Crossover Update for the
Apogee Stage
Full Range Ribbon Loudspeaker

StagesThis crossover design was developed by North Creek Music Systems as a "modernization" of the original Apogee Stage crossover, which was designed in 1988 by Apogee Acoustics in Boston, Massachusetts, and produced by Apogee and later Apogee/ADS from 1989 until the mid 1990's.

Background:

The Apogee Stage was already in production before I was privilege to join their engineering team in 1990. My only involvement with the product was as a member of the listening panel that approved the Kapton-based ribbon that superceded the original Mylar ribbon, and with the same listening panel that approved the stand. The original design was completed and original Stage was in production before I joined the team, and the development of the subwoofer for the Mini-Grand was done after I left.

The pair of Stage's for which I developed this crossover update is actually the very same pair that was reviewed in Stereophile in 1988. I lived with them as my reference system in 1991 through 1993, then set them aside for many years. I set them up again in 1999 in my fiance's listening room, and lived with them for six months in 2001 after we were married.

In September 2001, in response to many inquiries from the Apogee Users' group, I decided to work on a simple "modernization" of the crossover network, largely because the quality of crossover components has evolved so far in the last ten years or so.

In 2008, during development of the Fibonacci Technologies’ Vaya! Loudspeaker, I kept of a pair of upgraded Stage's and a pair of Duetta Signatures in my listening room as reference. I can say with no reservations, the Duetta Signatures are great speakers, but the upgraded Stage's blow them away.

  

Comments on the original design:

One thing about Apogee was that we were very particular about crossover component selection, and we selected only by ear. Apogee at the time of the Stage's production was quite possibly the only manufacturer in the world using 12 AWG inductors in their woofer circuits, and in fact 12 AWG inductors were not available on the world market until several years later. Likewise, the Sprague and ASC capacitors were chosen by ear simply because they sounded better than any other capacitors available in their day, and still sound better than the great majority of metallized capacitors in use today. Likewise, Apogee was the only high end audio company using Ohmite 1% mil-spec resistors; everyone else was using (and in most cases continue to use) sand cast resistors. Apogee's crossovers were probably the most expensive in production in their time, and would still cost an order of magnitude more than the great majority of crossovers produced today.

The original crossover design was a simple Linkwitz-Riley second order with the woofer in reversed phase. This type of network allows for the gentlest blending of the different drivers throughout the crossover region and also required the fewest crossover components. The second order high pass also protected the tweeter ribbon from overexcursion, and in fact the ribbon will begin to twist at high excursion long before it goes into a failure mode.

Both the planar magnetic woofer panel and ribbon tweeter were the most advanced produced at that time. There have been some advances in planar magnetic technology since then, largely to increase the efficiency of the drive unit. The tweeter ribbon and its longer siblings still represent the state of the art and remain the best ever made.

The Apogee Stage was superbly designed loudspeaker, considered among the best sounding loudspeakers available in its day, and still much better sounding than the vast majority of loudspeakers produced today. What really amazes me is that the loudspeaker was designed almost completely by ear; the time-windowing measurement equipment that is commonplace today had not been developed in 1988, and this loudspeaker is virtually impossible to measure by conventional means.

 

Comments on the updated design:

My design goals for the "updated" Stage network represent current thinking in crossover topology.

In 1988, the major crossover design goals were:

  1. flat frequency response at the listening position;
     
  2. minimum number of crossover components;

    In the late 1990's, largely due to the work published by Dr. Floyd Toole while he was with the NRC in Canada, three more design goals were added:
     
  3. the antiphase curve should approach a null;
     
  4. off axis frequency response can have dips but should not have peaks, and the off-axis frequency response should not peak above the on-axis frequency response;

    To this set of design goals I generally add two more:
     
  5. driver voltage plots should be monotomic (decreasing for low pass, increasing for high pass);
     
  6. minimum system impedance should never be lower than the driver DC resistance.

 From these parameters the new network was developed.

 

Description of the North Creek Crossover: 

The North Creek tweeter high-pass network topology and component values largely unchanged. I tried a few other topologies and varied the high pass Q, but in the end I kept returning to a Cascade Bypassed
67.1uF/1.5mH second order high pass. There are two reasons for this: 1) it is my feeling the current network pushes the tweeter ribbon as low as it will go while still remaining an effective transducer; 2) the failure level is so low that it may be the most dependable driver in history. The Cascade Bypass is accomplished with a group of Zen and Crescendo polypropylene capacitors. The 1.5mH inductor is 12 AWG.

I changed the trap quite a bit, as I felt it was still a bit too bright and also too narrow. This may have to do with the age of the Kapton tweeter ribbon, which is now extremely well broken in.

