Simple circuitry, low distortion, battery operated

Variable, low impedance output up to 1V RMS

 

Parts:

R1____________5K6  1/4W Resistor
R2____________1K8  1/4W Resistor
R3,R4________15K   1/4W Resistors
R5__________500R   1/2W Trimmer Cermet
R6__________330R   1/4W Resistor
R7__________470R   Linear Potentiometer

C1,C2________10nF  63V Polyester Capacitors
C3__________100µF  25V Electrolytic Capacitor
C4__________470nF  63V Polyester Capacitor

Q1,Q2_______BC238  25V 100mA NPN Transistors

LP1___________12V  40mA Lamp (See Notes)

J1__________Phono chassis Socket

SW1__________SPST  Slider Switch

B1_____________9V  PP3

Clip for 9V PP3 Battery

Circuit description:

This circuit generates a good 1KHz sinewave using the inverted Wien bridge configuration (C1-R3 & C2-R4). Features a variable output, low distortion and low output impedance in order to obtain good overload capability. A small filament lamp ensures a stable long term output amplitude waveform. Useful to test the Audio Millivoltmeter, Audio Power Meter and other audio circuits published in this site.

Notes:

• The lamp must be a low current type (12V 40-50mA or 6V 50mA) in order to obtain good long term stability and low distortion.

• Distortion @ 1V RMS output is 0.15% with a 12V 40mA lamp, raising to 0.5% with a 12V 100mA one.

• Using a lamp differing from specifications may require a change in R6 value to 220 or 150 Ohms to ensure proper circuit's oscillation.

• Set R5 to read 1V RMS on an Audio Millivoltmeter connected to the output with R7 fully clockwise, or to view a sinewave of 2.828V Peak-to-Peak on the oscilloscope.

• With C1,C2 = 100nF the frequency generated is 100Hz and with C1,C2 = 1nF frequency is 10KHz but R5 is needing adjustment.

• High gain transistors preferred for better performance.