Tobin Mods to Bozak 302A Urban speakers - definitely worth it!

Discussion in 'Audio Hardware' started by DrJ, Mar 21, 2006.

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  1. bsrecords

    bsrecords New Member

    Location:
    waverly ny
    Bozak 300A

    I stumbled across this forum on Bozak speakers and found it very informative. I purchased a set of B300's several years ago with the equipment cabinet, all very clean, however was not impressed with the sound. I later added a high frequency horn to the cabinet and the sound was much changed. I also heard things in the music I had never heard before and the sound spread out between the speakers was incredible. I left everything else as stock, just added the horn, glad I did.
     
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  2. DrJ

    DrJ Senior Member Thread Starter

    Location:
    Davis, CA, USA
    Interesting experience, thanks for relaying that!

    My next tweak is to experiment with modifying the crossover...I realize these are no longer going to be much like original Bozak speakers with all the changes, but I am learning a ton about speaker design from doing all this and there is nothing that I'm doing that cannot be undone if I ever decide the sound gets worse after a tweak, rather than better (hasn't happened yet).

    The stock crossover points in this N10102 crossover (1st order) are 800 Hz and 2500 Hz. Kind of narrow, and that leaves very little of the fundamental frequencies coming through the mids, most of the fundamentals are in fact going through the woofers - giving things a dark coloration - and the upper mids are all coming through the tweets, BUT (due to the soft 6 dB roll off in this first order crossover) partially doubled by the midrange drivers - which according to Pat Tobin continue to sing (albeit partially rolled off) all the way up to 10 kHz!

    My dad and I installed some L-pads in the crossover circuit while we are experimenting and what I find is that currently when I crank the midrange L-pad, trying to adjust a bit for the dark coloration of so much of the music going through the woofers, I get a harshness and over-emphasis of the upper mids that grates over extended listening (due probably to the midrange and tweeter drivers doubling in this range), and a boost in the fundamentals of just a few types of instruments/sounds (female vocals and violins and highs on the piano) but not much else.

    See the attached chart if you're interested in the typical fundamental and harmonic frequency ranges for various instruments, it was kind of an eye opener for me - a lot more going on below 1000 Hz than I had ever realized. You don't see saxes on this chart but fundamentals of alto and tenor run at just under 1000 Hz, guitars too.

    Anyway, this chart and my extended listening tests confirm that the B209A mids, with the existing crossover, are seriously underutilized. I'm not real clear on why Bozak used the crossover points he did.

    Modern first order crossover design would suggest a more ideal 3 octave separation between the lower and higher crossover points. So we are going to build a new crossover and experiment with it - 400 to 500 Hz or so for the low crossover and 4000 or so for the high crossover. This will expand the amount of critical fundamentals going through the midrange driver and I think allow a lot more control/tweaking of the sound to taste via the midrange L-pad (allowing you to adjust relative woofer/mid and mid/tweeter balance more adequately).

    The only fly in the ointment may be if the B209A aluminum mids don't have a wide enough frequency response range to handle dropping the lower crossover point - I have a question in to Pat Tobin about this since I don't have any manuals or anything with the B209A frequency response range. But I suspect it will be just fine and if so will post results once we have the new crossover hooked up for a test spin.

    BTW the more I listen the more I LOVE these woofers...they are unbelievably natural sounding, never had better bass. It's just that with the current crossover they are being asked, in my opinion, to do too much in the lower midrange by having them extend up to 800 Hz.
     

    Attached Files:

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  3. Fleetwood85

    Fleetwood85 New Member

    Location:
    Fayetteville, NC
    DrJ, great info on the crossover mods, I might have to try them. Here's the info on each Bozak driver from their catalog from May 1977. All the drivers are the Ceramic magnets, my guess is the Alnicos had the same parameters.
    Woofer B199Bc response 35 to 4,500 Hz, resonance less than 35 Hz in free air
    Mid-range B209Bc response 65 to 16,000 (working range 150 to 3,500 Hz)
    Tweeter B200Zc response 1,500 to 20,000 Hz
    Wide-Range B800Bc (used as bass or midrange) response 50 to 16,000 Hz
    Mid-range/Full-range B450 response 100 to 8,500 Hz
     
  4. DrJ

    DrJ Senior Member Thread Starter

    Location:
    Davis, CA, USA
    Thanks for the info Fleetwood85...I ended up finding an old 1968 Bozak brochure on the web that listed the frequency response range (working range is what is provided) for the Bozak B209A mid and B200Y tweeter:

    B209A mid - 200 Hz to 3500 Hz
    B200Y tweet - 2000 Hz 20 kHz

    So qsimilar to the frequency response ranges for the slightly later variants you listed!
     
