These are my own cardioid reference monitors. There were three major requirements:
good step response aka minimum phase behaviour
cardioid behaviour from the upper bass on
In the end I managed to achieve all these goals. Most importantly while doing so I was able to build a speaker with objectivist tendencies that manages to also impress in terms of sound. The transient response from this inefficient bookshelf speaker while managing to keep a certain smoothness needs to be heard. It also has a sound projection that is totally unlike anything I have experienced in this size range.
When trying it out I immediately realized that this 4" Al-Sandwich driver used here is incredibly capable, it just needs some special care with the crossover. I ended up using non-standard parts to get the response I want from it.
The driver combines many very desirable aspects of driver design:
A very thin, but stiff and still well damped driver diaphragm made of a honeycomb aluminium sandwich
A low-loss surround with edge-coating to absorb surround resonances and considerable Xmax
A magnet system that doesn't sound straining to the ears. Very low third order distortion across the board.
A flat diaphragm is actually a very desirable thing for dispersion and as such dispersion is very wide considering the driver's size
Last but not least: The resulting frequency response is quite smooth and can be further tweaked to perfection
It was clear from the start that for a cohesive sound I would need a metal woofer. Not very many metal woofers are up to the task and can perform in an enclosure the size I had envisioned, but there are two candidates that can be substituted for another with no change in baffle shape or crossover topology.
The woofer used in the Hathor may be rather inexpensive, but it punches well above its weight and has a very responsive and quick sound considering its sensitivity and size. It matches the tonality and timbre of the widebander very well and makes for a very seamless transition. In this cabinet impressive extension can be achieved and thus the desired full-range sound is a reality. No sub necessary.
A logical upgrade is the newly released Purifi 8" alu cone woofer. Its distortion levels match or exceed that of the widebander used here, a decent step up from the inexpensive alternative. This allows for more control and a more mature sound that rivals not only floorstanders of this size, but speakers with significantly larger woofers. Seriously, the Purifi stuff is that awesome, but the earlier paper drivers don't make for a good tonal match.
The ports facing towards the front make for a seamless transition to the woofers. They are tuned for a flat in-room response, not for a flat anechoic response. When placed near a wall impressive extension down to 20Hz can be achieved.
Not only does the amount and placement of the damping materials matter especially here, the material itself is very important, too.
Which is why natural sheep's wool in 4 very different forms is used here. For the woofer I use a special OOP combination of pure sheep's wool and polyester filling in addition to two types of sheep's wool on the prototype. Unfortunately since this material isn't available anymore I will need to find a substitute. My stack isn't that large.
The finish is white glossy paint on the rear and oiled wood on the front using a specially colored oil that preserves the color of the wood for many years to come. Different types of wood are available on request, but the inexpensive spruce wood has a very nice timbre and is thus recommended.
Special high end version: Hathor X
Purifi 8" Alu cone woofer
Only vintage paper in oil capactiors used
balanced crossover configuration
Mundorf copper foil paper coils
Nominal Impedance: 4 Ohm (minima: 3.2 Ohm at 27Hz & 3.4 Ohm at 106Hz)
Impedance rises smoothly towards high frequencies, slightly inductive load.
Frequency range (-10dB): 20Hz- 40kHz
Frequency range (-6dB): 35Hz - 35kHz
Frequency range (-3dB) 50Hz - 30kHz
Phase response: Excess phase is ± 1 degree 20Hz to 80kHz
Rear attenuation: -10dB at 100Hz, -20dB at 1kHz
Dimensions W x H x D: 250mm x 400mm x 400mm
Weight: 14kg (prototype)
2-way speaker with a 4.5" full-range and 8.5" woofer
Power rating; These aren't disco speakers, but they should be able to handle around 250W.
(Remember there is no high pass filter on the full-range driver)
Acoustical first order crossover at 200Hz, time aligned drivers
Port tuned to 28Hz
Bi-wiring terminals with an extra ground lug connected to the driver baskets
1.5mm² tinned copper in XLPO internal wiring
Large transformer core coil, only MKP capacitors and high quality wire wound resistors
HDF construction (prototype is MDF) and wooden front baffle.