Converting a Fisher X-10 to Bozak B-401 Rhapsody

Discussion in 'Speakers' started by sruddy, May 17, 2017.

  1. Retrovert

    Retrovert AK Subscriber Subscriber

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    All of those materials, alas, are far from ideal for sound deadening in an infinite-baffle application.

    The cabinet is small, true, but it is not too small; Bozak sold smaller bookshelf units, like the B-313 and B-502. While those were arguably too small for a B-199 (about 2 cu ft instead of the requisite 5 cu ft), Bozak used a similar padding/curtain layout, just scaled down. It will work, but the bass will not be as low as the larger variants.

    I suggest you discard the UltraTouch in favor of cotton batting. When Biggles had his problems with his B-302 project and was ready to give up, I suggested he replace all of the Ultratouch and move to a swinging curtain using upholstery batting. This completely solved the problems. Others have had similar results.

    The core issue here is that UltraTouch is far too dense and has no loft. I cannot say if this is a consequence of the factory compression for shipment, or inherent in a ground-up cotton instead of fiber approach, but it is a problem.

    Since UltraTouch is a dense mass it lacks interior voids crucial for sound deadening. It really is totally unsuitable for damping on a speaker cabinet, particularly an infinite baffle cabinet requiring a loose curtain . Again, Biggles will confirm this for you, as will others who had similar experiences.

    The curtain is a key part of the Bozak infinite-baffle cabinet.

    The outside of the flowerpot is less critical, but the goal is to remove all interior reflections and ensure the backwave is lost. This is why Bozak covered the crossover with a piece of batting. Many times these interior reflections will result in resonant peaks which cause non-linear behavior that degrades the quality of the audio, anywhere from suble to substantial.

    The short distance likely does not matter because it is at the top. As long as the curtain remains free to move, all should be good. The pressure waves will migrate around it. The point is to lose the backwave so it can't bounce off the sides and rears to return. That's what the curtain does.

    I suggest re-creating the interior of a B-302 and repeating your testing.

    Using software which will generate an SPL plot may show you details which you cannot easily distinguish by ear.
     
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  2. sruddy

    sruddy Well-Known Member

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    I must be missing something about the proper batting material. Your first post said cotton batting and that's what I purchased. Would you please supply a link to the material I should use?
    Thanks!

    When I get it done done correctly I'll do a frequency response check using a good mic and software. Last night I used an RTA app and it looked pretty good. I'll post a screen shot later.
     
  3. drbiggles

    drbiggles I like bacon Subscriber

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    Hey,

    I believe what Retrovert was referring to was the original Fisher batting and Ultra-touch. The new cotton upholstery batting is correct. The swinging curtain is your secret weapon. It's an integral part to a proper infinite baffle system. I used 3M spray adhesive on the flower pot to get the cotton batting to stick. I would suggest removing your newly installed cotton batting on the tops of your cabinets. Make a strip about the width of your cabinet, leave enough room for it to swing freely on the right and left sides, and bottom. Attach it to the top, leave enough length to run a tongue to the front of the speaker. Then, just cut a rectangle piece to attach from the curtain to the back of the cabinet.

    Biggles
     
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  4. drbiggles

    drbiggles I like bacon Subscriber

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    Oh, it's tough to know from here. But the new batting looks a little on the thick side, we're looking for a good 1" cover.

    Biggles
     
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  5. Retrovert

    Retrovert AK Subscriber Subscriber

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    Biggles is correct in that I was responding to your statement about using loose fiberglass, sheet fiberglass, or UltraTouch.

    The 1" cotton upholstery batting is what is required. It is a fiber padding instead of a ground compressed cotton sawdust like UltraTouch.
     
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  6. Retrovert

    Retrovert AK Subscriber Subscriber

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    The importance of the currtain cannot be strongly enough stressed. Elsewhere Biggles has photos of his UltraTouch curtain which was too dense, too thick, and too narrow. Ruined the sound to the point he was ready to give up. When the proper curtain was installed the sound profoundly improved.

    An infinite baffle system is very different than the cabinet types to which most people are accustomed. The backwave is eliminated. Not used as a restoring force as with Acoustic Suspension. Not delayed and emitted from a port like a T/L aka Acoustical Labyrinth. Not pushed around a shelf as in Bass Reflex. Removed.

    So internal reflections must be absorbed, the pressure wave must be allowed to expand (like an exhaust muffler) and migrate into a special region where it is lost.

    This is exactly correct. The curtain must swing. It must be loose and able to be moved by the pressure wave so that this will be absorbed instead of reflected.

    That is why UltraTouch is unsuitable. It is hard and reflecting, instead of being absorbing.

