PhonoFet2 - discrete preamplifier for MM cartridge
After the development of several phono preamplifiers based on operational amplifiers, I have decided, about 12 years ago, to design a preamplifier built with discrete components and to utilize experience achieved during design of discrete link level preamplifiers. Thus I have designed a phono preamplifier named PhonoFet. It was based on JFET and MOSFET transistors. This preamplifier has had a low-noise complementary-differential JFET input stage followed by the MOSFET folded cascode.
The new PhonoFet2 version has output HPF added to suppress unwanted infra-sound spurious voltages, resulting from turntable operation. This is followed by the buffer to eliminate influence of various load impedances.
The power supply for this preamp utilizes my ZT15VA power supply (separate board) and on-board constant current sources followed by a parallel voltage stabilizers. This circuit solution results in an excellent S/N ratio and suppression of HF interferences. The preamp is placed in a standard 19“ case 1U high and 280 mm deep.
Technical parameters
Measurements
PhonoFet2 frequency response
THD and THD+N with 10mV input voltage at 1kHz
THD and THD+N at 1.25V/5kHz output
We can see that distortion is quite independent on signal frequency.
THD+N vs. output voltage at 1kHz. Optimum is up to 2V, which will not be exceeded based on signal amplitudes coming from MM cartridges.
After the development of several phono preamplifiers based on operational amplifiers, I have decided, about 12 years ago, to design a preamplifier built with discrete components and to utilize experience achieved during design of discrete link level preamplifiers. Thus I have designed a phono preamplifier named PhonoFet. It was based on JFET and MOSFET transistors. This preamplifier has had a low-noise complementary-differential JFET input stage followed by the MOSFET folded cascode.
The new PhonoFet2 version has output HPF added to suppress unwanted infra-sound spurious voltages, resulting from turntable operation. This is followed by the buffer to eliminate influence of various load impedances.
The power supply for this preamp utilizes my ZT15VA power supply (separate board) and on-board constant current sources followed by a parallel voltage stabilizers. This circuit solution results in an excellent S/N ratio and suppression of HF interferences. The preamp is placed in a standard 19“ case 1U high and 280 mm deep.
Technical parameters
| voltage gain | 42 dB/1 kHz |
| output voltage noise | 19.4 uVA |
| max. output voltage | 9 Vrms |
| input impedance | 47 kohm |
| output impedance | 100 ohm |
| minimum load impedance | 600 ohm |
Measurements
PhonoFet2 frequency response
THD and THD+N with 10mV input voltage at 1kHz
THD and THD+N at 1.25V/5kHz output
We can see that distortion is quite independent on signal frequency.
THD+N vs. output voltage at 1kHz. Optimum is up to 2V, which will not be exceeded based on signal amplitudes coming from MM cartridges.
