Bozak Concert Grands midranges

[sbeineke]

A couple years ago I came across a pair of Bozak Concert Grand Speakers which came out of a Church that was being converted to something else. These are the 310 models with the array tweeter version. Needless to say these things sound just incredibly beautiful...except at high volumes there was a buzzing sound out of all 4 midranges. These are 16 ohm B209Bs.

I thought...oh, boy...where am I going to get replacements for these? After searching ebay and looking around the internet, I pretty much found nothing. So, I took one apart and lo and behold it is something very simple. On these midranges, which have a coated aluminum cone, they put a metal ring filled with a foam type material right over the surround. I am not sure what rings are for other than to prevent long excursion. No sure.

The foam is dried and brittle. So, I removed the ring and foam and they sound great now (have done one speaker but not the other). What i decided to do was just get some good RTV and fill the ring with that. It allows for excursion and prevents over excursion.

I don't normally play to very loud levels so I don't think this will be a issue at all.

Problem solved for less than 5 dollars.

 

[Sam Cogley]

Those aren't B209Bs. They're B209As. The B did away with the foam and the shell, supposedly redesigning the edge of the cone to eliminate the sonic artifacts that were the reason for the damping ring in the first place. Thing is, I can't find *any* difference between the B209A and B209B cone and surround. I suspect that Bozak just realized that it wasn't necessary, and marketing made up the "changes" so they could advertise the B209B as "new and improved."

You could use some soft foam to replace the old damping foam, as suggested by Bob Betts who worked for Bozak in that era. I think RTV would be too solid, and would overdamp the surround.

 

[sbeineke]

Well, then I will yank the two back out and remove the rings and just remove the rings from the other two.

 

[Retrovert]

Needless to say these things sound just incredibly beautiful...except at high volumes there was a buzzing sound out of all 4 midranges. [...] On these midranges, which have a coated aluminum cone, they put a metal ring filled with a foam type material right over the surround. I am not sure what rings are for other than to prevent long excursion. No sure.

The purpose of the foam in the B-209A is to damp resonance. Bozak subsequently redesigned the mounting at the surround to perform that function. Aluminum cone drivers are prone to this problem.

According to Bob Betts, who was Rudy Bozak's right-hand designer, it is required to properly damp the B-209A. It is possible to recreate the ring using an open-cell foam, as Betts has suggested, but I know of no one who has successfully done this and posted any measurements. He has stated that the B-209A was redesigned before the lifespan issue of the foam became known.

The foam is dried and brittle. So, I removed the ring and foam and they sound great now (have done one speaker but not the other). What i decided to do was just get some good RTV and fill the ring with that. It allows for excursion and prevents over excursion.

You put the silicone on the cabinet, not stuck to the driver, correct?

Just to clarify: the issue is not excursion, it is resonance.

The foam slightly touches the cone at the surround to damp out resonance. Silicone is very dense and may too strongly couple the driver to the cabinet. This will migrate cabinet vibration into the midrange causing distortion.

 

[Retrovert]

This is from a writeup I elsewhere posted on the subject.

Again, Bob Betts was a Bozak employee for many, many years and knows the truth.

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".

The foam was moved internal and it may also harden, causing problems:

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