Model 91 300B amplifiers built to order. Built using only the finest components. If you appreciate handcrafted components, ones that will give you satisfaction and for many years and never become obsolete, order yours today. Price dependent on components substituted and level of finish desired.For you intrepid souls who want to role your own, contact Angela Instruments for quality parts and plans.
- Italian double C-Core construction output transformers, 4/8/16 secondaries, potted in handmade enclousers.
- Chasis of solid 1/8 inch copper, hydraulically bent and holes machined on a Bridge Port milling machine.
- Expensive oil filled filter capacitors that will outlast your grandchildren.
- All wiring is point to point 16 gage solid silver.
- Exposed transformers and capacitors are bead blasted and painted in a blue-green hammertone finish.
Soon to be anounced, speakers that are tailered specifically for 300B triode tube amplifiers. These speakers will be based around the legondary 15 inch Tannoy Dual-Concentric drivers (description of their qualities below). They will have high efficiency, and employ an active vacuum tube electronic crossover with 24db/octive Linkwitz-Riley filter slopes. The enclosure will be sealed, aperiodically damped to help smooth and lower the bass impedeance peak, and have a low Qtc.
The 10 inch, 12 inch and 15 inch units use new pressure die-cast chassis forming the framework for the Dual Concentric. Employing innovative new thinking for mid/bass drive units, the new castings have a very open construction with a vented rear suspension. This improves thermal cooling, prevents major reflections and eliminates potential resonant trapped air cavities. Axial rigidity has been optimised to complement the new cabinet design philosophy. While the front surface profile, along with the purpose designed diffraction ring, blends the low frequency (LF) and high frequency (HF) wavefront with the front of the cabinet, our research has shown that, assuming the high frequency unit is well designed in the first place this is the single biggest factor in providing smooth high frequency radiation in Dual Concentric loudspeakers.
In all cases, the shape of the mid/bass cone has been calculated to match the HF hyperbolic waveguide ensuring that the wavefront remains spherical. The 15" drivers use traditional pulp cones with apex treatment and air dry felting. These materials have been carefully considered and chosen for their various performance characteristics when associated with the different sized designs. DMT has played a large part in the suspension/termination of the different cones. The characteristic cone termination impedance is matched by the surround material independently of the required suspension compliance. The unit system compliance is provided by the rear suspension where the best degree of mechanical control can be provided.
The motor system comprised of the magnet and voice coil is the heart of the mid/bass unit. Computer optimisation of the low frequency magnet gives linear flux linking to the voice coil using low carbon steel pole pieces and an anisotropic barium ferrite magnet. A specially designed pure copper stabilising ring fits over the outer pole of the 12 inch and 15 inch units where it reduces eddy current losses, lowers midrange distortion and increases thermal cooling by a massive 50 per cent. In this way both power compression and reliability are considerably enhanced.
Completely new high frequency units have been designed using a separate magnet assembly with easy field replacement for the HF diaphragm. A new design of waveguide has been arrived at by making extensive use of CAD (computer aided design). We call it a waveguide because there is a direct analogy with electromagnetic radiation in that characteristic impedances must be carefully matched without introducing standing waves. The Tannoy HF waveguide matches the acoustic source impedance at the HF diaphragm into the listening environment. The waveguide shapes the wavefront as it travels down from the diaphragm ensuring that path lengths are equal, the wavefront is perpendicular to the fixed surfaces and that the wavefront is spherical. In this way, transverse modes are minimised and high frequency dispersion maximised. Wavefront shaping begins at the diaphragm surface and because the compression ratio can be kept relatively low with this design, distortions due to air non-linearities are minimised. A hyperbolic flare has been chosen for optimum low frequency performance at this crossover point.
The HF diaphragms are 25 and 33mm new designs. The waveguide requires pure piston movement over the operating range since any break-up modes within the diaphragm will result in phase-shifted components at the start of waveguide propagation. The rigid piston diaphragm operating to above 25 kHz is made from an aluminium and magnesium alloy. Special tooling had to be designed and built by the engineering team at Tannoy to form and extrude the diaphragm, thus ensuring reliable handling for production and field service. The diaphragm assembly is suspended by a precision moulded inert nitrile elastomer surround. This has been designed and tooled using high precision, numerically controlled machining techniques with a narrow roll to eliminate resonances below 25 kHz yet provide a very stable and consistent mounting. The roll form ensures high excursions can take place if necessary and provides a fatigue indestructible assembly. The diaphragm is driven by a new design of voice coil assembly. High temperature polyimide insulated, copper clad aluminium rectangular ribbon conductor is chemically bonded onto a glass-fibre former that fits on to the outside of the HF diaphragm skirt.
This gives a high temperature (polyimide), very low mass (aluminium wire, glass fibre), high rigidity (rectangular wire, former to outside of diaphragm skirt), high reliability (nitrile suspension, copper clad aluminium) assembly. Leadout material choice is crucial for high frequency units and Tannoy's new design incorporates beryllium copper flat strip to eliminate fatigue breakages and prevent fusing on unsupported areas under overload conditions. The HF diaphragm assembly is factory mounted onto the waveguide by a newly designed high precision production process. This ensures that the spacing between the diaphragm and waveguide is consistent and the whole assembly self centres under all conditions when placed on the magnet assembly. Field replacement is therefore extremely simple and no soldering or special centring techniques are required. The high diaphragm very frequency linear, piston-like unit for the characteristics with very little variability due to design temperature importantly, it also provides good self damping.
With the addition of fluid damping and cooling, power handling is increased and the first break-up mode is pushed out of harm's way well above the audio band. The latest magnetic fluid formulation is used. The HF driver diaphragm assembly is a self-centring user replaceable part. A separate magnet assembly is also employed for easy field replacement.
Send E-mail to tonydean@hotmail.com
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Attn: Anthony Dean
Poor Design Group Inc.
2845 Harriet Avenue South · Suite 201
Minneapolis, MN 55407 · USA
(612) 871-3537 · Fax (612) 871-1530
E-mail to tonydean@hotmail.com
Copyright © 1998 by Poor Design Group Inc., Revised August 1998