a monitor IIa project begins

it's no secret i've been yearning for some walsh equipped infinitys. i've been particularly leaning towards some monitor IIa's. most of what i've found locally have been abused beyond salvation. i've come across a few decent out of state units, from people who wouldn't ship. the typical thing. doing lot of looking, but my walsh itch wasn't getting much scratch.

the other day these turned up in georgia (i'm in california). not only would the guy ship them, he had the original boxes, and would double box them. i couldn't pass that up.

yeah, the original woofers are MIA. that's certainly a bummer, but what's left is seeming like a good start for a restoration. pretty good cosmetically. all grills present, and they came with both tops. so, they got here today and i've extricated them from the packing. looks like fedex didn't totally destroy 'em (thank you factory boxes), so the project begins…

i know the original double-doped 12" woofers don't come up too often, but i'm telling myself to be patient. not so patient that i'm above groveling. PLEASE, if anyone has any leads on some, i'm in need. also looking for a set of original mounting screws for the woofs.

i'll pull the remaining drivers this week and see what i'm really starting with. in the meantime, its spring, and the siren's song of another vintage infinity project is calling.

in my book, that's a good thing.




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loquatious said:

it's no secret i've been yearning for some walsh equipped infinitys. i've been particularly leaning towards some monitor IIa's. most of what i've found locally have been abused beyond salvation. i've come across a few decent out of state units, from people who wouldn't ship. the typical thing. doing lot of looking, but my walsh itch wasn't getting much scratch.

the other day these turned up in georgia (i'm in california). not only would the guy ship them, he had the original boxes, and would double box them. i couldn't pass that up.

yeah, the original woofers are MIA. that's certainly a bummer, but what's left is seeming like a good start for a restoration. pretty good cosmetically. all grills present, and they came with both tops. so, they got here today and i've extricated them from the packing. looks like fedex didn't totally destroy 'em (thank you factory boxes), so the project begins…

i know the original double-doped 12" woofers don't come up too often, but i'm telling myself to be patient. not so patient that i'm above groveling. PLEASE, if anyone has any leads on some, i'm in need. also looking for a set of original mounting screws for the woofs.

i'll pull the remaining drivers this week and see what i'm really starting with. in the meantime, its spring, and the siren's song of another vintage infinity project is calling.

in my book, that's a good thing.




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Click to expand...

It's nice to have the originals. They're unusual looking.
These will work fine, perhaps better.


Have fun!

i'd noticed those. many have suggested the quantum jr. as a source for replacements, but the quantum jr. are 4ohm, whereas the monitor series uses 8ohm. i do know the following also used 8ohm double-doped 12" woofers:

monitor
monitor II
monitor jr.

1001
1001a

2000 II
2000 axt

would really like to get originals, as they were rated down to 22hz. none of the others are spec'd as going that low.

infinity (HK) lists 902-1187 as a (non-doped) replacement, though i seldom trust HK's claims re: vintage infinitys.

i came across a nice brochure for the IIa. don't know if its been posted before, but here it is.



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I think, if I remember correctly, the woofers to my Monitor IIas measured 3.2 ohms.
I would have been surprised had they measured 8 ohms as Infinity was basically a 4 ohm speaker manufacturer, most designs.

I've been wrong before, more than once
So I checked again. Measured 3.9 ohms.

The Monitor IIa's are wonderful speakers and certainly worthy of restoration. I have an original pair that I purchased in 1976 and completely restored around six years ago. They currently reside in my music room along with a number of other Walsh equipped Infinity speakers from the same era. Here's a link to a thread I created when I completed the initial setup.

http://www.audiokarma.org/forums/showthread.php?t=403889

I have subsequent to that thread finished all of the switching, made some other hardware changes, and added a pair of Monitor Jr's to the room. No Walsh's but still in the same family. If the Monitor IIa's look especially tall in the photo's in both threads, especially when compared to the 2000II's, it's because they are on 18" shot loaded steel stands.

http://www.audiokarma.org/forums/showthread.php?t=456213

The original doped woofers are definitely worth holding out for as they produce prodigious amounts of clean tight bass. In fact because my IIa's are in the corners of the room, an arrangement which I would not otherwise recommend, I had to tame the bass with foam bass traps carefully arranged behind them. In any case, a pair of 902-1187's would suffice to get your IIa's up and running but I would still continue the search for a pair of the original CTS woofers.

