LSTs sound/look different

[Mexicomike]

I recently acquired a pair of LSTs. In the rear they are not identical speaker cabinets - one has a chipboard crossover panel, the other has a plywood panel. The one with the plywood panel has the two speaker connections and the fuse; the other has an additional screw-type connector that has a black wire attached to it that connects to one post on the fuse. Additionally, they do not sound the same with both set on the same spectral balance position. One has to be set on "2" to sound like the other does on "1"

Does anyone have a detailed info that might be helpful here - the difference in the sound AND the slightly different configuration of the two speakers? I suspect that the chipboard crossover panel is a later version. Is there any other differences that might account for any sound difference? They seem a bit bass shy compared with my 3As though they are sitting more in the center of the room on the floor while the 3s are fairly close to the corners and on stands that raise them about a foot off the floor which may account for that.

Both speakers were refurbished recently with new Xover caps/woofer surrounds (not by me). I have not yet opened them up to look inside. The one with the chipboard has an AR paper showing what appears to be serial #0059. The other has no paper.

[dynaco_dan]

I recently acquired a pair of LSTs. In the rear they are not identical speaker cabinets - one has a chipboard crossover panel, the other has a plywood panel. The one with the plywood panel has the two speaker connections and the fuse; the other has an additional screw-type connector that has a black wire attached to it that connects to one post on the fuse. Additionally, they do not sound the same with both set on the same spectral balance position. One has to be set on "2" to sound like the other does on "1"

Does anyone have a detailed info that might be helpful here - the difference in the sound AND the slightly different configuration of the two speakers? I suspect that the chipboard crossover panel is a later version. Is there any other differences that might account for any sound difference? They seem a bit bass shy compared with my 3As though they are sitting more in the center of the room on the floor while the 3s are fairly close to the corners and on stands that raise them about a foot off the floor which may account for that.

Both speakers were refurbished recently with new Xover caps/woofer surrounds (not by me). I have not yet opened them up to look inside. The one with the chipboard has an AR paper showing what appears to be serial #0059. The other has no paper.

Hi there;

Can you take a photo of each rear cabinet, please and post them here.

With the sound difference, perhaps front un-covered drivers photos as well of both cabinets.

The extra terminal is not normal and a photo will help clear this up.

It may be a fuse bypass, still not stock.

Who did the upgrading for you?

Opening them up pretty much will void the warantee.

[Mexicomike]

Here are pics of the backs. Note the post connecting the "printed circuit" to the fuse one one speaker. THe other has a slightly different printed circuit config and does not have/need the post.

[RoyC]

Both speakers were refurbished recently with new Xover caps/woofer surrounds (not by me). I have not yet opened them up to look inside. The one with the chipboard has an AR paper showing what appears to be serial #0059. The other has no paper.

Mike,

Have you looked at the drivers that were used in the restoration? Woofer era and tweeter era/type/condition can easily cause differences.

Roy

[Mexicomike]

I have now pulled the front grills - all the drivers are AR oem drivers. The woofers have the typical flat section on the basket typical of the AR 12" woofs used on the 3a/LST. All of the drivers have some of the typical grease pencil "initials" I have seen on other ARs (like my 3As). I checked all the drivers and they all work and sound OK within each speaker. I ran a 10khz test tone through both and, with the same switch setting, one LST has considerably more output than the other. I'll pull the crossover panels and see if both are identical.

The 12" foam-surround woofs have a stiffer suspension than those on my cloth-surround Alnicos in my 3as. If you depress one of my 3a cones, it will take several seconds to return. The LST cones will return in maybe 1.5 sec or less. The cabinets didn't seem sealed as well as the 3As but that may be due to the fact that the LSTs use a gasket while my 3As use the old duct-seal material. One of the speakers is missing most of the crossover panel screws which makes the acoustic suspension on one unit essentially non-functional at the moment - air rushes out of the 8 vacant holes when I depress the woofer cone.

