4 & 8 ohm crossover in a T39

Message

fender3x · #1 Post by **fender3x** » Sat Dec 09, 2006 11:49 pm

Would it be possible to build a passive crossover for the T39 that could switch between 4 and 8 ohm tops?

Frankenspeakers · #2 Post by **Frankenspeakers** » Sun Dec 10, 2006 3:22 am

NO. Bi-amp, Bi-amp, Bi-amp!!

The simple answer is that no speaker or speaker system has a constant impedance. Impedance is dependant on the driver and how it acts and reacts with with the enclosure. 4 ohm system 'a' may have an impedance of 12 ohms at 100 Hz, 4 ohm system 'b' 5 ohms, and 4 ohm system 'c' may measure at 21 ohms. Same thing for 8 ohm systems. this is why loudspeaker designers and evaluators refer to the impedance curve.
You have to know exactly what the impedance is at a particular frequency is to properly design a crossover that will actually work. Now days, copper is so freakin expensive that the large heavy gauge inductors used in a passive crossover add up to nearly the price of another amp or electronic crossover. And (comma) with a DSP or even a simpler line level crossover, you can change the crossover frequencies to suit your current venue as you need it. Not so with a passive crossover. There are several recent threads discussing the ins & outs of various DBX Rolls, and Beringer products. These can be found cheap new, or dirt cheap used on craigslist or ebay. Hell, even HT rigs are starting to multi-amp. The .1 sub on all modern HT gear is just a line out that sums all the bass and routes it to one output (the sub) only difference is that with high and mid-grade gear you can choose the turnover frequency now instead of having to settle for what the manufacturer decides.

Sorry for the long-winded schpeil, but I always wanted to know why instead of 'because I said so' from a very young age.

Dave Non-Zero · #3 Post by **Dave Non-Zero** » Sun Dec 10, 2006 6:04 am

Frankenspeakers wrote:Sorry for the long-winded schpeil, but I always wanted to know why instead of 'because I said so' from a very young age.

+1

#4 Post by **Bill Fitzmaurice** » Sun Dec 10, 2006 9:33 am

fender3x wrote:Would it be possible to build a passive crossover for the T39 that could switch between 4 and 8 ohm tops?

Yes. An 8 ohm load uses a 125uF electrolytic and 10uF poly, a 4 ohm load uses a 250uF electrolytic and 10uF poly. You can use two 125uF electolytics, tied together at one end, with an SPST switch that ties the second to the first at the other end. Switch off you have an 8 ohm HP filter, switch on a 4 ohm HP filter.

#5 Post by **Bill Fitzmaurice** » Sun Dec 10, 2006 9:40 am

Frankenspeakers wrote:NO. Bi-amp, Bi-amp, Bi-amp!!

The T39 plans say exactly that, but also include schematics for a 1st order crossover for low-power situations. Bi-amping isn't a viable option for the bassplayer using a mono bass amp who's building a T39 to go beneath his 2x10, for instance. Even a 1st order crossover is a major improvement over how it's done by bass amp/speaker manufacturers, which is to just parallel wire multiple cabs with no regard to the consequences.

fender3x · #6 Post by **fender3x** » Sun Dec 10, 2006 10:27 am

Bill Fitzmaurice wrote:the bassplayer using a mono bass amp who's building a T39 to go beneath his 2x10

That would be me! Thanks.

(enjoyed reading the explanation above too...should be useful for PA types

fender3x · #7 Post by **fender3x** » Sun Dec 10, 2006 6:02 pm

Just to make this as idiot proof as I can...is this a fair rendition of the switchable crossover I'd need for a T39 iwth one BP102?

http://www.frappr.com/?a=viewphoto&id=1 ... myphotos=1

#8 Post by **Bill Fitzmaurice** » Sun Dec 10, 2006 6:51 pm

That will do it.

fender3x · #9 Post by **fender3x** » Sun Dec 10, 2006 7:34 pm

Great! One more question about this... the plans have parts numbers for the inductor...but I am easily confused about the the others.

