Circuit diagram:
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 Filament Lamp Bulb (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 adopting the inverted Wien bridge configuration (C1-R3 & C2-R4). It features a variable output, low distortion and low output impedance in order to obtain good overload capability. A small filament bulb ensures a stable long term output amplitude waveform. Useful to test the Precision Audio Millivoltmeter, Three-Level Audio Power Indicator and other audio circuits posted to this website.
Notes:
- The bulb 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% using a 12V 40mA bulb, raising to 0.5% with a 12V 100mA one.
- Using a bulb differing from specifications may require a change of 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 rotated fully clockwise, or to view a sinewave of 2.828V Peak-to-Peak amplitude on the oscilloscope.
- With C1, C2 = 100nF the frequency generated is 100Hz and with C1, C2 = 1nF frequency is 10KHz but R5 requires adjustment.
- High gain transistors are preferred for better performance.
