I built a wien bridge audio frequency oscillator using an op amp, a lamp for automatic gain control, and the required matched pairs of plastic film capacitors and resistors for the "bridge". I used a workbench, regulated power supply for the + and - 9V DC to the circuit.
Here is the schematic:
Here is a picture of the circuit on the breadboard:
(Yes, I am pretty proud of the soldering job on the lamp. )
The potentiometer on the right was not hooked up when I took the picture. When listening to the oscillator, I connected the pot to the output of the oscillator. I then fed the output of the pot to a small pair of self-powered computer speakers to listen.
(Sorry about the massive, 1,000 uF, 50V power supply filter caps. I didn't have any 1,000 uF caps handy that had a lower rated voltage.)
I tried both a TL072 op amp with JFET inputs as well as an NE5532 op amp with regular BJT inputs. (I only used one op amp on each chip.)
Each op amp produced a nice sine wave of 440 Hz.
The TL072, however, had a small amount of crossover distortion in its output. The NE5532 did not.
Here is the oscilloscope showing the output of the TL072:
The crossover distortion is small, but noticeable, right about on the zero crossing line. It is audible, I think. My hearing is not what it used to be, but I think I detect a small amount of audible distortion.
Here is the sine wave of the NE5532 shown on the oscilloscope:
Nice and smooth. No crossover distortion. Also, when I listen I think I can detect a slight improvement in sound over the TL072. Again, it is subtle.
So, the question: Does the NE5532 have some sort of class AB output stage? It does not say so in the datasheet. Here's a link to the datasheet:
http://www.ti.com/lit/ds/symlink/ne5532.pdf
Here is the link to the TL072 datasheet:
http://www.ti.com/lit/ds/symlink/tl072.pdf
The TL072, according to the datasheet, has low THD and is optimized for "hi-fidelity and audio pre-amplifier applications". Not if there is crossover distortion on the output!
I read on the internet that connecting a resistor from the output to the -V power rail might get rid of the crossover distortion. Didn't work for me. I also tried adding another resistor from the output to the +V power rail. That didn't help either.
Is there any way to get rid of this crossover distortion when I use the TL072 or am I just stuck using the NE5532? I don't mind using the NE5532 but it bothers me that I don't understand why the TL072 has the crossover distortion but the NE5532 does not.
Here is the schematic:
Here is a picture of the circuit on the breadboard:
(Yes, I am pretty proud of the soldering job on the lamp. )
The potentiometer on the right was not hooked up when I took the picture. When listening to the oscillator, I connected the pot to the output of the oscillator. I then fed the output of the pot to a small pair of self-powered computer speakers to listen.
(Sorry about the massive, 1,000 uF, 50V power supply filter caps. I didn't have any 1,000 uF caps handy that had a lower rated voltage.)
I tried both a TL072 op amp with JFET inputs as well as an NE5532 op amp with regular BJT inputs. (I only used one op amp on each chip.)
Each op amp produced a nice sine wave of 440 Hz.
The TL072, however, had a small amount of crossover distortion in its output. The NE5532 did not.
Here is the oscilloscope showing the output of the TL072:
The crossover distortion is small, but noticeable, right about on the zero crossing line. It is audible, I think. My hearing is not what it used to be, but I think I detect a small amount of audible distortion.
Here is the sine wave of the NE5532 shown on the oscilloscope:
Nice and smooth. No crossover distortion. Also, when I listen I think I can detect a slight improvement in sound over the TL072. Again, it is subtle.
So, the question: Does the NE5532 have some sort of class AB output stage? It does not say so in the datasheet. Here's a link to the datasheet:
http://www.ti.com/lit/ds/symlink/ne5532.pdf
Here is the link to the TL072 datasheet:
http://www.ti.com/lit/ds/symlink/tl072.pdf
The TL072, according to the datasheet, has low THD and is optimized for "hi-fidelity and audio pre-amplifier applications". Not if there is crossover distortion on the output!
I read on the internet that connecting a resistor from the output to the -V power rail might get rid of the crossover distortion. Didn't work for me. I also tried adding another resistor from the output to the +V power rail. That didn't help either.
Is there any way to get rid of this crossover distortion when I use the TL072 or am I just stuck using the NE5532? I don't mind using the NE5532 but it bothers me that I don't understand why the TL072 has the crossover distortion but the NE5532 does not.
Last edited: