North Creek Music Systems

A high output monitor developed specifically for the finest tube amplifiers

At right is the Catamount Revelator Signature, a slender tower loudspeaker built with our custom Scan-Speak woofers and available with a selection of tweeters. This loudspeaker was designed specifically for tube amplification and in particular SET's. The woofer loading is traditional aperiodic, so one can configure the bass response to optimally mate with the room and amplifier - bass anywhere from light and tight to aggressive and snappy. The midrange is rich and clean, the lower treble perfectly balanced without a touch of forwardness, and the top end has that beautiful softness characteristic of the best silk domes and true ribbons.

The cabinet width is kept to an absolute minimum, and along with crossover component and driver matching, the Catamount is capable of an extraordinary three-dimensional presentation. Microdynamics are exceptional, and the fine imaging and sound stage cues lost with most loudspeakers are clearly captured and expressed.

The Catamounts are also incredibly easy to drive. High power tube amps eat them up, and SET's love 'em.

The Catamount loudspeaker family was conceived as a cost-no-object, high sensitivity, high output monitor loudspeaker designed for the most demanding two-channel and home theater environments, and intended specifically for tube, SET and OTL amplification.

Unlike transistor amplifiers, which by virtue of their extremely low output impedance are relatively non-pulsed by frequency-dependent impedance variation, all traditional tube amplifiers are coupled to the loudspeaker via their output transformer. The most significant advantage of driving a loudspeaker through a transformer is that via impedance matching it allows the maximum power to be delivered to the loudspeaker with a minimum of stress on the output device (the tube). The disadvantage is that a loudspeaker is a frequency dependent – and frequency reactive – device. Loudspeakers designed for transistor amplification are rarely tested for their compatibility with tube amplification, and in many cases frequency dependent input impedance makes them a poor match.

Developing the Catamount called for a significant rethinking of the optimum characteristics of a high end loudspeaker that is coupled to the amplifier via a transformer.

The most overlooked aspect of loudspeaker design is that to perform at its best, it is essential to conceptualize from the beginning not only its specific application but also its expected room size and placement constraints. The Catamount is destined for mid-sized listening rooms (from 1500 to 3000 cubic feet). To get the most output in rooms this size, sustaining bass below 40 Hz at high SPL’s is unachievable without employing large woofers or subwoofers. Therefore, the Catamount’s low end corner is set around 45 Hz. Also because the room should be large enough to allow for considerable placement freedom, this loudspeaker is designed with its face a minimum of three feet from the back wall.

Because the Catamount is intended to be compatible with SET amplification (limited to about 11 Watts), sensitivity is a key issue. Many SET-specific designs use horn drivers (inherently high sensitivity) or high output full range drivers. Other than their high sensitivity, each of these approaches have performance limitations. Horn drivers in particular suffer from extremely limited bandwidth (requiring many drivers to cover a wide frequency range), directional dispersion, and inherent "throat compression" coloration. High output full range drivers suffer from both extremely limited low frequency response, require significant frequency contouring though the mid-band to correct for the cabinet diffraction field, and exhibit extremely poor performance at the top end compared to the best dedicated high frequency drivers.

As both the North Creek Borealis/Rhythm/Manifest full range family of loudspeakers and Vision MAPD monitors have performed well in many tube-applications (even though all were designed for transistor amps), it was clear that a conventional two-way MTM design, properly executed, could have considerable success in a tube-specific amplification. Therefore, the development direction of the Catamount was to borrow the applicable aspects of these designs and incorporate them into the new project.

The Catamount design goals are:

High Sensitivity
Ultra-Flat Frequency Response
High, Stable Input Impedance
Bass Extension to 45Hz
Configurable Low End Contour
Subwoofer Compatible

Because most tube amplifiers exhibit several ohms of output impedance at low frequencies, the same sealed box loudspeaker can exhibit variations in low frequency system damping that can make the system sound anywhere from thin to heavy, depending solely on the amplifier it is driven by. Vented box systems fair far worse, as with two impedance peaks bracketing the port tuning frequency, bass output can vary by as much as 6dB within a one octave window. Therefore, the optimum bass platform for tube amplification is the aperiodically damped infinite baffle, which with proper woofer selection can reproduce bass to 40 Hz cleanly yet offers unrivaled tuning flexibility.