    It is, again, important to ensure the batting remains in place for the surfaces to ensure no reflections. No matter where the backwave goes, it must be absorbed or passed into an area in which it can be absorbed.
     
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  7. sruddy

    sruddy Well-Known Member

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    It is Organic Cotton Batting Approximately 1 Inch Thick x 27 Inches Wide x 80-81 Inches Long. However it can fluff out much more than 1" and looks closer to 2" when installed. I can mash it and staple it down to get 1" if you think it's needed.

    Here is what I get with the mixed batting as seen in my last post. I'll install a curtain tonight and post the reading after that.

    [​IMG]
     
  8. drbiggles

    drbiggles I like bacon Subscriber

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    Hey,

    The graph looks interesting. Naw, don't mash it down. I'll sit here and wait for the curtain hanging!

    Biggles
     
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  9. sruddy

    sruddy Well-Known Member

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    Ha Ha, Ok I just have to find something that is 1" thick. When this stuff swells it will be too thick. I could try and pull it apart but it would be difficult to get even, and it may fall apart. Good thing is they sound ok to me, so waiting won't be painful at all!!! :)
     
  10. Retrovert

    Retrovert AK Subscriber Subscriber

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    Biggles is correct: do not mash.

    As I explained, the loft (how fluffy the batting is) determines the sound absorbing/deadening properties. The dense nature of UltraTouch is why it fails in loudspeakers.

    If you make the batting dense you will be adding a hard surface that cannot absorb the backwave.

    What you want is a light, fluffy, easily moved curtain which absorbs energy on both sides, and forces the backwave to migrate around it where it expands into the secondary region and cannot escape to put backpressure on the woofer.
     
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  11. Retrovert

    Retrovert AK Subscriber Subscriber

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    The material must have some coherence to ensure pressure cannot migrate through it like cheesecloth.

    Pulling it apart will create thin spots and will reduce the ability to absorb sound.

    The curtain will transport you to the land of ok to the land of awesome.
     
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  12. sruddy

    sruddy Well-Known Member

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    Well I wish I could use the material I have because based on your description it sounds perfect. Unfortunately it will be too thick for the limited depth of the X10 cabinet.
     
  13. Treyphan

    Treyphan New Member

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    Hello all in AudioKarma Land!! This is my first post, though Ive "lurked" for years and years, probably over a Decade. I must say, this combine knowledge of this site has been INVALUABLE to me during my Audio Travels. The very friendly helpful atmostphere is also refreshing on the Internet! So I must say, Im glad to now be an active part of the community.


    So, now that I got that done. My question is in Regards to Converting the Crossovers on my Newly Acquired Bozak 302A Century's, a speaker I have longed for sense the first time I set eyes/ears on them. I fell in Love instantly. Well to my luck, i looked on that big internet auction site, and found a set in basically MINT condition, only an hour from me. It was a local sale only....I jumped fast. Based only on the Pics, and the Sellers word that they worked.
    I got there and they indeed worked. And had the Aluminum Tweets and Mids. They are sequential serial numbers(which is always nice) and have a build date of Feb of 1973.

    I attribute their shape to being somewhat later in the 302 run, however someone really like these speakers and took great care. Ive been inside and everything looks perfect. The batting is all there, including the curtain midway back. It looks like it was just installed.

    Now my next move is to 'Tobinize' the x-overs. Ive re-capped and modded a lot of crossovers in my day. I also know my way around a soldering iron very well. I can read schematics 'pretty-well'.....for the most part.

    I do plan to go right to the L-Pads, in lieu of the 7.5 ohm resistor in the Zobel network. (I did order some just incase though, and to try it that way possibly). My parts list to PartsExpress was as follows:

    2 x 50uf Dayton DMPC 5% Caps
    2 x 8.2uf Dayton PMPC 1% Caps (actually I have these on hand, in my "stock")
    2 x 2 uf Dayton PMPC 1% Caps (Have these on hand too)
    4 x 4ohm Mills Resistors
    2 x 8 ohm L-Pad 100 watt
    2 x 7.5 ohm Dayton Resistors

    Im also getting the flower pots and batting for the Mid 'Covers'.

    I'm wondering if someone can please explain to me, how I wire the L-Pad into the X-over circuit. I realize it takes the place of the 7.5 ohm resistor. I just need to know which tabs go to where. I read earlier in the thread that only 2 of the 3 tags are used. The 8 ohm 'leg' and the negative, i assume? Forgive my ignorance. Perhaps @drbiggles or @Retrovert could help me? Or anyone with a pic of how they wired the L-Pad in. I'd be very appreciative!:)
    Also, before you ask why I paid for 100 watt L-Pads, I realize this is way overkill, however, I wanted the extra assurance. These will be used with a Pair of MC-240s, and Ive read if the L-pad were to fail....it could take a PT out on tube amps. Cant risk that, those McIntosh PT's are impossible to find, and I dont even want to know how much if I did. lol Perhaps its still overkill, thats ok to me....provided it doesnt effect SQ, if it does. I have no issue changing.