The North Creek woofer network is considerably different than the original network, as we have the ability to measure the woofer output to a very low frequency and make the empirical adjustments to the crossover accordingly. The measurement technology did not exist when the Stage was in production. Our topology both lowers the crossover frequency and increases the crossover slope, and calls for both a larger inductor and much larger capacitors. Here we use a 12 AWG inductor and some very large Zen capacitors. As this section of the network required considerable design time, we consider it our intellectual property and will not publish the component values.

Of course the new network is much larger than the original, so we are supplying it in a dedicated sub-enclosure that attaches to the back of the Stage in place of the original network. Installation requires disconnection and removal of the original network, some crimping and screw-turning. Soldering is not required but it is suggested.

 NCMXOandStock
Original Stage Crossover (upper) and North Creek Upgrade (lower)

 Comparison of the North Creek Crossover for the Apogee Stage to the original:

The first difference one notices after installation of the North Creek network is much cleaner and tighter bass. The improved low end sounds more like good acoustic suspension than dipole-panel. I attribute this to both the modern inductor manufacturing process and its significantly larger wire diameter, which minimizes the resistance present between the amplifier and the woofer panel and allows the amplifier better control of the panel.

The next improvement is how clean the lower midrange becomes. The original Stage was very good in this regard, but we found with some amplifiers with limited current capability, the lower midrange would become a little forward, and our measurements indicated a broad "hump" centered near 300 Hz (just above middle "C"). Also, slightly to the left or right of the optimum listening axis, the lower midrange of the closer loudspeaker would dominate. When measured, it appeared that even when the phase of the woofer panel was reversed from its original (antiphase) orientation, there was not a significant null at the crossover point. The new network, with its lower crossover frequency and increased slope, correct for both of these aberrations.

Moving up in frequency, the ribbon tweeter takes over. Here advances in crossover component technology over the last decade are evident, as Cascade Bypassing with film-foil capacitors yields a significant improvements in resolution, clarity and detail, whilst the modern 12 AWG inductor creates a cleaner path to ground and results in a much lower noise level. The midrange always was very good but with the upgraded crossover is just superb.

For the high frequency level, we made the trap a little wider and a little deeper. This was done by targeting flat frequency response above 5kHz, then adjusting by ear. We also use a Crescendo bypass capacitor in the trap, making the top end both sweeter and more detailed than it was before.

All told, the frequency response for the upgraded Stage measures a remarkable 2dB from the low frequency limit to 18kHz.

Image9

As part of our manufacturing process, all crossover components are provided in 1% hand matched pairs. This is much tighter than the 10% specified by the manufacturer (although Apogee was usually closer to an excellent 5%). Hand matching the left and right crossovers directly yields more focused images in a much larger sound stage.

The biggest overall difference between the original Stage and the North Creek version is simply how much more musically involving. The original's minor weaknesses - a slightly forward lower midrange with some amplifiers; the "one speaker effect" when listening off axis, somewhat soft bass (compared with the best acoustic suspension speakers), and high noise floor - have all been minimized, while those things that the Stage does so well - a lush midrange, a sweet top end, enormous soundstage and great focus - have all been improved.

 

Availability:

The North Creek Crossover for the Apogee Stage is available fully assembled and tested in matched left-right pairs. All connections are fully hardwired and silver-soldered. All wiring is point-to-point. The subenclosure and mounting hardware are included.

North Creek Crossover for the Apogee Stage ….$  Retired 2016


Guarantee:

North Creek Music Systems guarantees the original purchaser of the crossover will be fully satisfied, or the purchaser can return the crossover any time within sixty days of the original purchase date for a full refund of the merchandise charges. The North Creek network is fully uninstallable so the loudspeaker can easily be returned to its original condition. Shipping charges are not refundable, and return shipping charges are the responsibility of the purchaser.


Warranty:

North Creek Music Systems warranties the Crossover Upgrade for the Apogee Stage for a period of three years from date of purchase to be free of manufacturing defects. The warranty is transferable.

Installation Instructions

For further information, please contact NorthCreekMusic@gmail.com .

 

A Comment from Mark Nazar, formerly Chief Engineer at Apogee Acoustics

 

To: George Short

George,

How are you doing? By the looks of your web site, quite well.

I couldn't help but to read through the discussion of the Apogee Stage and
the good work you did on a revised crossover. It would be a thrill to hear
it. I wanted to make a couple simple points on your discussion: the Apogee
Stage was developed using some rather sophisticated time windowing
techniques using a B&K spectrum analyzer with special test setups made by
Steve Temme. We looked specifically at major and minor reflection effects
from cabinetry, etc. Also, we were very concerned with providing uniform
group delay and precise impulse response. The crossover was designed using
one of the most sophisticated electronic CAD simulation packages available
at that time called MicroCap. The listening evaluations were used to fine
tune the musicality and select brands of crossover components.

Drop me a line sometime.