Traditional aperiodic designs (configured for transistor applications) require a woofer with a Qts in the range of 0.6 to 0.7. For tube applications, where the high output impedance will raise the effective system Q, a lower Qts is required. Because aperiodic systems do not offer the overexcursion protection of a sealed box, extremely long throw is also a required. Because the Catamount is an MTM, high DCR is required. Lastly, to get the deepest bass, the lower the free air resonance, the better.

The woofer we selected for the Catamount is a custom 5" built on Scan-Speak’s 15W Revelator platform, called the North 15W-8530K00SC. This driver is built with beautifully cast and finished "grasshopper" frame, an enormous, stacked motor structure and unique, patent-protected pole with multiple Faraday rings. This configuration allows the voice coil to travel in a linear magnetic field that is constant regardless of excursion, resulting in the lowest harmonic distortion of any mid-woofer available.

The 15W cone is hard paper which is sliced at computer optimized angles to eliminate the channels that allow wave reflection within the cone body. This and a variable thickness cast rubber surround allow undistorted piston cone motion well into the lower treble and far above the system crossover frequency. The drivers’ smooth roll off at the top end clearly indicates there is no energy stored in the cone, rather it is perfectly absorbed by the surround.

Lastly, no compromises were made to limit dynamic range. The 15W’s have linear excursion of 13mm and maximum excursion of 19mm peak-to-peak (nearly three quarters of an inch!). This is more than three times the excursion capability of conventional 5" woofers.

Driver parameters are Qts = 0.45, fs = 38 Hz, Vas = 12 liters. DCR is almost 6 Ohms In the MTM configuration, system sensitivity is 91dB.

As the Catamount is in two performance levels, there are two tweeter choices.

The first step North D28-06S. A North Creek exclusive, the D28 features a hand coated 28mm silk dome, stacked shielded magnet structure, aperiodic rear chamber, and a beautifully machined aluminum face plate with an automotive quality silver metal flake finish. Unique among tweeters, the North D28 is constructed with an overhung voice coil, extended pole and a copper Faraday Sleeve that extends above and below the top plate well beyond the dome’s maximum excursion. The North D28 exhibits extremely low harmonic distortion and the lowest intermodulation distortion of any dome tweeter on the market. In terms of sound quality, it is incredibly smooth, and remarkably detailed without a hint of sibilant emphasis, and a joy to listen too for long periods.

For our Signature performance level, the Scan-Speak D2905/9900 Revelator is the tweeter of choice. It features a 28mm hand coated silk dome, aperiodic rear chamber, overhung voice coil with a Faraday cap at the top of the pole, a rugged ½" thick machined aluminum face plate with an optimum contour to control vertical dispersion, and a unique machined top plate that channels reflections from beneath the surround away from the dome. The Scan-Speak D2905/9900 Revelator is considered by many to be the best sounding soft dome ever made.

As stated above, the Catamount is designed for tube amplification and mid-sized listening rooms. The caveat is the placement constraint – with the face of the loudspeaker a minimum of three feet (one meter) from the back wall, and the inside edges at least six feet apart. This is intended to be compatible with most projection televisions, audio equipment racks, and the aesthetic requirements. With this much space behind it, a properly designed loudspeaker can be capable of creating a very deep sound stage and great image depth – those aspects in which the best tube amps and particularly microdynamic-rich SET’s excel.

Voicing the loudspeaker entirely by ear while making extensive measurements to confirm free-field frequency response remains ruler flat leads to a midrange that is rich and detailed, and will not get forward or hard even at extremely high volume levels, and that the harmonic balance is correct throughout the entire loudspeaker’s bandwidth . This was accomplished by employing our unique progressive-slope crossover, which precisely mates the drivers’ natural frequency responses and the effects of the acoustic field.

