Fig. 1: Full schematic of the new phono stage Memphis MC 1 (click to enlarge).
At first glance the schematic looks like a PNP overall concept with the negative voltage of -36V on top and the positive +12V at the bottom (fig. 1). That results from the Sziklai configuration of both the first and the second stage. However, at a closer look the more crucial transistors are of NPN polarity. In the schematic the input stage is set to suit a Benz Micro Gold cartridge (output voltage 0.4mV). However, with a very few tweaks the input stage can be modified easily to suit any other MC cartrige.First StageA Lundahl LL1933 step-up transformer multiplies the MC output voltage by the factor eight. Basically it serves two purposes:•transforming the tiny 0.4mV signal to a more convenient 3.2mV without adding noise and•to protect expensive cartridges from being burned out by front end failures.Furthermore, the transformer allows a symmetric connection between the MC cartrige and preamplifier if needed. R3 across the primary coils has to be matched to suit the MC cartridge impedance (500Ohm [my preference] to 1k for the Benz Micro Gold). A differential input design is necessary to cancel out the base currents of the input transistors to keep the transformer free from DC.The two dual input low noise transistors 2N3810 (PNP) and MAT12 (NPN) are expensive but still reasonable compared to New Old Stock (NOS) tubes. They are closely matched and of superb linearity. They are connected in a Sziklai configuration. The current distribution through both transistors is determined by the ratios of the resistors R17/R18 and R19/R21 respectively. The MAT 12 dual transistor Q6 / Q8 is a current mirror with a 1:6.5 ratio. Q7 (BC550) is a folded cascode idling a current of 7.9mA. Its current is fed into the frequency depending load L6, R1, R2, R14, C1, C2. The requested high gain of the input stage requires a high load impedance at the collector of Q7. The 21k resistor (R14) alone would limit the collector current to a value below 1mA with respect to its voltage drop. This would severely compromise the overload capability of the input stage. Therefore a choke (L6) is in parallel with the 21k resistor. Its inductance is 810H, its copper resistance is 2.4k. Thus the DC load at the collector of Q7 is 2.4k in parallel with R14 while the AC load is almost 21K at the low end of the audio spectrum which then decreases with ascending frequency according to the RIAA equalization curve.The LL 1667choke can be used at higher DC currentsas the specified 5mA for its maximum AC voltageswing of 390V RMS is not required. Jac van de Walle(JacMusic) published a very useful application noteabout DC currents and corresponding maximumAC signal levels for theLL1667 choke (tab. 1). In thephono stage the current through the choke is setto 7.3mA.Raw gain of the input stage (RIAA network R1, C1, C2 disabled) is 70dB plus the eightfold voltage step-up of the LL 1933 transformer. The input stage can easily be adapted to different gains by adjusting the degeneration resistors R22 and R23. Furthermore the step-up transformer can be changed from a 1:8 to a 1:16 winding ratio if real low output MC cartridges are favored. Figures 2 shows the first experimental setup.Passive RIAA EqualizationContrary to standard voltage driven passive networks (fig. 6) the RIAA network of the Memphis MC 1 phono stage is current driven, i.e. its signal comes from a current source with (almost) infinite impedance. Thus the traditional circuit for a passive network has to be modified as shown in figure 7. Details on the RIAA equalization network including a precision anti-RIAA circuit for SPICE simulation is described on the Laboratory Page of this web site. Deviations from the ideal RIAA with reference to 0 dB at 1kHz is between +0,17dB and -0.1dB between 30Hz and 40kHz (fig. 4). At 20Hz the response drops to -0.48dB below the ideal RIAA, at 10Hz it is -2.1dB due to the limited inductance of the LL1667 choke.Fig. 6 and 7: The impedance of the voltage source V1 is close to zero while the impedance of the current source I1 is close to infinity. Thus C1, R1 and C2 "see" the same impedance of R3.Second StageThe second stage (fig. 1, fig. 3) is just an impedance converter with unity gain topass the audio signal as unaltered as possible from the RIAA network to the outputand to enable the phono stage to drive even critical loads. The PNP/NPN Sziklai pairinput impedance is of several Megohm at the base of the transistor Q23 and doesnot alter the RIAA equalization. The low-pass filter at the input (10K parallel with10mH choke) suppresses RF oscillation (fig. 8).Years ago Nelson Pass wrote in his article on cascode amp design: „ . . in real life, the gain of a transistor, tube or FET changes as the voltage across the devices changes and as the current through the device changes. As these conditions fluctuate, the device creates distortion, but if we hold these conditions to a constant, the device becomes distortionless.” Therefore the p-channel MosFet on top is configured to bootstrap the transistors. Its source follows the output signal. Thus the emitter / collector voltage across the Sziklai transistors is kept constant to reduce distortion. For the same reason the output stage idle current is set to a relatively high 65mA to reduce current changes with the output signal to a miniscule fraction of the idle current. Figure 5 shows the maximum voltage swing of the phono stage which is around 30Vpp before clipping. Safety measuresIn the unlikely event of a shortcut or a failing transistor the accumulator based power supply will not limit currents. As a worst case scenario the accumulators may cause fire by feeding a hundred amperes into the failing circuit. Therefore slow blow fuses of 1A were incorporated (fig. 9, red arrows).Fig. 9: Experimental setup of the Memphis MC 1 phono stage. The red arrows show the current limiting fuses. Overall voltage of the fully charged power supply is 50.6V.Fig. 10: Fully wired Memphis MC 1 phono stage.Fig. 12: Not designed to win a beauty contest but to play music. The case comprises a simple MDF panel plus four aluminum profiles. Behind the accumulator based power supply.
Fig. 5: Maximum voltage swing before clipping is 30Vpp (red arrow indicates +/- 15V on the screen).
Fig. 2: Experimental setup of the input stage. A small heat sink is mounted on the cascoding BC550 transistor (power dissipation around 180mW).
Fig. 3: Experimental setup of the impedance converter.
Fig. 4: Measured deviations from the ideal RIAA response (db) between 10Hz and 40kHz with MC input transformers and output capacitor included. For more details see the Laboratory Page.
Tab. 1: DC current versus maximum AC level for Lundahl LL1667 chokes