Your friend,

Mark

Mark Nazar
Director of OEM Automotive Product Development
Boston Acoustics

 

Owners’ Comments: 

George,

I wanted to send you a quick note to let you know that the Stage crossover upgrade I purchased nearly two years ago still continues to impress me. The speakers get better with every improvement I've made to the electronics.

Rick

 

Hi George,

I have been so consumed by work that I never got a chance to write anything up.  Hope all is well.

Everytime I turn on my stereo I keep reminding myself to thank you.

Here are some notes on my feelings about the Stage, pre & post NorthCreek Crossover.

It's not an excited auditioner with a new toy, it's true.

Stock:
Metallic strings
Brass sounds thin and squeaky
Hashy sound when cranked up
No matter where you place it there is always a hump and a trough in the bass
Sibilant sounds annoying
images, while very well defined an separated
have no body, no bloom of each instrument

Post NorthCreek
Fast bass, with the right amount of undertones but not boomy like the
Duettas (even when I had KSA300S) which
go deep but tend to overhwhelm the room with certain frequencies
Still sound a bit drier than my reference Eidolons but its close
Piano music is simply magical, no more honky tonk xylophone sound
Crank it up, and the speakers disappear and your ears won't bleed :)
Details details details, the closest to my ESL989s than anything else!
Body! Images are stable and full-bodied without being muddy.

I simply cannot thank you enough!!!

-Johnson

 

Dear George, Windy et al,

The crossovers are a complete success. I've been using some 'substitute'
Acurus amplifiers and generic wires while waiting for upgrades on the Van
Alstines. I thought the sound was recessed and maybe too reticent after the
modifications, but after some break-in time, the magic is back again -- even
more so, thank you again. They will be appreciated in our home indefinitely.

Bill and Sandy

Dear George,

Firstly I would like to apologize for taking so long to let you know if the crossovers arrived safely. 
They did! On the 11th of October I collected them at the post office where
I paid 200 + euros import duties or taxes.

I balanced them on the back of my cycle as my daughter had borrowed our car for two weeks.

Home safely I unpacked the huge box and they were undamaged.

In one of the crossovers, see image called DSC02440JPG, there seems to be an extra capacitor.
Was this added to balance the sound, to match the crossovers or to make up for some short comings?
Or for a difference in the output/frequency balance of the crossover?

My old crossover is in not in to good a state with tape used to hold the woofer inductor together. See image.

I decided very stubbornly not to lay the speakers flat to install the crossovers because I don't like moving them too much.  This meant that it was more difficult to install them on my own. But by putting some books underneath the crossovers while I connected them I was able to manage. Only one of the screws I couldn't quite manage to get it lined up or in the hole. The screw is also quite blunt the end is not ending in a point, probably because they are custom shortened to suit the size they need to be. I don't think that one screw is going to make a difference.

The sound was a bit disappointing as expected. Recessed although the lower frequencies seemed much more present, better and refineder straight away. More details were present all round and the stereo image was better.  I could hear the possibilities and have been burning in the crossovers for something like 50 hours now.

> The last thing to get better were the high frequencies, it was still sounding brittle.   Now they make everything sound live. More differentiation of the colour tones of instruments, especially obvious with brass sections in big bands.  The Stages were already better in this respect compared to my old B&W 801 mk II's. Great dynamics, were already better from the first hours of play.  More body in the sound. Double base instruments are now fascinating to listen to.

I have a damaged ear nerve and now it doesn't irritate even when music is played louder. Clarinets are still annoying if played too loud. Probably due to the damage I have sustained when I used to play the clarinet. 

Today on Sunday, the electricity supply seems to be better, it always does sound better on Sundays, the Stages sound almost perfect with huge dynamics at a normal volume setting. The base seems to be limited as the Stages only go so low but I hardly miss anything especially on Sundays. Maybe I can arrange a separate earth pin in the garden sometime for the Hifi.

Always something to improve. Perhaps the base will improve even more with more hours of burning in.  Maybe the Krell amplifier has it's limitations as it is an old design and the parts are old + Sphinx pre.

The main point is I am very satisfied and find it value for money and can look forward to many hours of music pleasure. Thanks!  The Stage is certainly a remarkable speaker and your crossover is also very special.

I couldn't recommend a better improvement for a Hifi for the price.

I am also proud to own a unique hand made crossover from a respected name in Hifi also having had your experiences with Apogee.

Greetings, Russell.


Hello!

First of all, I am very very glad to hear all arrived in good order and the installation went well.  And we are overjoyed to learn that you are so pleased with them!

They are a little different from the photo, the capacitors are updated and also the power handling has been increased (if you look closely, you will see there are three resistors where there used to be two).  The inductor was also changed to match the upgraded capacitors.  This is the latest version (officially 2012 revision), same as mine.

They will continue to get better, it takes about 300 hours for the tweeter capacitors to break in.

My audio system also seems to be best after midnight and on Sunday.

Please keep me updated!

-George