PETRE TZV PETROV
PRATIMA
KUMARI
T
he buffer circuit described here can be used to en- hance the AC input imped- ance of audio amplifiers that are used with pickups in musical instruments.Some signal sources for pre- amplifiers, passive pickups for electric guitars or sensors based on capacitors require very high- impedance of over 5-mega-ohm. This can be achieved easily with junction field effect transistor (JFET) but may require special design of the printed circuit board (PCB), appropriate con- struction techniques of the box, proper cables and connectors.
This circuit provides a solution for a high-impedance, low-cost, low- quiescent current, buffer-follower based on PN4393 JFET.
Circuit and working
The circuit of the high-impedance audio buffer with JFET is shown in Fig. 1. It has almost unity gain and relatively-low-output impedance. The buffer can be used with any ap- propriate JFET including J201, J202, J113, PN4391, PN4392, PN4393, 2N5457, 2N5458, 2N3819, BF245B, MPF102 or similar. Although there are some differences between these JFETs, all of these can do the job depending on the input sig- nal range. However, care must be taken because some JFETs are not pin-to-pin compatible.
Audio input is applied to connec- tor CON1. Input impedance depends mainly on the values of resistors R2 and R8 and is around 5-mega-ohm. If resistors R8 and R9 and capacitor C6 are omitted, input resistance will de-
pend mainly on R2 and will be around 10-mega-ohm.
Value of R2 can be increased but it may be difficult to maintain the high input resistance during the practical usage of the circuit. Resis- tor R1 and capacitor C1 are used for protection and filtering functions for the input of the buffer, respectively. C1 can be omitted but not R1. Values of R1 and C1 can be changed as per requirement. Capacitor C2 is used to remove DC input components.
Resistors R8 and R9 between gate G and drain D of transistor T1 are not always needed. So resistor R8 is not necessarily equal to R2. Also, R8 will reduce the input impedance. Use of high-value resistors R2 and R8 may increase input noise, so care must be taken while selecting their values.
Resistor R9 and capacitor C6 are for filtering noise from the power source. These can be omitted if the power source is clean. But filtering capacitors C3 and C4 should not be omitted.
PARTS LIST
Semiconductors:
T1 - PN4393, n-channel JFET LED1 - 5mm LED
Resistors (all 1/4-watt, ±5% carbon):
R1, R9 - 10-kilo-ohm R2, R8 - 10-mega-ohm R3, R4 - 3.9-kilo-ohm R5 - 5.6-kilo-ohm R6, R7 - 560-ohm VR1 - 10-kilo-ohm potmeter Capacitors: C1 - 50pF ceramic C2 - 0.1µF ceramic C3 - 33nF ceramic C4 - 220µF, 25V electrolytic C5 - 47µF, 25V electrolytic C6 - 10µF, 25V electrolytic Miscellaneous: S1 - On/off switch BATT.1 - 9V, 6F22 or PP3 battery CON1 - Audio jack connector CON2-CON4 - 2-pin connector SJ1 - Shorting jumper HP1, HP2 - Headphone
Fig. 1: Circuit diagram of the high-impedance audio buffer with JFET D S G T1 PN4393 R1 10K R2 10M R33.9K R43.9K R5 5.6K R6 560E R7 560E R8 10M JACK1 AUDIO IN S1 ON/OFF VR1 10K C1 50p C2 0.1u C3 33n 220uC4 C5 47u C6 10u CON2 BATT.1 CON4 HP2 CON3 HP1 LED1 POWER SJ1 DC BIAS R9 10K GND 25V 25V 9V CON1 HP1,HP2=HEADPHONE 25V FOR
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Fig. 2: Actual-size PCB layout for the high-impedance audio buffer with JFET
Fig. 3: Component layout of the PCB
Petre Tzv Petrov was a researcher and assistant professor in
Technical University of Sofia (Bulgaria) and expert-lecturer in OFPPT (Casablanca), Kingdom of
Morocco. Now he is working as an electronics engineer in the private sector in Bulgaria power consumption. Bias depends
on the input signal, parameters of the JFET and power supply.
JFETs have wide tolerances, which is the main issue during their application. Fortunately, this is not a problem here. If a single circuit is required, choose an appropriate value for R3 depending on the JFET. In that case, SJ1 and R4 can be omitted. If some other JFET devices are required, connect R4 to source S of the JFET through SJ1 without changing the value of R3 to adjust the circuit to the parameters of the JFET.
This buffer has two outputs available at connectors CON3 and CON4. The outputs can drive loads of 10-kilo-ohm or more, simultaneously. Higher impedance loads are preferred. The loads can be reduced to 2-kilo- ohm without overloading the circuit, but amplitude of the signal will drop. Usually, that is not a problem. The circuit can drive two high-impedance 2-kilo-ohm headphones (HP1 and HP2) connected to CON3 or CON4 (a total load of 4-kilo-ohm).
Signal level on output CON3 is not adjustable but signal level on
CON4 is, using pot- meter VR1.
Choice of power supply depends on the peak-to-peak amplitude from the signal source. One or two 9V batteries of type 6F22 can be used. These provide 9V or 18V of power supply voltage, cover- ing practically all pas- sive pickups for the musical instruments and other high- impedance sensors. Some JFETs allow higher power supply. A well-filtered DC wall adaptor can also be used.
Construction
and testing
An actual-size, single-side PCB for the high-impedance audio buffer with JFET is shown in Fig. 2 and its component layout in Fig. 3.
This circuit may require appro- priate selection of values of resis- tors R3 and R4 in order to optimise the peak-to-peak amplitude of the output signal. It is appropriate for battery operation, which is impor- tant for portable applications.
The circuit can be mounted in a small box near the pickup or attached to the strap of the string instrument. Due to its high input impedance, input cable should be shorter than one metre and shielded or, at least, twisted. The gain is lower than unity but that is not a problem because most pickups pro- vide strong signals.
A. SAMIUDDHIN
DWIVE