D2-JFET discrete preamplifier (2010) with 1.5W/32ohm output capability
In this thread I would like to mention my Dispre 2-JFET (D2-JFET) preamplifier, that was introduced in 2010 and hundreds of PCB boards and assembled modules were sold to DIY community. In the meantime, my measuring system based on the sound cards available in 2010 was not able to measure distortion levels of this preamp properly, so now, after many years, I have wiped out the dust of the first prototype and measured it with my current system based on Topping D10s DAC and E1DA Cosmos ADC.
Dispre 2 – JFET could be also used as a headphone amplifier, without any components changes needed.
Dispre 2 – JFET had 1 single-ended stereo RCA input and 1 pseudo-balanced XLR (or single-ended RCA) stereo output.
Dispre 2 – JFET had complementary differential circuit topology with self biased JFET input stage (this topology was introduced by John Curl). This maximizes linearity of the input stage and distortion suppression does not rely only on an overall negative feedback. To remove output DC voltage component, DC servo was used.
Output stage idle current was set at about 40mA, and it worked in a class A up to 8V into 100 ohm load. For standard link load 600 ohm and higher, the output stage worked in a class A for the whole range of output voltage up to clipping.
Dispre2 - JFET had wide bandwidth and high slew rate.
Power supply
2 x 15 V stabilized. Current consumption per board is 2 x 120mA (standby), power supply should be able to deliver at least 2 x 200mA. The ZT15VA power supply with 70053K transformer was designed especially for this preamp.
Technical parameters (as originally declared)
New measurements (December 2022)
D2-JFET was re-measured with my system that consists of Topping D10s DAC and E1DA Cosmos ADC. D2-JFET was powered from 2x15V stabilized power supply.
Output voltage noise is 9uV/22kHz BW
Voltage gain is 5x (14dB)
Measurements with 870 ohm load
Why 870 ohm? Because it is the input impedance of Cosmos ADC at 10V DIP setting and L+R inputs in parallel SE input. Below please se a photo of my setup.
THD+N vs. output voltage at 50Hz, 1kHz and 6kHz
31-tone multitone at 1V/870 ohm
Measurements into 32 ohm load
I think this is a more interesting part. First, spectra at 2V/32ohm
Multitone at 750mV/32ohm
THD+N vs. output voltage into 32ohm at 50Hz, 1kHz and 6kHz
THD+N vs. output power into 32ohm at 50Hz, 1kHz and 6kHz
I think this is not bad. This discrete preamp is able to give up to 1.5W into 32 ohm load through the audible band. Please compare to the SOTA preamplifiers tested here, like
www.audiosciencereview.com
by @Thorsten Loesch .
In this thread I would like to mention my Dispre 2-JFET (D2-JFET) preamplifier, that was introduced in 2010 and hundreds of PCB boards and assembled modules were sold to DIY community. In the meantime, my measuring system based on the sound cards available in 2010 was not able to measure distortion levels of this preamp properly, so now, after many years, I have wiped out the dust of the first prototype and measured it with my current system based on Topping D10s DAC and E1DA Cosmos ADC.
Dispre 2 – JFET could be also used as a headphone amplifier, without any components changes needed.
Dispre 2 – JFET had 1 single-ended stereo RCA input and 1 pseudo-balanced XLR (or single-ended RCA) stereo output.
Dispre 2 – JFET had complementary differential circuit topology with self biased JFET input stage (this topology was introduced by John Curl). This maximizes linearity of the input stage and distortion suppression does not rely only on an overall negative feedback. To remove output DC voltage component, DC servo was used.
Output stage idle current was set at about 40mA, and it worked in a class A up to 8V into 100 ohm load. For standard link load 600 ohm and higher, the output stage worked in a class A for the whole range of output voltage up to clipping.
Dispre2 - JFET had wide bandwidth and high slew rate.
Power supply
2 x 15 V stabilized. Current consumption per board is 2 x 120mA (standby), power supply should be able to deliver at least 2 x 200mA. The ZT15VA power supply with 70053K transformer was designed especially for this preamp.
Technical parameters (as originally declared)
- Output voltage max. 26 Vp-p / 600 ohm (9.19 V ef)
- Load impedance - can drive 32 ohm headphones
- Freq range (-3dB) 0.1 Hz – 220 kHz for potentiometer set at 100%
- Freq range (-3dB) 0.1 Hz – 150 kHz for potentiometer set at 50%
- THD at 1Vrms/600 ohm - less than 0.001% through audio range
- CCIF IMD 19+20kHz at 1Vrms/600 ohm - less than 0.001%
- Gain - 5x (14 dB)
- Output impedance - 10 ohm
- Supply/consumption - 2 x 15V stabilized, 2 x 120 mA
- Power supply requirements - 2 x 15V stabilized, at least 2 x 200 mA
New measurements (December 2022)
D2-JFET was re-measured with my system that consists of Topping D10s DAC and E1DA Cosmos ADC. D2-JFET was powered from 2x15V stabilized power supply.
Output voltage noise is 9uV/22kHz BW
Voltage gain is 5x (14dB)
Measurements with 870 ohm load
Why 870 ohm? Because it is the input impedance of Cosmos ADC at 10V DIP setting and L+R inputs in parallel SE input. Below please se a photo of my setup.
THD+N vs. output voltage at 50Hz, 1kHz and 6kHz
31-tone multitone at 1V/870 ohm
Measurements into 32 ohm load
I think this is a more interesting part. First, spectra at 2V/32ohm
Multitone at 750mV/32ohm
THD+N vs. output voltage into 32ohm at 50Hz, 1kHz and 6kHz
THD+N vs. output power into 32ohm at 50Hz, 1kHz and 6kHz
I think this is not bad. This discrete preamp is able to give up to 1.5W into 32 ohm load through the audible band. Please compare to the SOTA preamplifiers tested here, like
ifi Zen CAN Review (Headphone Amp)
This is a review and detailed measurements of the ifi Zen CAN balanced headphone amplifier. It is on kind loan from a member and costs US $150 from Amazon including Prime shipping. The design is identical to new line of budget products from ifi: No, it is not just the pictures. The silver...
www.audiosciencereview.com
