Help with Bozak B-302a Century modded with JansZen 65 Electrostats? Thoughts on how to proceed?

[mgolan]

Hello -

I'm a longtime reader, but this is my very first post... please bare with me as I'm a beginner.

I just picked up a pair of what I believe to be Bozak B-302a's that have been modified - the original mid driver and tweeters had been removed, and replaced with a JansZen 65 in each of the cabinets.

I've been looking to upgrade my speakers from a pair of Klipsch RP-160M bookshelfs to a more substantial set speakers for my living room. Although I've never been fortunate enough the hear either Bozaks or JansZen electrostats in person – based on comments I've read from AKers over the years – I was excited when a pair of these Bozaks popped up locally and I grabbed them right away.

I got home... plugged them into my Marantz 2245 and played a few records. All I can say is... WOW! Even though they're not working 100%, I was immediately amazed, they just sound lovely.

I was hoping some folks might want to chime in with some thoughts and point me in the right direction to get these in top working order.

First off, I'm curious on anyone's thoughts on the Frankenstein Bozak/JansZen combination. I have no frame of reference for how the stock Bozaks would compare sonically, but I am sure impressed with what I'm hearing.

Second, only one of the JansZen appear to be outputting any sound, so my first step will be to try and identify what's causing the issue and make this repair.

I have not yet opened the cabinets to see what changes were made to internal components, crossovers, etc, and I'm not sure I'd know what I'd be looking at, but I am hopeful to learn more on this project. I have been practicing my soldering skills, so I am confident I can make the repair, but I am a complete beginner when it comes to troubleshooting/technical side. Does anyone have any recommendations on how to get started with repairing JansZen 65 tweeters?

Lastly, I wanted to provide some of my thoughts on the sound of these speakers themselves, and see if anyone has any suggestions in that area as well.

Since there is only one set of tweeters running at the moment, my first step will be to get both in working order and spend some time listening to these so I can form a better opinion. That said, I do feel that there may be a bit lacking in the upper mids... perhaps because the mid driver was removed. Again, I have no reference for what the stock cabinets sounded like so its hard for me to say.

I will say that the quality of sound coming out of these is already incredible to me - the presence and detail, just perfectly balanced and tight bass, created an amazing sound stage with my eq settings mostly flat - definitely not what I am used to. I felt like I was sitting in the room with the musicians, and after several hours of listening I did not miss the upper mids I felt were lacking initially. My hope is that getting the other tweeters functional will only help in this area.

That said, I was thinking that perhaps it may be worthwhile to take it another step - and possibly add a passive mid driver back to the cabinet somehow. Any recommendations if this would be a worthy addition? What mid driver would you recommend? Should I try to source Bozaks? I'm curious what you guys think!

Anyways, thanks in advance to those that chime in. Any help is greatly appreciated. Pic included below.

Mike

 

[Retrovert]

Welcome to AK!

Nice score! Those were the ones from Cleveland, or something like that? Saw those, but I'm not allowed to buy any more large speakers.

The JansZen are doing the work of the midrange and tweeter, which gives rise to some question as to the crossover points. You'll need to replace the capacitors, and maybe make some other modifications because this is a very non-standard configuration.

Does the speaker have an N-10102 crossover in it? Or something else?

If the speaker started life as a three-way B-302, with a B-209 midrange and dual tweeters on a bracket in front of the B-199 woofer. (The woofer-tweeter combo is a B-207.)

But if it started life as a two-way B-300, without a B-209 midrange (just a hole covered with plywood) it would only have had a capacitor for the tweeter crossover, the woofer being mechanically limited to whatever it did. The B-300 was Bozak's entry-level unit, with a later (and expensive) upgrade of a crossover and a midrange.

The normal crossover point for the three-way B-302 is 800 Hz and 2,500 Hz, and for the two-way B-300 it is 1,250 Hz. So you can see why it's important to know.

BUT, it gets more complicated than that.

The Tobin modification for the B-302 normally drops the woofer down to 400 Hz and runs the midrange from 400 Hz to 2,500 Hz instead of from 800 Hz to 2,500 Hz. That won't work for you because the frequency response of the JansZen 65 is 700 Hz to 30,000 Hz, so the low-end can't go down to where Tobin places it. As a result, crossing at 800 Hz is likely the lowest one would want to go and it's above the lowest reproduced frequency.

But that's no guarantee that if this was a B-300 the same tweeter capacitor was re-used as the crossover, so ti's crossing at 1,250 Hz, not the standard 800 Hz of the N-10102.

Plus there's the fact that the JansZen 65 normally included a second-order LC crossover. So that might be there, crossing it at 700 Hz.

