The ALL-JFET “SUPER BUFFER” (1) is a very high quality low-noise, wide-band amplifier. The PCB is 50x145mm and contains two completely independent buffers. The buffers can be supplied from two independent power supplies. The schematic is shown in fig. 1.
Figure 1.
The input JFET-Cascodes, consisting of Q1/Q3 and Q2/Q4 are operated at 2 mA. Q1 and Q2 have to be matched to 10% of Idss. The second stage Cascodes, consisting of JFETs Q5/Q6 and Q7/Q8 are operating at just below 10mA. Q5 through Q8 have to be selected for Idss>10mA. Note that this also provides matching between the N- and P-channel JFETs. The output devices: Q9/Q10 and Q11/Q12 should be matched to 10% of Idss and have an Idss range of 8-10mA.
The layout is made for two pairs of JFETs in the output (Q9/Q10 and Q11/Q12), this configuration is used for special applications and Q10/Q12 are not included in the kits. R13 and C12 are also not needed for normal buffer application and are not included in the kits.
Typical specifications with R11=47K5:
O.L.G: 67dB
O.L. Freq. Response: 17kHz
Unity gain freq. Response: >1MHz
Unity gain rise time: 100nsec.
Unity gain output impedance: <2 Ohm (without R14)
Input noise: 260nV, 20-30kHz (-129.5dBu)
Input capacitance: approx. 1.5pF
Minimum load impedance: 1kOhm
Set-up Procedure.
If possible test each buffer module separately before installing it in the chassis. This simplifies measurements, adjustments, and if necessary, component changes. If you have access to a scope connect it to the output of the buffer and check whether radio frequency (RF) oscillations are present. If you have a complete audio instrumentation in your workshop perform the usual gain, frequency response, noise, Total Harmonic Distortion (THD) and Intermodulation Distortion (IM) measurements. +INP should be connected to SGND under DC measurements/adjustments.
Before testing set P1 to mid-position. Connect +INP to SGND. If the buffer is used in unity gain configuration ¨CINP is left open. If it is used in 6dB gain configuration, then connect ¨CINP to SGND.
Connect +/-24V regulated supply to the board.
Connect a DVM across R3 (R4) and measure the voltage drop. It should be 2.8-3V. If the voltage-drop is more than 3V, increase the value of R5 and R6. If this voltage is correct, then the current in the second stage is automatically correct (check the voltage drop across R9/R10, it should indicate a current of approx. 10mA in the second stage). Total current consumption of the buffer should be ¡Ü 25mA.
Connect the DVM to the output of the buffer and adjust the DC offset to zero Volt with P1 .
If you have audio instrumentation connect an oscillator, set to 1kHz/1VRMS to the +INP and connect a mV meter/THD analyser to the output. The output should be 1VRMS in unity gain configuration and 2VRMS with 6dB gain. Check the THD, it should be less than 0.002%.
This completes the set-up procedure.
1. Erno Borbely: The All-FET Line Amp, AudioXpress 5/02. See also our homepage under "Articles"
ALL-JFET “SUPER BUFFER”
