Sunday, January 13, 2013

Bozak Rebuild Project - Part 3

The next steps are to address the sibilance inherent with the B-200Y tweeters and then add a supertweeter to do the job right.  The entire B-200 series tweeters are not uniform SPL drivers and have an annoying rise between 4KHz and 7KHz that drives one nuts after moderate listening to vocal material. 

Rudy Bozak matched the sound pressures of two 12” woofers to one 6 ½” midrange pretty well.  Each 12” woofer realistically needs about 8 cubic foot of box volume to operate optimally and anything smaller restricts the deep bass response (for the B-302A, this is about 50Hz at –3dB).  My system uses two 12” woofers in an isobaric arrangement to lower the box volume requirement to 4 cubic feet and it is ported to enhance the deep bass.  To better match the midrange to the isobaric woofer, I first attenuated the midrange driver about -3dB.  The results were extremely satisfying smoothing out the minor dips and peaks.
Here is the RTA measurement at this point of the modifications.  The green lines show the crossover points and the yellow line shows the point at which sibilance begins (4-6KHz).  7KHz is where I plan to cross in a super tweeter:
Stock Tweeter reversed polarity, Midrange Attenuated
I was encouraged from what the graphs showed and what I heard with my ears matched what I saw.  I was delighted to hear that the transient response was still very snappy, something I suspected may suffer.  While extreme dynamics were a little compressed from the addition of the second order network to the midrange (I no longer jump out of my chair when an unexpected explosion appears on a video), the system will still impress the heck out of you.  The system still handsomely demonstrates what low-mass drivers can do right and high-mass drivers cannot (the old F=MA law of physics rears its head here).
As you can see in the above RTA analysis, the double peak of the tweeter was there as anticipated. So the next step was to tame it with a first-order choke hoping that the choke would also eliminate the peak without impacting the lower frequencies. I tried several different values from those calculated and settled on a 0.31mH.  BUT the addition of the choke introduced insertion losses to that part of the network and reduced the SPL output of the tweeter.  So I removed the extra –3dB attenuation returning the tweeter array attenuation to -12dB.  Also remember that my system has three Bozak B-200Y tweeters in each channel so the attenuation used by Mr. Tobin in his modifications should work fine for a 2-tweeter B-302A.
B-200Y Tweeter with LPF
As you can see, the sibilance peaks although much better are just not going to go away and the tweeters should be replaced with something a little more linear.  Finding drivers this efficient is the problem.  There are a few around and the super tweeter I found was one that also fit in the existing hole (I did not want to build another cabinet).  So I moved on to adding a super tweeter to bring twinkles to the top octave.  I bought some Russian polystyrene capacitors for this job.  I like the way they sound at the very high end of the audio spectrum and played with a few values trying to add sparkle without adding sibilance.  1.12uF turned out to be a good option and the resulting RTA is shown next.
Adding the Super Tweeter
I suspect that the microphone, although “calibrated,” is not telling the whole story in these RTA images.  It appears that at the extremes below 150Hz and above 6KHz, the microphone is just – well lacking.  But RELATIVE differences between RTA measurements can be perceived and this is where I focused my attention.  One day I will get a better unit…after the trip to Colorado and after the new telescope and after the OPPO Blu-Ray player and…well you get the idea.  The point is that you CAN do a lot of good to your system with even modest tools and a very limited budget if you are patient and interpret your results based on the limitations of your equipment.
So this uniformity of sound pressure was getting pretty close to what I was hoping for at the start of this project back in January of 2012.  In that 12 month period, I took almost one hundred RTA measurements of different network configurations and tried many approaches to solving problems.  I succeeded at one point and then took two steps back at another, but in the end persistence prevailed and I believe Rudy Bozak would approve of what I’ve done.
The sound of instruments, vocals, and synthesizers is even smoother than ever and the characteristic Bozak dynamic sonic signature remains.  However, they do sound quite different from the stock B-302A in that the bass is MUCH deeper and the sound is much smoother.  The sibilance of the Bozak B-200Y tweeter is tamed but not completely gone but I am not a believer in notch filters at a place where your ear is most sensitive to them.
Below is a picture of the externally-mounted crossover network on these speakers.  As you can see, the system is bi-wired from the amplifier.
The Current Network
Presently I am in a prolonged listening and evaluation period to the system to see what my ears tell me I should do next.  It sounds really good but as it is with all audiophile gear there is probably something else that you can do to make it better.  Here, it would be to replace the tweeters and super tweeter with a really good planar driver that is also very efficient.  This is almost an oxymoron but there are a few out there if you search long and hard.
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 picture of it in Part 4 (although this may never happen).  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 turntable…and that to the preamp…
The Satellite
See also Part 1 and Part 2

Copyright © 2015 by Philip Rastocny. All rights reserved.

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