There was an Alnico CTS woofer that was used in the 2000a that showed up in a pair of Monitor Jr's that I got for some reason or another a few years ago. Infinity might have used them in the early Monitors before they got the woofers for that series sorted out but they aren't doped and as I don't recall ever hearing them - I just came across them a few months ago when I was sorting out my "extra" Infinity cabs in the garage - I don't know if they would make acceptable substitutes for the woofer we expect to find in the Monitor series. Here are links to a couple of photos of one of them:

http://www.audiokarma.org/forums/attachment.php?attachmentid=407641&d=1359936794

http://www.audiokarma.org/forums/attachment.php?attachmentid=407640&d=1359936794

As far as the woofer screws go, it has been my experience that they are the usual black 1/4" hex headed wood screws that Infinity used as their default in the 70's at least. I've come across a few that were round headed phillips heads but I suspect they might have been replacements. Are the hex heads what you are looking for?

Anyhow, as I have great affection for Walsh equipped Infinity's, I am always pleased to hear of another pair in the hands of an owner, especially an AK'er, who is dedicated to restoring them and keeping them alive and well.

thanks for checking your woofers, tubed. always nice to see some proven spec's.

seeing everybody's pics of doped woofers laying around, gives me hope that some will eventually find their way into my hands. (i hope people are not just hoarding these).

i think we are all familiar with the impressive "coastsider collection" of vintage infinitys, and there is no doubt his high praise of the monitor IIa influenced my decision to track down a pair.

in my critical listening area i have a couple of pairs of infinity reference series, with emits and img drivers. to my ears, thats a combination thats hard to beat. they spec at 35hz – 40khz.

last spring i restored a pair of column II's (35hz – 20khz), and they have been doing duty in my living room for the last year. they have the two 10" woofers (front-firing plain cone, down-firing doped cone) and put out very pleasing bass (if not particularly low), but i've found the motorola piezo tweeters (one front-firing, one rear-firing) a bit anemic. while a very decent sounding speaker, the column II's are better suited to a conversation area, than a critical listening area.

i'm hoping the monitor IIa's, with their impressive 22hz – 28khz range will prove more satisfying.

coastsider, the screws i'm hoping to use for for this project are the indented hex head type. its funny, i had a set left over from my column II project, but gave them to a fellow AK'er for his restoration project. hopefully when i track down some proper doped woofers, i can get the proper mounting screws as well. i'm a restoration guy, and originality is important to me.


back to the topic of the woofers, i am curious about some things. all the following appear to use the same woofer, yet the spec's vary quite a bit.


the monitor IIa are rated down to 22hz.

the monitor jr. are rated down to 32hz.

the 1000 are rated down to 30hz.

the 1001a are rated down to 33hz.

the 2000 II are rated down to 28hz

the 2000 axt are rated down to 30hz

(just for the record, the quantum jr. only went down to 40hz)


obviously the cabinets play some role in how deep the bass can dig and still remain clean, but it makes me seriously question if these doped 12" woofers are, in fact, all the exact same units, even though they superficially might appear to be, and whether they are indeed interchangeable. for example:

to understand what the doping on these models was done to achieve, i'll defer to the infinity literature.

the 1001a lit says, the woofer "undergoes two separate treatments. firstly, the soft paper cone is permeated with a plasticizing agent which adds enormous stiffness to the cone. secondly, thick asbestos (!) compound is applied to damp out spurious modes produced by increasing stiffness."

in the monitor IIa literature, the woofers are said to have "FIVE TIMES the stiffness-to-mass ratio of other woofers." these woofers are rated to go far lower than any of the others.

