Since retiring in 2013, electrical engineer Larry Cicchinelli has provided technical support at an educational radio station. For audio circuit debugging and testing, he uses a DIY battery-powered oscillator/volume unit (VU) meter. Details follow.
Originally, I was only going to build the audio source. When I thought about how I would use the unit, it occurred to me that the device should have a display. I decided to design and build an easy-to-use unit that would combine a calibrated audio source with a level display. Then, I would have a single, battery-powered instrument to do some significant audio circuit testing and debugging.
Cicchinelli describes the Twin-T Oscillator:
The oscillator uses the well-known Twin-T configuration with a minor modification to ensure a constant level over a range of power supply voltages. The circuit I implemented maintains its output level over a range of at least 6 to 15 V. Below 6 V, the output begins to distort if you have full output voltage (0 dBu). The modification consists of two antiparallel diodes in the feedback loop. The idea came from a project on DiscoverCircuits.com. The project designer also indicates that the diodes reduce distortion.
Figure 1 shows the oscillator’s schematic. Header H1 and diode D1 enable you to have two power sources. I installed a 9-V battery and snap connector in the enclosure as well as a connector for external power. The diode enables the external source to power the unit if its voltage is greater than the battery. Otherwise the battery will power the unit. The oscillator draws about 4 mA so it does not create a large battery drain.
The standard professional line level is 4 dBu, which is 1.228 VRMS or 3.473 VPP into a 600-Ω load. The circuit values enable you to use R18 to calibrate it, so the maximum output can be set to the 4-dBu level. A 7.7 (3.473/0.45) gain is required to provide 4 dBu at the transformer. Using the resistors shown in Figure 1, R18 varies the gain of U1.2 from about 4.3 to 13.
You may need to use different resistor values for R18, R19, and R20 to achieve a different maximum level. If you prefer to use 0 dBm (0.775 VRMS into 600 Ω) instead of 4 dBu, you should change R20 to about 5 kΩ to give R18 a range more closely centered on a 4.87 (2.19/0.45) gain. The R20’s value shown in Figure 1 will probably work, but the required gain is too close to the minimum necessary for comfort. Most schematics for a Twin-T oscillator will show the combination of R3 and R4 as a single resistor of value Rx/2. They will also show the combination of C1 and C2 as a single capacitor of value Cx × 2. These values lead to the following formula:
As you can see in the nearby photo, the Twin-T Oscillator and VU meter contain separate circuit boards.
This article first appeared in audioXpress January 2014. audioXpress is one of Circuit Cellar‘s sister publications.