1st order crossover capacitor value question

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Guest101

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My question concerns a pair of 1976 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. (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.
http://www.parts-express.com/pe/showdetl.cfm?partnumber=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?
http://www.parts-express.com/pe/showdetl.cfm?partnumber=270-018
If yes what xover capacitor value?
 
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What about if I use the existing 8 ohm cone tweeter? The existing 2uf cap seems like it crosses it over way to high. More like 10khz and not the 3khz according to the stated Altec specs. This is according to all the on-line xover calculators for a 1st order crossover design.
 
Is the tweeter actually 8 ohm? What's the VC resistance your multimeter reads? If everything is original, I'll trust the Altec engineers. Doing some measurements could confirm if the crossover point is correct.

If the xover is 3000Hz, and your actual crossover is close to 10000Hz, the speaker should lack of many frequencies, and you won't need a microphone to check that, it should sound bad. If the speaker sounds good, the crossover is correct with that 2uF cap, and I don't think a 1976 woofer can reach 10000Hz easily... So the tweeter impedance is probably more than 8 ohm, what lowers the xover point...

When changing the tweeter, if the new tweeter is approx the same impedance than the old one, I'd keep the same cap value, and do some testing adding some capacitance, 1uF more, and listen.

If the new tweeter is half the impedance, I'd double the cap value (after checking the tweeter can be crossed at that freq.). If the new tweeter doubles the impedance, half value cap, and listen. And tweak it later. If you hear too much mid-range, reduce the cap value. If mid-range is too low, increase the cap value.

I use to connect some wires from the tweeter, with some alligator clips, outside the cabinet, so I can change the capacitor, add caps in parallel, and so, while listening.
 
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Both of the tweeters listed are 90dB and the model 1 are 84dB total. The replacements may be a bit hot.
Also, why do you think that running the bass uncrossed is typical of Altec? I have several models and they all have xovers. Maybe this applies to some of the computer type speakers, which aren't really the same Altec company at all.
 
I'm sorry to say that on-line calculators are completely useless. They are written by people who have no actual experience designing crossovers, and are based on simplified math equations that have long since been replaced. Really and truly, you are best off totally ignoring them.

In order to make a good sounding crossover, you need to know the complete impedance curve of the tweeter, not just a single number like, "4 Ohms" or "8 Ohms." Decent tweeters will come with suggested crossover circuits for using them. Or, it is usually not difficult to find an internet design that uses the tweeter you like, which will serve as a guide.

I have done hundreds of speaker designs, and the size of the caps used on tweeters varies from about 2uF up to about 10uF. Also, the presence of an L-Pad, (which is probably not even a proper L-Pad, and is just a potentiometer or series resistor), complicates things. How anyone here can advise you what value to use is totally beyond me. That's a total shot in the dark, and the results are very unlikely to be the best the tweeter is capable of.

-k
 
The impedance graphs from several tweeters I've seen were quite flat after the Fs peak, that's the reason I've suggested to compare the new tweeter nominal impedance with the old one, and modify the cap value with that data, as a rough approx to tweak later "by ear ".

Of course, with the proper measured data, you could design a completely new crossover to get the best of your drivers.

I had success replacing a tweeter in an existing speaker, tweaking the crossover after taking some measurements. The results exceeded my expectation.
 
Have you tried just replacing the 2uF capacitor with a new 2uF capacitor of the same type? ...after 37 years that old cap may not be in the best shape.

There are also lots of people here that get good results using deoxit to clean up old potentiometers.

Do you have any pictures of this speaker and its internals?
 
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And remember, electrical crossover frequency isn't always the same as acoustic crossover frequency..

Old cone tweeters tend to have a boost at resonance frequency, so it's needed to compensate that in crossover design.
 
kyle said:
Both of the tweeters listed are 90dB and the model 1 are 84dB total. The replacements may be a bit hot.
Also, why do you think that running the bass uncrossed is typical of Altec? I have several models and they all have xovers. Maybe this applies to some of the computer type speakers, which aren't really the same Altec company at all.
Click to expand...

My Altec Model 15s and now my Model 1 ran their woofers direct to the binding posts w/o a xover. Both are mid '70s speakers.

And for that matter neither did my Dynaco A25xl speakers. Both the Altecs and Dynaco ran their woofers full range and used its natural roll off. Must have been a '70s thing. Either way much purer for bass/mids.
 
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elnaldo said:
Is the tweeter actually 8 ohm? What's the VC resistance your multimeter reads? If everything is original, I'll trust the Altec engineers. Doing some measurements could confirm if the crossover point is correct.