  5. DrJ

    DrJ Senior Member Thread Starter

    Location:
    Davis, CA, USA
    So next crossover tweak now completed: I lowered the lower crossover point from 800 Hz to 400 Hz. BIG improvement, less dark coloration to the sound, bass stayed excellent but the mids now sound like true mids. These speakers still have a vintage sound magic but with a touch of the modern sheen, a nice compromise.

    Next tweak coming: will raise the upper crossover point from 2500 to about 3300 Hz (can't go up to 4000 as I'd initially planned as the rated frequency range for the B209 mids is only up to 3500 Hz)...this will be the last tweak for these speakers and then I'm done. It should minimize the problem of the midrange driver and the tweeters doubling in the 2500-3200 Hz zone in this first order crossover with its rather gentle roll-off slopes. That's about the only ongoing problem I can hear with these speakers now - when the tweets and midrange driver are otherwise nicely balanced with each other and the woofer, there's just a bit of artificial brightness in the middle highs that I can't get rid of without having to adjust the tweeter L-pad down to the point where I then lose too much from the upper highs.

    Will report back on this final tweak once finished, hopefully by next week.
     
  6. SamS

    SamS Forum Legend

    Location:
    Texas
  7. DrJ

    DrJ Senior Member Thread Starter

    Location:
    Davis, CA, USA
    I'm not as familiar with that model as with the Urbans...seems a little pricey to me, though not totally unreasonable, might want to try and barter a bit.
     
  8. Fleetwood85

    Fleetwood85 New Member

    Location:
    Fayetteville, NC
    Sam, that price is not bad, the B305's had (2) woofers, each had the dual tweeter array mounted in front of the woofers with (1) 209 midrange. Like Tony did with his 302, you might want to re-located the tweeters. Also have the guy check to see if the tweeters and the midranges are paper cone or aluminum.
     
  9. Mad shadows

    Mad shadows Forum Resident

    Location:
    Karlskrona- Sweden
    It´s very interesting to read about your project. Ambitious is the only word I can say.

    There´s nothing like big paper cones. Bought a pair of big Tannoys (Arden) the other day.
    The realism is stunning.
     
  10. DrJ

    DrJ Senior Member Thread Starter

    Location:
    Davis, CA, USA
    Stick a fork in me (but not my Bozak paper woofers!) - I'm done and they're done.

    I have now raised the upper crossover point to just under 3300 Hz and I'm now 100% happy with the sound of these speakers. The artificial brightness from the tweets and mids doubling in the lower highs is now gone and the tonality of instruments sounds just right. I'm in hog heaven and done tweaking!

    Hope all this info has been of use to everyone. It seems like a lot of work reading about it, but actually it ended up pretty simple. As much as the original Tobin mods were wonderful in getting me to realize what these speakers were capable of, there would have been a more direct way to get to where I ended up, I now realize, so to recap my "Suggestions for Hot-Rodding Your Vintage Bozak 3-Way Speakers" (Urbans for sure, and probably applies to some of their other 3-way models though no personal experience with modifying those):

    1) I suggest making sure you have the 200Y aluminum tweets and B209B aluminum mids. To my ears they are superior to the older paper drivers (especially the tweets). The original wool 12" woofers RULE though.

    2) Consider moving the tweets up to the top of your cabinets - much more modern, appropriate location. Easy to do by building a simple wood shop project type box for them and running some wires up through the back of the cabinet.

    3) After going through my odyssey with these speakers, I'd now suggest, rather than messing around with modifying the original Bozak crossovers, that you simply make your own crossovers and choose 400 Hz and 3300 Hz as your crossover points. Using the on-line crossover design calculator I mentioned you can easily calculate the capacitors you need and also which inductor coils to get (when you do this, punch in 3400 for the upper crossover point, which is a little more than 3 octaves up from the lower point, and it calculates out around 3300). These home made classical first order crossovers are dirt cheap and they sound every bit as good, to my ears, as the modified first order Bozak crossovers I went with initially. All you need to do this is the crossover calculator, the parts (you can order from places like Parts Express), and soldering skills.