the 2000 axt literature says the woofer has "a full one inch movement capability. its cone is treated twice – once to increase stiffness-to-mass ratio by a factor of THREE, while the second treatment ensures proper cone damping to compliment the added stiffness.

in all 3 of the above mentioned models, the woofers are in a "loaded in a transmission line enclosure," indicating, in addition to the doping, infinity was optimizing the cabinets to squeeze out as much bass as they could, yet results vary considerably. the above indicates each of the permutations may not have gotten the same doping application. this raises more questions:

did all variants have a full 1-inch of movement?

did all use the same size and strength of magnet?


just as an aside, it is interesting to note the doped, down-firing sub-woofers for the column II model were only doped over the voice coil area, and the dope (roofing tar, no asbestos) was strictly to add 3X mass to that slot-loaded woofer, not stiffness. a different design theory altogether, as those woofers were working in unison with the untreated front-firing (mid-bass) woofer in a staggered-resonance configuration in a fully sealed cabinet.


to change the subject, i pulled the walshes, and they both have continuity. they will need foam caps for sure. i'm a little surprised that, in removing the remnants of the original foam, i find quite a bit of difference in the amount of poly-fill in the cones. one fully filled, one 2/3rds filled. i'm wondering which is "correct."



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Doped CTS woofers

I've seen quite a few of these 12" doped CTS woofers that Infinity used in the Monitors as well as the other models in which they were installed in my quest to properly equip the three pair of Infinity's with them that are currently in service in my music room and to have an appropriate number of spares on hand just in case, and while I can't claim to have compared the magnets on all of them side by side as to size differences such as exist in the different types of Watkins Woofers Infinity deployed over the years, they all look pretty much identical to me with the exception of one early CTS variant pair that have very different magnets and that I mentioned before in an earlier thread.

http://www.audiokarma.org/forums/attachment.php?attachmentid=385997&d=1351387481

They are not in service specifically because I wanted all of the woofers in the active collection to be as identical as possible.

As far as the doping is concerned, I have seen some variation as to where specifically on the cone the heavy doping stops and the lighter coating begins and my theory is that this was something that Infinity did in the production process to meet their specs on each batch of woofers that they received from CTS but not specific to any particular model in which they were to be installed. Some of the woofers in very early Monitors even had a material similar to caulk stuck to the cone around the dust cover presumably in an effort to add mass although I have never seen a pair of these in person, only in photos. The ones I have installed in my active collection match as closely as any that I have in my possession with the remainder stashed away as spares, the number of spares being consistent with the number of active pairs and so not considered by me to be a hoard. I just wanted to make that clear in my own self-defense while also admitting that the closet that contains my spares stash is a very scarey place.

The doping on WTLC 8" drivers is another whole ball of wax relative to how much was applied to the front firing vs the down firing pair and I won't even get into that discussion but I think it adds to the theory that Infinity just played it by ear (no pun intended) as to the application of this doping material on the drivers they sourced from manufacturer's to get them where they wanted them spec-wise.

As far as the difference in low end frequency response ratings from model to model, I suspect that it has more to do with the dimensions, internal volume, and port placement than it does to any differences in the woofers although there really is no way to know of any design changes CTS might have made over the four years or so that Infinity used this basic 12" driver. The Jr's and 1001a's are much smaller cabs of course and I don't know how the internal volume of the 2000II's and AXT's, which I believe have similar dimensions, compare to the IIa's, but current room placement aside I can honestly say that my Monitor IIa's produce more bass than do my 2000II's and the woofers are as identical as any two pair that I own. Maybe the fact that the IIa's port is located near the top on the back of the cabinet and the 2000II's at the bottom in the front contributes to the disparity. It's anybody's guess.

So that's my take on the subject and it certainly is open to discussion and/or dispute.