The measurements of the Catamount - North D28 are shown below:

Note that the scale is 5dB per major division, twice the resolution of most published graphs. The single most important attribute of a great loudspeaker for 2-channel applications is that it must be a joy to listen to for extended periods regardless of the type and scale of the music being played or its volume level. By choosing to build a monitor loudspeaker (one with limited bass extension), we were able to concentrate on the most critical region of the frequency spectrum - the midrange. Our target was a frequency response that measured better than ± 2dB from 300 Hz (just above middle "C") to 6000 Hz (the mid-treble), a bandwidth of nearly 5 octaves. We also required nearly flat power response, perfect constructive interference through the crossover transition region, and no peaks in the off axis response.

The Catamount's measured frequency response is ±1.5dB from 50 Hz to beyond 20kHz and actually ±0.5dB from 150 Hz to 6.5kHz, simply spectacular for a tower loudspeaker placed well into the listening room. The 1dB rise in the top octaves is actually a product of the voicing, as virtually every amplifier we tested was a bit soft in this region, and the loudspeaker is designed with just a touch of sparkle to compensate.

The antiphase null is greater than -30dB - so perfect it is actually off the scale! It is also beautifully symmetric, indicating there are no frequency response peaks off axis (regardless of the axis selected). This stabilizes the sound stage and improves image focus even in rooms with difficult reverberation fields.

This waterfall plot is taken of the Catamount Revelator Signature at 1 meter, 5 degrees on the horizontal axis. Note that the scale is 30dB, offering twice the resolution of typical waterfall plots. We can do this because the decay is so fast and so clean there is very little decay information above the -30dB threshold.

The entire tweeter decay takes place in less than 1msec, indicting the diaphragm is well damped, there are no reflections off the baffle, and the high pass crossover is very low Q. Note that the decay is an almost perfectly straight angle across the noise floor. What this means is that the phase is carried smoothly from the woofer to the tweeter, that the mid-woofer's cone resonances are well controlled.

The photo at right are the Catamount Revelator Signature networks. The woofer network is on top, the tweeter on the bottom right. The tweeter in the photo is the Scan Speak D2905/9900 Revelator, along with its frequency response curve. All North Creek drivers are serial numbered, tested and had matched.

Note that the 8 Gauge woofer inductor in larger than the 5" diameter Revelator faceplate, larger than the woofer cone! Also note all network connections are hard-wired, point-to-point, and all connections are crimped, then soldered with Kester silver solder. All terminals are gold-plated.

Even though the network is built with as few elements as possible and is a relatively simple topology, a pair of Catamount Revelator Signature crossovers weigh in at a very hefty 32 pounds.

 Input Impedance

This measurement was also taken with the equivalent of a 2.83V input, so the low frequency characteristics are properly documented.

Above is the Catamount Revelator system input impedance, with the Scan-Vent sealed. The fundamental resonance is 53 Hz. This is perfectly suitable for most tube amplifiers as one can incorporate a high pass filter to block the amplifier input below 40 Hz, essentially doubling the amplifier power.

Other than the 45-Ohm peak at 53 Hz (the woofer tuning frequency), the impedance measures an ultra-stable 6.00 Ohm ±2.50 Ohm and above 250 Hz, it is actually ±1.25 Ohm. Even the most modest tube amplifier can drive the Catamounts with ease.

Above is the Catamount Revelator with the Scan-Vent adjusted to "soft", compared to the sealed system. The change in the impedance maximum at resonance indicates the system is much more strongly damped, optimized for amplification with high output impedance.

Subwoofers

Definitely. 7.2 with SET's driving the top seven and dual Thunder subs.... Yum. The Kitty Kats are in the woodshop as of the time of this writing.

The most amazing quality of the Catamount family is their ability to resolve seemingly endless layers of nuance and subtlety that only the most uncomplicated tube amplification can capture. These are not bold, aggressive loudspeakers; rather, they have a delicate touch and excel at those things which tube amplification also does best. Microdynamics, harmonic balance, and a natural sense of timing and fluidity. Even when driven by as little as three Watts, the Catamounts do not sound like they are reproducing music; rather, they seem to embrace it.