If the xover is 3000Hz, and your actual crossover is close to 10000Hz, the speaker should lack of many frequencies, and you won't need a microphone to check that, it should sound bad. If the speaker sounds good, the crossover is correct with that 2uF cap, and I don't think a 1976 woofer can reach 10000Hz easily... So the tweeter impedance is probably more than 8 ohm, what lowers the xover point...

When changing the tweeter, if the new tweeter is approx the same impedance than the old one, I'd keep the same cap value, and do some testing adding some capacitance, 1uF more, and listen.

If the new tweeter is half the impedance, I'd double the cap value (after checking the tweeter can be crossed at that freq.). If the new tweeter doubles the impedance, half value cap, and listen. And tweak it later. If you hear too much mid-range, reduce the cap value. If mid-range is too low, increase the cap value.

I use to connect some wires from the tweeter, with some alligator clips, outside the cabinet, so I can change the capacitor, add caps in parallel, and so, while listening.
Click to expand...


Tweeter resistance is 5.9 and 6.1 so they probably are 8 ohm impedance.
They released a Series II which changed the xover from 3khz to 3.5khz, probably to lower the peak of the cone tweeter. The 2 versions are the same with the series II having an additional 130 cu in of cabinet space and greater efficiency.
I've recently thought about replacing the tweeter w/ a semi-modern Vifa D19TD dome tweeter and cross it at 3.5khz.
 
bazz_77 said:
And remember, electrical crossover frequency isn't always the same as acoustic crossover frequency..

Old cone tweeters tend to have a boost at resonance frequency, so it's needed to compensate that in crossover design.
Click to expand...

Yes I hear the peakiness and it bothers me.
 
elnaldo said:
The impedance graphs from several tweeters I've seen were quite flat after the Fs peak, that's the reason I've suggested to compare the new tweeter nominal impedance with the old one, and modify the cap value with that data, as a rough approx to tweak later "by ear ".

Of course, with the proper measured data, you could design a completely new crossover to get the best of your drivers.

I had success replacing a tweeter in an existing speaker, tweaking the crossover after taking some measurements. The results exceeded my expectation.
Click to expand...

Depends on inductance, ferrofluid, etc.

But, at any rate, the Fs region is generally right where the crossover is functioning. What the impedance is above that is fairly immaterial.

-k
 
ken kantor said:
Depends on inductance, ferrofluid, etc.
But, at any rate, the Fs region is generally right where the crossover is functioning. What the impedance is above that is fairly immaterial.
-k
Click to expand...
I've alway thought it best to crossover a tweeter at 2x the Fs to make sure that the resonant frequency is rolled of below the xover point. At least that is what I do.
 
I'd say "2 octaves " instead of "2x".. Anyway, that "safe zone " depends of the xover slope. Manufacturers use to suggest the lower crossover point.
 
mjcmt said:
I've alway thought it best to crossover a tweeter at 2x the Fs to make sure that the resonant frequency is rolled of below the xover point. At least that is what I do.
Click to expand...

In an ideal world, you are correct. However, there are three mitigating factors.

First of all, most dome tweeters have a resonance between 2.5 KHz and 3.5KHz. Doubling that would be far too high to take the woofer in a 2-way design.

Second of all, even if you have a midrange driver, or extended range woofer, crossing over at 5 KHz, or above, is going to lead to terrible interference patterns, and poor imaging.

Finally, a tweeter with a significant impedance peak at 3 KHz is still going to play havoc with a low-order crossover at 5 or 6 KHz.

When I have done 1st order crossovers on tweeters, it has >always< been with FF tweeters, because the resonance is highly damped. FWIW.

-k
 
ken kantor said:
In an ideal world, you are correct. However, there are three mitigating factors.

First of all, most dome tweeters have a resonance between 2.5 KHz and 3.5KHz. Doubling that would be far too high to take the woofer in a 2-way design.

Second of all, even if you have a midrange driver, or extended range woofer, crossing over at 5 KHz, or above, is going to lead to terrible interference patterns, and poor imaging.

Finally, a tweeter with a significant impedance peak at 3 KHz is still going to play havoc with a low-order crossover at 5 or 6 KHz.

When I have done 1st order crossovers on tweeters, it has >always< been with FF tweeters, because the resonance is highly damped. FWIW.

-k
Click to expand...

Not necessarily true. Many dome tweeters have resonant frequencies of 1100-1500hz. And the Vifa d19td has resonant frequency in the 700hz range.
 
mjcmt said:
Not necessarily true. Many dome tweeters have resonant frequencies of 1100-1500hz. And the Vifa d19td has resonant frequency in the 700hz range.
Click to expand...

Some do, for sure. Various companies have made them for me over the years, including Vifa.

Ask me how many d19's I've blown out trying to get them below 1,500 Hz.

Don't ask.....

-k
 
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