300Hz, 1KHz, 3KHz “output stable” test equipment tool
P1_______________4K7 Linear Potentiometer R1______________12K 1/4W Resistor R2_______________2K2 1/4W Resistor R3,R5,R7_________1K 1/2W Trimmers (Cermet) R4,R6____________1K5 1/4W Resistors R8_______________1K 1/4W Resistor R9_______________4K7 1/4W Resistor R10______________3K3 1/4W Resistor R11______________2K7 1/4W Resistor R12____________300R 1/4W Resistor R13______________1K 1/4W Resistor (Ωptional) R14____________100K 1/4W Resistor C1______________22nF 63V Polyester Capacitor C2_______________3µ3 63V Polyester Capacitor C3,C6__________330nF 63V Polyester Capacitors C4______________56nF 63V Polyester Capacitor C5______________22µF 25V Electrolytic Capacitor D1,D2_________1N4148 75V 150mA Diodes D3_______________3mm Red LED (Ωptional) Q1,Q2,Q3_______BC550C 45V 100mA Low noise High gain NPN Transistors IC1____________78L05 5V 100mA Regulator IC J1_______________RCA Audio output or 3mm or 6.3mm Mono Jack sockets SW1________________1 pole 3 ways Rotary or Slider Switch SW2________________2 poles 3 ways Rotary or Slider Switch B1________________9V PP3 Battery Clip for PP3 Battery
This circuit was designed to provide a valuable test equipment tool for sound reinforcement systems like guitar amplifiers and the like. Used in conjunction with an ac voltmeter or other ac measurement tools, it can be of considerable help in setting and controlling levels through any amplifying chain.
Please see the Precision Audio Millivoltmeter, Peak Level Indicator and Three-level Audio Power Indicator designs, all available on this website, to complete the equipment.
The circuit is based on a simple, two-transistor oscillator circuit, featuring very high frequency and amplitude stability but avoiding thermistors, bulbs or any special amplitude-limiting device.
Three different output frequencies, namely: 300Hz, 1KHz and 3KHz, are provided by switching SW1.
The sine wave output amplitude can be varied continuously (by means of P1) in two ranges: 0 – 77.5mV RMS (219.7mV peak-to-peak) and 0 – 0.775V RMS (2.191V peak-to-peak).
A regulated supply is necessary to obtain a stable output waveform. D1 and D2 force IC1 to deliver 6.2V output instead of the nominal 5V.
- THD at maximum output voltage: 0.11% @ 1KHz, 0.23% @ 300Hz, 0.05% @ 3KHz.
- Total current drawing (LED pilot light omitted): about 4.5mA.
- D3 LED pilot light and its current limiting resistor R13 are optional.
Circuit set-up using an oscilloscope:
Connect the oscilloscope to J1, rotate P1 fully clockwise and switch SW2 to 0.775V position.
Adjust R3, R5 and R7, after switching SW1 to the correspondent position, to obtain output sine waves having 2.2V peak-to-peak amplitude.
“By ear” circuit set-up:
Connect an amplifier to J1, switch SW2 to 0.775V position and rotate P1 and the amplifier controls to obtain a moderate output sound level.
Carefully adjust R3, R5 and R7, after switching SW1 to the correspondent position, until the output sound will stop; then turn back the trimmer very slowly and stop adjusting immediately when the sound will start again.