If you can provide the model number from the sticker on the back AND a photo of the interior, that would be helpful.

Given the above guesses and facts, I'd speculate this started as a two-way B-300 crossed at 1,250 Hz and the modification was purely to remove the tweeters from the bracket and install the JansZen 65, everything else is the same. So the woofer is mechanically limited to whatever it does, and the JansZen 65 kicks in at 700 Hz. If that's accurate, you'll likely want to drop the woofer point to 700 Hz using an inductor to remove the woofer's contribution (distortion) to the midrange, and replace the capacitor in the JansZen 65 as it is degraded and muffling your midrange/tweeter signal.

I have a pair of JansZen 1-30s for my Bozaks, but the power supplies need to be rebuilt and I was going run the normal Bozak tweeters to 8k Hz and then switch to the JansZens. That should preserve the original Bozak voicing. This is why I've carefully considered all of the above.

The Bozak rebuild threads here do not necessarily apply to your unit, but you might find some of the discussion helpful.

Again, excellent score and welcome to AK!

 

[Retrovert]

One more thing.

The JansZen 65 power supplies are either bad or going bad, and will need a rebuild. That's likely why you didn't hear any sound from one side. Nothing fancy, just replacing the selenium rectifier with a silicon diode and a dropping resistor, and replacing the small filter capacitor with a new film capacitor. That's in addition to replacing the capacitor in the JansZen LC crossover network.

 

[Retrovert]

I don't think you'll be able to stuff in a 6 inch B-209 midrange unless you lose one of the panels. Just a lack of room. That should be fine, as the SPL from one panel should exceed whatever a B-200Y tweeter could provide.

The other, and likely superior, option is to move the JansZen 65 outside and to the top of the cabinet (it's a dipole, which has advantages) and put the B-209 into the freed-up space. This would get the tweeter closer to your ears. Many Bozak owners move the B-200Y tweeters higher up.

Again, many options.

 

[mgolan]

Sweet! Thanks so much @Retrovert. Yes, these were the one's located just outside of Cincinnati actually...

Ok, sounds like my first steps will be to determine if this started as a B-300 or B-302, by way of closely inspecting the model numbers on the rear labels (which are quite faded), and then opening these up to see if these have the N-10102 crossover (or not) and taking a closer look at the cabinet fronts. At that point, I'll grab some additional photos of the crossovers, cabinet interior, etc.

After that, I'll look at making the repairs to the JansZen 65 power supplies and LC crossovers. I presume that would make sense to go ahead and repair both units at this time. I will be looking for previous threads related to rebuilding the JansZen 65's, to see if I can locate a specific parts list for the components you mentioned, and possibly the best place to order them...

I am very curious on whats going on inside of these. On a side note, the guy I purchased these from said he acquired them at an estate sale a few years back - they were owned by an electrical engineer / professor at the University of Cincinnati. That makes me think that original owner may have considered the crossover points when he added the JansZens, so I have to wonder if he made additional modifications to the crossovers at that time.

Either way, I really appreciate you getting me steered in the right direction. I'll post back with pictures as soon as I can.

- Mike

 

[Retrovert]

If you need the schematic for the JansZen 65, let me know.

It will be interesting to see what happened with the crossover points. I don't know if he knew enough about Bozaks, and retained the standard two-way tweeter crossover point of 1,250 Hz, and none for the B-199 other than its mechanical limits, as originally used by the B-300 . I'd further bet that he didn't touch the JansZen crossover, and consequently let the two drivers overlap, as they did in the original two-way. Just a hunch.

One more thing. The Bozak speaker requires lining of the cabinet and an internal curtain to function. The curtain essentially creates a second chamber, such that the backwave enters that chamber and is then lost. Without the curtain much of the low-end response is sacrificed. Neither fiberglass nor UltraTouch, will work.

Getting the backs off will be the first step.

 

[tonyk]

 

[Katalyst]

I am glad someone got those. Looking forward to your progress.

 

[mgolan]

Sure @Retrovert, if you have the schematic for the JansZen 65 handy that would be really helpful... thanks.

- Mike

 

[Retrovert]

Here you go.

 

[Nat]

They are somewhat reminiscent of the Janszen/AR combo, which morphed into a Janszen with someone's 12 inch woofer in a larger than AR cabinet (or on a panel to stick in your own enclosure or into the wall). The Bozak woofer might be a very nice complement to the Janszen tweeters, as long as the cabinet is big enough. But I'm wondering if the sort of matte finish to the sides of the tweeters is caused by some kind of damped vents, which would load the Bozaks in a somewhat unpredictable way.

