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OLA crossover change in mid 70's


Brad1234
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I have 4 OLA's I am getting ready to recap and found out they changed the crossover between my 2 models and cant figure out why?  Older Advents have 2 16uF caps, while the 1975 or so later ones have a 16uF and an 8uF?  I recapped a pair previously with the double 16uF caps and they sound amazing!  Is there any reason to not recap the later model with dual 16uF's?  Everything else is the same in the xover, and it sounds amazing that way. Any advice?

On a related note, should I stuff the newer advent that had foam with fiberglass insulation?  I have a bunch of it sitting around.  I posted this on FB Advent group as well, sorry for double post. 

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PeteB is an expert on this. Maybe he’ll chime in. When I restored some OLAs I used the latest xo design and replaced the foam with fg. Also used better inductors per Pete’s advice 

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PeteB is an expert on this. Maybe he’ll chime in. When I restored some OLAs I used the latest xo design and replaced the foam with fg. Also used better inductors per Pete’s advice 

PS: found the link http://baselaudiolab.com/ADVENT_LA_XO.html

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6 hours ago, JKent said:

PeteB is an expert on this. Maybe he’ll chime in. When I restored some OLAs I used the latest xo design and replaced the foam with fg. Also used better inductors per Pete’s advice 

PS: found the link http://baselaudiolab.com/ADVENT_LA_XO.html

Thanks JKent!  I knew I had seen the website for the Advent xovers but couldn't find it. I am planning to swap the foam for fiberglass insulation because I have it and spent around $40 for the roll, so want to use it. It is pricier but better sounding than foam or polyfill. Maybe I can sell the original foam on ebay for $10?!

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I have examples of both versions and to me, there really isn't much difference in sound between the two. I believe Advent switched to the 8uF cap. more to make it easier on the tweeter than to affect the sound. I don't know if Pete agrees with this or not.

 

BTW, the example you show with the green tweeter, assuming that's original, it was made during a relatively narrow window from the middle of 1975 to ,late 1975. The all-metal woofer would be replacements. Nope, see next paragraph. :D

 

Actually, looking at the photos again and assuming they are of the same example, it's kind of a hybrid with a very early crossover and replacement drivers including the green tweeter.

Doug

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Ok Doug, you caught me, good eye!!  The green tweeters are from a beveled walnut cabinet with 2nd gen crossover, I swapped them with the red tweeters from the pre 1974 vinyl wrapped cabinet in the pic. I have a matching set of OLA beveled walnut speakers from pre and post 1974 and wanted the tweeters to match. The green tweeter magnets measure 3" wide so they were built for the large advents, just part of that anomaly era from advent. Are the green tweeters more rare and "valuable" and I should swap them back?  I dont know, the question vexes me. Should I keep the original tweeters with original speakers or does it not matter??

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Aha! the dichotomy is revealed!

The green tweeter thing is this: the green cones were originally for The Smaller Advent speaker, most likely for easy identification because the Smaller Advent had a 2.5" square magnet instead of the 3" magnet of the original Advents and it was much easier for factory people to just look at the cone color to determine which was which instead of judging or measuring the magnet size.

 

In 1975, Advent decided to make all the tweeters the same with 3" magnets and use a crossover change to match the tweeter/woofer efficiencies in the smaller Advent. So, from then on, all the tweeters were the same but Advent had a bunch of green cones left over and they just used them up. So, for a while, both Smaller and original Advents got green tweeters until the green cones were used up and then they both got orange/red tweeters and this may have been random so you could probably get red or green tweeters in both models during this time period

As far as performance, the two are the same. Only the earlier smaller magnet tweeters are different because of the smaller magnet.

The green.large magnet tweeters are certainly more rare but as far as value, probably not much difference. The only real determinant as to what tweeters to put in what speakers would just depend on your level of desire for originality.

Doug

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Thanks Doug for that detailed history! I'm glad to hear the tweeters are the same, their magnets both measure 3" wide. The green color has kind of grown on me now so I have decided to reswap the red and green tweeters back to their original cabinets. Plus it will be kind of cool to have matching OLA cabinet pairs with different color tweeters.  Thanks for all of the help everyone!

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The difference between the 16/16/3 and 16/8/3 crossovers only matters, with regard to the caps,

in the "Extended" switch position.  The speaker is too bright even when set to "Normal", so the 8uF

cap is better because it makes "Extended" less harsh sounding.

I've said this many times, the tiny Red Dot inductors in the 16/16/3 crossover tended to burn out,

there was a service bulletin about testing them.  I suggest replacing them with 18 or 20 ga air core.

Also, Rockwood resistors tend to fail and I always replace them when rebuilding.

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They are the worst!

How are you doing Roy, have not seen you since that Frankenfest years back?

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On 11/27/2022 at 12:42 PM, Pete B said:

How are you doing Roy, have not seen you since that Frankenfest years back?

All good here, Pete. I try to visit this forum on a regular basis. I haven't seen you here much, but have recently consulted a number of your insightful posts in other audio forums.