    4) Most importantly, strongly consider adding L-pads to your mid and tweeter circuits, so you can dial in the balance among your woofs/mids/tweets to YOUR taste. You can mount them inside or outside the cabinet - again in separate boxes if you don't want to mar your speaker cabinets. Once you do this, trust me, you will wonder how you ever got by without having this degree of control over your speakers. Without it, you're "stuck" with the sound the speaker designer likes, not what YOU might like. Even if the speakers sound great to you before you mess with this, I will pretty much guarantee you will tweak the driver balance somewhat to get them sounding EXACTLY as you want them to sound. Very cool!

    I'd do things in that order exactly - Steps 1 and 2 first, then listen, and then do Steps 3 and 4 at the same time and again listen. Assuming you would need to buy the aluminum tweets and mids as I did, total cost of doing these mods I would estimate at about $250 on the very high side and probably as little as $150 if you're patient - that is mostly driver costs, the modification parts like L-pads and caps and inductor coils are peanuts. If you already have the Bozak aluminum drivers - well it will cost next to nothing to do all this.

    You'll be amazed at what these 40+ year old speakers can do with these mods!
     
  11. pocketchange

    pocketchange New Member

    Location:
    TEXAS
    If you go with Mr. Tobins XO's you get the choice of having a higher or lower point for the tweeters.
    In my case, I have a Sonic Frontiers Line 3 Pre which has the upgraded IC's and I use the lower of the two setting for the 302's.
    I would like to suggest spending time on the 12's and reseal the cloth surrounds with some SuperFlex if you can find it. Take your time and only use enough to cover the old surrounds with a minimum of SuperFlex, this will take you into future decades of musical bliss and you won't have to worry about the old cloth surrounds...
     
  12. DrJ

    DrJ Senior Member Thread Starter

    Location:
    Davis, CA, USA
    Yes, Pat has a workaround for dealing with the overlap in tweets and mids that he shared with me - it involved changing which of the taps to one of the inductor coils that is used (going from the 8 ohm tap used at the factory to a 16 ohm tap that was originally intended for use in the 16 ohm version of the speakers). But my understanding is that changing the tap you use doesn't actually change the crossover point, rather it simply changes relative attenuation of the tweets vs mids. I could be wrong though, and if using the different tap does modify the upper crossover point - or if there's some other tweak Pat developed for changing it that I'm not aware of - then that's great and another reasonable approach.

    Nice thing about making your own crossovers though is that you can dial in whatever crossover points you want, high AND low, just by varying the capacitors you use. And it is totally easy - trust me, if I can do it, anyone can do it. As long as what you're doing is within the operating frequency range of the drivers, it's really cool to have that flexibility and again to my ears there was nothing special that the vintage Bozak crossover was adding to the sound that I'm missing by having moved away from it to home mades (as opposed to the Bozak drivers which are awesome and have a gorgeous sound). I guess the only issue would be if you want to keep everything as close to original as possible, but in my opinion the Bozak Urbans are pieces of vintage gear that don't really shine until a lot of polishing and judicious modernizing has been done.

    pocketchange - can you clarify what issues tend to arise with the cloth surrounds on the woofers? Mine have no deterioration I can see and they certainly sound fantastic. Would be good info to know about if you're willing to elaborate - thanks!
     
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  13. pocketchange

    pocketchange New Member

    Location:
    TEXAS
    Air tight... they ain't, hold them up to a light and you will notice the old rubberized dope is going away. Small holes are all along the suround. RTV or SuperFlex and some time and patience will take care of the problem. Use no more than is necessary to fill the surround, wiping away any excess to insure that you only use a minimum of material. Check with Mr. Tobin, maybe he will send you some pics of the results... they do look much better than the untreated surrounds..pc
     
  14. DrJ

    DrJ Senior Member Thread Starter

    Location:
    Davis, CA, USA
    Cool, thanks so much for the tip on this issue.
     
  15. Pat Tobin

    Pat Tobin New Member

    Location:
    Los Angeles CA USA
    Tobin Bozak Crossover Mods

    Hi Tony and other Bozak friends. I finally got off the dime and joined the group. Haven't yet scanned all Bozak posts, but will do so soon.

