This network was designed by John Warren of North Reading Engineering, based on extensive testing he has done with the V-Trac horn, BMS 4592 driver, Beyma CP-25 tweeter, and a Klipschorn bass horn.

The KV1 is the base network (shown above) and the KV2 adds a notch filter to the bass section, which will be described in more detail below.

The KV2 is shown below (with the bass notch filter).

Either of these networks is considered to be a premium option for the full Klipschorn upgrade package from Volti Audio, including the V-Trac horn, BMS 4592ND-MID midrange driver, and Beyma CP25 tweeter.  No other network we offer more perfectly balances the components in this upgrade package.

This network is comprised of two sections, one for the bass horn and the other for the upper horns.  A 4th-order bandpass filter is used on the midrange, and a 4th-order hi-pass filter is used on the tweeter.  Jantzen caps and inductors are used throughout.

A notch filter is utilized in the current Klipsch stock AK-5 crossover network, and the notch filter offered with the KV2 from North Reading Engineering very closely simulates the results of the Klipsch filter.  The purpose of using a notch filter for the bass horn of the Klipsch Khorn is to smooth out the peak in frequency response that occurs between 100Hz and 350Hz.  For more detailed information, please scroll down towards the bottom of this page.

These North Reading Engineering crossovers are sold exclusively by Volti Audio.

KV1 - $1,250/pr.
KV2 - $1,750/pr.

How do these networks compare to the VTK networks?

Comparison to the Volti Audio VTK400 network is inevitable.  I knew just as soon as I offered this new network for sale, that I would be asked the question, "so which network do you like best"?  There is no easy answer.  Both networks have their strengths, and the price will play a part in your decision making.

When John first approached me with the idea of designing a crossover network for the V-Trac horns, I was very excited and cautious at the same time.  Excited, because of the extensive testing that I knew John would do on the V-Trac upgrades, which is something that I am not capable of myself, and would cost thousands of dollars if I hired it out.  Cautious, because of the conflict it might cause with marketing John's networks along with my own, given price differences, and not knowing how the sound quality would be.

But I'm always interested in and I keep an open mind to anything that might provide a higher quality sound for my system, so I provided John with a horn, BMS driver, and Beyma tweeter (John has Khorn bass horns) and waited for the results.  After a couple of months John started sending me graphs and technical descriptions from his testing, and it all looked good.  Soon after, he gave me a pair of KV2 networks to try in my own system.  In fact, John came up to my home and we listened to my fully upgraded Khorns with the VTK400 networks, and then switched over to the KV2 networks.  As with so many things in high-end audio, the differences between the networks were not immediately apparent.  We could hear differences, but they were very slight, and it was difficult to pinpoint and describe them.  John left the networks with me, and over the course of two weeks, switching between the KV2's and the VTK400's, using different power amplifiers, listening at various volume levels, and listening to lots of different music, I was finally able to discern and describe what I was hearing.  I'll boil it down.  One thing that stood out with the KV2 networks was the clarity through the high-mids and high frequencies.  I noticed this particularly with piano recordings, where I thought the piano was more "believable" or "true" as compared to the VTK networks.  With the KV2 networks, I also noticed more separation and depth of sound within recordings.   One last thing with the KV2's, is that I noticed a bit more detail in the high end.

Overall, if cost were no object, I would choose the KV2 or KV1 networks for my own system over the VTK400 networks.  I still love the VTK400's in my system though.   During my testing, I wrote repeatedly in my notes that both networks sound fantastic with this system, and it's true, they are both great networks that compliment these upgrades perfectly.  The VTK networks do provide more flexibility in being able to balance the system, with easy changes to midrange output level, and the choice of three different upper-bass filters.

To help with the decision making, and perhaps even cut down a bit on the inevitable email questions (lol), here's a bullet list of positives for each of the two networks I offer for the V-Trac horn upgrades:

KV1/KV2 Networks

• Design based on technical analysis of the components
• Steeper slopes used on the upper horns provides a bit more clarity in the high end
• Bass notch filter option
• Overall, best sounding on my own system

VTK400 Networks

• Aesthetics
• Input binding posts
• Bi-wireable
• Three different upper-bass settings
• Autotransformer midrange attenuation
• Higher quality parts (Sonicraft caps and Solen open-air inductors)
• Lower price

THE V-TRAC KLIPSCHORN NETWORK (KV1 & KV2)

