Diffusori 3 vie Bozak B 302 A Century

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I haven’t seen too many comments on these forums about Bozak speakers. I recently acquired a pair of Bozak 302A Urban speakers from circa 1960. These are big, infinite baffle cabinet design speakers, 2 ft x 2 ft x 2 ft cubes, beautifully simple, “early 60s modernistic” styling, and the single prior owner kept them as new. Here’s an image from the web of the 1960 ad brochure for Bozak’s line, the 302As (with matching cabinet that I also have) are at top: http://www.hifilit.com/hifilit/Bozak/1960-4.jpg

I wanted to sing the praises of these speakers a bit here and talk about some very simple mods that can be made to the older version of this speaker – which I acquired – to bring them into the big leagues. I’m so happy with these speakers after mods that I have stopped worrying about spending big dough to buy Tannoy Concentric Golds off eBay that I wouldn’t even be able to listen to first. I still hope to pick up Tannoys someday, but now there’s no hurry, and I can wait until I can find a pair at a garage or estate sale for $100 like Steve did! 🙂 (I can dream, can’t I?)

I WILL POST PHOTOS LATER TODAY of the speakers and the mods when I’m home. I’m NOT generally a guy who goes into making major mods to vintage gear – my MC30s and Marantz 7 are stock, stock, stock and will stay that way – but the Bozaks are a bit different. First off they are less expensive to begin with, even if stock, so resale value isn’t a big deal. Second, the original Urbans I bought have some known major limitations that need to be addressed, but once they are, you get a world class speaker. So I took the plunge. Here’s what I changed, working with my dad over the weekend:

1) First off, the older/original version of this speaker had all paper cones – each speaker had one 12″ woofer (B199A), one 5″ mid (B209), and two small (2 1/4″ I believe) tweeters mounted side by side (B200X). The 12″ paper woofer is one of the things Bozak was famous for. It is superb – more on that later. The paper mids weren’t bad but not stellar either. The paper tweeters are frankly terrible.

Enter Pat Tobin, probably the greatest expert on Bozak speakers. His e-mail, in case anyone is interested, is audio.consultant@verizon.net VERY approachable, nice person who is very willing to help anyone get their Bozaks singing. He was for example cited as “the” Bozak speaker expert in a recent Stereophile article about the Bozak flagship model, the Concert Grand B410 speakers (massive suckers, take most of a wall up!): Concert Grand article ( http://stereophile.com/historical/1005bozak/index1.html )

Tobin has posted widely on many audiophile forums about the superiority of the slightly later aluminum tweets (B200Y) and mids (B209A) that Bozak developed, which are interchangeable in size with the older paper versions. I saw these posts in doing Web searches on Bozak speakers and then contacted him and after speaking was convinced this would be worth pursuing. So first thing I did was pick up a pair of Bozak 1970s 313s with whipped external cabinets – but which had pristine aluminum mids and tweets – and this weekend dad and I pirated those and installed them in the Urbans and took out the paper mids and tweets.

2) Second, a couple of crossover mods are optimal in improving the sound of the Urbans. First, the old Bozak N10102 crossover in the original Urbans has a huge cap box with 6 big paper caps. These are all out of spec and really unnecessary nowadays – there for a couple reasons originally that are no longer salient. 4 of the caps were for the midrange network, 2 for the tweets. Working in the cap box would have been a nightmare and I doubt worth the effort. Tobin agrees, and recommends eliminating this cap box and replacing the 4 mid paper caps with a single 25 uF Dayton Audio grade caps and the 2 paper tweeter caps with a single 8.2 uF cap. We did that. Much cleaner.

3) Finally, the biggest issue with going to the aluminum tweets is that they are much “hotter” than the original paper tweets, about 9dB louder. So to balance them with the rest of the cones a tweeter network mod is required, basically installing a 2.0 uF cap and some resistors (attenuation circuit). This mod also allows the tweets to extend above 10 kHz – in the original crossover, the highs roll off rapidly at 10kHz, a purposeful limitation within the crossover and not an inherent limitation of the aluminum tweeters. We used Tobin’s instructions to make this modification as well.

4) So in summary, the circuit diagram below shows the entire modified speaker circuit, including Tobin’s tweeter network mod WITH TWO SMALL ERRORS – first, the 8.2 uF cap in the tweeter network should obviously come BEFORE the attenuation mod. We wired things correctly, just didn’t have a chance to redraw the incorrect diagram. Second, the numbers of the cones my dad put next to the speaker symbols are the older paper ones rather than the updated aluminum ones.

5) BEWARE! We also found some wiring mistakes FROM THE BOZAK FACTORY! Polarity was reversed on one of the speakers in the wires going from the back of speaker hookup terminals to the crossover network input! We fixed that. I’d suggest always checking Bozaks for stuff like this, apparently there were some issues.