    Regarding Tony's mods to my mods: for the lower xover freq., Bozak used 800 for the (single woofer) 302A systems and 400 Hz for the larger Symphony (two woofs per box) and Concert Grand (four woofs per box). The reason for using 800 on the 302A is not clear. I believe it has to do with the use of the B-207A 'coax' as the woofer-tweeter unit. This brings the tweeters down in front of the woofer and away from the midrange. I feel that 800 Hz might have served better for that driver relative spacing, altho I can't swear to it. The 400 Hz lower xover freq. was available as an option for the 302A, altho not promoted.

    If my theory is correct, 400 Hz is better when the tweets are 'liberated' from down low, in front of the woofer.

    Now for the upper xover freq. What Tony is alluding to is that the world-beating B-209B midrange sings quite happily up to 10 kHz. (The upper bandwidth limit could not be lowered in the physical construction of the driver without degrading transient response.) Rolloff is theoretically by means of the inductor in series with the midrange. Problem is, any driver has a rising impedance characteristic in the top portion of its range. So just as the inductor would be rolling off response above 2,500 Hz, the rising impedance of the midrange keeps the response almost flat. This is a given: Using the B-209B with the N-10102A xover, the midrange will double with the tweets from 2,500 Hz to about 10kHz. There is no question that Rudy knew about this. But let's jump in the time capsule and go back 40 years. Most speaker systems of every stripe had a boosted high end. It was glitzy and impressed customers in the demo room. Rudy probably felt that this doubling was in a range that was very appealing to most users.

    But it is not my job to help Bozak sell speakers in the '60s. It is my task to revise the xovers (and other elements such as cabinet design) in order to allow the world-class drivers to sing in their true, natural voices.

    Here's what works best to curtail that upper end doubling: On the N-10102A xover, at terminal strip A, move the jumper from the N-101/N-104 terminal to the other one, marked N-102. That doubles the inductance feeding the midrange, which happens to lower the drive to the midrange in the 2.5 kHz ~ 10 kHz range, exactly what we need. (For technocrats: that inductance was used for an 800 Hz rolloff for the 16-ohm B-305 midrange.)

    As for using a 3,300 Hz upper xover: that's OK but there are a couple of drawbacks. For one, when the frequency is high (short wavelength) compared with the diameter of the radiator (cone), the upper frequencies get very directional. That might not be noticed very much, but it could actually reduce the acoustic output at a considerable angle off straight ahead. Secondly, needless to say, at any given frequency, the transient response of a small cone is superior to that of a large, heavier cone. And if I had to pick the one most outstanding property of Bozak drivers, it would have to be transient response.

    That's my two cents worth for now. If I can answer any questions, fire away.

    I want to sincerely thank Tony Jerant (DrJ) for disclosing my xover mods to the group, and for his most gratifying compliments.

    Best wishes to all,

    Pat Tobin
     
  16. DrJ

    DrJ Senior Member Thread Starter

    Location:
    Davis, CA, USA
    GREAT to see you here Pat!!!

    Interesting comments you make about the upper crossover point. So far at 3300 Hz I haven't heard any problems with extreme directionality or loss of acoustic output. I have to add that my listening room is quite small - 12 x 16 - and is dedicated to listening. So I'm always right in the sweet spot and it would probably be unlikely I'd hear those potential problems. I would imagine in a bigger room with more distance between the speakers, off-angle seating, etc it could become an issue.

    I also have to add that I tend to like a midrangy sound and having more music now coming through the wonderful B209B mids is suiting my taste there quite well!

    Still, I am probably going to experiment a bit more based on your input - after listening for a while with the 3300 Hz crossover point I will change the capacitor in my homemade Xovers back to what I had and relisten with the 2500 Hz crossover point again for a while, then make a final decision about where to leave it. The beauty of these wonderfully simple first order crossovers is the ease with which such switches can be made - a quick capacitor change and voila!

    If I did go back to 2500 Hz for the upper crossover - in a homemade Xover, how would one achieve the roll-off of the 2500 Hz-10 kHz in the mids (to avoid the doubling problem) that you achieve in the Bozak crossovers by going to the 16 ohm jumper? Obviously that isn't an option for me now, but I would imagine there has to be some other way of doing this in a home brew Xover?

    Anyway, thanks again for everything Pat - your info and encouragement are the reason I am now enjoying these fantastic speakers so much.
     
