Hybrid Phono Preamplifier MK II
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 160V power supply voltage only. A
special feature is a battery grid biasing of all tubes. With no current flow except for minuscule grid leakage currents,
battery life span will be limited predominantly by self-discharge. With lithium ion manganese oxide (LMO) batteries
life span may be up to 10 years. Interesting articles on triode battery biasing can be found here and here.
The MK II Circuit
Improved Power Source Rejection Ratio
The MK II Power Supply
The MK II Circuit
Figure 1 shows the schematic. The drain of the 2SK369 input jFet (J1) is loaded by a current source (J2, R9, R10), set to
5mA. Upstream a EC(C)88 (U3) provides a constant supply voltage of 21V. Both, J2 and U3 form a cascoded current
source and isolate the input jFet and the following BJT from potential noise on the the power supply rail. Any current
change of the input jFet is forced through the base-emitter diode of the following BC560C BJT (Q1, BC557C in the
schematic). The amplified signal current appears at the cathode of the second cascoding triode (U1) and thus at its
plate. 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.
A plate choke is used instead of a plate resistor. The common 7mA idle current of Q1 and U1 passes through the
LL1667 choke, while any signal voltage is fed into the RIAA network and the second stage via C2. 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 more information). The choke and the resistor R4 form a low pass
filter (except for very low frequencies due to C2) and reduces noise from the power supply. A drawback is the need of
a large coupling capacitor C2. The capacitances right channel / left channel should be matched for the capacitors are
RIAA critical. 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 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 C2 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 with the interstage transformer and the 47k
load taken into consideration.
The second (output) stage uses the more powerful 5687 twin 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. Instead of the usual cathode resistor and a large bypassing electrolyte capacitor also the 5687
triode grid is battery biased at -3V. R5 provides some degeneration and allows easy measurements of the triode‘s idle
current. At the output an interstage LL1660 transformer is used to avoid ground loops and to allow a balanced
output if needed. With an input voltage of 0.4mV @ 1000Hz the phono stage‘s output voltage is 1.3V in the
simulation. The phono stage easily can take 10mV at the input which would be 35V at the output.
Fig. 3: Plate resistance of the ECC88 @ Fig. 4: Plate resistance of the 5687 @ idle current of 20mA
idle current of 10mA
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Improved Power Source Rejection Ratio
Simulation of PSRR were carried out on the MK I input design with a plate resistor (fig. 5) and the MK II design with a
plate choke (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 less power supply voltage and thus
should carry less 100Hz (or 120Hz) hum. Additionally the RIAA network will add some improvement on PSRR at
higher frequencies.
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.
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The MK II Power Supply
The power supply for the MK II hybrid phono stage is “old school” designed. The mains transformer is a grossly
oversized Lundahl LL1651 delivering 2 x 250V at its secondary coils. The two-phase rectifier for the high voltage and
the bridge rectifier for the heater voltage use fast recovery diodes and Schottky diodes respectively. The two 10nF
capacitors across the diodes D1and D2 slow down voltage peaks from the rectifier switching noise transferring the
energy towards lower frequencies. At the bridge rectifier (D3 - D6) a snubbing network (C17, R19) minimizes the
switching noise.
Fig. 9: Power supply of the MK II hybrid phono stage. Transformer, rectifiers, chokes and most capacitors are mounted in a
separate housing. The information on the primary coil inductance of the LL1651 mains transformer is taken from the
Lundahl website and can be found here. All voltages and currents are taken from LT SPICE simulation.
Ripple currents are powerful sources of hum and buzz. Therefore, reservoir capacitors are omitted to avoid sharp
current pulses. Instead, the rectified currents are fed into reservoir chokes (LL2733, LL2743) as the storages of
energy. Chokes try to maintain a constant current and block the AC components. They even provide some voltage
regulation. This is explained nicely in Merlin Blencowe’s book on Tube Preamps.
When the rectifier diodes switch off the chokes will generate flyback voltages. Therefore, two film capacitors (47nF
and 1µF) are connected immediately before the chokes. Each choke comprises two coils on a common core and are
wired in a common mode rejection configuration. Common mode noise may leak from other transformer coils via
stray capacitances and appear equally on both wires. The arrangement of the two choke coils block common mode
noise to a certain extent (click here and here for more information).
The high voltage rectification comprises an additional flyback diode D7 parallel to the 47nF capacitor. It absorbs the
flyback voltage from the choke in case of a fuse blow. The bridge rectifier does not need an extra flyback protection.
Downstream the chokes simple RC filtering is added for further smoothing – not particularly efficient but effective.
R25 simulates the resistance of all paralleled heater filaments, R24 and R26 simulate the 5687 triodes, the current
sinks I2 and I3 simulate the cascoded ECC88 triodes.
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to be continued . . . .