A long time reference is Bonnie Raitt ballad "Louise" (Warner Bros. 9-26242-2), a complex mix with two lead guitars and vocals floating top dead center. One can clearly discern the individual instruments' character and the musicians' fret styles, and Ms. Raitt never sounded better.

Putting on Putamayo "Women of Spirit" (PUTU1137-2) Sibongile Khumalo, "Untold Story", the scale of the entire piece is perfectly represented while ambient acoustics float through the entire room. Ani DiFranco "Cradle And All" (Putomayo "Women's Work" PUTU128-2), a very well mixed live recording, her message is frankly conveyed and the power in her voice enthralling.

Digging out the classics, the Yes-Years remastered classic "Heart of the Sunrise" form "Fragile" (Atlantic 19132-2, 91644-2 remastered) is remarkably well preserved for 1972. Here the layers and layers of the overdub-happy 70's are beautifully captured and the texture of Chris Squire's masterwork is crisply reproduced.

The Gratful Dead "Workingman's Dead (1970 Warner Bros 1869-2) "Uncle John's Band" the harmonizing is just great, each voice separate and all together. "Cumberland Blues" is also a lot of fun to listen to.

Little Feat "Roll Um Easy" ("Dixie Chicken" Warner Bros W2-2686), simple but the layer's of complexity in Lowell George's voice clearly communicated.

The Catamounts are a good match with transistor amplification above 80 Hz. In the deep-bass, due to the absence of an output transformer, they are very detailed but definitely not robust enough to be considered a full range. With the cabinets sealed the system Qtc is 0.70, which is considered the optimum tuning for a sealed loudspeaker that is mated to a subwoofer. They are superb when matched with a good transistor amp and dual subs.

Internal wiring is 10 AWG OFHC copper to each woofer and 14 AWG Tef-Flex AG silver-clad OFHC copper to the tweeter. Binding posts are North Creek Big as Texas Posts. All connections are gold plated. Calibrated Scan Speak Scan Vents, Big Toes and Very Big Toes with inserts and lock nuts, mounting screws, gaskets, etc. are also included, and no soldering is required. In short, everything except the wood.

Catamount - North D28 Unlimited ....$ retired
Click HERE for cabinet drawings

The crossover network is fully assembled (hard wired) from our finest, hand-matched components. All inductors are 8 AWG North Creek Music Coil inductors; all resistors are NORTH non-inductive wire wound. The tweeter network features fully cascade-bypassed Crescendo capacitors also bypassed with a special 0.1uF, 1kV military film-foil cap of which we have a very limited supply. The woofer section uses Zen capacitors cascade-bypassed with Crescendo capacitors. All told, the crossover network uses ten 1.0uF Crescendo capacitors and ten 0.1uF bypass caps, and weighs in at about 30 pounds.

Internal wiring is 10 AWG OFHC copper to each woofer and 14 AWG Tef-Flex AG silver-clad OFHC copper to the tweeter. Binding posts are North Creek Big Posts. All connections are gold plated. Calibrated Scan Speak Scan Vents, Big Toes and Very Big Toes with inserts and lock nuts, mounting screws, gaskets, etc. are also included, and no soldering is required. In short, everything except the wood

Catamount Revelator Signature ...$2,999 per pair

So for my third attempt, I had to very seriously rethink what this loudspeaker could and could not do, and also what the associated amplifiers' limitations were. The conclusions were that (1) high output impedance of all tube amplifiers makes most vented loudspeaker systems a poor match; (2) variations in output impedance makes a single bass tuning good for some amps and bad for others; (3) the only way to get high efficiency with small cones is to limit deep bass response; (4) a loudspeaker with limited bass response must be easily mated to a subwoofer.

There are a lot of ways to limit bass extension (3). The approach taken here is get the lightest 5" cone with the softest suspension and hugest magnet we could find, then house it in an sealed box. The suspension element introduced by the sealed box is more linear than any mechanical suspension, so at large cone excursion system output remains clean. Because the time constant of the aperiodic vent is much longer than the frequencies being reproduced, there is no compromise in the system's physical linearity.