EDIT: There is also the school of thought that the amount of polyfill (usually copious as was Infinity's habit) and its arrangement in the cabinet of any given mid-late 70's model, especially the larger ones, also affects the amount of bass produced. I thought I'd just throw that in.

coastsider, i think you fall into the archivist camp, rather than the hoarder camp.

thanks for your thoughts and insights on the woofers. you have more hands-on knowledge from side-by-side comparison than most do, and first-hand experience carries considerable weight over hearsay and speculation. it would certainly make finding replacements easier if they were all the same woofers (with the same treatment) in the mentioned models. still, i'm hoping to be able to source OEM replacements from another pair of IIa's.

in the meantime, i'm itching to get at the crossover so i can order caps. figured i better ask another question.

coastsider just mentioned: "there is also the school of thought that the amount of polyfill (usually copious as was infinity's habit) and its arrangement in the cabinet of any given mid-late 70's model, especially the larger ones, also affects the amount of bass produced."

the brochure also says the enclosure is packed with a "uniquely configured material" that provides a transmission line load to relieve air pressure. this suggests the fill in the cabinets is not installed uniformly. my cabs appear to just be stuffed to capacity. from what is exposed, i can see no unique "configuration," but i can't be sure if the polyfill has ever been removed from my cabs in the past. before i start pulling out fistfulls of polyfill willy-nilly to get at the crossovers and wiring, has anyone verifed if there is indeed a specific (as per the factory) configuration to the fill in the IIa cabinets? i have found the wiring to the woofers is wrapped with polyfill coiled around them. i haven't seen that before.


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I think that Infinity's brochure description of their "uniquely configured material that provides a transmission line load to relieve air pressure", is a fancy brochure way of saying that they stuffed as much standard polyfill into the cabs as they would take, or at least that's been my experience with the number of IIa cabs from which I've pulled drivers. They are always just packed to the gills. I've tried moving the fill around to no significantly noticeable effect except in the area of the rear facing port which I feel does effect the bass depending on the amount and tightness of the fill directly behind it and likewise in the port tube itself since it would seem that is where the air being displaced by the movement of the woofer cone is most constricted. I read once that the way to modify the bass in JBL 4310's, a pair of which I have in my office for background music when I'm working or otherwise occupied in there (I'm not a JBL guy), was to stuff a pair of socks into the port, so I guess the theory is essentially the same and probably just as scientific.

When I recapped my IIa's, after removing the woofers I also removed enough of the fill to expose the crossovers then shoved the rest of it up into the cabinet to get it out of the way and then reversed the procedure when I was done. I don't think the fact that the woofer wires are kind of threaded through the fill has any significance but as I have never read any information specific to exactly what methodology Infinity employed when packing the IIa cabs with fiberfill or whether or not there is any great mystery to how it was done, I consider myself no expert on the subject and just try to figure it out as I go along. When it sounds as good as I can make it sound, I try to stop screwing around with it.

i got to spend some time with the IIa's this morning.

i found the black vinyl coverings on the cabinets was kind of sticky from age and accumulated schmutz. this had been the case on the column IIs i did last year as well. i've found a good going over with a scrap of towel soaked in a product called "goof-off" takes care of that annoyance quite well. the vinyl looks (and feels) like new again.

the veneer is in surprisingly good shape and will only need a treatment with good ol' howard's restore-a-finish to bring out their best. the cabinet bases are a little rough. after 38 years of head-to-head battle with an elecrolux, i'm guessing thats only fair. they will need some attention.


i pulled the polyfill to uncover the crossovers. pretty tight quarters in there. none of the caps appear to be electrolytic, but think i'll replace them anyway. for those of you who've recapped their IIa's, did you choose to pull the crossovers from the cabs, or recap in-place?

i want to put together a cap order, but i find the values on the caps in mine unreadable.

since we know we can't always trust the factory schematics, does anyone know if the cap values on the schematics for the monitor IIa are correct?

i have an original infinity drawing, and the later HK schematic. the first thing i notice is the tweeter circuit is different between the two drawings (reversed + for –). other than that they seem to agree. i tend to want to believe the old hand-drawn one, as the later HK ones are so often wrong. are there any insights anyone can share on which is correct?

i'm hoping my wiring is as it should be, so for now i'll leave it alone. it looks like the spec's for the inductors match what i have. so how about the cap values?





