[FredT300B]

I've just completed the best sounding single driver speakers I've ever built. Much better octave-to-octave tonality than the Fostex, Tang Band, and Audio Nirvana single driver speakers I've built in the past. The design is a few years old, by Jim Griffin, using CSS FR-125SR full range drivers, and I added a rear mounted HiVi RT1C-A Planar tweeter. The front and rear-mounted full range drivers are wired in parallel, with no compensation network, and the tweeter is wired to reinforce only the highest frequencies with a 1.0uF Audiocap Theta film capacitor in series.

I had built a pair without the supertweeter a few years ago, enjoyed them for a while, then sold them. Soon after I regretted selling them and finally built another pair late last year, but after listening to line arrays with Fountek ribbons and a 2.5-way using Scan Speak Revelator drivers I found something was missing. Adding a cheap Parts Express neo tweeter to the front panel regained some of the air that was missing, but a cheap front mounted tweeter doesn't mesh too well with a high quality full range driver like the CSS.

I recently read Danny Ritchie's advice about adding a supertweeter to a full range speaker (mount it on the back), and I took that advice. As usual, Danny knows what he's talking about. The tweeter adds that extra bit of "air" without standing out from the full range drivers. These little speakers sound fantastic, with absolutely none of the shout I'm accustomed to hearing with high sensitivity light-cone drivers like Fostex and Lowther. However, these are not high sensitivity speakers and they need the conrol of a medium power amplifer to manage the bass. I've found my Virtue One works fine, and a Krell integrated is even better. Haven't tried them with tubes but I anticipate any EL34 pp amp would be fine. Definitely not a candidate for a low power SET amp. Sorry tube guys, you can't have everything in one speaker.

http://homepage.mac.com/tlinespeakers/FAL/downloads/BipolarMLTLDesignPak.pdf

Jim Griffin has since designed other bipole MLTLs using Jordan and Mark Audio  drivers. I haven't heard these designs and I'm sure they surpass the CSS, but for the money this one is hard to beat. Here's a link to a thread about the Jordan and Mark Audio driver designs:
http://www.diyaudio.com/forums/full-range/137585-mark-audio-alpair-10-mltl-design.html

[Duke]

Very nice, Fred!

I like your incorporation of the rear-firing supertweeter.   Often a fullrange driver has decent highs on-axis, but off-axis the highs are weak.  Adding a rear-firing supertweeter improves the spectral balance of the reverberant energy.  I bet they sound rich and lively.

My first commercial effort was a bipolar based on Fostex drivers, a rear-firing supertweeter, and a built-in powered subwoofer section.  I figured adding all that extra stuff to a fullrange driver was cheating, so I called it the "Cheetah". 

[Jim Griffin]

Great work Fred!  I'm glad to hear that you still like that bipolar design.  Lots of sound for a very simple configuration.

My position on adding a tweeter to a bipolar pair of full range drivers is that Duke's approach (two-way woofer and tweeter on front and back baffles) along with a proper crossover is the ultimate solution.  Ask yourself whether a full range driver would do a better job of covering the whole band (including the upper ranges) versus a multi-way where-in a high quality tweeter covers the top 2 to 2.5 octaves or so.  Proper spacing between drivers and a crossover network that achieves the correct amplitude and phasing will yield optimal results.

But Duke points out, you can get magical sound with a rear mounted 'super tweeter' that adds ambience and airyness that improves the overall performance.   The supertweeter works without mucking up the full range driver sound. 

I like what you did and in fact I'm planning on trying out the planar tweeter (I'm trying a later planar version--the Hi Vi RT1.3) on the rear baffle of my prototype bipolar speakers.   If that sounds good, I'll next develop a proper crossover to compare to the  rear supertweeter version.

Jim

[Danny Richie]

Nice job Fred.

Of all of the full range drivers out there that I have been able to get my hands on and measure, I think that the FR-125 is the most accurate and among the best sounding.

