1st order xover question

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My question concerns a pair of Altec Lansing Model 1 acoustic suspension 2-way bookshelf loudspeakers.

It use a 8" woofer running full range w/o any xover, in typical Altec design.
The 2" paper cone tweeter w/ metal dust cover uses a simple 1st order xover w/ a single cap and variable L-pad.
Both drivers are 8 ohm and Altec's spec say the xover is at 3 khz.
The single cap is 2.0 uf. All the on-line crossover calculators figure about a 6 uf cap. The reason for my question is that I would like to try a soft dome tweeter. (Or at least replace the cap because while the original tweeters work they sound distorted and harsh to me.)

I've found a budget dome tweeter that is a drop in.
Goldwood GT-510 1" Soft Dome Tweeter 270-176
Should I replace the xover cap with a 2uf or go with the calculators that say about 6 uf?

Or is there any benefit to replace with this paper cone tweeter?
Goldwood GT-25 4" Cone Tweeter 270-018
 
How sure are you that the existing Capacitor is actually 2.0uF?

When you say Altec says the crossover is 3000, who says that? Did you look that up on-line? Did you contact Altec? Are you sure they new specifically what Model you were speaking of?

According to the Crossover Calculator I used, 2uF crosses to 8 ohms at 9500hz. That seem unlikely.

How sure are you that the existing tweeter is really 8 ohms? If the tweeter were 16 ohms, for a 3khz crossover, then the capacitor would need to be about 3.3uF. WIth a 16 ohm tweeter, a 2uF capacitor would give a crossover of about 4500hz.

Do you have an ohm meter to measure the Tweeter. The DC Resistance is about 2/3rds to 3/4ths the rated impedance. That might give us a clue.

Again, does the capacitor say clearly and indisputably 2uF?

I think if you use a new tweeter, you use the calculated value appropriate for that tweeter. It is best if you can find a tweeter that has a full SPEC Sheet. That way you can design the crossover to the specific impedance of the tweeter at the crossover frequency.

Dayton and other high brands typically do have full Spec Sheets available. Goldwood ... not so much.

The Dayton DC28F-8 1-1/8" Silk Dome is popular, and reasonably priced, though certainly there are many others. Here is a link to the full spec sheet -

http://www.parts-express.com/pdf/275-070s.pdf

If you look at the Impedance Graph, at 3khz, the actual impedance is about 6.2 ohms.

Just a few thoughts.

Steve/bluewizard
 
How sure are you that the existing Capacitor is actually 2.0uF?
I looked at both crossovers and they each have a 2uf 50v cap

When you say Altec says the crossover is 3000, who says that? Did you look that up on-line? Did you contact Altec? Are you sure they new specifically what Model you were speaking of?
Altec Lansings spec sheet say 3khz
http://www.lansingheritage.org/images/altec/catalogs/1975-home/page09.jpg


According to the Crossover Calculator I used, 2uF crosses to 8 ohms at 9500hz. That seem unlikely.
Exactly my sentiments

How sure are you that the existing tweeter is really 8 ohms? If the tweeter were 16 ohms, for a 3khz crossover, then the capacitor would need to be about 3.3uF. WIth a 16 ohm tweeter, a 2uF capacitor would give a crossover of about 4500hz.
That is what I came up with. The measured resistance of each tweeter is 5.9 and 6.1ohms

Do you have an ohm meter to measure the Tweeter. The DC Resistance is about 2/3rds to 3/4ths the rated impedance. That might give us a clue.
I've read specs on other tweeters and it seems like many have 6.2 ohm resistance for a 8 ohm tweeter

Again, does the capacitor say clearly and indisputably 2uF?
Yes clearly with out a doubt. I'll take a photo tomorrow.

I think if you use a new tweeter, you use the calculated value appropriate for that tweeter. It is best if you can find a tweeter that has a full SPEC Sheet. That way you can design the crossover to the specific impedance of the tweeter at the crossover frequency.

Dayton and other high brands typically do have full Spec Sheets available. Goldwood ... not so much.

The Dayton DC28F-8 1-1/8" Silk Dome is popular, and reasonably priced, though certainly there are many others. Here is a link to the full spec sheet -
http://www.parts-express.com/pdf/275-070s.pdf
I've thought about that too. I want to make sure it has 4" dia so I can replace original tweeters if I sell later on.
If you look at the Impedance Graph, at 3khz, the actual impedance is about 6.2 ohms.

Just a few thoughts.

Steve/bluewizard

Steve, thanks for all the thorough questions I hope I answered them all for you.
 
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I never was a fan of Cone Tweeters.

If you can find a drop-in replacement in a dome tweeter, I think it would be an improvement.

How are the surrounds holding up on the Tweeter? Any signs of wear, tear, or deterioration?

Can we get a date on these, any idea when they were manufactured?

Can you tell what kind (electrolytic, non-polar electrolytic, Film) capacitors? If any kind of electrolytic and if they were manufactured a few decades ago, then likely they have deteriorated. A Poly or Mylar or Metal Film Capacitor would be a big step up and last for many more decades. Quality capacitors need not cost a fortune, but I recommend only using the non-polar electrolytic if you are on a starvation budget. Otherwise, the various film capacitors are you best choice.

Just a few more thoughts.

Steve/bluewizard
 
Thanks Dissi.

I can't find it now but in my research I noticed the peak of paper cone tweeters. That is the irritation I must be hearing. I was wondering if a 6 db roll off of a 1st order crossover can lower it enough.