  17. 5th beatle

    5th beatle Forum Resident

    Location:
    atlanta, ga, usa
    Tobion Mods definitely worth it!

    Dr.J

    A tip-o-the cap from all of us Bozak heads for your detailed experiences you have shared. :righton: Spreadding the word is a good thing. Here's a question regarding 400 vs 800 crossover point. Your post mentions "gives less dark coloration, mids sound like true mids."

    Always thought Rudy engineered for no coloration, striving for the closest thing to "live" reproduction. Would another person listening to your new settings observe a "bright" sound much like JBL product? Or do they still retain that "warm glow? :laugh:
     
  18. DrJ

    DrJ Senior Member Thread Starter

    Location:
    Davis, CA, USA
    I think they still sound very warm (not overly warm - but a warm-sounding recording will come across that way and not sound like it was EQ'd to be brighter, if you get what I mean). Definitely not bright or cold or sterile. If there is any coloration it would be a TAD of midrange coloration - which I personally like. But it is a TAD - very, very subtle.

    Originally, it was more a problem (to my ears) of having everything going through the woofs up to 800 Hz with the original crossover set up imparting an overly dark coloration than these now sounding on the bright side. They are nice and neutral to me now.

    Example: a couple of the CDs I've been using heavily to adjust the driver balance are Steve's EAGLES GREATEST HITS and Doug Sax's SACD mastering of ART PEPPER MEETS THE RHYTHM SECTION. Both are as you know fantastic sounding discs. With my current settings, the tonality of these CDs sounds just right, letting the great mastering shine through. However, a bright CD - say, some of Ron McMaster's discs of vintage Blue Note material or the fairly recent JVC (Japan) studios remastering of Benny Carter's JAZZ GIANT or the remastered 4 CD Ella Sings Gershwin box - will definitely sound a tad bright, though definitely not unlistenable. Before I made the crossover changes, even CDs that I knew were bright (from listening on other systems) sounded on the overly warm side, so there was definitely some major coloration going on.

    Another example: Steve posted at one point in the past about how the original Dylan JOHN WESLEY HARDING CD is pretty much a flat transfer from the masters. He pointed out some spots to listen for that reveal this and what one should hear, and said abiliity to hear these things will require a revealing system. I picked up this disc and have also been using it as a test disc, and I can hear all of the stuff he mentioned easily - the bottoming out of the electric bass at the start of "I Am A Lonesome Hobo," the varying EQ on Dylan's vocal mic between tunes cut at different recording sessions, etc.

    So these speakers are pretty darn neutral and revealing!

    Hope this helps!
     
  19. Pat Tobin

    Pat Tobin New Member

    Location:
    Los Angeles CA USA
    Hi Tony - Thanks for the warm welcome. I forgot that you built 'roll your own' xovers rather than modifying the N10102(A) series. Nothing wrong with that - with the right values, home brew xovers are just as good. To lower the upper frequency output of the B-209B, all you need to do is change the midrange L (inductor) to 1.1 mH (1.0 would be plenty close enough). I have checked this thoroughly on the bench with instruments, as well as in the listening room. This one of those synergystic happenstances where it just happens to come out perfectly. I have never heard a better top end from a Bozak system than with this mod.

    Pat Tobin
     
  20. Pat Tobin

    Pat Tobin New Member

    Location:
    Los Angeles CA USA
    Bozak Midrange and General Sound

    Well, here I am starting at the beginning of time. (Well, April.)

    Regarding the comments about "The Bozak Sound:" The early Bozaks ('50s) were never an easy first sale. I worked as the repair tech in a prominent stereo store in the '50s (yes, I was barely out of diapers) and saw this first hand: In A-B comparisons in the listening room, most first-time buyers would opt for a competing system that had more flash bang in the sound. The Bozaks are just too true, while the competing iron jumped at you. Bozak was under a lot of pressure from the dealer network to make the Bozaks sound more like the inferior competition which was selling better. Most first-time buyers, after learning how little they liked hearing the same flash and bang in everything they played, would come back to the store in a few months and say, "Uh...could I hear those Bozaks again?" But this was little consolation to the dealers, who wanted Bozak to compete for first-time sales.

