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I've built two diode stabilised variable frequency wien bridge oscillator circuits according to this schematic:

enter image description here

(source). Both are on the same stripboard and connect to a single LM324.

One of the oscillators works beautifully, the other doesn't. The oscillator which doesn't work connects to the opamp with around 3cm of shielded wire (my original stripboard layout wasn't designed for two oscillators, so this was a hack).

I have checked my breadboard around 20 times and am fairly certain I haven't just made a silly mistake in the non-functioning oscillator.

Is there a possibility that the length of these wires may be preventing the oscillation? I understand that the successful functioning of the wien bridge can be sensitive to things being just so, but I don't know if the inductance in shielded wire this length could be enough to stop it from working.

Unfortunately I don't know enough about electronics yet to properly troubleshoot the circuit apart from simple continuity and resistance tests. FWIW, the dual gang pot for the dud circuit is better matched than the functioning circuit (~1Ω vs 10Ω).

Update

I'm using the same power supply (two 9V batteries) for both oscillators and have connected the +9V to both circuits, as per the awful diagram below:

enter image description here

(the -9V and ground connections have also been made directly to the batteries in both circuits).

Is this naive? Have I now made the two oscillator circuits dependent on each other as the two 10k resistors are now connected?

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  • \$\begingroup\$ What is the purpose of the 10k resistors between the output and the supply voltage? \$\endgroup\$ – LvW May 15 '16 at 8:48
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    \$\begingroup\$ According to the source, it's to eliminate crossover distortion. TBH, I don't fully understand the nuances. \$\endgroup\$ – nullPainter May 15 '16 at 8:52
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    \$\begingroup\$ OK - I know what you mean. Such a resistor will turn the output transistors into a class-A stage with less cross.over effects - however, this works for single supply only. Therefore: (1) Try to eliminate the 10k resistors and see if you can live with the signal quality or (b) switch to another opamp type (my recommendation) or (c) use single supply only. \$\endgroup\$ – LvW May 15 '16 at 10:32
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    \$\begingroup\$ For R1/R2/R2a, what tolerance resistors are you using? \$\endgroup\$ – WhatRoughBeast May 15 '16 at 12:07
  • \$\begingroup\$ Thanks @LvW. This is just for an audio synthesis application so I'm sure I can live with some distortion. I'll give bypassing the 10k resistors a go this evening. \$\endgroup\$ – nullPainter May 15 '16 at 19:59
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While this is not certain, there is a very good chance that you've been burned by resistor tolerance.

A Wein bridge oscillator requires a gain of 3 at the resonant frequency, and for your configuration that means a 2:1 (or better) ratio between R1 and (R2 + R2a). The nominal values you've got give a gain $$G = 1+\frac{R1}{R2+R2a} = 1 + \frac{4.7}{2.3} = 3.043$$ which provides a slight gain margin over your minimum of 3.

However, you've used 5% resistors, so the actual value of R1 could be as low as 4.465, and your R2/R2a could be as high as 2.415. This would give a gain of $$ G = 1 +\frac{4.465}{2.415} = 2.849$$ and this would explain your failure to oscillate.

Replacing your resistors with 1% units is a good idea, although it's still possible that the unit won't oscillate. If this happens, try removing R2a and see what happens. You might also measure your existing resistors and see just what values you really have (do it with the resistors disconnected from the circuit).

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  • \$\begingroup\$ I replaced those resistors with 1% resistors and the symptoms unfortunately continued. Bypassing R2a resulted in a rustly sound when the output was connected to headphones and removing it entirely resulted in the same audio as with it in (oscillation at a few lower frequencies, but faint whistling at higher frequencies). Clearly something more drastic is amiss, but I'm marking this as the accepted answer as it is a very sensible conclusion to draw and a good thing to check. Thanks! \$\endgroup\$ – nullPainter May 23 '16 at 7:49

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