That XBL^ motor structure actually gives the driver a little stronger output in the lower ranges compared to the upper ranges. The good thing about that is that it tends to mirror image much of the baffle step loss that you get from using those narrow baffles. Every other full range driver needs a compensation network to correct for the loss. With sensitivity already really low, you hate to have to loose even more from a compensation circuit. The FR-125 can get by without it.

I'll share one other little trick with you guys that I'd almost rather not... It is a really good one. I have found that a single FR-125 can only handle peaks of about 30 watts to reach full X-max using most music and playing them full range. So with this trick you can easily double power handling and then you can then put enough power on these to reach reasonable output levels.

Fred, for you, you can do this to each side.

Add a second woofer and bypass the lower woofer with a cap value that is between 47uF and 75uF. I usually end up with a 62uF or  75uF myself.

So wire it like this:

+_________________+
                                     }
                                     } top woofer
                                     }
                                    -
                                    I
                                    I
                   _________+
                  I                   }
                cap                } lower woofer
                  I                   }
-________I__________-

If you think it through then you will see that the upper frequency ranges pass through the cap and to the upper woofer only as that is a path of much less resistance for the upper ranges. But, as frequency decreases then the cap is more resistive and both woofers share the low frequency load together. Since with woofers are in phase and working together then they can share the same air space too. his will double power handling without causing any comb filtering effects that would disrupt the response.

I must also agree with Jim. As good as the FR-125 is, it still gets really outperformed by a well designed two way.

[planet10]

Definitely not a candidate for a low power SET amp.

Not a candidate for any SE amp... tube., SS, or high powered. Seems that something about SE and the less than graceful limits of XBL drivers (at least in my experience) push them over the edge and the VC dramatically hits its stops.

dave

[FredT300B]

My position on adding a tweeter to a bipolar pair of full range drivers is that Duke's approach (two-way woofer and tweeter on front and back baffles) along with a proper crossover is the ultimate solution.

Jim and Danny, I agree. Every year at the LSAF I'm able to compare the sound of single drivers to the sound of two-ways, and I always prefer the two-ways. Compared to a well designed two-way, single drivers have great sounding midrange, but their great midrange is often compromised by attenuated low bass, wooly mid bass, peaky lower treble and attenuated upper treble. Like an attractive woman who's great in bed but can't cook, single drivers can be lots of fun but for me they're better as an occasional partner than as a wife.

These speakers with the CSS drivers, driven within their power handling limits by the right amplifer, seem to overcome all the disadvantages of single drivers I mentioned above (except for the attenuated upper treble). Adding the supertweeter solves the upper treble issue without detracting from any of the other good features. The bipole design does result in a soundstage that's deeper than a monopole but not as precise, but it sounds more like live music.

Jim, if you care to comment I would be interested in how some of your more recent designs using Mark Audio drivers compare to this one.

[Jim Griffin]

Danny,

Just a reminder that the original speaker design is a bipolar with two FR125's per box.  The power handling is inherently doubled plus you don't have the worry about baffle step comp with both front facing and rear facing drivers.  Your thought about using a capacitor to bypass one of the drivers in the box could work out to mitigate the wraparound dip (I measured about 2-3 dB) in the 400-500 Hz range with the bipolar design.  You could select the value of the cap to roll off the rear driver starting about 400 HZ so the dip is reduced.

Jim 

[JLM]

I love my EnABL'd Fostex F200A drivers.  IMO they represent the best compromise between the power/full range attributes of multi-driver designs and the purist single driver designs. 

Admittedly they cost 5 times what the CSS FR-125SR do (but less than many others).

[Duke]

Your thought about using a capacitor to bypass one of the drivers in the box could work out to mitigate the wraparound dip (I measured about 2-3 dB) in the 400-500 Hz range with the bipolar design.  You could select the value of the cap to roll off the rear driver starting about 400 HZ so the dip is reduced.

With the addition of an inductor, the circuit could notch out the wrap-around dip.  Mirage notched out the wrap-around dip on their first generation bipolar, but not on subsequent models, to the best of my knowledge.  Since the wrap-around dip effectively disappears in the power response anyway, in my opinion we're better off leaving it in there, but it's a judgment call.  I tried it and prefererred the sound with the rear woofer un-notched.  The impedance curve is affected by notching out the wrap-around, in case that's a consideration.   

