The S2 1.5" Pressure Unit is a landmark product in the history of the British audio industry. The beautiful simplicity of its design has stood the test of time and it remains in production today, almost unchanged, more than 60 years after its conception.
The original design brief was extraordinarily challenging: to combine the delicacy, clarity and linearity of response demanded by the most discerning audiophiles, with the durability, serviceability and flexibility required by public performance venues. So successful were the design team in meeting this brief, that examples from the very earliest days can be found in some of today's finest user-built systems, whilst current production pieces power some of the world's most renowned "statement" loudspeaker products.
The diaphragm is remarkable. Aluminium alloy only .002" thick is formed into a short tube with a closed domed end, which by a special process, is made extra stiff. A purpose built machine is used to wind the coil onto the tubular surface of the dome and to secure it to a flexible surround which positions the coil in the magnet gap. With the coil wound directly onto the dome there is direct drive - no intervening adhesive joint - and damping of eddy currents. The surround is formed from polyester, a material virtually immune to vibration fracture, and is stiffened by means of heat-formed tangential ribs which allow free movement of the dome over the required excursion.
An intense magnetic air gap, in which the coil works, is powered by a heavy Alcomax III AlNiCo magnet positioned centrally in the magnet assembly. This allows the magnetic lines of force to be conducted upwards through a hollow centre pole, down and around, to complete the circuit through a magnet pot and top plate. A non magnetic location device ensures accurate concentric positioning of the top plate and the centre pole, which houses the phase corrector in one assembly.
Because a lot of acoustical power is required to provide a wide dynamic range, the area of the dome must be large. This conflicts with the need to reproduce high frequency sound of which the wavelength is very small. The phase corrector ensures that a pressure wave created on the inner surface of the diaphragm dome is still recognisably the same by the time it reaches the start of the horn which is at the base of the centre pole. It takes the form of concentric tapered plugs fitted into the conically dished top of the centre pole, forming three concentric air channels angled to join each other in the heart of the unit.
To say that the engineering of the unit is exquisitely precise and the performance powerful and accurate would be absolutely true, but it would only go part of the way to explaining what it is that really separates the S2 from its competitors. The quality that makes it truly great, the very essence of the S2, is musicality.
Fitted to a suitable dispersive horn, and properly mechanically and electronically integrated into system, the S2 makes the most sublime music.