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It's been so long since I recapped my IIa's that I had to go back and revisit the schematic that I used and on which I took notes but that only served to cause confusion which then prompted me to drag out one of the pair of IIa cabs that I have in the garage in which both the mid dome and dome tweeter are still installed as well as the crossover, all unmolested from the day that I brought them home. I'm going to have to leave further investigation until tomorrow because it too late and I'm too tired to sort things out properly but I will say that while the capacitor values in the "official" schematic" appear to be correct - that stack is the 20.4ufd cap - there are some other anomalies relative to the wiring that I want to have a clear and awake mind before trying to sort out. I already took a photo and will pull the remaining drivers tomorrow since the schematic and crossover appear to contradict Infinity's usual practice of using the solid color wires as the positive leads and the color/black striped wire as negative and there are a few other strange things that could be real or just the product of a tired and confused mind. So stay tuned and I'll try to figure out what's going on tomorrow.

Of course this now has me wondering how I have my active pair wired since the IIa's were my first go at a recap and I was more likely to use the schematic as gospel than I am now. See what you've started.

Oh, and I did do the recap with the crossovers in place which is a bit of a tough go since they are not directly behind the woofer holes. I guess I wanted to make my first try as difficult as possible.

Monitor IIa crossover

loquatious -

So . . .. . after careful examination of the crossover in a spare Monitor IIa that is stored in my garage as well as the connections going to both the dome midrange and dome tweeter drivers, and being truly convinced by physical evidence that no modifications had ever been made to the speaker prior to it's coming into my possession, I have reached the following conclusions:

First, observing that the four wires that are tied to the negative input are white, yellow, blue/black, and red/black the error in the tweeter wiring on the schematic became apparent.

Second, as you suspected the hand drawn crossover schematic you posted is correct indicating that the midrange and tweeter are both wired out of phase with the Walsh and Woofer. I am attaching a copy on which I have added the wire colors to make it more complete and easier to use in a real world situation.

Finally, the "official" HK schematic is correct with the exception as indicated earlier, of the errors in the tweeter wiring which is completely messed up. Rather than attempt to explain the anomalies I am attaching a copy of that schematic that I corrected to reflect the actual crossover configuration and connections. You can compare it to the original version from HK to see what the errors were.

So it does appear that I was correct in my assumption that Infinity's convention was to always connect the black striped color wires to the negative terminal of a driver, even though at times such as the case with the Monitor IIa it carries the positive signal.

Now I am wondering if, when I recapped my active pair of IIa's, I even disconnected the wires to any of the other driver besides the woofers as I would have had no reason to as they were all working properly. The exception is the Walsh's which I did pull to refill the cones with foam, but since the wiring to them is correct on the schematic, I would have reinstalled them correctly. Today I am going to pull one dome tweeter to check the polarity of the connections and if it is correct relative to the corrected schematic, I'm home free.

Finally, as I said in my earlier post, I am confident that the cap values stated on both schematics are correct and agreed with the values I found on the old caps in the crossovers in my original pair.

thanks coastsider. i appreciate the time and effort it took to do that. dragging out and climbing into those heavy spare cabinets, and doing corrected diagrams too! as a retired technical/scientific illustrator, i do like a good diagram. kudos to you sir!

you said, "see what you've started?" yeah, i seem to open up a can of worms with every restoration project. i admit i have a habit of asking a lot of questions, and (maybe) a tendency to over-think things, but i find catching discrepancies, and documenting facts, to be quite rewarding. i'm not afraid to learn. when i restore something i like to put it back as intended, and besides, these archives are here for posterity, so why not try to get it right?

reading the crossover description in the brochure (that i posted above), this revised diagram makes more sense to me, and jibes more with the brochure's claims that the monitor IIa used the same time-coherent spacial phasing theory (gotta love that marketing-speak) that was developed for the monitor jr. i've attached a monitor jr. schematic for comparison (although its an HK, so it may be wrong too), and it would seem to bear this out.

as an aside, i conclude that means that the original monitor, and the monitor II (i couldn't find schematics for those), used different phasing than the "evolved" phasing of the monitor IIa and the monitor jr. i did find the schematic for the similar 2000 II, and it's phasing is different (with the walsh and midrange out of phase with the woofer and tweeter).



