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Large Advent - Crossover change for matched 4CH setup?

Macdude

Macdude

Hi /on/ Fidelity
Here's a strange question. I have a decent quadraphonic system and for years I've been using 4 Large Advent speakers with it. Sadly, they are not matched. The front pair is a later version with the drivers spaced closely together. Their crossover is the NLA, simple design. The rear set is OLA version, with farther driver spacing and a more complex, earlier crossover, which I rebuilt years ago. The woofers are all the steel frame style.

Problem: The rears sound terrible. The bass response is poor and they sound lower in volume. I've read the NLAs may be a slightly more efficient speaker, which would explain the volume difference.

Here is my question. A friend has offered me a third pair, however, they're also the OLA style like my current rears. Let's say I re-built the crossovers in the 4 OLA's to the simple design used in the NLA, with the 13uF cap/1 Ohm resistor on the tweeter and the .3mH inductor on the woofer.. eliminating the switch and putting them permanently in "Normal" mode. Would this be a no-no? Am I trying to interchange drivers and crossover characteristics that shouldn't interchange? Will the drivers being farther apart not agree with the later crossover design? Am I really over-thinking this?
 
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Some really good questions for which I do not have the answers. I would get the pair from your friend and compare them to your current pairs of speakers. If they sound the same as your OLA, then modify one pair to match the NLA and see the result. Of course, do not throw away the old crossover components, in case the modification does not work out. Have you replaced the foam surround on your original pair of OLA's? Bad foam could explain some of the poor sound. Good luck with your project.
 
I did do the foam on both front and rear pairs back when I got them. The other pair is coming for sure, so maybe I will do what you suggest and change one set.
 
I did see that. I guess to sum it up, what I should have asked was: If I’m going to use 4 of the OLA tweeters, will they mind being driven by the NLA crossover? As I understand it, the OLA and NLA tweeters aren’t completely the same. Maybe I’m splitting hairs and it won’t be that noticable.
 
Macdude said:
If I’m going to use 4 of the OLA tweeters, will they mind being driven by the NLA crossover? As I understand it, the OLA and NLA tweeters aren’t completely the same. Maybe I’m splitting hairs and it won’t be that noticable.
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Shouldn't be a problem. The NLA tweeters are crossed at a higher frequency (1.5 kHz vs 1 kHz) and have ferro-fluid cooled tweeters - both of which for added power handling.

Using the higher xover with the originals will be "easier" on them.
 
No you should not use the simpler crossover. The OLA XO was second order
because the tweeter did not have ferro fluid resulting in a peak in the input
impedance and far more motion at lower frequencies that will cause driver
failure if you use the simpler crossover. Use the OLA crossover.

Also from memory the fundamental resonance of the OLA tweeter is about 800 Hz
whereas the NLA is about 1800 Hz give or take. The OLA tweeter needs the
inductor to more rapidly attenuate program content that will excite the 800 Hz
resonance. The tweeters are obviously very different.
 
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I am surprised at the poor bass performance from you OLA's as compared to the new. Have you reversed the old and new front to back? Exact spot for exact spot?
I have OLA's. I can't describe the bass performance as poor.
 
I guess the answer isn't so simple.

I did swap them front to back and it still sounded lacking in low response, however I didn't physically move them around. I'll try that out. Even in my last place I always found myself moving the fader to the rear to compensate for their lower output, so I didn't figure the placement had much to do with it, but I'll give it a shot.
 
How much of that ferro fluid still exists after all these years? Is it like the fluid filled earpads in my Koss headphones (aka dust in 2019)?
 
Ferro fluid tends to harden as you've probably seen in many threads here where
it has to be replaced when disassembly is possible. NLA tweeters are glued
together and I've not seen anyone replace the FF in them.
 
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OLA's and NLA's both have a system resonance in the low 40s usually 41 - 42 Hz
when the correct foam and original spider are used with a Qtc of about .8. Every
properly done woofer in system that I've seen has measured roughly the same and should
sound the same. The OLA has a larger woofer inductor which makes them sound
better with a bit more baffle step than the NLA.

My guess is that the incorrect foam was used, and/or that the correct rest position
was not established resulting in the coil being in a weaker part of the gap. It is also
possible that the spider has lost the correct rest position.
 
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Makes sense. I can’t say for sure, but the foam may have been generic MAT Electronics foam. It’s been so long. The odd thing is I used the same foam to do the other pair and they sound great. I swapped woofers front to back and the OLA still sound lackluster.

I see the woofers inductor value changes between crossover designs. Could I be hearing that?
 
So by swapping woofers you've determined that it is the crossover or box.
I wonder if the inductors have failed for some reason, some of the connections
are crimped and can become corroded. It would be good to solder them all.

How are they stuffed, both with fiberglass or?

Did you recap them and test the resistors?

Have you tried moving the boxes to the fronts so that they are on the same
amp channels?
 
Oh .... wait ... this is a true Quadraphonic system?
If so the rears are supposed to play at a much lower level and I
would not expect them to produce much bass. Often rears are
smaller less capable speakers because not much content goes
to them.
Or are you setting the system so that the rears get the same signal
as the front, some sort of 2ch mode instead of 4?
 
In my opinion, in a quad system, all speakers should be identical, regardless of what literature of the day stated. That was all marketing nonsense so people would be convinced all they had to do was buy two new speakers and use them with their existing pair.

My vintage system has four original Advent stacks and it sounds terrific. Trying to mess around with changing components and other things to try to match them up is just as likely to result in disappointment than not.

Also, some matrix quad records were recorded with more ambience material in the back but not most, by far. That is why you need full range in all speakers.

Doug
 
The problem follows the cabinets. Using my poorly trained ear, upper end response does appear to match the fronts, but the bass seems to roll off higher than the woofers in the front (NLA) pair. It's not the woofers, as I swapped them around between cabinets. Maybe the inductors went bad, but it seems unlikely. I'll have to pull them apart and check them out. I'm not ruling out that I may have messed up the crossover work back when. I replaced the caps with Dayton brand and also changed the resistors. I would have used the original values. I should mention that the front set (NLA) have never had their crossovers rebuilt.

This is a true quad system centered around a Sansui QRX-7001. These tests are being made in 2CH + REAR mode, so the same level and response is delivered to all four channels. The poor performance of the rear makes the Hall and Surround modes somewhat worthless without fading a little to the rear. I have about 60 quad LPs and a few Q8 tapes, as well as a fully functioning CD-4 demodulator, so this is occurring with discrete 4ch material as well. Unlike modern surround systems, I was always under the impression that 4 identical speakers were to be utilized in these legacy configurations.
 
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