Bozak Rebuild Project - Part 4

Working with inexpensive tools can only provide data to the level of the accuracy of those budget tools.  While relative changes between two measurements can be useful, absolute levels are essential for fine tuning.  The RTA Pro app I have been using on my cell phone is now falling into the relative measurement category and I must find a better piece of equipment to continue my tuning efforts. 

After contacting the developer, a text file calibrating the contour of the feeble cell phone microphone is forthcoming in a new release of this fine software.  Until such a time that this new version becomes available, my search is on for a suitable or temporary substitute.  In other words, I can no longer use the cell phone app to tweak the crossover network until I am confident that what it tells me is actually what I am hearing.  But work in areas other than this can continue.

Once completing the steps in Part 3 of this series, I discovered that the sibilance problem inherent with the B-200Y tweeters and the addition of a supertweeter was going to be a complicated and prolonged issue to resolve.  Individual driver and crossover component measurements revealed many things that - assumed to be handled in the original 1958 Bozak design - were unfortunately not addressed.

For example, the rising impedance of any dynamic tweeter is normal and to be expected, hence the typical use of Zobel networks to prevent a driver from contributing acoustic energy above the crossover point.  The B-200Y tweeter and the B-209A midrange drivers are no exception to this rising impedance characteristic.  What I measured explained part of the problem I heard.  The minimum Z ot the tweeter, presumed to be close to the rated Z of 16 ohms, was 1/3 that of the Z at 20KHz (yikes!).  And the Z at the crossover point was over 2x the minimum I measured.  Careful measurements helped me to correct for this issue although internal resonances of the sealed tweeter combined with its age are proving to be a challenge to address.

I disassembled one of the blown tweeters to see what the heck was going on with the measured 4KHz peak.  Once disassembled, I noticed that the tiny holes observed from the front of the aluminum diaphragm were indeed holes completely through the diaphragm and not delaminated layers of the aluminum-to-acrylic cone.  This means that any tweeter with these tiny specs will have tiny holes in it.  Of the six drivers in my system, three have many, many specs (holes).

Disassembling A Tweeter Revealed Internal Problems

I also found that the rear of the diaphragm had fused itself to the sound absorbing material used to control internal resonances behind the aluminum cone.  This means that any of the six tweeters I am using could have this same problem and totally change the way they performed today as compared to that of new.

Diaphragm Fused to Damping Material

 Also stated in Part 3, I believed Rudy Bozak matched the sound pressures of two 12” woofers to one 6 ½” midrange but this also proved to be an erroneous assumption.  After further measurements, a -5.5dB attenuation of the midrange driver better matches the acoustic level of the 12” woofer in the stock B-302A cabinet.

The last modification still ongoing is to minimize first reflections back to the tweeter and supertweeter.  After building the satellite box, I realized that I forgot to build into the design the recess for the tweeter and supertweeter frames (duh!).  What I heard lacking was the deep and wide sound stage from previous designs and attributed this defecit to my oversight.  Setting out to test my theory, I fashioned a smooth transition between the front baffle board and the frame edges with painters tape.  The results were astounding!

Testing First Reflection Theory

Not only did the tape improve the depth and width of the sound stage, it also helped reduce the 4KHz peak in the system.  I was pleased from the results of this test so I removed the tape and treated the edges with a liberal coating of black RTV.

Adding RTV

So I hope you enjoyed this dissertation and can benefit from what I have tried.  I will make the network pretty once I stop tweaking it and maybe add a final picture of it in Part 5 (although this may never happen, but then again I said that Part 4 would not happen either but here it is).  As I said, I am still writing and I have a life.  Besides audio I also love the beach, astronomy, and bicycling with my wife.  I have another novel and a self-help book in the works so time allocated to my blog is very limited.

Bear with me, keep checking back for updates, and you will see I have much more to share.  Hmmm…what if I do this to the tonearm of the turntable…

See also Part 1, Part 2, and Part 3

Yours for higher fidelity,
Philip Rastocny

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