Creating a variable wien bridge oscillator

[ARNAB KUMAR MONDAL]

I had been trying to build a variable wien bridge oscillator . The schematic is shown below - I wanted to change the frequency by using a pot . But it starts to distort after 12 Khz. Also when i am reducing both the capacitors to 1 nF I find the waveform to be triangular more than sinusoidal. Also if I want to drive 0.5 w 8 ohm ospeaker should i use an op amp or an lm386 ? Also how do i change the frequency with one control pin?

 

[Mooly]

You need a faster opamp than a 741 for high frequency use. The Wien bridge has two frequency determining networks and so normally you would use a dual gang pot.

Controlling an oscillator with a single input usually implies a 'voltage controlled oscillator' whose basic waveshape is often triangular. This is then shaped via a diode network to approximate a sine wave.

http://www.ti.com/sc/docs/apps/msp/journal/aug2000/aug_07.pdf

 

[ARNAB KUMAR MONDAL]

I want to only detect the audio rangge of frequencies . So i could be using a 741c

 

[Mooly]

I think you need something faster than a 741.

Also when i am reducing both the capacitors to 1 nF I find the waveform to be triangular more than sinusoidal.

That is a classic sign of slew rate limiting... the opamp output can't keep up.

 

[ARNAB KUMAR MONDAL]

Can i drive a 0.5 watt speaker using this circuit with the addition of the dual gang pot

 

[Pafi]

Use a stereo potmeter! It cannot be a problem.

Also using a faster OPA shouldnt be a challenge.

 

[forr]

This may help :
'Oscillators with single-element frequency control'
Electronics World August 2004, pp36-37.

 

[Pafi]

Of course there are oscillators with 1 adjustment element, I designed one also, but not wien bridge. Not this simple or not low distortion.

 

[mlloyd1]

in addition to the input from Mooly regarding the opamp speed vs quality of signal, you should also calculate the current needed to drive the load that you wish to drive; then look at the data sheet for the available output current from the opamp.

mlloyd1

 

[ARNAB KUMAR MONDAL]

Of course there are oscillators with 1 adjustment element, I designed one also, but not wien bridge. Not this simple or not low distortion.

I had built the oscillator and now I want to drive it using a speaker . So i Had used an emitter follower . BUt somehow the speaker is giving a very reduced voice output. How do i increase the power more. Also can you give the schematic of your oscillator

 

[JMFahey]

To drive a speaker you need WAY more current than what an emitter follower can supply.

Shortest path is to attenuate output signal and drive a chipamp, from LM386 to TDA 20xx to ....

 

[Mooly]

^ LM386 is perfect for 9 volt battery operation. Follow the applications in the data sheet.

 

[Tedknowlegy]

The values of 10K and 10uF look like they are too much of a load for the opamp. I would go with something like 100k dual pot and 0.1uF.

Use a buffer amp for the speaker...LM386 or TDA2002, or similar.

 

[Pafi]

I had built the oscillator and now I want to drive it using a speaker . So i Had used an emitter follower . BUt somehow the speaker is giving a very reduced voice output. How do i increase the power more. Also can you give the schematic of your oscillator

Attached a simple square+triangle+sine generator. The sine is very rough, THD is about 2...5%.

For higher power you can use any power amp IC, or if you like emitter follower, then it should be complementary, with a little higher quiescent current.

 

[Elvee]

Documents of interest:
http://www.sdiy.org/colbecklabs/spec_sheets/AN-263.pdf
ftp://ztchs.p.lodz.pl/PRP/PRP_2008_2009/LB-16.pdf
And in the context of speaker driver applications, this one too:
http://www.diyaudio.com/forums/chip-amps/254384-automatic-resonance-frequency-finder.html