    The early Bozak systems were perfectly balanced - woofer to midrange to tweeters - and everything in phase. So in the early '60s, when the aluminum-coned B-209A/B midrange and B-200Y tweets came on stream, Bozak really fouled up the xovers. For starters, the polarity of the midrange was reversed. This resulted in a mid sound that was incredibly prominent without actually being louder. With the mid reversed in polarity compared with the woofs and tweets, the xover regions were cancellation notches. This 'isolated' the midrange in a manner similar to a picture which is surrounded by a wide, plain matte - it makes the subject more prominent. But the down sides were degraded overall smoothness and seriously degraded stereo imaging. Without any doubt, the reversed polarity on all Bozak N-10102A xovers should be corrected. (It is not crossed on the N-10102 xover, used with the early paper-coned B-209 midrange and B-200X tweets.

    The other issue was the Y tweets. They are about 9 dB hotter (louder) than the woofer and midrange (and the previous paper-coned X tweets). Presumably this was to enable inclusion of a Brightness control which could reduce tweeter level or make it higher than flat. Only a very few systems actually had the Brightness control, mainly the early Symphonies and a few B-302A systems. After just a year or so, the Brightness control pots were eliminated. A simple network consisting of a 25 ohm resistor paralleled by a 2.0 uF capacitor was put in series with the Y tweets. The results, by today's standards, are pretty awful. It results in a big hump in the mid highs, 5.0 kHz to 10.0 klHz, allowing the natural rolloff of the Y tweet above 10 kHz to go unaided. Can you say, "Disco?"

    It is not my job to help Bozak sell speakers in a tough '60s and '70s market. What I am doing is re-engineering the xovers to remove the strange tweaks and allow the world-class drivers to sing in their full, true voices, unhampered by the craziness of the '60s and '70s. To that end I designed a much better circuit to drive the Y tweeters. It is more complex than the bozak one, and works much better. It reduces the extra 9 dB level to match that of the woofs and midrange, flat, without the hump. Then, with 9 dB of 'extra' level to play with, it is used to extend the extreme high range, boosting from 10 kHz up to the tweeter's normal limit of about 16 kHz. The result is highs that are very smooth; no peaks, no dips, and a smoothly extended upper range.

    (The later xovers, from about '95, need even more help.)

    You wanna know what the punch line is to this: Dig: No one who has never heard their Bozaks with the '60s ~ '70s craziness exorcised, has any idea how great they can really sound! That sounds radical, but is very, very true.

    When the corrections are made, here is what you get, echoing Tony's comments:

    The TRUE Bozak sound is neither mellow nor sharp. It is just incredibly accurate. Let me magnify that point. Little-known fact: The B-209B midrange driver uses the same 'motor' as the 12" woofer! Same magnet, same post and plates, same voice coil diameter. Yet the ultra-light aluminum cone must cover a range of only about 2-1/2 octaves (with 400 Hz lower xover). Driving that with the woofer motor is a little like putting a hemi V8 in a VW. The B-209B, with enough driving power, will reproduce transients that many other mids don't even see.

    The woofer cone doesn't look particularly light, but the entire cone assy., including voice coil and vc tube, inner 'spider' suspension and outer suspension, all together weigh 39.5 grams. That translates, again, to superb transient response. The gutty string sound of a well-recorded bass can be so enjoyable on a Bozak woofer.

    Here's another important point. Quick transients in good-quality recorded audio can easily reach 10 dB (voltage) above average level. That's a factor of 20:1 in terms of power. Therefore, if your average level is 3 watts, the amplifier must be capable of a clean 60 watts if all the transients are to be heard without clipping. Sometimes it is said that Bozaks will play comfortably loud from a pair of 6AQ5s or the like - call it 15 watts. It is highly unfortunate when that is done, because few high transients are being heard.

    Rudy Bozak, in his many tutorial writings in brochures over the years, spoke about the 'educated listener.' It is clear what he means: those who have never heard Bozaks, properly configured and with adequate driving power, literally don't know what they are missing. They are judging from their experience base of speakers of less capability.

    Bozaks do have a sound character, and this is it: Properly set-up Bozaks reproduce, accurately, every kind of sound you can throw at them. They are great on classical, great on jazz and great on rock. And they don't give a hoot what is driving them - VT or SS - so long as the power is ample and very clean. There is nothing to support the opinion that Bozaks are best with VT amplifiers. I listened to my early mono systems with an MC-30 for years. When I converted to stereo I went with a clean SS receiver and have never looked back. Actually, the damping factor of the Macs is very moderate - around 12 - very kind to cranky, hard-to-drive complex xovers. The Bozak bottom end actually appreciates the much-higher DF of good SS amplifiers.

