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DIY HiFi Forum Welcome! Need support, you got it. Or share your ideas and experiences. |
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In Reply to: Eureka! 100db from a 30L Cabinet - 65hz to 25khz posted by bcherry on May 10, 2010 at 00:21:45:
Each of the parts making up the Crescendo Speaker system have been selected to be outstanding in concept and quality. Yet by careful design we have achieved a whole that is more than the already considerable sum of the parts.Offering 100dB/2.83V/1m Sensitivity in room (106db/2.83V/1m if we account for the fact that we use a stereo pair of speakers) the Crescendo is the highest efficiency speaker available from Diyhifisupply and it combines this with also some of the highest practical power handling and dynamic range.
Successfully blending a hornloaded and high efficiency HF system with a cone midrange is a challenge, finding a midrange that can match the electrostatic like clarity of a Ribbon Driver such as used in the Crescendo is even harder. Then bridging the gap to a Subwoofer is another challenge.
We needed to create the perfect combination of Drivers, Enclosure and Crossover to achieve the kind of performance the Crescendo offers.
The Crescendo Standmounting speaker was designed to offer the highest possible sound quality and high efficiency from a compact speaker. The Crescendo was designed mainly with the aim of partnering low powered valve amplifiers, however due to the exceptionally well controlled directivity an application in home cinemas is also possible.
The only limiting fact that was accepted from the beginning was that the speaker would be aimed at providing an 70Hz lower frequency limit to be used with either the dedicated Stand/Subwoofer of our own design or with any high quality commercial subwoofer system.
Crescendo LF Driver
--------------------------------The Crescendo low frequency driver is a 8" driver combining an oversized massive 5.5" diameter vented magnet system and cast frame, to avoid resonances and lack of precision induced by using stamped steel frames. The cone surface is about the same as that of two common 6.5" HiFi drivers, yet reserved only for midrange and upper bass.
In order to allow for a light but sufficiently stiff cone the Crescendo LF driver uses a carbon fiber matrix reinforced paper cone with a dustcap made from lacquer impregnated cloth. The surround is formed by double roll lacquer impregnated cloth.
With a 1.5" Voice coil and a very light cone (19.5g which is only a little more than the 14.7g for Lowther drivers!) the Crescendo LF driver shares much with the high efficiency full range drivers that have become fashionable in recent years. It offers a similar combination of a strong motor, light cone and large surface area provide a perceived "speed" sufficient to match the Ribbon/Horn assembly used for the high frequencies. Yet by designing a driver that strictly aims at the lower midrange and upper bass, rather than attempting to produce a fullrange driver the performance can be maximised for the desired frequency range, without having to be compromised in the interest of high frequency reproduction.
Crescendo LF Enclosure
-----------------------------------The Crescendo enclosure are modified GHP enclosures originating and somewhat common in the German audio scene. This combines in our case a vented enclosure with a precisely adjusted RC circuit in series with the driver. This RC circuit alters the tuning and driver phase in band to produce more low frequency sound pressure in the desired band and acts as excursion limiter for the Crescendo LF Driver below the range it is meant to handle.
This enclosure is not the commonly used vented box with series high-pass capacitor (which produces significant frequency response peaks before the roll-off and often fails to protect the driver from large excursions) but a more advanced system that allows the maximum SPL (at the limits of Hoffmans "iron law") and bandwidth as well as a smooth integration with Subwoofers.
The resulting Systems with the Crescendo LF Driver offers a tuning to 60Hz (minimum movement point of the driver) with a -3dB Point of 70Hz and 98dB/2.83V in half space and a realistic SPL of 100dB/2.83V inside an enclosed space such as a listening room. Overall cone-movenent is controlled very well and is held below X-Max at all frequencies up to around 25 Watt input power.
For use with much higher power levels (e.g. Home Theater) using "Bass Management" with the speakers set to "small" is recommended.
Crescendo HF Driver
--------------------------------We use an quite extraordinary magnetostat ribbon with a surface area twice as large as the surface area of a 3" midrange dome, yet with a frequency response up to 25KHz, coupled to an exponential waveguide (horn). This unit is derived from ribbons for professional sound reinforcement duties with huge SPL capabilities thanks to horn loading and high power handling. Using it in a HiFi speaker means the driver is very much underexploited, which repays with low distortion and a freedom from compression. It also allows us to use a very low crossover frequency (600Hz) and low order crossover (1st order or 6dB/Octave) without concerns of damage to the driver or acoustically overloading the ribbon driver.
The Crescendo HF driver combines a cast aluminum 60 by 20 degrees waveguide and a cast aluminum rear chamber with neodymium magnets and a kapton diaphragm to produce a high frequency unit without parallel. Covering 600Hz - 25KHz (+/-3dB) with a real 100dB/2.83V/1m and 6 Ohm impedance it is the ultimate driver for the upper midrange well into the ultrasonic range.