 

[Retrovert]

They are somewhat reminiscent of the Janszen/AR combo, which morphed into a Janszen with someone's 12 inch woofer in a larger than AR cabinet (or on a panel to stick in your own enclosure or into the wall). The Bozak woofer might be a very nice complement to the Janszen tweeters, as long as the cabinet is big enough.

The electrostatics require an internal enclosure, otherwise the woofer's backwave would blow out the panels. Which gives a clue as to the construction of this cabinet modification. If you examine the rectangular region around the panels I think it is clear that the Model 65 was not removed from its case, only the front grill was removed. I think the electrostatic box sits on a shelf. That must prevent leakage, which would ruin the effect of the infinite baffle. But I suspect that was internally done, because it looks like the tweeter was front mounted into that box. The rear panel, was remounted on the Bozak's rear. Which makes me wonder if the 65 is still sealed at the back. But, again, I suspect it's in its own box.

The cabinet is large enough to lose a small amount for the tweeters and not have the bass significantly impacted. Provided the curtain remains, of course.

I think this is a modified B-300, so it had the curtain, just no crossover and no midrange.

But I'm wondering if the sort of matte finish to the sides of the tweeters is caused by some kind of damped vents, which would load the Bozaks in a somewhat unpredictable way.

That's a good observation, but I think that matte area is nothing more than a trick of the light. Look at the swath of light across the front, and the lighter area on the right side makes perfect sense. It's just a little bleached out from the light, but not the left.

The electrostatics don't want to be ported, anyway. Being dipoles, the backwave is either absorbed (as in the case of the JansZen 65 or 1-30) or reflected off a wall, as in the case of the KLH Model Nine or similar panel speaker.

Awaiting the view from the interior.

 

[Nat]

The original 12 inch dynamic electrostatic Janszens that I remember probably had a shelf separating the electrostatic elements from the acoustic suspension woofer. And I suspect that the elements radiator's backwave simply was dissipated into the space behind them.

 

[Retrovert]

The original 12 inch dynamic electrostatic Janszens that I remember probably had a shelf separating the electrostatic elements from the acoustic suspension woofer. And I suspect that the elements radiator's backwave simply was dissipated into the space behind them.

Right. That's my recollection, too.

I looked it up, and as far as I can tell, the JansZen speakers did place the panel into its own box. Stands to reason. The backwave from the woofer would destroy that thin film.

JansZen sold those same rectangular boxes for use as reinforcing speakers for large venues. Two panels per box. Not as fancy as the Model 65 or 1-30 units.

The lining is fiberglass which isn't as good as cotton batting for dissipating the backwave, but it's not at particularly high pressure, of course.

 

[mgolan]

Thanks for the schematic retrovert... And I think you are correct about these being B-300s. Upon closer look at the label it does faintly appear as such.

I only had time to take a quick peak into the cabinet by removing the janszen rear cover. I can confirm that the internal curtain is in place, but was not able to confirm if the 65s we're in an enclosure just yet. It makes sense based the comments, so I would also suspect that to be the case.

I'll have to figure out the best way to get the backs off so I can really see what's going on. Until then I'll also take a closer peak at the front panels as well to see if I can determine how these may have been mounted. There appears to be a 18 in or so board that was mounted vertically across the center, top to bottom, with a rough hole cut out for the existing woofer.

Peculiar... I'd not seen anything like the section of clear tubing with inline resistors like that. Doesn't seem right to me, especially with the electical tape and the fact it was not connected to anything... but like I said, I'm a beginner, but thinking this may be a part of the issue?

Meanwhile here are some additional pics, not the greatest sorry...

 

[Retrovert]

Pics are fine, no worries.

So we've confirmed that it's a modified two-way B-300, with the JansZen 65 instead of the B-200. Again, highly likely that the crossover is the internal JansZen crossover at 700 Hz, and the woofer doing whatever it does until it doesn't anymore. Sound should dramatically improve with a crossover cutting over at 800 Hz. Best to move a bit above the 700 Hz low end of the JansZen. The B-199 was normally crossed at 800 Hz in nearly all of the Bozaks, so Rudy Bozak considered it to be a good point. 400 Hz is both better and worse, as would be the case with any large driver vs. a smaller driver, but sound quality at 800 Hz will be excellent.

I think that board you see is the rear of the JansZen box.

The tubing is for the high voltage running to the panels. Some of the JansZen panels, BTW, have problems with corrosion where the wires are attached. This is fixable.

My guess about the lack of sound is you either have a bad selenium rectifier or a bad tweeter capacitor which is leaky or open, possibly both. Regardless of the cause, I suggest fixing the power supplies on both before using these speakers any more. It's not an expensive fix. You do not want to have low frequencies in that electrostatic as it can damage the panel, just as they would damage a tweeter.