However, the effect of the aperiodic vent on the fundamental system resonance (around 55 Hz) is to drop the impedance peak by 50% or more - so it is easy to find the "sweet spot" where the amplifier is most comfortable. I don't know why I did not make this connection ten years ago.

Strangely enough, with one amplifier in particular (Anthem Amp-1), the Cats driven via the 8-Ohm taps were extremely lean even with the Scan-Vents sealed. In this regard, they sounded a lot like they were being driven by transistors at the low end and tubes from the midrange on up. Since the Scan-Vent and North Creek 3" x 5" flared port share nearly identical cut outs, I could not resist popping out the Scan-Vent and installing the port. Tuning it to 45 Hz, the Cats had excellent bass extension and plenty of gusto. I freely admit I had no idea why this works, but it does, so we also provide the Catamount loudspeakers with the port tubes for those who with to try it out.

Dr. Craig M. B.

For about a week now I have been listening to the Catamounts and playing with the venting options and room placement. These things sound great. The pure, accurate sound reproduction is clearly evident when listening to individual instruments (piano, guitar, human voice, etc.). One must keep reminding oneself that these are speakers and not the real performers in the room, they are that good. The Catamounts seem to prefer tube amps over transistors but even sound great with a transistor amp (circa 1980 high end) and just come alive with tubes. My old Dynaco Mk III's (considerably updated) never sounded better and now I am inspired to do the rebuild of my Dynaco PAS2 that I have been putting off. I am still playing with room placement and have yet to find the ideal location, but I know it's there.

Thanks for a great design. Some photos are attached.

Frank D. H.

The original design was targeted to be an 8-ohm plus system, to be compatible with typical tube amplifiers and SET’s as well as the latest generation of OTL’s. As there are virtually no true 8-ohm woofers, my first attempts were to connect a pair of 4-Ohm woofers in series (Scan Speak 15W4530G00’s). This was successful to some extent – certainly in terms of high system impedance and bass extension, but compared to my reference systems, the two-woofers-in-series prototypes did not remotely approach the level of midrange clarity of any other North Creek loudspeakers…perplexing as the 15W4530’sound great when driven solo. Among my reference systems are the Okara II – 9500 and the Eska mini-tower, both of which are built with Vifa P13 5" woofers, where the Okara II is a single and the Eska is an MTM with the P13’s in series. Directly comparing the Eska to the Okara II, there was no loss of midrange purity when going from the single driver to the series-wired MTM. Hmmmmmmm…….

Both the Scan Speak 15W4530 and Vifa P13 are manufactured by Danish Sound Technology and the build quality is comparable, but there are significant differences in technological innovation. Other than the enormous motor employed by the Scan-Speak driver, the major difference is three Faraday rings that encase the Scan-Speak motor’s center pole. Faraday rings perform an interesting function – they protect the motor’s center pole’s magnetic field strength from being modulated by the current through the voice coil, which greatly reduces midrange intermodulation distortion. However, the voice coil current is still modulated by voice coil motion, voice coil inductance, and the driver’s low frequency resonance, and it is this modulated current that drives the second woofer down the line.

The only conclusion I was able to reach regarding why it was not audible through the Vifa P13’s, yet clearly audible through the Scan Speak 15W’s, is that because the Vifa P13’s do not have Faraday rings the modulation is symmetric (that is, they modulate both themselves and the next woofer), where with the Scan Speak 15W’s do have Faraday rings so the modulation is asymmetric (that is, they modulate only the second woofer).

After discussing this with another loudspeaker designer, who observed the same phenomenon with SEAS Excel drivers (which incorporate a similar motor to Scan-Speak’s), both of us concluded that it is Faraday-ring related. Unfortunate as to the best of my knowledge, this is the first time a Faraday ring has proved to be detrimental to system performance.

At the same time, I was trying to run a string of Aurum Cantus G1's in series as the tweeter line, and they also failed to approach the performance of a single Aurum Cantus G1 (in fact, they did not even come close). This I attributed to all of those impedance matching transformers run in series.

Because of this, North Creek has dropped the development of a Scan-Speak - Aurum Cantus based line array.