Parts express has what look to my untrained eye like several "metalized polypropylene" caps with 10.0 uF values... but the prices vary wildly. Jantzens cost about $4 (p/n 027-932). Solens (p/n 027-568) are almost as cheap. Daytons are $6 (p/n 027-244). But then Kimber Kaps jump to over $25 (p/n 027-888) and AudioCaps go up to almost $50! (027-740). How does one choose? I hope not by "you get what you pay for" logic... <gulp!>

The range of choices at PE for 100V non-polarized caps seems much more limited...so I am guessing these will do?: p/n 027-340?

Sorry for all these questions. I am barely competent to solder these things together, and left to choose...will choose wrong, I bet!

fender3x · #10 Post by **fender3x** » Sun Dec 10, 2006 7:51 pm

...and while I am going crazy with the T39 quex... are these the casters that you used, Bill?

http://www.partsexpress.com/pe/pshowdet ... N=19101618

http://www.partsexpress.com/pe/pshowdet ... N=19101618

azaural · #11 Post by **azaural** » Mon Dec 11, 2006 9:39 am

fender3x wrote: Parts express has what look to my untrained eye like several "metalized polypropylene" caps with 10.0 uF values... but the prices vary wildly. Jantzens cost about $4 (p/n 027-932). Solens (p/n 027-568) are almost as cheap. Daytons are $6 (p/n 027-244). But then Kimber Kaps jump to over $25 (p/n 027-888) and AudioCaps go up to almost $50! (027-740). How does one choose? I hope not by "you get what you pay for" logic... <gulp!>

You can use the Jantzen's, Solen's or Daytons. All are fine quality. The Kimber & AudioCaps are "audiophile" snake oil. Definitely not necessary here.

fender3x · #12 Post by **fender3x** » Mon Dec 11, 2006 9:44 am

That's what makes this site so great... separating the legit expenses from the snake oil. Thanks!

Mon Dec 11, 2006 10:33 am

I use Dayton caps. They might not be 'audiophile' grade but they do the job. Like wire once you get past a certain price point the cost/benefit ratio drops at an astounding rate.
I used 3" casters similar to the first ones listed, bought from HD or Lowes they're probably cheaper once you factor in shipping. The recessed Penn-Elcoms are the ones I used, those aren't available from local sources.

Frankenspeakers · Mon Dec 11, 2006 12:35 pm

Polypropylene caps are about the best bang for the buck. Make sure they are non-polar. Stay away from the plain electrolytics there really is a difference betweeen those and poly caps. Anyone that is selling you a cap for $20.00+ may be selling snake oil, anyone who selling a cap for more than that is definatly in the snake oil business. As an aside: the old(er) timers will remember a thread posted last year about PE selling some electrical outlets and plugs for nearly $100 each! For a freaquin plug! The hardware store sells hostital grade plugs for $20, but it's amazing what the audiophiles will pay for common electrical items! (To go with the chrystals to hang on your speakers to 'lift the veils' and improve openness!) C'meere, I's gots youse a schweet deal on this 'ere Brooklyn bridge, papers 'n all- wot 'choo say?

fender3x · #15 Post by **fender3x** » Mon Dec 11, 2006 1:39 pm

Thanks, Bill, now I have the info to complete the order. I was particularly concerned about the electonic parts...but I think I have it now. For other noobs, I have added part numbers to the x-over pic that I posted earlier at Frapper.

Would be grateful to have some tech-savvy person check it to make sure that I did not transpose a number or something

Here's the link:

http://www.frappr.com/?a=viewphoto&id=1 ... myphotos=1

I appreciate all the posts tryin' ta ejucate me!

BillFitzmaurice.info

4 & 8 ohm crossover in a T39

4 & 8 ohm crossover in a T39

Re: 4 & 8 ohm crossover in a T39