As long as you're ordering parts for that, you might want to order the woofer inductors and get it all done at the same time. No point in listening to a speaker with considerable overlap between the woofer and midrange, particularly when the woofer cannot properly reproduce those higher frequencies.

 

[mgolan]

Again, highly likely that the crossover is the internal JansZen crossover at 700 Hz, and the woofer doing whatever it does until it doesn't anymore

Ok, if I'm understanding this correctly, since these are B-300s there should be no crossover to the B-199s. I can also confirm this to be true, they appear to be simply wired from the terminals to the B-199.

That said, so I would be looking at building a crossover for the B-199s at 800 Hz? Or would I try to source these somewhere, since it sounds like some factory bozak crossover are at 800 Hz as well? Not sure how readily available these may be found .

With regard to the JansZens, I had a little bit of time to get at them from the front. I disabled all connection from the rear in hopes to be able to get the tweeters loose from the front. The way it was constructed made this a bit trickier than I expected...

I ended up having to remove the lower panel which had the B-199 attached, and then was able to get at the JasZens.

You all were correct... they are in a small enclosure, which was essentially suspended in place by way of a 2 piece surround which fit into a groove in the tweeter enclosure. There was some baffling material covering the underside of the tweeter enclosure, which looks similar to the bozak but feels a bit more like fiberglass .

Anyways, I was not able to get them out fully. I'm not exactly sure what was restricting the tweeter enclosure from being removed, perhaps the baffling was affixed or there was some other attachment point I could not easy determine .

Either way, I had limited time and thought it best not to force anything, so a grabbed a few more pics and will have to tackle this when I have more time and probably a better setup that in the living room

My next step it to search for threads that discuss the best approach for getting the backs off the cabinets, etc, and then try to get the JansZens out so I can get a better look at the components that need to be replaced.

Here are a few more pics.

Mike

 

[Retrovert]

Good work on the disassembly and photos.

Ok, if I'm understanding this correctly, since these are B-300s there should be no crossover to the B-199s. I can also confirm this to be true, they appear to be simply wired from the terminals to the B-199.

Right. So no crossover is present. All is as was expected, so no surprises.

That said, so I would be looking at building a crossover for the B-199s at 800 Hz? Or would I try to source these somewhere, since it sounds like some factory bozak crossover are at 800 Hz as well? Not sure how readily available these may be found .

You can do this two ways.

(1) Purchase an 8 Ω Bozak crossover of some sort, but that's a needless expense. The common Bozak N-10102 crossover has an inductor for the woofer, an inductor and capacitor for the midrange, and a capacitor for the tweeter. Since your unit uses the electrostatic as midrange and tweeter, you wouldn't use anything from the N-10102 except the woofer inductor. All of the capacitors in the Bozak crossovers must be replaced, anyway, as they were awful when new, and now are are long dead or dyring.

(2) Purchase an inductor for the woofer and replace the capacitor in the JansZen crossover. Done.​

I think (2) is the better option.

You do not need the Bozak inductors. Nothing magickal there. I suggest some of the high-quality copper inductors. 14 gauge is perfect. Low resistance. Check Parts Express.

I also suggest bypassing the crossover capacitor for the tweeter with a 0.1 uF and a 0.01 uF placed in parallel. The time for charge distribution across the plate varies with plate size and frequency, and smaller capacitors have smaller plates and can thus better respond to rapidly varying AC signals like audio. It's an inexpensive addition and one which is widely perceived as an improvement.

You all were correct... they are in a small enclosure, which was essentially suspended in place by way of a 2 piece surround which fit into a groove in the tweeter enclosure. There was some baffling material covering the underside of the tweeter enclosure, which looks similar to the bozak but feels a bit more like fiberglass .

Yeah, that's fiberglass. Bleh. It can be replaced with cotton batting if you like. Since the cabinet is sealed the fibers aren't going to escape.

Anyways, I was not able to get them out fully. I'm not exactly sure what was restricting the tweeter enclosure from being removed, perhaps the baffling was affixed or there was some other attachment point I could not easy determine .

Might be screwed to something. Need to remove the rear of the B-300 cabinet.

 

[mgolan]

Well I'm somewhat at a pause in getting the rear panels out.

It appears that forcefully prying may be my only option?

I just don't want to destroy anything needlessly. Not finding much specifically for removal of back panels through cursory searches.

Any tips would be greatly appreciated...

Mike

 

[Retrovert]

You mean the rear of the B-300 cabinet? You removed all the screws?

The smaller cabinets used tongue-in-groove joints that were glued together, and the backs must be cut off. But not the B-300 or B-302.

It may be the rear slightly swelled and is a tight friction fit.

Got some photos?