The preamplifier consists of two stages. At the entrance, the volume potentiometer is equipped with three sockets for physiological correction (loudness). The intervention curves are visible in the first graph.
Philips considers the frequency of 800 Hz to be the center of the audible band, so the treble and bass controls are calculated to enhance the frequencies above and below that value. The choice of this frequency is very important for the musicality and the pleasantness perceived by listening to the device.
In the second graph the frequency response of the preamplifier, with the tones enhanced to the maximum. You notice the different equalization you get by listening to the AM bands (Long Waves, Medium, Short) or listening to the FM band and PHONO.
The final stages are two distinct, one for the low frequencies and one for the high frequencies. These amplifiers control separate speakers, a 26 cm woofer for the bass and three speakers for mid/high frequency, with the 17 cm front speaker disconnectable for 3D effect.
The most interesting feature of these final stages is that they are without an output transformer.
The feature that makes this device very interesting is that each final stage uses a pair of EL84 / UL41 in an ironless configuration, ie without an output transformer (the known critical point of the final stage).
The output stage uses the operating principle called Shunt Regulated Push-Pull (SRPP) used today in HIGH-END amplifiers.
The output transformer limits the bandwidth of the entire amplifier. The bass is limited by the inductance of the primary winding, the treble is limited by the parasitic capacitance resulting from the alternation of the primary and secondary windings. The alternating winding of the primary and the secondary windings is also necessary to improve the coupling and thus reduce the dispersed flow.
The reduction of the dispersed flow increases the damping. This is essential in the loudspeakers mounted inside the cabinet of the radios, which do not work in hermetically closed boxes, and which are therefore free to move almost without resistance from the surrounding air.
The two output tubes (EL84, UL41) of each channel, four final valves in total, are connected in series with DC-DC coupling. Each UL41 is configured as a triode.
The output is taken from the cathode of the UL41. The speaker has an impedance of 800 ohms, identical to the average impedance that occurs on the catode, whereby direct coupling (without transformer) is possible between the final stage and the loudspeaker.
The Philips Saturn 653 is therefore totally free of all the problems deriving from the output transformers.
The damping of the loudspeakers is kept high by the low dynamic resistance of the feedback loop.