Reverse Engineer

[AlexQS]

Hello, I'm new here, and have enjoyed some reading. Such a wealth of knowledge.

I have a pair of speakers that my Dad built in the mid 1970's. They were a proven design of which he followed the blueprints and purchased specific driver according to some stereophile magazine (I think). He has talked before about them being pink-noise tested (but I don't know exactly what that means.) He said that he "hand wrapped the cross-overs" too.

They are a three-way. Transmission-Line. 8" drivers, 4" mids, and a tweeter.

There are two knobs on the back that adjust the crossovers, and 35 year old pots would sound noisy and scratchy if they ever needed to be adjusted.

Sometimes the tweeters sound a little crunchy (on certain songs). I had once unscrewed the speaker line input to see if I could determine which was posative and negative, as the old markings had worn off years ago. The wire inside the cabinet looked green with oxidation.

The thing I love about these speakers is that they have a very full range responce, and things like the human voice come through especially well. Excellent imageing, despite the fact that they are closer than I would like to the rear wall as they are a transmission line.

My thought is to replace all the drivers, cross-overs, etc; to refresh them and make them like new. The cabinets themselves I think are excellent (very heavy) and they also have a lot of sentimental value to me.

Would I lose their "magic" if I were to change anything. I had once owed some 2 way speakers that had a soft-dome tweeter, and I thought they sounded good -is this a more modern thing...possible upgrade?

Getting to the question here: Dad had once said that the 8" driver and the distance of the travel of the speaker cone was all carefully planned to match the volume of the box, and the distances in the transmission line (so I don't want to mess that up)

Could I measure the inside dimention of the top cabinet and choose a more modern mid and tweeter. Find a 8" woofer for the bottom cabinet that would work with the cabinet (having the right distance of cone travel). Have the right cross-over to make it all magic and well balanced again?

Another part of my goal, besides replacing worn out drivers, and oxadized wires inside,... is to at a later date build a matching center channel,.. matching meaning that it would at least use the same tweeter, perhaps even an MTM that I could put on it's side under my TV, using two 4" that match the Mids in the aforementioned speakers. I think I could build a cabinet if I had the proven design. I think at some point I'll need to learn to build a cross-over (am open to book suggestions)

I think one of my goals is to have a fairly high SPL rating (is that a good goal?)

I didn't mean to write a book.... Any other info you may want? I have not yet taken the speakers out to measure the inside of the cabinet. Outside dimentions are roughly 12 x 12 x 12 -housing the tweeter and mid, that sits on top of a 12 x 12 x 36 tall which has 8" and transmission line. Then there's a big cabinet surrounding those two inner cabinets making it one. I hope these word of mouth details from my dad who built them years ago help you to envision what I am working with.

What would you do to salvage these old cabinets, and then to build a matching center in the future. Driver selection and crossover is where I need the most help.

Thanks in advance for any suggestions.

[AlexQS]

I made an error in descriptiion above. Outside dimentions are 12 wide by 12 deep by 48 tall, that's all I know for sure about dimentions. The insides are obviously then a little smaller. I am fairly certain that there is an enclosure for the mid and tweet, then a seperate enclosure for the woofer at the top of the transmission line. All enclosed by another cabinet, this additional layer making it heavy and sturdy.

[mdocod]

Sounds like you have a lot of work cut out for you, but don't fret. Hopefully a TL guy can help you determine what driver characteristics are necessary to work well in the box, and then once a woofer is chosen, a mid range and tweeter that would complement the woofer well can follow, followed by an x-over.

[GloBug]

Pictures!

[bear]

Post an image, especially one that shows the drivers without a grille cloth if at all possible.

What is wrong with the woofers now? The midrange? You said the tweeters were not sounding right?

The noise on the adjustment pots is probably not important IF they are wirewound with a power rating suitable for this application, probably 10watts or more... easy to tell when you look at it.

High SPL? not likely. The SPL is a function of max power rating and the sensitivity of the speakers. Most 8" LF woofers are not going to be terribly high power AND high sensitivity at the same time, if any.

Change the sound, yes changing the drivers will change the sound.

It may be possible to identify the original drivers.

Change only the capacitors in the xovers (assuming they are correctly done for the
drivers you have).

Using Pink noise is a way of testing the frequency response - your Dad probably used an "RTA" or Real Time Analyzer, which is bargraphs centered on various frequencies. Today you can download much more sophisticated test software that runs on your computer (good idea).

There are threads here on TL and variations. Many websites on that design too...

But start with posting some pix of the speakers and you'll likely get more information back than you can handle...



_-_-bear

[scott17]

It would be good if you could accurately provide internal dimensions and possibly photos of the cabinets. Are the cabinets constructed of standard 3/4" material? Based on the info so far, it looks like you have a great set of cabinets that will be relatively easy to update.

[planet10]

Originally Posted by GloBug

Pictures!

Yes, pictures!!

[AlexQS]

Working on the photos. Thanks guys!!

[AllenB]

This has mostly been said, but any changes you make will likely be significant, and not always positive. You mentioned that this was a proven design and that you felt it sounded good.

Pictures will help the experts here decide on the best course of action and I would advise you to take any driver and crossover changes slowly, preferrably one at a time, for example. There is little hope of both solid and fast results if you are unable to take your own measurements.

[phivates]

You're in the Audionics diaspora.