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.
The schematic proposed below has frontend similar to John Curl’s beloved topology, used in his famous Vendetta preamplifier as well as other Parasound amplifiers. JFET input stage is based on self biased symmetrical topology with 2SK170/2SJ74 complementary pairs. This ensures schematic’s low-noise and high slew-rate. Since the matched quads 2SK170/2SJ74 are hard to obtain and expensive nowadays, ability to use symmetrical frontend with BJTs is provided on the PCB. It is just a matter of mounting different set of parts on the board.
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 for most of available headphones. The quiescent currents can be raised to 60 mA, but the appropriate heatsinks should be used. In case the schematic is used only as a preamplifier, the quiescent currents can be lowered to 20 mA.
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 fed directly from power supply rails, the schematic’s max. power supply voltage is limited to the OPAmp’s one.
Schematics measured preformance:
– Voltage gain: ~ 8 (can be lowered to 4-5)
– THD at 1 kHz: ~ 0.001%
– S/N ratio: ~ 120 dB
– Slew-rate (JFET frontend): ~ 60 V/uS
– Slew-rate (BJT frontend): ~ 45 V/uS
– Bandwidth at -3 dB (JFET frontend): ~ 850 kHz
– Bandwidth at -3 dB (BJT frontend): ~ 750 kHz
On the pictures are given some of schematic’s measured parameters.
Power supply requirements:
Power supply voltage: +/- 15V to +/- 18V (depending from DC servo OPAmp type).
Power consumption (with output transistors biased at 40 mA): max. 200 mA.
Even though schematic’s PSRR is very good (due to it’s full symmetry), the best results were achieved with stabilised 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.
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 which helps for shorter tracks and optimal layout.
Забележка: This listing is for PCB only. Pictures of populated PCB is for information purposes only.
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