I replaced the woofer gaskets with duct seal and the woofer in one cabinet is slower to return now so I think the original gasket was not sealing as well as it should have. Of course the other one, with the missing screws hasn't improved so I need to find replacement screws. They appear to be standard machine screws so I should be able to find them even here in Mexico!

[Mexicomike]

Here are pics of the Xovers...

Are these originals but different versions or one original/other not, or both not?

[dynaco_dan]

I have now pulled the front grills - all the drivers are AR oem drivers. The woofers have the typical flat section on the basket typical of the AR 12" woofs used on the 3a/LST. All of the drivers have some of the typical grease pencil "initials" I have seen on other ARs (like my 3As). I checked all the drivers and they all work and sound OK within each speaker. I ran a 10khz test tone through both and, with the same switch setting, one LST has considerably more output than the other. I'll pull the crossover panels and see if both are identical.

The 12" foam-surround woofs have a stiffer suspension than those on my cloth-surround Alnicos in my 3as. If you depress one of my 3a cones, it will take several seconds to return. The LST cones will return in maybe 1.5 sec or less. The cabinets didn't seem sealed as well as the 3As but that may be due to the fact that the LSTs use a gasket while my 3As use the old duct-seal material. One of the speakers is missing most of the crossover panel screws which makes the acoustic suspension on one unit essentially non-functional at the moment - air rushes out of the 8 vacant holes when I depress the woofer cone.

I replaced the woofer gaskets with duct seal and the woofer in one cabinet is slower to return now so I think the original gasket was not sealing as well as it should have. Of course the other one, with the missing screws hasn't improved so I need to find replacement screws. They appear to be standard machine screws so I should be able to find them even here in Mexico!

Hi there;

Before you go any further, buy replacement bolts so that you can test the speaker, sealed.

You have more like a bass reflex speaker with so much air relieving itself through those holes.

Having 8 vacant holes is a lot of open cabinet, several square inches, I believe.

You wrote earlier that the previous owner had the caps replaced, they look un-matched to me.

Caps are usually about 20% +/- tolerance.

What are the specs written on those caps?

Can you zoom in on the two rear mounted panels, please.

[Diamonds&Rust]

Here are pics of the Xovers...

all the drivers are AR oem drivers

Are you saying they all say "AR" on the face plates, or are you saying you can identify them from the 3a restoration document or what? There are a lot of AR OEM drivers that don't belong in an LST... and not because they don't look right.

The woofers have the typical flat section on the basket typical of the AR 12" woofs used on the 3a/LST.

I don't mean to sound frustrated, but every AR 12" woofer had that from 1954 to sometime in the 1980s, or maybe 90s. Which version of the flat-sided woofer you use makes a difference. There are a lot of them that don't belong in an LST and unlike some people, I don't think an AR91 woofer belongs in an LST or vice-versa. They are different.

Ditto the midrange drivers, many of which look "almost like" the others.

The crossovers, if they are still using those wax box capacitors, need attention.

I know I get as weird about this as Vern does about fusing, but Mike... there is no way in even the very most optimistic person's most wishful thought that those caps are still good. Testing them with $100 capacitor tester won't cut-it. For those to be good Peter Pan would have had to be storing them in Neverland where nothing gets old.

Yes, that (the caps) could account for all your trouble, BUT until you let us see the drivers, we're all flying blind and might as well stop guessing.

Bret

[Mexicomike]

Here is a pic of one of the LSTs - the other looks the same with the same drivers except that while this speaker has one mid with the red glue on the perimeter of the mesh, the other speakers mids all have the red glue around the mesh perimeter. I haven't pulled the FG out of the cabinets yet to check the driver part numbers but I will do that. Based on appearance, all drivers look to me like they match the drivers pictured in the AR3 restoration doc as being correct for the LST. Of course, not being an expert on this sort of thing, I may be "seeing" it wrong.

[KlausDK]

Here are pics of the Xovers...

Are these originals but different versions or one original/other not, or both not?