    I have unlimited admiration for the late Rudy Bozak. Nothing I do is intended to 'correct' any of Rudy's work. Were it not for the intense pressure from the dealer network, I am fully confident that the sound with my mods is exactly the sound Rudy would have liked. It took the speaker industry years, even decades to adopt some of the principles Rudy originated in the 50's. My work is dedicated to Rudy Bozak, in hopes that modern listeners can hear his genius through his world-class drivers as they were designed to be heard.

    There's lots more to say - about enclosures, bi-amping, tweeters a lot. This essay will serve as foundation on which to base any further (hopefully brief!) comments.

    With best wishes to all,

    Pat Tobin
    [email protected]
     
  21. Peter Breuninger

    Peter Breuninger New Member

    Location:
    Philadelphia
    Tony,

    Your Bozak mods are right on. Its far easier to do your own X-overs thus preserving the orginals for resale purposes.

    Pulling the tweeters and boxing them up on top of the 302 cab makes for a $5,000 speakers at $300.

    We did not want to go into the DYI aspect of Bozak mods in the Stereophile article since its a new thing for the mag.

    Hats off to you! (and Pat)

    Sincerely,
    Peter Breuninger
    Stereophile
     
  22. 5th beatle

    5th beatle Forum Resident

    Location:
    atlanta, ga, usa
    Tobin Mods to Bozak 302A Urban Speakers

    Dr. J,

    Thanks for the further clarification. Very reasoned and well thought out. Love Pat's lengthy tutorial. More info is better :righton:

    5th Beatle
     
  23. Pat Tobin

    Pat Tobin New Member

    Location:
    Los Angeles CA USA
    Oops - Misteak

    In my lengthy diatribe yesterday I stated that peaks of 10 dB (voltage, what most meters and 'scopes read) requires 20 times the power. Wrong - 10 dB voltage is equivalent to a power change of 10 times, not 20. My conscience got the better of me last night and I hit the calculator.

    However, don't extrapolate from that, that a voltage change of 6 dB equates to a power change of x 6, 15 dB voltage equates to a power change of x 15 etc.; it doesn't. It just so happens that the 10 dB voltage equates to power change of 10 times.

    This is what I get from releasing fingers before engaging brain. If you look at the time stamp of yesterday's post, it will give a clue to brain fade!

    But peak power requirements should be taken very seriously. Average power of 5 watts (a fairly loud normal listening level with speakers around 89 dB efficiency, as are the Bozaks) still requires a 50 WPC amplifier to pass all the transient peaks un-clipped.

    Peak clipping is not audible in good amplifiers. That is not happenstance - it is careful design. There have been plenty of mediocre designs in past years where even a brief overload would put the amplifier into a conniption that would magnify the problem. Modern good design allows brief clipping of of the amplifier to pass without affecting the rest of the audio. The ear is very insensitive to brief peaks that are clipped cleanly, which is how low-powered but well-designed amplifiers get away with it. But it is the accurate reproduction of transients, on Bozak systems driven with plenty of clean power, that gives music the incredible texture not heard on many other systems.

    Pat Tobin
     
  24. dnewma04

    dnewma04 New Member

    Location:
    Michigan
    Pat, I wholeheartedly agree with your comments on the required power to handle transients, and even though you mixed up some numbers, your original numbers were probably closer than the corrected ones. It's not at all unusual to have crest factors of 15-20dB or more. 10dB would be on the low end of what a typical classical recording would have, for isntance. Crest factor seems to be very related to type of music, however.
     
  25. Pat Tobin

    Pat Tobin New Member

    Location:
    Los Angeles CA USA
    Couple of points here. In the mid '60s Bozak began reversing the polarity to the midrange. While that does give a much more prominent midrange which isn't actually louder, there are a couple of severe side effects. One is that overall smoothness is degraded, the other is stereo imaging which is REALLY degraded. Imaging is excellent once the midranges are un-reversed. Second, about 'quiet tweeters,' many of the old paper cone B-200X had one side (one tweeter) burned out. Since the two are in parallel, the other continued but the noise was down 6 dB. The Y tweets are almost impossible to burn out because the aluminum cones are the world's best heat sink for the voice coil. Best wishes to everybody, Pat Tobin
     
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