Bozak Crossover Differences

[Sam Cogley]

Not 100% certain, but if memory serves - per Pat Tobin, for Bozak drivers labeled "BC", the B pertains to mounting design (to baffle): B = Back; both are defintely ceramic.

"B" was just a revision indicator. The B209A had a foam ring that had a habit of quickly shrinking and vibrating against the cone. The B209B was the version without the unnecessary and problematic foam ring. B209C was the front-mount version, if memory serves.

 

[Retrovert]

"B" was just a revision indicator. The B209A had a foam ring that had a habit of quickly shrinking and vibrating against the cone. The B209B was the version without the unnecessary and problematic foam ring. B209C was the front-mount version, if memory serves.

Uh-oh. My B-305, which has a B-209 mid driver, dates back to 1957, so it likely is the A model.

So I Google a bit and here's what I learned (NB: line breaks added to text to improve readability):

www.lencoheaven.net/forum/index.php?topic=2932.45
(Quoting Bob Betts from the Yahoo Bozak group)
The foam used to damp the 209A does have a limited life. I have seen good ones come in for testing that were almost 20 years old - late 1970s, but have seen some that were only 10 or 12 years old that were dried up. Once again, the environment you live in can, and will, have an affect.

Actually, a clever experimenter could fix those rings by replacing the foam with a medium density polyurethane (open or closed cell) material, or even a soft latex foam. The density is really not too terribly critical as long as it isn't too soft or too hard.

It's purpose is to dampen the free-mode ringing that occurs when the cone comes to rest between excursions. It must compress against the cone edge suspension without actually displacing the cone (travel). Without it, the energy wave tends to travel up toward the suspension (that's normal) but then back down the cone due to the lack of damping (not normal). Peaks and phase distortion can result.

The "A" model did not last long before being replaced with the "B". Actually, the "B" came along, long before we knew that there was a life-expectancy problem with the "A".

www.audioasylum.com/cgi/t.mpl?f=vintage&m=60379
The original B-209 (no B or C suffix) midrange is also paper cone. Easy to spot even in a photo because in their old age, the cones have faded to a lighter color, often tan.

In '60 the B-209A appeared (before the Y version tweeters). The B-209A has the 'new' aluminum curvilinear cone, black latex covered. But the A version requires a separate damping ring mounted just in front of the driver. It is a metal ring supporting a ring of foam which presses against the outer suspension of the driver. So the B-209A is really a two-piece driver, and would be under-damped if the damping ring is missing.

In '63 the B-209B appeared, not requiring the seperate damping ring. The midrange, certainly one of the best ever made, using the same magnet and basic motor (voice coil and magnet assy.) as the woofer, remained unchanged until used without the latex coating in the LS-330A and other systems, in the early '80s.

The B-209C is the same as the B-209B except in a squarish frame rather than round, intended to be easier to mount from the front.

www.audioasylum.com/cgi/vt.mpl?f=speakers&m=333366&VT=T
The other Issues I have had with older Bozaks are several:earliest Bozaka used a paper cone for the midrange, and then switched over to aluminum. The painted black aluminum mids have a foam rubber dampener under the dust cap.

In several units, I have found that that mid drivers suffer from limited movement because the foam has hardened. You can assess this by gently pushing the cone at the edges and then at the center and evaluating the range of movement.

If stiffened, you can cut out the dust cap, and find a cardboard blank across the VC gap, which you will also have to carefully cut out. Remove and vacuum the foam out and replace parts. You can use aftermarket dust caps with little issues.

A replacement foam ring is shown here:
www.lencoheaven.net/forum/index.php?topic=2932.60

 

[Sam Cogley]

Uh-oh. My B-305, which has a B-209 mid driver, dates back to 1957, so it likely is the A model.

Unless it was swapped at some point, it should be the paper B209. The B209A wasn't introduced until 1960, at least. I've always read that the B209A followed the B200Y, but that quote you posted has the timeline reversed. Doesn't really matter, no matter what it was at least 1960.

Shine a light through the grille cloth. If it's brownish tan, it's the B209. If it's black, it's aluminum and will be a B209A, B209B, or B209Bc. The only way to know at that point is to look at the back of the driver.

The information about the latex coating being deleted in the 1980s is untrue, unless it happened after 1985. My B209Bc mids have mid-1985 date codes, and still have the black latex on the front. It's also not accurate that the drivers remained unchanged, all were switched to ceramic magnets in 1970 or 1971.

The paper B209 is actually a very nice driver. It's not quite as good as the B209B (or the B209Bc, which I prefer), but it doesn't give up much. The paper tweeters were...not good...and that seems to have spilled over and harmed the B209's reputation.

 

[Retrovert]

Unless it was swapped at some point, it should be the paper B209. The B209A wasn't introduced until 1960, at least. I've always read that the B209A followed the B200Y, but that quote you posted has the timeline reversed. Doesn't really matter, no matter what it was at least 1960.

Shine a light through the grille cloth. If it's brownish tan, it's the B209. If it's black, it's aluminum and will be a B209A, B209B, or B209Bc. The only way to know at that point is to look at the back of the driver.

Aha! Paper! Success! Thanks for the quick test.

The paper B209 is actually a very nice driver. It's not quite as good as the B209B (or the B209Bc, which I prefer), but it doesn't give up much.

What's the big difference in frequency response? I searched, but turned up nothing substantive.

The paper tweeters were...not good...and that seems to have spilled over and harmed the B209's reputation.

Yeah, those must be replaced. With what, however, is the question.

I don't know if I'll use the 200Y or if I'll install some nice ribbon tweeters. The problem with the Y tweeters is removing the deteriorated foam. I was thinking of putting in a vertical line array of some aftermarket tweeter. Yeah, I know, it's not pure Bozak, but I read a lot of caveats about tweeters issues because of age, be it the flaking latex which results in ringing and the deteriorated foam, to make me think of another solution.

 

[Sam Cogley]

What's the big difference in frequency response? I searched, but turned up nothing substantive.

Just going by my ears, the frequency response is pretty close. The aluminum version seems to be a bit more efficient, and a bit more "sharp" sounding.

I could tell the difference when I dropped the B209Bcs in, but in retrospect I could have lived with the B209s and been happy.

 

[Retrovert]

Just going by my ears, the frequency response is pretty close. The aluminum version seems to be a bit more efficient, and a bit more "sharp" sounding.

I could tell the difference when I dropped the B209Bcs in, but in retrospect I could have lived with the B209s and been happy.

Could this be another psychoacoustic issue of relative volume, because the improved efficiency make it louder and our ears are very good at selecting the louder source as better?

Also, how is the coating on your aluminum drivers? I've read that once the latex coating starts to flake off that the driver suffers from resonance effects, but I haven't seen any quantification of that.

I wonder if there is much different response outside of the volume and in the normal listening-volume range.