Regarding Doug's comments on tweeter changes, I will be acquiring a couple of first generation Small Advents as a result of the imminent demise of a brick and mortar shop for which I've been repairing speakers. They have been knocking around the back of the shop for a long time, having convinced the shop owner not to pass them on to the "recycling" guy. The original green tweeters are missing.

Some years ago I noticed an obvious difference in sensitivity between a smaller magnet green tweeter and a larger magnet orange dome version on the bench. Additionally, based on the last orange cone tweeter on the parts shelf, the cabinet holes of my early specimens will need to be enlarged to accommodate later versions of the tweeter. I'm also aware the Small Advent crossover was changed along the way, presumably to compensate for tweeter differences. Do you guys have any other SA experience or schematics to share? I've worked on a number of these over the years, but too long ago to remember specifics.

Roy

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I've also seen those schematics Roy but never kept the links to them.

If I understand you, you want the large magnet schematic?

I'm looking for a pair of SA woofers if you come across some.

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At one time, I knew the changes they made in the Smaller Advents to accommodate the large magnet tweeters and they have been discussed in various threads on forums but I never made note of the changes. Smart, huh?

Doug

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On 11/26/2022 at 11:10 PM, Pete B said:

The difference between the 16/16/3 and 16/8/3 crossovers only matters, with regard to the caps,

in the "Extended" switch position.  The speaker is too bright even when set to "Normal", so the 8uF

cap is better because it makes "Extended" less harsh sounding.

I've said this many times, the tiny Red Dot inductors in the 16/16/3 crossover tended to burn out,

there was a service bulletin about testing them.  I suggest replacing them with 18 or 20 ga air core.

Also, Rockwood resistors tend to fail and I always replace them when rebuilding.

Hi Pete, 

Thanks for the info!  I have heard about the potential problem with the red dot inductor but have not tested inductance before. I have a simple $30 multimeter tht I don't think can test inductance. I have really just done caps and resistors in my amateur speaker restorations.
 

As far as finding a replacement 18-20 ga air core inductor, do you have suggestions on where to shop for them and how to choose/test for proper inductance when installing the new one?  I usually buy from Parts Express for my caps/resistors. 

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Do you plan to just make them 16/8/3 with the new inductor?

The other off white inductor is only used in the decrease position, and doesn't need to be replaced

mainly because that position is useless with modern recordings.

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On 11/28/2022 at 10:22 PM, Pete B said:

I've also seen those schematics Roy but never kept the links to them.

If I understand you, you want the large magnet schematic?

I'm looking for a pair of SA woofers if you come across some.

I'll check for woofers when I return to the shop next week, Pete.

I was referring to a schematic for the large tweeter magnet version, but after reading the old thread Doug found (thanks!) I believe I have enough information to proceed.

Roy

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On 11/30/2022 at 5:44 PM, Pete B said:

Do you plan to just make them 16/8/3 with the new inductor?

The other off white inductor is only used in the decrease position, and doesn't need to be replaced

mainly because that position is useless with modern recordings.

 

On 11/30/2022 at 5:44 PM, Pete B said:

Do you plan to just make them 16/8/3 with the new inductor?

The other off white inductor is only used in the decrease position, and doesn't need to be replaced

mainly because that position is useless with modern recordings.

Hi Pete, I think the normal and increase xover settings are most important. I am about to place an order for caps from parts express and want know if I need to include inductors to replace the "red dot" inductor, and what mH and ohm to buy to replace red dot inductor?

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Build it as 16/8/3 and use this inductor to replace the one with the red top:

Dayton Audio 0.45mH 20 AWG Air Core Inductor Crossover Coil

Part #257-034  

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4 hours ago, Pete B said:

Build it as 16/8/3 and use this inductor to replace the one with the red top:

Dayton Audio 0.45mH 20 AWG Air Core Inductor Crossover Coil

Part #257-034  

Thanks Pete, sorry to add more complications and confusion, but I double checked my 1976 OLA's and they dont have the red dot, they have a different inductor that I think is the 0.45mH inductor based on the Rev 2 xover. Based on the Rev 1 xover for the pre 1974 Advents, it looks like the red dot is a 0.8mH inductor that I need to replace in my pre-1974 OLAs?  See pics of my 2 sets of Advents w Rev 1 and Rev 2 xovers. 

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Hi Pete, I had not even thought of converting the Rev1 xover to a Rev 2 with just changing the 16 to 8uF cap and changing the red dot to 0.45 mH inductor, is that all it takes?  Is the Rev 2 xover better, I guess it probably is since it is a newer design. I was just planning to restore like with like between each pair, but if the newer xover is superior, I can convert my pre-1974 pair to new xover. 

Does the white dot inductor 0.2 mH need to change to a 0.065 mH as well to fully match Rev 2 xover?  I think you said that inductor is only needed in the decrease position, which is not really ever used. But while I am elbow deep in the cabinet I might as well replace it too?

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