my example, you build a simple 6db/oct passive crossover which is a capacitor in series with the tweeter and an inductor in series with the woofer.

The capacitor allows high frequencies to pass with ease, yet its reactance starts to increase the impedance at lower frequencies. This continues as the frequency gets lower. The increasing impedance lowers the current flowing, thus lowering power delivered to the tweeter at lower frequencies, and thus resulting in a desirable response.

The inductor on the woofer does the opposite by allowing lower frequencies to pass easily but increasing the impedance as the frequency increases, thus reducing the current flowing (and power) and thus resulting in another desirable response.

If you choose the correct values (simple tables exist to choose those values from) then the high-pass capacitor will add 8 ohms to your 8 ohm tweeter at the crossover point, making it 16ohms there. The inductor will add 8ohms to the 8ohm woofer at the crossover point, making it 16ohms as well. 16 ohms and 16 ohms in parallel is 8 ohms. At a different frequency one might be 10 ohms and the other 40 ohms and those in parallel equals 8 ohms. Both tweeter and woofer with their respective crossover components have impedances whose values work out on a smooth curve so in the end they equal a nominal 8 ohms across the entire frequency range. Pretty slick and really easy to build.

And if you've chosen drivers with matched sensitivity, the outputs should sum to an acoustic response that is relatively flat. Hopefully after all that, it sounds good too. Happy building.

Cliff

 

[chroot]

From the sound of it, the "built-in crossover" in your tweeter will only affect the tweeter.

- Warren

 

[Cliff_J]

lol, I thought the idea was to provide information to lead to an answer, not the answer! :) j/k

Its suprising how many OEM or inexpensive aftermarket tweeters have a 'built-in crossover' that is a merely an electrolytic cap wired in series with the tweeter. Some hide it in a piece of shrink wrap and tuck it away neatly, but its still there.

Even better is the 'bass-blockers' that stereo shops sell to assist people who add an aftermarket subwoofer system and have problems with their factory speakers handling a little more bass boost. That 'bass-blocker' that sells for $10 is again a simple electrolytic cap wrapped in shrink wrap. Kudos for good marketing of a simple product to address a simple need and make it conveinient (the shack is getting disappointing for parts in the last couple years) but its neat IMO to understand what is behind the scenes.

I'm surprised darkegale hasn't responded. I think speaker building is a great hobby and hope my explanation helped this person move beyond just the simple series/parallel impedance concept to bigger and better things.

Cliff

 

[darkeagle]

Hey there, sorry about the delay.. I was suffering some minor technical difficulties (AH-HEM... system crash..;) but... I am back on track now, and can continue to focus on my audiophile dilemma... Thank you kindly, Cliff, for your much-more-than-expected response... And after all of that, I went and came across an amplifier capable of running 4ohm loads... however, I'm going to take another run to radio shack and see if I can't find the necessary parts to build a crossover. I agree with you, that the shack is definitely starting to lost considerable face over cheapness.. they used to be "the place to go" for audio supplies... I do have another question though...

I am building the boxes for these beasts myself, and the shack didn't provide me with much information, regarding recommended enclosure sizes, and port tuning... etc. Just wondering if anyone could steer me in the right direction when it comes to building the boxes.. I don't want to waste all that time/money and find out they sound like a tin can with a beetle trapped in it... Thanks again...

-bryan

 

[Macgyver]

This is probably an obvious sugestion but if you search through a few audiophile web sites you should be able to find a speaker box design and tips that will work for you.

As for the electronics parts, when the shack fails I usualy search for a consumer distibuter online. I have yet to find one that has both a good selection and no minimum order.

Good luck.

 

[Loren Booda]

The following has worked for me:

Use the thickest practical particle board for the walls. Try not to size any dimension of the box near an integral multiple of another. (You may find an truncated pyramid shape to resonate less than a box.) Seal with latex caulk and wood screws, while packing the completed box with glass wool, installing crossover and drivers (with foam gaskets) last.

Familiarize yourself with the frequency and phase response of the drivers. If you add a port for efficiency, try estimating an interference between the port and mid-bass driver(s) to be perceived as relatively flat and in-phase for the listener. The port may be further tuned with foam rubber.

The crossover should be tuned so the upper drivers are not overloaded, and the lower drivers not muddy the sound. As a rule, two crossover points maximum. A 6dB/octave crossover is preferable to a steeper dropoff. Make sure the electrolytic capacitors you get are bipolar.

When considering materials, I think of the box's design as partially reversing "imperfections" in the the drivers.

 

[Cliff_J]

partsexpress.com has good selection of speaker building supplies and have had good service. They carry some closeout and some dimestore stuff, but a careful shopper will find their better supplies.

For building speakers, a great read is the "Loudspeaker Design Cookbook" by Vance Dickason. He covers every aspect of speaker selection and integration and does it using plain english and formulas that are easy to plug into excel or perform on a regular calculator.

For electronic parts without minimum orders, mouser.com has been good to me. Much easier to use their paper catalog (whose isn't) but their website has lots too. Parts Express has a better laid out selection of crossover inductors and electrolytic caps (and the mylar and solen caps too) and would be an easier source to use.

For the box and port dimensions and tuning frequencies, what are the Thiele-Small parameters of the drivers, and what type of equipment do you have access to? With a tone generator (could be a computer with a small amplified speaker you could use to drive the speaker you're measuring) and a multimeter, you can measure most of the T/S parameters yourself, and with a calibrated weight or enclosure measure all the important T/S parameters. The LDC explains all this in detail.

The shack seems to have made a business decision that they make their money selling cell phones and helping people with easy electronics and batteries, not supporting some little group of hardcore individuals. Makes more sense to play to your strengths and stay in business, but I still complain because it used to be more conveinent. :)

Cliff

 

[smell_you_later]

Series or Parallel, Ohms?

Im setting up a sort of homemade P.A

but i don't know if it will work or not (i don't want to try it without getting some advice cause well i might blow up quite a few amps)

The mic. is being Plugged into a (40watt) guitar amp, and then that is being connected to an old HiFi system, (it will be recognised by the HiFi as a radio or CD player etc. depending on where i plug it in)

But what i want to know is will it blow up or not as.

I am using 2x40 Watt speakers (both 6 ohms) and
2x70 Watt speakers (both 8 ohms) i also have the option of
2x28 Watt speakers (both 6 ohms) but i want to get the loudest sound i can, without blowing anything up.

The old HiFi says it has an Ohm Resistance (or limit, can't remember what) of 16 ohms (meaning that if you put any more in it, then it won't work...i think)

and the speakers way the speakers are plugged in looks like...

R L

1 + - + -

2 + - + -

(and it says, 1+2=16 ohms OR 1or2 =8 ohms)

if anyone can help me, and save me blowing up a lot of equipment i would be very appreciative.

 

[frogdogbb]

I have built my own tri-amped system crossovers, amps, preamp, etc based on Rod Elliot's information and it is awesome. Check out www.sound.au.com[/URL] there is a wealth of information on his site its free and ligit I know since I have talked many times with him and have bought stuff as well. Series speaker=bad and as was suggested look and see what this crossover device is if it is just a cap you probably want to learn about some speaker theory.
I use MDF for the speaker cabinets it does not flake like particle board and is more dense pre-drill the holes and if you want a nice finish countersink them and wood putty over them or hole plugs or many options. There are vary thoughts geometry and side ratios Try the ajsealed box designer [url]http://www.ajdesigner.com/speaker/index.php[/url] There is no adware/spyware it is free and accurate. I have tested its math and it allows you to use the golden rule of side ratios and then if you need to you can constrain one or more of the sides and enter your desired dimension.
Good Luck