Bozak 302a Tobin Mods 2015

[Omegaman]

I perform the Tobin crossover modifications to my 1968 Bozak 302a Centurys

Here is what I base my changes on

From Pat Tobin:
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"




The cabinets were stripped and I used Watco Danish Oil on them


B-200Y tweeters
B-209B mid
B-199A Woofer



Pat Tobins latest crossover schematic


The original B-302a crossover. If you can make out the writing this original schematic shows the original coil connnections are on the lower connector
L1 - tapped at 509mH but provides a 1.1MH tap (Upper connector)
L3 - tapped at 1.58MH but provides a 3.15MH tap (Upper connector)

When performing the Tobin mods, use both upper connectors


Parts express order list (I did not order the 25uf caps) as I used 50uf caps


These images were collected from those who have gone before

 

[Omegaman]

Teardown and rebuild

More from Pat Tobin:
"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"





Here is the original crossover


With the crossover disconnected from the drivers, Ohm out the drivers




I just discarded the old can. Some re-stuff it and that is to be commended but I went for utility
Plus I cheaped out and did not use connectors, just silicone to glue the caps down

I just soldered everything together and did not use screw connectors

The tweeter circuit

 

[Omegaman]

The B-200Y tweeter circuit

Not fancy, but functional

 

[Omegaman]

The B-2009B mid-range and B-199A Woofer circuits

The B-2009B was wired from the factory backwards so that gets corrected per Tobin mods
edit: Not backwards, just one way and Tobin mods suggest another way per schematic

Plus the coil tap was moved to the top connection per Tobin Mods

The image also shows the B-199A coil tap moved to the top connection

 

[Andyman]

The B-2009B was wired from the factory backwards so that gets corrected per Tobin mods

Plus the coil tap was moved to the top connection per Tobin Mods

The image shows the B-199A coil tap moved to the top connection

Nice job on catching that. When I built mine I had to do some digging to find that out.

Whenever folks post that schematic, they should annotate it to let other know the 3.0 mH coil is acquired by simply moving the lead to the rear tap. Tehre's no need to buy a new coil as some have done.

Let us know how you like the mods.

 

[Omegaman]

Finshed project

Rinse and repeat...and we are done

I also glued a copy of the schematic to the bottom of each crossover.

 

[Omegaman]

They sound fantastic, total success

 

[Brian]

Rudy took a lot of pride in his crossovers as to meticulness of layout and parts. He hated the sloppy ones the likes of AR and KLH were hacking together. Wonder what he would say of yours? He did not miswire them if you are talking of the midrange. It was done on purpose under protest after being pressured by the investors who felt it made them sound more "modern" and competitive in the market.

When I discovered it with my set on querying the gurus they said to reverse the wires to get back to the original design. It was a remarkable improvement, like night and day. His investors should have stayed in a corner wearing their green visors running their fingers through the coinage like Scrooge McDuck and left Rudy alone.

 

[Omegaman]

Thank you Brian

 

[twiiii]

You guys need to read the story about midrange polarity on Bob Betts Bozak web site. He was Rudy's head engineer when all the flap was going on. Just Google Bozak. The debate still goes on at the Bozak owners web site on Yahoo. All my Bozaks polarity are as original, is my answer to the debate. 2 Grands, Tri- amped with super tweeters, 2 Symphonys, one bi-amped with super tweeter, one Symphony stock. 2 B-313's stock. All have new caps at original values.

Grands, L&R, one Symphony C, One Symphony rear, B-313 sides.

 

[Omegaman]

Thank you Twiii

I was not aware of any controversy over the mid-range polarity.

This thread is about Pat Tobin's crossover modifications and his schematic says to wire it this way

 

[Andyman]

I was not aware of any controversy over the mid-rage polarity.

This thread is about Pat Tobin's crossover modifications and his schematic says to wire it this way

Yep. Tobin says to switch them, others, like Brian, feel it's not right.

They're your speakers; do with them as YOU please since you're the one listening to them.

 

[Brian]

No, I found switching the polarity on the original improves the originally intended sound. I have never looked at Pat's crossover so do not know if the midrange polarity is opposite the woofer and tweeters or not. He may have built his design to switch them. I just ran into this situation with a pair of Klipsch Heresy II crossovers. One had the + going to the - terminal. This was the ealry hardwired crossover. The later pcb based crossover switched the polarity on the board so the +on the board went to the + on the tweeter.

 

[lunchbox22]

The speakers look great! Glad to hear that the mods worked out for ya.

 

[amante]

Looks great! Omegaman, I may send pics of my crossover work to you just for double checking if ya don't mind. Going to use your parts list, too.

 

[tensleep]

Your crossover modification looks great! I really like my modified 302 as well.

 

[litefootdan]

"Rudy took a lot of pride in his crossovers as to meticulness of layout and parts. He hated the sloppy ones the likes of AR and KLH were hacking together."

Brian, Why the jab at AR and KLH????

 

[Andyman]

No, I found switching the polarity on the original improves the originally intended sound. I have never looked at Pat's crossover so do not know if the midrange polarity is opposite the woofer and tweeters or not. He may have built his design to switch them.

Sorry, I misunderstood your post earlier. As you can see in the pic below, the midrange is opposite the woofer and tweeter with the green lead from #2 going to the caps rather than the coils. Tobin moves the green back to #2 input and the coil.

Original crossover.

Tobin schematic and modded crossover

 

[bshorey]

Thank you Twiii

I was not aware of any controversy over the mid-range polarity.

This thread is about Pat Tobin's crossover modifications and his schematic says to wire it this way

I've been reading a lot about these lately, as I'm going through the Tobin mods right now myself.

I believe Pat recommends switching from the reversed polarity mids to the straight through mids. If I'm not mistaken, this is what Brian said he did, and liked the results.

bs

 

[Sam Cogley]

Early Bozaks had the mids in the same polarity as the other drivers. The switch to reversed polarity occurred sometime in the early 1960s, I think.