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You are welcome. It was kind of fun sorting things out. Of course now I have to check not only the dome tweeter in my Music Room IIa's, but also everything in the 2000II's and Monitor Jr's from which I did pull most of the drivers when I restored them to make sure everything is up to snuff. The midrange in the 2000II is different then the one in the IIa - it is a Peerless KO40 paper cone driver which may be why the in-phase/out of phase setup is different in those. It's a great midrange. They were used in a number of highly regarded Gale speakers back in the late 70's including the GS401.

http://0339436.netsolhost.com/WordPress/gs401-speaker/

Funny that the GS-401 also used doped woofers from CTS, although they were 8".

I had a pair of Monitor II's before I got the IIa's and had them both for a number of years but divested myself of the II's when I moved to Northern California in 1988. They had a different dome midrange than the IIa's have and in fact I have a pair in my spares stash for no particular reason - they must have been in something I bought to get something else. They are round and have a cage over the dome. The original Monitor was only a three-way with just the 12" woofer, the caged mid-dome, and the Walsh tweeter. Because I wasn't doing anything to my speakers in those days for lack knowledge and information I never had any schematics or other such documentation for any of them.

So I guess I've got some work to do today. I'll see what I can get done without making a big mess and wonder if, as good as the IIa's and 2000II's sound in the room right now, what will happen if I find something wrongly wired out of phase. The adventure continues.

You are welcome. It was kind of fun sorting things out. Of course now I have to check not only the dome tweeter in my Music Room IIa's, but also the 2000II's and Monitor Jr's which from which I did pull most of the drivers when I restored them to make sure everything is up to snuff. The midrange in the 2000II is different then the one in the IIa - it is a Peerless KO40 paper cone driver which may be why the in-phase/out of phase setup is different in those. It's a great midrange. They were used in a number of highly regarded Gale speakers back in the late 70's including the GS401.

http://0339436.netsolhost.com/WordPress/gs401-speaker/

Funny that the GS-401 also used doped woofers from CTS, although they were 8".

I had a pair of Monitor II's before I got the IIa's and had them both for a number of years but divested myself of the II's when I moved to Northern California in 1988. They had a different dome midrange than the IIa's have and in fact I have a pair in my spares stash for no particular reason - they must have been in something I bought to get something else. They are round and have a cage over the dome. The original Monitor was only a three-way with just the 12" woofer, the caged mid-dome, and the Walsh tweeter. Because I wasn't doing anything to my speakers in those days for lack knowledge and information I never had any schematics or other such documentation for any of them.

So I guess I've got some work to do today. I'll see what I can get done without making a big mess and wonder if, as good as the IIa's and 2000II's sound in the room right now, what will happen if I find something wired out of phase. The adventure continues.

This sure brings back memories. I bought a pair of Monitor Jr. new back in the mid ‘70s. The bass performance of that woofer and the way Infinity loaded it was outstanding. The system was rated down to 32Hz and it did every bit of that.
To this day I think that the Monitor Jr. had the best bass I’ve ever heard from a speaker of that size.

Coastsider said:

So I guess I've got some work to do today. I'll see what I can get done without making a big mess and wonder if, as good as the IIa's and 2000II's sound in the room right now, what will happen if I find something wired out of phase. The adventure continues.

Click to expand...

loquatious -

My work is complete and I am all good - Monitor IIa's, 2000II's, and Monitor Jr's all checked and verified as to proper driver connections.