6) Finally we also replaced the fiberglass baffling inside the cabinets with new fiberglass insulation material. The old stuff was crumbling to even gentle touch, a mess. Nice and clean now.

SO HOW DO THEY SOUND? In a word, unbelievable! The 12″ paper bass cones Bozak made were amazing. I’m still learning and have a lot of speakers to hear, but the transient response on these woofers is amazing. Plus I’m coming from much smaller speakers with 8″ woofers, so to me the visceral impact is simply amazing with the Bozaks, drums sound like drums and bass like bass, and it’s not bloated or rounded off bass – you hear the growl and twang of string bass on jazz recordings, and it sounds different than the round fatness of the electric bass on a disc like Steve’s BOB JAMES IN HI-FI – just as it should be! The aluminum mids are to die for – you listen to ELLA AND LOUIS and they are standing there in your listening room (particularly when these are paired with MC30s!). I am hearing vocal nuances I have never heard. And the aluminum tweets are a HUGE improvement over the paper versions, which hissed and spat in response to anything in the 5-10 kHz range. With the aluminums, it’s smooth as silk and Tobin’s crossover mod allows you to get extension above 10 kHz. I haven’t done any test tone testing yet but I’m guessing they go all the way up to 20 kHz as modern speakers do.

The only remaining weakness I can see with these speakers is the tweeter placement. Tweets are placed in front of the bass cone as you can see from this brochure (it’s the 207A woofer/tweet combo:


…and the bass cone is mounted down low in the cabinet. This places the tweets only about a foot off the floor, below the midrange cone. No good. Next project which will be easy will be to dismount the tweets from the woofer bracket and build a very small enclosure for them to sit on top of the Urbans. We’ll run the tweeter wires through the back of the cabinet and up on top. This will get all the cones properly oriented and at that point we’ll be done. Make no mistake even with this flaw the highs are gorgeous, but clearly this needs to be addressed based on my listening tests (sitting on floor versus in usual chair – highs better when I’m on the floor).

So anyway, wanted to share this for a few reasons:

1) Bozaks are fantastic speakers, don’t let anyone tell you otherwise. Maybe one of the best bargains in vintage speakers. Altogether I paid about $500-700 for these speakers, including buying the upgraded aluminum cones and all the crossover mod parts – PLUS I also got the matching audio component cabinet Bozak made to go with them. I can’t be exact about price because I bought these as part of a larger sale where I acquired an entire single prior owner vintage system for a lump sum. Bozak was notorious for not publishing speaker sensitivities but I’m guessing based on where I have to put my volume knob on the preamp, as well as the era of manufacture and use of paper and very thin aluminum cones that they are in the 94-96 dB/watt range. Great for vintage tubes.

2) Bozak Urbans will likely require some work to get them up to snuff. If you plug them in as is, stock, you’ll be disappointed, especially with the older all-paper cone versions. Very boxy, dull sound with terrible highs. The contrast after the modifications is amazing, they become world class speakers. I don’t think you need to worry much about resale if you do these mods – in fact, I’m guessing the value of mine has not been hurt and may actually have gone up with the “Tobin mods.” But I could care less because I’m not selling! 😀 The combo of the paper 12″ woofer and the aluminum mids and tweets is to die for.

3) Even with the mods, these speakers still have a wonderful, “vintage” sound – and it’s primarily the amazing midrange cones that provide this. So don’t worry about making them into something they shouldn’t be, I view this as bringing out their best. They sing with all kinds of music but I have to say, they sound most magical with well-recorded material from the 1950s-60s.

4) Anyone who can read a circuit diagram and solder can make these modifications! Totally simple with Tobin’s help/guidance. I’m the poster child for “if I can do this, anyone can do it!” And if you’re still gun shy about doing the mods, Tobin will make the crossover mods for you for $60 if you send them to him. A really, really great guy who in my book has great ears – what he said the mods would do to the sound is exactly what I’m hearing.

5) Finally: if you buy the slightly later Urbans that already have the aluminum mids and tweets, beware: you will STILL need to make the crossover mods Tobin recommends because Bozak did something weird to the tweeter network on that later model – generating a really off-putting “hump” at 5-10 kHz which sounds harsh and fake, plus the highs still roll off sharply at 10 kHz. Good news is that the later N10102A crossover in these later Urbans can be modified just exactly as the older N10102s can – they end up looking and sounding just the same as you will rip out the crappy sounding stock tweeter network.

Happy to field questions/comments – and I’m hoping there are at least some other Bozak fans out there.

Di cosa si tratta: Diffusori 3 vie

Produttore: Bozak

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Diffusori 3 vie Bozak B 302 A Century