    I hope Im not beating a dead horse here, with questions about the Tobin Mods. I just want to try and get these right during the first go. Or close to the first try.

    Thanks!!
     
    Last edited: Jun 25, 2018
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  14. drbiggles

    drbiggles I like bacon Subscriber

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    Hey man,

    Welcome to AK!

    OoOoOOoO, tasty bits. This is an easy one. Your l-pad has 3 prongs. Put one lead from your meter on the center prong. Put the second lead on another. With your meter set to ohms, move the l-pads knob back and forth. Does it go from 0 to 8ohms? Those are the two prongs you use. If it it doesn't, leave the center lead attached and try the other prong. Then just wire the l-pad in where the 7.5ohm resistor would have gone. 2 leads on the resistor, 2 leads on the l-pad. That's it!

    Biggles
     
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  15. Retrovert

    Retrovert AK Subscriber Subscriber

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    Howdy. Welcome to Bozak Cult.

    Come on in. Bozak party still has plenty of punch and canapes.

    Looks like you have it covered. Only addition I would make is to suggest you bypass the capacitors with 0.1 uF and 0.01 uF placed in parallel. This helps to improve the response on higher frequencies.

    Congratulations on bringing your A game out of the chute. You clearly did your homework. Impressive.

    Looks like Biggles has you covered on the wiring.

    No worries, it does not affect sound quality for this application. As a general rule, however, larger resistors have lower Johnson-Nyquist noise than smaller ones. But that noise is sooooo tiny it won't matter for this application. The benefit comes from using larger sizes in an amplifier.

    The I think you're discussing is if the wiper shorts out, and the impedance goes to zero. Because of the nature of an L-pad that is improbable. If the L-pad was overdriven the resistance element would burn out and open. Current would continue to flow through the other drivers so the amplifier would not see an open circuit.

    Various protective tricks exist, such as paralleling a high value resistor with the outputs or using flyback diodes, to protect output transformers. Subject for another thread.

    Outstanding speaker choice, BTW.
     
  16. Treyphan

    Treyphan New Member

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    Well Thanks for the warm welcome!!


    Thanks!! If I might be more bother, prong goes to where? Thats the only thing I think Im still wondering. Like the Center goes to xxxx, and the 8 ohm prong goes to xxx. I really appreciate it! Parts should be here today. So ill be starting hopefully this evening and hopefully have the time to complete the mods by this weekend and be listening to these beauties!!


    Thanks!

    I like the idea on the 'bypass' caps. Being such low values, high end brands are still relatively affordable. So I'll be placing an order for them soon. Anything that'll help!

    I try to do as much research/homework as I can before asking lots of obvious questions that have been answered previously. Helps speed things up, and I don't have to bother people as much. lol

    Regarding the speaker choice, thanks! Im glad I passed on a few other Bozak's I saw. Some of the bigger ones, while gorgeous, are just too big for my home unfortunately. The 302 Century's are just the most gorgeous speaker Ive ever laid eyes on. So to have a shot a pair like this, well I feel honored to have such a piece of Audio History. I mean they have a few small 'issues' on the cabs, but nothing worth re-veneering which I've done to many speakers. I just cant do it to them. Ill restore the original finish.....they'll be perfect!

    Thats why the Tobin mods appealed so much to me. They were done within the parameters Rudy originally could've/would've/did use had he been able to freely make Speakers for Speakers sake and not have been pressured by Marketing and Bean Counters! From the research Ive done anyway, thats what it seems happened with some of the crossover points and whatnot.

    I think he would've approved whole heartedly of the Late Mr.Tobin's Mods, and the updates guys like You and DrBiggles bring to them! I think he'd be on the forum 'playing' along with us all, thrilled people love his speakers over a half a century later!!! But who am I to assume what Rudy would've thought. Thats just mho.

    Anyway, Im off to get some work done. I'll be back to update with pics and questions I'm sure. Probably post pics of the x-overs when I've got 'em ready to go, imo anyway. See what the real experts think. Again, I appreciate the help so much from both of you!!!

    Also, is it worth replacing the inductors? With Modern ones?
     

     

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  17. drbiggles

    drbiggles I like bacon Subscriber

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    Oh yes, way easy enough to get it done by the weekend.

    Open up your n-104T-Tobin.jpg, or whichever jpg holds your 104T schematic.

    Look to the left, the tweeter's circuit that's labelled B-200Y.

    Notice the little squiggly line that's labelled 7.5. That's the 7.5ohm resistor.

