Hybrid Phono Preamplifier MK II  
The Circuit The Hybrid Phono Preamplifier MK II is an advanced version of its predecessor MK I. It has full MC capability, an improved power source rejection ratio (PSSR), lower output impedance and needs 120V power supply voltage only. Figure 1 shows the schematic. The 2SK369 input jFet (J1) is loaded by a current source (J2, R9, R10) set to 5mA. Upstream the cascoding EC(C)88 (U3) holds a constant supply voltage of 21V. Both, current source and cascode isolate the input jFet from noise and hum on the the power supply line. Any current change of the input jFet is forced through the base-emitter diode of the following BC560C (Q1, BC557C in the schematic) BJT. The amplified signal current appears at the cathode and thus at the plate of the second cascoding triode (U1). The BJT / triode idle current is set to 7mA by R2. Two 9V batteries (V2, V3) elevate the ECC88 grid voltages to 9V and 18V respectively. The only battery loads are the minuscule grid leakage currents. Instead of the conventional plate resistors  LL1667 and LL1668 plate chokes are used in a parafeed design. The 7mA idle current is fed into the LL1667 choke, while the signal voltage is fed into the load resistor R4 via C1. According to the manufacturer the nominal choke idle current is 5mA. However, for the sacrifice of voltage swing headroom the idle current may be exceeded up to 8mA (click here for further information). Choke, capacitor and the grounded load resistor R4 form a low pass filter. They reduce hum and noise from the power supply line. A drawback is the need of a large capacitor C1. Gain of the input stage is 67db (RIAA disabled). Fig. 1: Schematic of the MK II phono stage with an anti-RIAA filter at the input. The plate resistors have been replaced by    chokes. The two systems of the 5687 twin triode are shared by the left and the right channel. All voltages and currents are    taken from LT SPICE simulation.  The first stage is followed by a passive RIAA network. The load resistor R4 as well as C1 are RIAA critical. Details on the RIAA equalization network design can be found on the Laboratory Page of this web site. The simulated RIAA response is within +/- 40mdb between 20Hz and 40kHz (Fig. 2). Fig. 2: Simulated RIAA response is within +/- 40mdb between 20Hz and 20kHz. The second (output) stage uses the more powerful 5687 triode in a plain common cathode configuration. At 20mA idle current its plate resistance is around 2000Ohm and thus 50% lower compared to the ECC88 (fig. 3, 4). Still the gain is sufficient. The bypassing capacitor C3 inhibits current feedback induced by the cathode resistor R5. Thus, C3 increases gain and lowers the output impedance of the second stage. With an input voltage of 0.4mV @ 1000Hz the phono stage‘s output voltage is 1.5V.    Fig. 3: Plate resistance of the ECC88 @                         Fig. 4: Plate resistance of the 5687 @ idle current of 20mA                idle current of 10mA Power Source Rejection Ratio (PSRR) of the MK I and the MK II Design Analyses of PSRR were carried out on the MK I design with plate resistors (fig. 5) and the MK II design with plate chokes (fig. 6). Sinusoidal interference voltages of 1V were were added to the supply voltages as frequency sweeps from 50Hz to 5000Hz. Although the MK II design has twice the gain, its PSRR is considerably improved (fig. 7, 8). In real life the result might be even better, because the MK II design requires half the power supply voltage only and thus should carry only 50% of the 100Hz (or 120Hz) hum. Additionally the RIAA network will improve PSRR at higher frequencies in both designs.       Fig. 5: MK I input stage without RIAA network                                    Fig. 6: MK II input stage without RIAA network                Fig. 7: PSRR of the MK I input stage is -0.3db with 1V AC interference voltage added to the power supply voltage (50Hz to 5000Hz). Fig. 8: PSRR of the MK II input stage is between -23db and -51db with 1V AC interference voltage added to the power supply voltage (50Hz to 5000Hz). At the crucial 100Hz PSRR is -29db. Power Supply Current Load Chokes try to maintain a constant current through themselves by storing and releasing energy. This takes stress from the power supply because it has to handle less current change at a given signal amplitude. The figures 9 and 10 show the output stages of the resistor loaded (MK I) and the choke loaded (MK II) design. For demonstration both circuits are fed with an input voltage that delivers 20V at the output. Figures 11 and 12 show the corresponding current swings on the power supply line.      Fig. 9: Resistor loaded MK I output stage                                                                   Fig. 10: Choke loaded MK II output stage                Fig. 11: Power supply current of the MK I output  stage is 9.3mA at idle. With 20V output  voltage into 47k current swing dI is 3.7mA Fig. 12: Power supply current of the MK II output  stage is 19.3mA at idle. With 20V output  voltage into 47k current swing dI is 38µA to be continued . . . .