The controlled and narrow dispersion means that more of the sound from the recording is radiated towards the listener and less towards the floor, ceiling and other walls, where they would cause reflections and conflicting spatial cues. This directness of sound radiation with it's freedom from room sound clutter helps producing a more holographic sound-field than most speakers, even in room situations that are less than ideal for music reproduction (modern style living rooms for example).
Crescendo Crossover
--------------------------------The Crescendo crossover combines extreme simplicity with the highest quality possible. The crossover between the Crescendo HF Ribbon/Horn combo and the Crescendo LF section is at 600Hz with first order slopes.
The Crescendo HF Driver is fed though a Obbligato Oil Capacitor with a small amount of attenuation to match the SPL of the Crescendo LF Driver implemented using Kiwame carbon resistors in series with the capacitor. A set of Obbligato Tinfoil Capacitors with a Silver/Mica bypass capacitor act as "speedup" capacitors, connecting directly to the Crescendo HF Driver and binding posts, bypassing the Obbligato Oil Capacitor and the attenuation for very high frequencies.
The Crescendo LF Driver uses the RC low frequency tuning network described in the enclosure section. This is realised with three Obbligato Polypropylene Film Capacitors (100uF/400V) and Kiwame resistors.
The lowpass is realised using our own copper-foil & oiled paper inductors. A further copper foil choke is used for a antiresonance RLC circuit which corrects the drivers impedance and frequency response in the crossover region.
This is it. As minimal as possible with no attenuation and only minimal equalisation to avoid sucking the efficiency and life out of the speaker , yet at a level of parts quality only found in the most esotheric High End designs.
Crescendo Subwoofer
-------------------------------The Crescendo subwoofer was designed to match the SPL produced by the Crescendo main module when driven by normal Amplifiers up to around 15 Watt. This covers most single ended tube amplifiers. For Home Theater use or when using high power tube amplifiers it is recommended to utilise two Crescendo Subwoofers (one above and one below the Crescendo Satellite) to gain extra SPL capabilities, to keep step with the high efficiency of the main module.
Subwoofer Driver
-------------------------------The driver in the Subwoofer we designed to match the Crescondo main module uses our long standing Dynafone Driver. This subwoofer driver was designed ground-up with the aim to support high efficiency speakers in the low bass AND work in small cabinets due to the specified low VAS (~30L).
The Dynafone Drivers uses a double stack if 5.5" diameter magnets to produce a strong motor. A cast frame is mandatory given the forces generated when the cone is forced to great excursions. The 2" Diameter voice coil is wound on a aluminum voice coil former to allow greater power handling. A double-spider system is used to ensure the cone movement remains linear even at high excursions.
The diaphragm is an inverted dome made from aluminum and coated with a special oxide to damp the internal resonances of the aluminum diaphragm. The result is a woofer that can play up to beyond 1KHz without resonances and falls off smoothly past that.
Subwoofer Enclosure
-------------------------------The Crescendo Subwoofer uses a Mass Loaded Transmission Line Enclosure (MLTL). This is a modification on the traditional transmission line enclosure that combined the quarterwave resonance of the transmission line with a special loading system that tunes the line to a much lower frequency than that of a traditional Transmission line of the same length.
This allows a compact enclosure tuned to 25Hz with considerable gains in LF extension and output at very low frequencies compared to sealed boxes, yet without the usual compression, port chuffing at high volumes and boomy bass that characterises common reflex enclosures.
When combined with the active crossover required for the Crescendo (4th order lowpass at 50Hz) the bandwidth of the system is 20Hz - 60Hz (-3dB points). The maximum power handling is 250W/8Ohm down to 25Hz while remaining within the linear capabilities of the Dynafone Driver. However, the system is able to safely handle over 500W without damage.
Subwoofer Amplifier/Crossover
----------------------------------------------The Crescendo Subwoofer is driven by the Reckhorn A1000. The A1000 combines an adjustable 4th order lowpass crossover with a 800W Class D Mono Poweramplifier. It is the kind of "powerhouse" Amplifier that seems to deliver limitless power that is really ideal for power hungry subwoofer applications.
In the Crescendo system it provides 400W to one Crescendo Subwoofer, or 600W to a pair of Crescendo Subwoofers.
This translates into around 114dB/1m at 30Hz (inside a room) for a single Crescendo Subwoofer when driven hard.
Using two in a stereo pair with a pair of Reckhorn A1000 Amplifiers as would be common with the crescendo system, a pair of Crescendo Subwoofers can produce 120dB/1m at 30Hz.
If two pairs of subwoofers are employed with two Reckhorn A1000 Amplifiers theoretically an SPL of 125dB/1m at 30Hz can be attained.
This is more than enough to keep up will with the Crescendo Main Module.
DIY - Done Right!
Edits: 05/31/10
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Follow Ups
- Driver, Crossover and Cabinet Details - bcherry 21:20:30 05/31/10 (2)
- RE: Driver, Crossover and Cabinet Details - jusbe 14:22:47 06/03/10 (1)
- RE: Driver, Crossover and Cabinet Details - bcherry 20:25:44 06/03/10 (0)
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