Hi Mike

Could you follow the black wire connected seperately to the fuse on the older (the lower) x-over, to see where it goes? Does it go to the same place as the black wire on the newer (upper) x-over? It probably goes directly to the Woofer.

Also could you check the polarity and value of the four capacitors on the older x-over, as well as the big blue capacitor on the upper x-over.

I think you have a very early LST, the one with the four seperate caps instead of the bigblue, if you look up the schematic for the LST in the library, you will see that it has 4 polar capacitors of 5000 myF/50V in a parallel/seriel mix, like yours seems to have.

Electrically they look pretty equal to me, except that the different kind of capacitors used; this fact, as well as an aging of the capacitors, could account for the differences in sound you experience. Here I fully agree with Bret's post above, especially the yellow wax caps drift quite heavily as they age and dry out. I would try to update the capacitors to new and equal types/value for the two speakers, and then see what it sounds like.

I have a post where I describe my experiences with x-over update of a set of LST's you can use for inspiration:

http://www.classicspeakerpages.net/IP.Boar...amp;#entry63384

BRgds Klaus

[Mexicomike]

Klaus,

The black wire connected seprately to the fuse on the older Xover goes to woofer, just as the black wire from the fuse does on the newer Xover

Each of the 4 caps are a Sprague 5000MFD 50VDC cap. The Big Blue on the other Xover is a Sprague 2500mfd 60VDC cap

Re an earlier question about drivers - I have removed the FG from the cabinets and they are (re the 3a restore appendix) the A.19 tweets, A.12 mids, and A.3 woofs. All mids are stamped with dates from Jun-Sep, 1971.

[KlausDK]

Each of the 4 caps are a Sprague 5000MFD 50VDC cap. The Big Blue on the other Xover is a Sprague 2500mfd 60VDC cap

All mids are stamped with dates from Jun-Sep, 1971.

Hi Mike

Are the four spragues polar (with a + and -) or nonpolar = bipolar, ie without + and -?

I have two 5000myF/60V bipolar in series in my LSt's which equals 2500myF/120V. If you are interested I can get you two brand new 2700myF/63V bipolar caps from Jensen in Denmark, they are about 40 US$ each plus shipping. The rest of the caps you can get locally or from the US.

It seems that both of your LST's are pretty early, but the 4 cap version is for sure the earliest. Interesting paper from the BAS (Boston Audio Society) from 1975 indicates, that the earliest LST's didn't have the big caps at all:

Ron Dunlap – 7506b: In testing an AR LST, the instrument indicated an effective impedance of 6 to 8 ohms over most of the audio frequency range except in the bass; below 100 Hz the apparent impedance was said to be about 2 ohms, descending toward a short circuit below the audible range. (The autotransformer in the LST is effectively a short-circuit at dc, rising to a respectable impedance in the audio range; but because this reportedly caused failure in some amplifiers, current LST's contain a large input capacitor).

BRgds Klaus

[Mexicomike]

The four spragues are polarized caps (+/-).

Here's a pic looking into the cabinet from the rear. All of the respective drivers in both cabinets look exactly like those you can see in this pic.

[Diamonds&Rust]

Here's a pic looking into the cabinet from the rear. All of the respective drivers in both cabinets look exactly like those you can see in this pic.

Mike,

Thanks for all the photos.

Honestly, I can't see the midranges well enough to be able to swear in a court of law that those are correct, but they "look like it from here." If all the drivers are working about equally well, then I see no reason to suspect drivers.

That means the difference in sound you are getting must be something I can't see.

I can't see the condition of the capacitors.

I also cannot follow the wiring, but I know you can do that - making sure that the same taps from the transformer are connected to the same places on the switch, etc.

Klaus does nothing in half-measures. He'd be a good person to listen-to. He's pickier than I am, and that's not easy.

I'm going to "go quiet" and watch with interest as this saga unfolds.

I feel you surely will be pleased with the way they eventually sound... when you get them to sound pleasing, the way I feel you eventually surely will.