Pcb For Preamplifier Headphone Amp With Jfet Or Bjt Frontend & Diamond Buffer

US $84

Plovdiv, default, Bulgaria
Jun 9th

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1. Schematic 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 not less than 40 mA which is ehough to maintain work in class A for most of available headphones. The quiescent currents can be raised to 60 mA or more, but the appropriate heatsinks should be used. In case the schematic is used only as a preamplifier, the quiescent currents can be lowered to 15 - 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 feed 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 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 (see my other item for sale: item number: 131075354870) was used as a power supply there was no audible hum even with high-sensivity headphones as Sennheiser HD650 or AKG K701. 2. PCB PCB dimensions: 145.5x95.2 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. Two ventillation holes for each heatsink are provided on the PCB for better cooling. There are two pairs of output terminals, connected to the Diamond buffers outputs - one for the headphones and the other - for the LINE output. User can choose between two types of terminals: Molex Headers & Wire Housings 2.54mm or Molex Fixed Terminal Blocks 3.5mm for amplifier input and outputs (see pictures). Power supply terminal is Molex Fixed Terminal Blocks 3.5mm. On the PCB is provided ability for mounting two types of input potentiometer: a cheaper one - Song Huei (RK-2122G) or ALPS RK27 Blue Velvet (recommended). In order to further minimise received hum, the potentiometer's body can be grounded with piece of wire (see pictures). Note: Pictures of populated PCB are for reference purposes only. The price is for PCB only (non-populated). Of course, I can populate it for you but the price will be different. The package will contain: - PCB: 1 pcs. Detailed schematics, Parts Lists and instructions for building will be sent via e-mail immediately after payment, so please make sure a working e-mail address is provided along with PayPal transaction. I always ship with Registered Airmail, so tracking number will be sent. Combined shipping with other my items is welcome - just use "ask a question" at the bottom of the page or "request a total from seller".