Low Budget MM Phono Preamplifier(The Blues Boy)This preamplifier has not been built. Is is just a conceptual idea.
Sometimes circuit designs may be restricted by tight financial limits. The low budget Blues Boy phono stage was designed for moving magnet (MM) cartridges with an estimated budget of 20€ for the electronic components.A single power supply of +24V was chosen, because simple 24V transformers are cheap and common or may be just around in the hobby room. With a bridge rectifier, two capacitors, one +24V regulator, a mains switch and a fuse holder the power supply is complete. Estimated costs are:•Voltage regulator +24V0.29€•bridge rectifier0.17€•transformer 10VA 24V5.60€•mains switch0.69€•fuse holder0.26€That adds up to 7.01€ for the power supply components. However, any voltage between +24V and +36V is suitable for the phono stage (R5 has to adjusted to keep the idle currents of J1 and Q1 at 5mA). All prices were taken from the Reichelt Elektronik website.Fig. 1: Schematic of the the moving magnet phono stage (values in brackets are for optimized RIAA). The LT op amp was chosen because of its availability from the LT SPICE libraryFigure 1 shows the schematic. A 2SK369 jFet at the input is cascoded by a PNP BJT. R12 sets the idle current of the jFet to 5mA and R5 sets the idle current of the BC560 to 5mA. Simple single ended input stages are highly susceptible to ripple and noise from the power supply. The folded cascode allows to cancel out ripple or noise because the base and the emitter of the BC560 are both connected to the positive rail. The BC560 feeds its current signal into a passive RIAA equalization. A passive network was chosen to avoid a frequency depending feed-back loop and thus allow adjustment of the gain of the operational amplifier over a wide without interfering with the RIAA response. Estimated costs for the circuit components for each channel are:•12 resistors1.00€•1 330nF0.27€•1 1uF0.58€•1 10uF0.17€•2 1000uF0.64 €•1 2Sk3691.40 €•1 BC 5600.05 €•1 opamp OP 070.40€That adds up to 4.51€ for each channel. Thus components costs for both channels and the power supply are around 16€ . Four cinch sockets can be purchased for 0.64€. That leaves around 3€ for some kind of housing. Again all prices were taken from the Reichelt Elektronik website.Fig. 2 and 3 show the RIAA response with the standard values (2.2k, 1uF) and the optimized values (2.26k, 970nF). Depending on your budget you may swap components, use a good (shielded) transformer, real good operational amplifiers like the LME49720, tightly matched jFets and KP foil capacitors (or even better) for the RIAA, add a choke to the power supply and the BluesBoy might blossom into a high class musician.Fig. 2: RIAA response with the standard values of the passive network. The deviation is within 0.3db.Fig. 3: RIAA response with the optimized values of the passive network. Between 20Hz and 10kHz the deviation is within 0.05db. At 20kHz the deviation is within 0.15db.
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