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ZirconiumZephyr 30th August 2009 03:22 AM

Renovation Challenge
 
3 Attachment(s)
Hello all

As a complete newcomer to DIY audio having just finished a gainclone amp the other week, I'm looking to my next challenge.
I have an old pair of handmade speakers from 20 odd years ago running in my room atm and am wondering if there is anything I can do to rebuild them.
Ideally I'd like to reuse the drivers only and start again from scratch if it seems worth it. I'm quite happy to keep the current ones as the sound is still miles ahead of my plastic computer speakers ;)

Anyway, onto the details.
The speakers have 3 drivers, 1 Magnavox ?? 8 ohm woofer and 2 identical Magnavox 15 ohm, (3tc Mk 11) tweeters. That's all I could find written on the drivers themselves. I have little idea about the crossover network but it seems very basic; all +ve terminals wired together with 2 ?capacitors? between the woofer and 1st tweeter and all -ve terminals wired together. Excuse my terminology, trying to learn by doing ;)

So, my question is, what can I do to make a nicer pair of speakers? Nothing too big (though thin floorstanders may work) and not too complicated :) Would simply designing a new x-over improve them?? I'm open to practically any suggestion as this is simply a learning experience.

To finish, some pictures of the insides, hopefully they can explain anything I've left out.

Thanks in advance

planet10 30th August 2009 04:03 AM

Ignoring that they are made of MDF, it looks like the execution on the boxes isn't bad.

Route off the front protrusion, add some bracing, treat the cones on the drivers, play with the XO.

dave

wintermute 30th August 2009 04:33 AM

wow looks like a little brother to my 3ways. The box construction method must have been common in the late 70's / early 80's. Mine originally had Plessey 12" woofers (quite efficient, but not particulary great in bass extention). with a 5" plessey midrange with a wizzer cone... they now consist of a vifa 10" phillips Dome mid and phillips dome tweeter, with a jaycar 3 way crossover... completely non-optimal, and hopefully soon to be usurped by my MTM project. Bit like grandfathers axe really, only the box remains. The other thing is I finally painted the boxes after 25 years.. nice flat black ;)

anyway here is a pic of one of them ---> http://www.diyaudio.com/forums/showp...1&postcount=16 you might see the similarity (if you ignore the drivers).

One thing I suspect is that appart from the recesed baffle causing diffraction effects, the Mids being set back with that flare could quite possibly be causing some nasty diffraction effects.. I know it was a bad Idea when I tried it for the tweeter of my MTM prototype. flush mounting was definitely the way to go there.

If you can measure the T/S params of the woofers you will probably find that the tuning of the box is far from optimal too... the port looks like mine was. very large diameter and very short... not what you would expect :)

Tony.

rjb 30th August 2009 04:35 AM

As Dave says, the boxes seem to be better built than average.
That bass speaker is not too bad, I'd keep it. You would need to measure its parameters before designing a new box, but I suspect it would be of similar volume to the present one. In fact you could use that volume in a new shape if you prefer.
The mid-range tweeters are not up to modern standards, particularly at the top end. However the fact that only a capacitor was used as a cross-over, and the bass run full range, indicates they are all smooth enough in the main listening frequencies. They probably sound a bit mellow compared to modern designs.
If you can hear higher frequencies, why not add a tweeter.
That means a better (three way) cross-over of course.
Beware, once you start down this track, you will get hooked on experimentation.

ZirconiumZephyr 30th August 2009 05:59 AM

thanks for the quick replies :)

so, unfortunately at the moment I don't believe I have to knowledge or expertise to measure the parameters of my loudspeaker so perhaps keeping the same volume would be the way to go.

The best option to me sounds like rebuilding the enclosure, or at the very least redesigning the baffle and probably adding a tweeter. When you suggest this do you mean instead of the two mids or as well as? Then lastly a good 3-way crossover.

Are there any resources one could recommend for a total newcomer to speaker design. Apart from the very basics about what different parts do, I'm mainly clueless about how to design crossovers (i.e. what frequencies to choose) and choosing drivers. Could anyone recommend a good tweeter, cheap as possible really as I'm on a student budget ;) Also, what did you mean by treating the cones?

ZirconiumZephyr 30th August 2009 06:07 AM

just after a quick look, would this tweeter and xover combo work?
http://jaycar.com.au/productView.asp...T&SUBCATID=459
http://jaycar.com.au/productView.asp...T&SUBCATID=534

wintermute 30th August 2009 08:45 AM

I've used the cheaper Jaycar 3 way crossovers in the past and they were pretty terible. The ones I have (jaycar no longer sell something like them) were rather expensive I think about $70 each (around 20 years ago), they have polypropylene capacitors for all but the biggest (which is an electrolityic). However even with expensive pre made crossovers, they are a BIG compromise. The chances of them matching your speakers, no matter what you have is very low.

You will probably end up with something that sounds better than the original, but it will be far from optimal. However this is how I started, and I've been listening to mine (with various mods) for the last 20 years.

The one you linked to is a first order (only 6DB/octave) certainly I guess crossing at 5Khz to a decent tweeter will help the top end, and the original woofers were effectively running full range, so It probably won't be any worse than what you had, provided the tweeter can handle a first level crossover (which looking at the specs it would seem it can at the freq that crossover operates at.

I've not played with ribbon tweeters before so not sure about them.

Anyway If you want to do something on the cheap what you have linked to will probably fit the bill, but don't be too surprised if the result is not as good as you would have hoped...

Tony.

ZirconiumZephyr 30th August 2009 09:36 AM

after a bit more than 5 seconds research I'm liking the look (and price) of the HiVi range of drivers over at darcher audio.
As for a crossover, how would I go about making my own. The main problem I can see encountering is not knowing the parameters of my woofer. I may be able to test them if I can source some equipment. As I have my speakers running through my computer, can I just generate a sine wave from there to test them?

Thanks everyone for your help btw.

Moondog55 30th August 2009 10:52 AM

I can think of a couple of things to do if you have a router or can borrow one.

Router a half round on the front of the baffle using a bit with a guide wheel after making sure all the screws are out.

put felt on the baffle itself, put an inductor on the woofer, approximately 0.47mH, to roll off the woofer a little earlier and use a tweeter that can play down to 3k, using a second order XO use it as a sealed box; try it first by blocking the hole with a plug of an old sock stuffed with fiberglass

Moondog55 30th August 2009 11:04 AM

How big are the tweeter holes?

If you want to go 3-way you will find a lot of helpful advice on this forum, also a lot of people who will say "Don't try it "

You will learn a lot if you start, but be warned:-) speaker building is frustrating and addictive


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