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 The plate resistors have been replaced by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 . . . .