A three-way crossover network for the V-Trac modified Klipschorn loudspeaker system has been developed and is described here.  The V-Trac consists of a large format wood construction tractrix horn manufactured by Greg Roberts at Volti Audio.   It is driven by the BMS ND4562 MID neodymium, 2" throat, polyester diaphragm, compression driver.  High frequency content is handled by the Beyma CP-25.   The network has been developed by extensive iteration between computer model simulations using both LTSPICE and MATLAB simulators and experimentally determined acoustic responses.  The design approach considers the acoustic response limitations of each component and seeks a network configuration that establishes as near a flat, on-axis, near-field, frequency response as is possible within the design practice allowances for useable bandwidth, phase distortion, physical size, part count and cost.   Network simulations are terminated with loads that accurately capture both impedance magnitude and phase information of the bass, mid and high frequency horns providing accurate predictions of purpose specific design revisions.  Near and far-field impulse, frequency and phase data files and curves are generated using CLIO (v.7.3 and 10.31) acoustic analyzer.

Note regarding acoustic response curves - Real, unsmoothed acoustic response curves are not always pretty and are almost never shown simply because the data likely fails to substantiate the performance claims made by the manufacturer.  We have characterized, in great detail, the acoustic response of this system.  We have used computer simulation to develop the crossover network.  The computer models were then calibrated to the actual acoustic response measurements.  This provides us with a powerful tool to refine the networks and establish the desired acoustic response.  The only thing that matters is the acoustic response of the system.   We have refrained from showing the predicted response from simulation software because predictions have no value to the end user.  That said, all claims are substantiated with performance data.

DESCRIPTION

The KV1 consists of two sections, a bass horn filter and a V-Trac filter.   The bass horn filter section sends low frequency signals to the Klipschorn folded bass unit whilst the top section filter sends signals to the BMS midrange and Beyma high frequency drivers.  Capacitors and inductors are Janzen manufactured (Capacitors: polypropylene Cross-Caps, Inductors: #18 air-core and #15 iron-core).   Additional we have incorporated one design feature found on the current factory network, a large notch filter in the bass horn section.  We have however, altered the shape of the response to suit our design requirements.  The filter with the LF notch filter is designated the KV2.

HIGHLIGHTS

Compatible with any reasonable quality audio amplifier (minimum real component of impedance 4.2Ohm).
Midrange horn output level user modified by switching out optional L-pads.
Midrange filter is 4th order band-pass, establishes well-defined acoustic center at critical mid-band frequencies
High frequency filter is 4th order high-pass.
Bass horn filter is 2cd order low-pass.


BASS HORN FILTER RESPONSE

The Vittora bass horn near-mouth, acoustic response, is shown as a function of various filter configurations in the plot below.  The horn near-mouth response is measured by placing the microphone in the geometric center of the upward facing bass horn mouth.   Shown in the plot are a third-order (green), second-order (blue) and no-filter (purple) responses.  The V1 bass filter utilizes the second-order filter.

The Vittora bass horn ground-plane magnitude response is shown in the plot below at 0, 2, and 4m (top to bottom.  The responses were measured outdoors (restricted access parking facility) with the measurement microphone place at ground level and centered on the bass horn mouth.  The measurement conditions used provide an ideal environment to assess the low frequency response of the bass horn section.  Responses are DFFT of MLS stimulus, raw data from analyzer (no averaging).  The responses shown are with the V1 bass filter.

Cumulative Spectral Decay (CSD) response of the bass horn with no-filter (top) and second order, V1 filter, (bottom) shown below.  Responses are 2m ground-plane measurements.

Note the attenuation of prominent resonances at approximately 480Hz and above 800Hz with the V1 bass filter.

MID AND HF HORN SECTION

Plot below is the near mouth response of the MID horn (green) and HF unit (purple) connected to the V1 network.  Responses derived using DFFT on MLS impulse using a rectangular window.

MID-RANGE AND TWEETER CROSSOVER QUASI-ANECHOIC RESPONSE

The measurement microphone was placed approximately 1m from the center of the V-Trac horn mouth section and an MLS impulse applied and response recorded.  The mid-range and HF compression drivers were connected to the V1 network.  The quasi-anechoic, frequency response shown is valid for frequencies above approximately 400Hz.  Orange curve is mid horn response, red is high frequency response and purple is the system response.  Frequency domain information derived from DFFT of MLS impulse.  Blue plot is excess phase rotations determined by subtracting numerically derived minimum phase response associated with measured frequency response from the total phase response measured.

