In 2008 we designed a new phono pre-amplifier based on newly available "audiophile" operational amplifiers (op amps). Although we are quite happy with the P-13 phono pre-amp, with tubes, we wanted a pre-amp with a noise level far below the 200 µV some cartridges are offering.
When doing so one has to be very careful with the power supply. Although op amps "seem" to have adequate suppression of any noise from the power supply when looking at the specifications, this not realistic. When the frequency increases the figure of the CMRR (Common Mode Rejection Ratio) decreases! So we still have to take good care of the power supply not to produce noise at higher frequencies.
The "easy" way to create a constant supply voltage is to use a "series regulator". But ...... such a regulator has a transistor between input and output. That transistor has some capacitance (Cce) which will let through higher frequency noises. In the P-14 (as in most of our amplifiers nowadays) we therefore include a shunt regulator after the series regulator. This shunt should be a fast one in order to suppress noises whatever the frequency.
The P-14 circuit, when connected to a low impedance MC cartridge, shows no discernable hum or noise. When listening with your ear against the loudspeaker (!) you'll hear some very far off noise. This is far better then any tube type or a transistor pre-amplifier.
Above is the block schematic. This shows that there are 3 switches at the input as mentioned below. The RIAA correction is a passive circuit.
The first amp is "loaded" with a current source. This will use some current from the output of the op amp and in this way that amp is somewhat further in "class-A". Also there's some warming up of the die.
Positioning the input switches (on the PCB)
Normally all switches are positioned in the OFF position.
S1, the input capacitance
All off = 0 pF
1 = 150 pF
2 = 150 pF
3 = 150 pF
1+2 = 300 pF
1+2+3 = 450 pF
S2, the input impedance
All off = 47 kOhm
1 = 47 Ohm
2 = 100 Ohm
3 = 1 kOhm
1+2 = 33 Ohm
S3, the sensitivity (for 400 mV at the output)
All off = 3 mV
1 = 1,5 mV
2 = 600 µV
3 = 400 µV
2+3 = 300 µV
The complete kit including the cabinet and including our protractor now comes at a price of 650 Euro.
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