    You can see two very small black dots on the right and the left of the 7.5ohm resistor. Those black dots denote where the resistor is connected into the circuit.

    Instead of the 7.5ohm resistor, wire in the l-pad.

    The center prong on the l-pad goes to one of those two connections. The prong that gives you a variable 8ohm reading, goes to the other. It doesn't matter which wire from the l-pad goes onto which.

    I hope that helps!

    Biggles
     
  18. Retrovert

    Retrovert AK Subscriber Subscriber

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    You won't need to spend money on fancy capacitors for bypass; no point at all. The standard PP film sold by Parts Express under its Dayton house brand are perfect. The ESR of the film is nearly non-existent to start, and bypassing drops it to nothing. The other properties are excellent as well. The goal is to speed up the dipole alignment at higher frequencies because it takes time for the propagation of polarity to occur.

    That's an excellent practice as having done the homework you are able to understand the answers.

    Far too many purported audiophiles purchase giant speakers which cannot properly converge in the available space. The Concert Grand is one of them. Needs a giant living room. Even the Symphony, half a Concert Grand, requires about twenty feet. Biggles can tell you that his Symphony project was not a success because he had to put his back against the wall to get even quasi-convergence. Most of us do not have a living room that is deep enough.

    Infinite baffle is a sound that few have heard because the size is deprecated. Interesting how everyone went from low-power amplifiers and giant speakers to high-power amplifiers and tiny speakers.

    The originals should be fine provided you have air cores and copper wire.

    Many of you are old enough to remember how the rise in copper prices led to aluminum house wiring, and how the poor contacts and high resistance led to ohmic heating and home fires. While the speaker crossover won't burst into flame and burn down the house, it does impact the sound.

    In the early 1970s Bozak switched to aluminum wiring and ferrite-core inductors to reduce the copper content and save manufacturing costs. The aluminum wiring may be identified by the the pigtails crimped onto the inductor, since soldering to aluminum is problematic because of the surface oxide layer. A ferrite-core inductor may be identified by the grey slug in the center of the inductor. These lower-quality inductors were used across the board, even in the Concert Grande's crossovers.

    Worse yet, the later crossovers often placed both inductors adjacent and in the same plane, which causes inter-inductor coupling, just like an air-core transformer, where each inductor induces a current in the other.

    So the later crossovers may require complete rebuilds to remove capacitors and inductors, and then re-alignment of inductors to not be planar.

    The advantage of modern inductors is lower DCR which reduces IR losses at high power. That's not going to be an issue with the Bozaks, as the drivers are very efficient. No reason to shell out for those as the originals will be perfect. Certainly no need to purchase ribbons.
     
  19. Treyphan

    Treyphan New Member

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    Cool! That was my exact question....if it mattered which wire went to where on the resistor. So that helps immensly! Its the little things. lol

    Yeah, the mods don't seem to time consuming at all. Just hoping I can squeeze it in, however, if my previous ventures mean anything.....ill MAKE SURE to find time. Even if I have to skip sleep...who needs it anyway??! ;)

    Great info!! Thanks so much! I'm not old enough to remember the rise in copper prices back then. Wish I would've been around for the "Golden Age" of audio! That's why as a 38 year old I'm trying to re-create it! I love new equiptment too, no doubt, but something about the 'Classics' pulls me in! I also think when the Equipment is brought to spec, or better, and well maintained and SETUP, it can hang with the best of todays "Top" stuff.(maybe not 6 figure stuff) Again, thats just MHO.

    Well, I cant wait...nearly foaming at the mouth!!

    Question on Speaker efficiency, now that you bring it up....do you know what the 302a's Efficieny is? I haven't found that info yet. I'm a generally curious fella, but also want to know because I'm wondering if 1 MC240, restored and with strong output tubes at around 55wpc, is enough to really drive these Bozaks's to where they need to be? I'm in the process of finishing a 2nd 240, that I had planned to use in another system or sell/trade for an MX110 pre, but am now thinking I'm gonna run both in mono for about 85-100 McIntosh tube watts per Bozak. Unless that's overkill...well 2 Mc240's is overkill....but you know what I mean.

    Or would these do better with a bigger power SS amp, say a Carver M1.5t at 350 x 2? Decisions decisions.......
     
  20. Retrovert

    Retrovert AK Subscriber Subscriber

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    Pat Tobin had stated that the Bozaks are 97 dB/W.

    Ummm, yeah. You're more than covered. Most listening is done at 1 WPC, and the rest is headroom for transients.

    The advantage of bi-amping is eliminating cross-modulation and much of the crossover. Midrange can be done as first-order, instead of second-order.

    I wouldn't worry about having enough power with 55 WPC.
     
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