North
Creek Music Systems
Crossover
Update for the
Apogee Stage
Full Range
Ribbon Loudspeaker
This
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Kapton-based ribbon that superceded the original Mylar ribbon, and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:
flat frequency response at the
listening position
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:
the antiphase curve should
approach a null
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:
driver voltage plots should be
monotonic (decreasing for low pass, increasing for high pass)
minimum system impedance should never be lower than the
driver DC resistance
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.
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
± 2dB from the low frequency limit
to 18kHz.
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 focussed
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 ….$1149 per
pair.
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 six months 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:
Installation.htm
For further information, please contact Info@NorthCreekMusic.com.
To order, please contact Sales@NorthCreekMusic.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
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
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>
> 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 beter 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 pen 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
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