Topology with symmetrical JFET frontend is well known from Erno Borbely’s articles and is used with great success in numerous DIY and professional schematics.
Since the matched P-N JFET quads are very rare and expensive nowadays, here is proposed an alternative variant with matched BJT transistors in the input stage. BJTs possess a higher transconductance compared to JFETs and hence a higher slew-rate can be achieved. On the listening sessions some people unequivocal preferred this version instead of the version with JFET frontend.
The input symmetrical BJT differential amplifiers are fed by Constant Current Sources which lowers distortions and ensures a high PSRR. The output stage is Diamond buffer – another topology with very good reputation among audio community.
The driver transistors of the buffer are biased through the constant current sources, which ensures stability of the output transistor’s currents.
The output transistors are biased at approx. 40 mA which is enough to maintain work in class A even if schematic is used as a headphone amplifier.
As can be seen from schematics, there are no capacitors at the signal path. That’s why in order to maintain the zero voltage at the output, a DC servo chain is implemented. It should be noted that since the DC servo’s OPAmp is feed directly from power supply rails, the schematic’s max. power supply voltage is limited to the OPAmp’s one.
PCB dimensions: 145.5×95.2 mm.
PCB thickness: FR4 1.55 mm. Copper thickness: 35um (1oz). Surface finish – Nickel Gold (ENIG). PCB is two-layers design (Top and Bottom). Both sides have solder masks. Silkscreen is placed on Top layer.
There are ground planes on both layers. Special care has been taken in the ground plane’s design – as it’s seen on the photos, each ground plane is separated to 3 areas – input, middle and output in order to minimise parasitic current flows between amplifier stages.
3. Fully populated board
The board is fully populated, tested and measured. As can be seen from the pictures, only high quality components are used – ALPS RK27 “Blue velvet” as an input regulator, BC550C/560C as small signal transistors, KSA1220/KSC2690 as output transistors, Nichicon KW and Panasonic FC series as filtering caps, Vishay Dale CMF55 resistors in the signal path, Yageo MF0207 in the rest of the places. Non-polar capacitors are ceramic NP0/C0G, Vishay MKT370 and WIMA. For ease of cable mounting Molex Fixed Terminal Blocks 3.5mm are used for input/output connectors as long as for power supply connector.
4. Schematics measured performance:
– Voltage gain: ~ 8 (can be lowered to 4-5)
– THD at 1 kHz: ~ 0.001%
– S/N ratio: ~ 125 dB
– Slew-rate: ~ 73 V/uS
– Bandwidth at -3 dB: ~ 0.85 MHz
From pictures below you can see THD measured at 1kHz – on the soundcard and on the preamplifier – they are almost indistinguishable (Do not take into account 50Hz and 150Hz spikes which are due to the some ground loop in the measurement equipment). The noise of preamplifier (red) compared to the noise of the soundcard itself (blue) is given on another picture. On the last picture is shown result from test with 100kHz square wave signal – magnified view. Measured slew-rate is ~ 73 V/uS (220 nS for 20V p-p).
Note: All measurements has been conducted with 56 Ohm load at the output.
Power supply requirements:
Power supply voltage: +/- 15V to +/- 17V.
Power consumption: max. 200 mA.
Even though schematic’s PSRR is very good (due to it’s full symmetry), the best results were achieved with regulated power supply. When modified Sulzer regulator was used as a power supply there was no audible hum even with high-sensivity headphones as Sennheiser HD650 or AKG K701.
What the buyer will receive:
The package will contain 1 pcs. Fully assembled an tested preamplifier – BJT version PCB.
The unit will be shipped with Priority AirMail Registered (or equivalent courier service where applicable). The tracking number will be sent to the customer after shipment.