I looked at the Phenolic Ring tweeter to keep my Altec vintage, yet upgrading, and I like the look. As I began my search I got swayed by soft dome tweeters because I'm familiar with their sound.

The Phenolic Ring tweeter's resonant frequency is 1084 so I can easily cross it over at 3khz or 3.5khz. Would you consider 5.5uf-6.5uf cap to cross over it? Or where do you recommend I cross it at?
 
I never was a fan of Cone Tweeters.

If you can find a drop-in replacement in a dome tweeter, I think it would be an improvement.

How are the surrounds holding up on the Tweeter? Any signs of wear, tear, or deterioration?

Can we get a date on these, any idea when they were manufactured?

Can you tell what kind (electrolytic, non-polar electrolytic, Film) capacitors? If any kind of electrolytic and if they were manufactured a few decades ago, then likely they have deteriorated. A Poly or Mylar or Metal Film Capacitor would be a big step up and last for many more decades. Quality capacitors need not cost a fortune, but I recommend only using the non-polar electrolytic if you are on a starvation budget. Otherwise, the various film capacitors are you best choice.

Just a few more thoughts.

Steve/bluewizard

Speaker manufactured in 1976. Tweeters look like there is some wear, surrounds are good but paper cone a bit distorted.
Solen makes caps inexpensive caps. What you say? Any recommendation of other brands of metalized film caps?
 
Re: ' Both drivers are 8 ohm " - no they're not, read my sig. Without measurements, it's all guesswork...
PeteMcK,
There's no way I have the ability to do this. I'm just trying to make a little improvement to the tweeters of my 1976 Altec Lansings. Not really trying to redesign them. I am concerned that the stock 2uf cap is to low for the stock tweeter. While I'm at it maybe switching the original cone tweeter for a dome or Phenolic tweeter, w/ the correct cap for a 3khz 1st order xover.
 
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The 2" paper cone tweeter w/ metal dust cover uses a simple 1st order xover w/ a single cap and variable L-pad.http://www.parts-express.com/pe/showdetl.cfm?partnumber=270-018
The L-pad presents an opportunity to test for issues with the tweeter at resonance. As you attenuate it, the impedance peak will have less interaction with the capacitor.

What you basically do is disconnect the woofer, turn the L-pad to full (0dB), and listen for the problem in the tweeter. Now turn the L-pad down and the amplifier up until it's at the same level as before and see if the problem is less obvious.

If this proves true, you can use a filter to remove the peak altogether.
 
I think that's too high and 3.3 uF should be right, but you have to try. Another possibility to consider: The distortions could be caused by bad contacts (oxidation) in the L-pad.

Is this the speaker we are talking about? SEVENTIES STEREO: Altec/Lansing Model One
Yes this is it.
I did deoxit the variable L-pad and speaker binding posts. I think what I'm hearing is the ever present peakiness of the paper cone tweeter.
Maybe cap replacement should be my first step before I throw out the baby w/ the bath water. Should I replace w/ a 2uf or 3.3uf? Would you use Solen or another inexpensive brand?

The L-pad presents an opportunity to test for issues with the tweeter at resonance. As you attenuate it, the impedance peak will have less interaction with the capacitor.

What you basically do is disconnect the woofer, turn the L-pad to full (0dB), and listen for the problem in the tweeter. Now turn the L-pad down and the amplifier up until it's at the same level as before and see if the problem is less obvious.

If this proves true, you can use a filter to remove the peak altogether.

I did deoxit the variable L-pad and speaker binding posts.
As I adjust the L-pad it gets bright and clearer and as I lower it gets muffled, but there is the present resonant peak and raspyness. I was thinking of replacing the cap, So maybe this should be my first step.
 
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As I adjust the L-pad it gets bright and clearer and as I lower it gets muffled, but there is the present resonant peak and raspyness.
Did you disconnect the woofer for this test? If you didn't then is it possible this is a woofer issue.

I was thinking of replacing the cap, So maybe this should be my first step.
The quality of the cap may make a small difference. It sounds as if this isn't a small issue? It is sometimes possible to get surprisingly good sound out of a tweeter connected through even a cheap electrolytic capacitor.

Regardless it would probably be best to apply a filter to tame the resonance impedance peak before using a larger value of capacitor.

The better option (I'm assuming) would be to make the tweeter second order and cross the woofer.
 
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It's very likely that the harshness comes from a peak in the tweeter.

I suggest you look at this polydome Vifa D19TD-05 tweeter. It's only $16.88 at PE. It is an excellent candidate for 1st order. You may not even need a LCR.

Thanks for the confirmation as I just put that one at the top of my list. What cap value is usually used with this tweeter in a 1st order crossover?
What do you mean by LCR? I only know of it as Left/Center/Right speakers and the Altecs in this system is a vintage analogue system.
 
Did you disconnect the woofer for this test? If you didn't then is it possible this is a woofer issue.

The quality of the cap may make a small difference. It sounds as if this isn't a small issue? It is sometimes possible to get surprisingly good sound out of a tweeter connected through even a cheap electrolytic capacitor.

Regardless it would probably be best to apply a filter to tame the resonance impedance peak before using a larger value of capacitor.

The better option (I'm assuming) would be to make the tweeter second order and cross the woofer.

I did not disconnect the woofer but with my ear at the woofer it is clear and smooth, with my ear at the tweeter there is the peakiness. The more I listen there is added sibilance and edge with vocals so I think that the peak is the problem I'm hearing. Other than that the speakers sound very nice and involving so I don't think I want to mess w/ the woofer.
 
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