[FredT300B]

Here's a picture of one speaker after final cosmetic finishing, with the baffle painted.

[versus rider]

nice work, I am  a fan of the css fr125 myself and have 2 pairs of standmounts in regular use.

these I built two years ago and they are currently being driven by a 300B set amp and sound excellent.

[JimJ]

Alien-looking

[motosapien]

I've just completed a Coniston^2 full range build.  It uses 2 CSS EL70 drivers that are 4ohms.  The lower mids and bass are as good as anything I have heard.  Simply intoxicating!!  I had them wired up in series but found the upper mids and high's somewhat muddy.  They sound better in parallel but this is a 2 ohm load.  I noticed Danny's trick above about removing the highs from the lower driver (in my case it's lower and on the inside edge of the cabinets ).  I ordered a 68 uF cap to give that a try.  I also ordered the above referenced ribbon tweets and the 1 uF caps to go with it.

My difficulty is I don't understand how to wire this up.  I can give Danny's trick a try easily since I have leads coming off all four drivers and will just do it bread board style on the floor with alligator clips.  What's giving me trouble is how to wire in the ribbon tweet. 

[motosapien]

I finally got the parts.  I'm listening now to the speakers wired in series with the 68uF cap across the second drivers.  It makes a huge difference!!  These are a whole nuther animal now.  And I'm pretty certain I won't need the ribbon tweeters anymore.  Once in a while you make a little change and it pays off big time.  Too bad I don't understand how the capacitor accomplishes this or where the frequency is tapered off but perhaps enough will soak in over time so I do.

Thanks Danny for sharing this.  And I'll be keeping my N2X's for when I want to listen to Niel Young's weld and such.

[Danny Richie]

I'm glad that worked out for you.

I don't think people realize just how destructive to the response it is by running full range drivers together like that. That simple cap trick solves the whole problem.

Try running the ribbon tweeter on the back side of the speaker or aiming it towards the ceiling. You might be surprised at what it does.

[motosapien]

I hooked up the ribbon tweets and you are right.  They add significant air and depth to the top end.  The caps did that for the lower registers balancing them nicely between higher mids and top which were too forward before the change.   Now I've got it on the top as well.  I threw an under handed pitch at them by playing the Cowboy Junkies Trinity Sessions first.  If there is a disc for demonstrating a properly integrated ribbon tweeter, this is it.  Today I'll pull the drivers and get it all soldered and mounted up proper.

So with two capacitors and a tweeter I have essentially created a three way speaker. So be it.  The sound is deep and sweet with nuance and detail I have not often heard. And there is a surprising range of music that play well on them.  They bloom early giving their gifts at lower volumes which make listening easy.

[Danny Richie]

So with two capacitors and a tweeter I have essentially created a three way speaker. So be it.

You still have no low pass crossover anywhere and no real crossover point. So you could still technically call it a 1.5 way design.

Regardless of how you want to think of it, the configuration does work very well.

Enjoy!

[JLM]

Help!! People are pissing in the pristine pool of single driver speakers under false pretenses.   

(Now you guys are discussing 4 drivers in a situ 3-way design, that's getting a long way from the prescribed home base.)

[rajacat]

Help!! People are pissing in the pristine pool of single driver speakers under false pretenses.   

(Now you guys are discussing 4 drivers in a situ 3-way design, that's getting a long way from the prescribed home base.)

+1

I agree. Once you start talking about multi-way designs and caps, etc.,  it will degenerate to active crossovers and so on. Seems to me that this circle should be limited to cabinet design, tweeks and driver choice/design. However, supertweeters could be allowed.

-Roy

[motosapien]

Well, this is only two CSS EL70's and a super tweet.  You really can't listen to the two EL70's in the same cabinet without taking the highs away from one of them.  The box by Scott of Woden designs is spot on as are his stuffing recommendations.  So I turned one of them into a woofer with a cap.  My comment about it being a three way was a simplistic observation about the three drivers with different frequency ranges that combine to make something very musical and involving.

Give them a try.  They are wonderful.