This was an interesting and informative discussion. I hope you will keep the forum informed of further progress with your Monitor IIa project.

here is a project update.

i've been on a quest for a suitable foam to top off the walsh tweeters. its proving an interesting challenge. a lot of criteria to meet. the original foam was 1/2" thick. the foam needs to be an open cell type, so it is permeable to air. it needs to be flexible enough to conform to a dome shape, yet resilient enough to hold the cone in place. it needs to be open enough to be acoustically transparent, but dense enough to damp the waves at the top of the cone. a lot of variables, but there is a lot of foam out there to choose from. i've been looking at quite a few different specialty foams but have, so far, rejected all of them for one shortcoming or another. i'll keep looking, and when i find one that best fits the bill, i'll pass the info along.

in the meantime, i sourced some 12" woofers to use as placeholders while i search for some doped originals. the stand-ins, which i bought loose, ostensibly came from some quantum jr's., which many say are suitable donors. the tech sheet for the quantum jr. list 902-1187 as the part number for the woofer, the same number listed as a replacement part for the monitor IIa woofer.

as usual, there's an anomaly. they don't have numbers that i've seen before. not the 902-0018 or 902-1187 numbers usually i see associated with quantum jr. woofers. but they do look like the woofers i see in most photos of production quantum jr's.

these have paper cones that are treated with something (clear acrylic?) on the front side of the cone as a stiffening agent, and have big 4.5" dust caps. though cosmetically identical to each other, one magnet has 95141078 stamped on it, the other has 45141278, and both paper cones have 776902 printed on the untreated side.

they do read 4.1ohms on the multimeter, which is pretty close to the 3.9 reading that tubed got from his doped monitor IIa woofers. these look very much like slightly bigger versions of the woofers in my column IIs. certainly they come from the same manufacturer. they fit the openings in the monitor IIa perfectly and the mounting holes line up.

the terminals are not marked + or –, but one is dabbed with red ink. i've seen this before on photos of other infinity drivers. can someone verify whether the red marked terminal would indicate the positive or the negative terminal of the woofer?





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loquatious said:

here is a project update.

i've been on a quest for a suitable foam to top off the walsh tweeters. its proving an interesting challenge. a lot of criteria to meet. the original foam was 1/2" thick. the foam needs to be an open cell type, so it is permeable to air. it needs to be flexible enough to conform to a dome shape, yet resilient enough to hold the cone in place. it needs to be open enough to be acoustically transparent, but dense enough to damp the waves at the top of the cone. a lot of variables, but here is a lot of foam out there to choose from. i've been looking at quite a few different specialty foams but have, so far, rejected all of them for one shortcoming or another. i'll keep looking, and when i find one that best fits the bill, i'll pass the info along.

in the meantime, i sourced some 12" woofers to use as placeholders while i search for some doped originals. the stand-ins, which i bought loose, ostensibly came from some quantum jr's., which many say are suitable donors. the tech sheet for the quantum jr. list 902-1187 as the part number for the woofer, the same number listed as a replacement part for the monitor IIa woofer.

as usual, there's an anomaly. they don't have numbers that i've seen before. not the 902-0018 or 902-1187 numbers usually i see associated with quantum jr. woofers. but they do look like the woofers i see in most photos of production quantum jr's.

these have paper cones that are treated with something (clear acrylic?) on the front side of the cone as a stiffening agent, and have big 4.5" dust caps. though cosmetically identical to each other, one magnet has 95141078 stamped on it, the other has 45141278, and both paper cones have 776902 printed on the untreated side.

they do read 4.1ohms on the multimeter, which is pretty close to the 3.9 reading that tubed got from his doped monitor IIa woofers. these look very much like slightly bigger versions of the woofers in my column IIs. certainly they come from the same manufacturer. they fit the openings in the monitor IIa perfectly and the mounting holes line up.

the terminals are not marked + or –, but one is dabbed with red ink. i've seen this before on photos of other infinity drivers. can someone verify whether the red marked terminal would indicate the positive or the negative terminal of the woofer?





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You'd think the red mark signifies positive
I always double check with the "C" battery test.
When the woofer pushes outward, that's the correct polarity.