CUMULATIVE SPECTRAL DECAY MID-RANGE TO TWEETER

The Cumulative Spectral Decay (CSD) plot of the response shown above is provided below (top plot).  Bottom plot is higher resolution plot of the CSD at the MID to HF crossover frequency located at 3800kHz.  Note smooth transition between the MID and HF unit at the crossover frequency.  Note also the exceptionally uniform decay at crossover frequency

EXCESS GROUP DELAY

Excess group delay plots provide a quantitative assessment of the time dispersive characteristics of a multi-way loudspeaker system.  For example, if a loudspeaker system is claimed to be time coherent that implies is a flat excess group delay plot across the useful bandwidth of the system.  Large format horn systems exhibit excess group delay values that can be large especially when compared to well engineered direct radiator systems.  Thus, given the constraints imposed by the physics of the system, the design of the V1 network attempts to establish a well-defined acoustic center in the critical MID range.  This is accomplished using a 4th order MID band-pass and 4th order hi-pass on the tweeter.  As evidenced in the plot below, the magnitude of the excess group delay of the MID lags the tweeter unit by approximately 1.2ms down to about 900Hz.  At lower frequencies, the magnitude then increases with frequency, a typical observation.  The large peak in the plot is associated with the crossover performing the filtering function.  Excess phase derived from DFFT of MLS impulse.

IMPEDANCE

The impedance modulus (Z) of the Vittora loudspeaker system is shown below (blue), the phase angle is shown also (purple.  The measurements shown include the resistance associated with the 6ft of #16ga hook-up wire used to connect each of the three drivers to the V1 network.

The Real component (red) and Z (blue) plotted below.  A minimum in Re is observed at approximately 150Hz (Re~4.2Ohm.

Low Frequency Notch Filter

Please note that the following information has been written based on testing for the KS2 networks.  However, the KV2 notch filter is identical to the KS2 and the results are the same.

A NEW LOW FREQUENCY FILTER FOR THE KLIPSCHORN FOLDED BASS HORN (KS2 LF) A second order low-pass filter with frequency domain correction is described here. The filter has been developed to improve the linearity of the Klipschorn bass horn by attenuating the exaggerated output measured between 100 and 350Hz. The addition of a notch filter is placed in parallel with the LF driver operating in the folded assembly. Photos of the new notch filter are provided in the thumbnails (click on image).

FREQUENCY RESPONSE OF KLIPSCHORN FOLDED HORN (BASS SECTION ONLY) The GREEN plot in the graph below is the acoustic response of the Klipschorn bass horn with no filter (no low pass or notch) measured using a method developed specifically for this loudspeaker and to be described in a separate article. The BLUE plot is the response of the horn with a second order low-pass only. The primary function of the low-pass is to provide a significant roll-off of output above about 300Hz as is seen when compared to the unfiltered response. Note that the low-pass also attenuates output across all frequencies.

NOTCH FILTER DESIGN AND PERFORMANCE The next plot shown below is a simulation and shows predicted voice coil current as a function of frequency for the low-pass filter (light blue) and the low-pass with notch filter active (black). As is shown in the plot, the drive current with the notch active crosses the no-notch response at approximately 90Hz and remains lower until about 340Hz. The simulated filter is loaded by a circuit constructed to simulate the impedance and phase responses of the Klipschorn driven by the Klipsch K33E, 15" diameter, sq. magnet, stamped basket driver made by Eminence. Simulation performed with 1.0V constant voltage source.

The acoustic output of the bass horn under the same conditions simulated above (low-pass, low-pass + notch filter) is provided in the plot below. The notch filter attenuates the large peak observed between 100 and 200Hz.

In the plot below, the output of the Klipschorn bass filter (RED), the mid filter (GREEN) and HF filter (BLUE) output responses are shown. The mid and HF drivers are the OEM drivers sold by Klipsch for the Klipschorn (K55 and K77, respectively). The mid-horn is the K401, an injection molded (plastic) horn. As evident in the response plots, the filter provides well balanced outputs. The three-way filter is described here and is designated the KS2.

IMPEDANCE MAGNITUDE OF NEW LF FILTER The magnitude of the impedance of the Klipschorn bass unit, over the operating range of the new LF filter, is shown in the BLUE plot below. The RED plot is the folded bass horn, no filter. The lowest value measured occurs at 59Hz with magnitude 3.8 Ohms. The impedance was measured with the analyzer connected directly to the filter amplifier input terminals and thus does not include added resistance associated with speaker cables. The bass horn with the new filter is suitable for any reasonable quality amplifier rated for 4 Ohm loudspeaker loads.

COMPLEX IMPEDANCE OF NEW LF FILTER The complex impedance of the Klipschorn bass unit between 30-1000Hz, both with the filter (BLUE) and without (RED), is shown in the two plots below. The plot at left is the complex impedance calculated from magnitude and phase data collected from the analyzer. The plot at right is a closer view of the behavior over the stop band of the notch. As is evident in the plot many of "loops" associated with reflections in the bass horn are eliminated using the filter described here.