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I keep getting oscillation / uncontrollable noise with the following circuit. My input is a balanced audio signal which goes through a TL082 op amp configured as a differential amp. The output of the op amp feeds an STA540 power amp IC. What's frustrating is if I use either circuit independent of the other, I have no issues. I get a great, clean signal through headphones at the output of the op amp, and I get great, clean signal if I feed it directly into the STA540 and bypass the op amp. BTW - the balanced inputs arrive at the op amp decoupled. The output of the previous stage (not shown) already has decoupling capacitors.

I'm stumped. Any help is appreciated. Schematic

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    \$\begingroup\$ You probably need a lot more decoupling on your Vcc rail in order to break the feedback that's occurring between the two stages. \$\endgroup\$
    – Dave Tweed
    Feb 22, 2015 at 19:43
  • \$\begingroup\$ I've gone as high as 2,940uF smoothing (between VCC and Ground) on VCC without any improvement. Is that what you mean? \$\endgroup\$
    – JoeBob_OH
    Feb 22, 2015 at 20:09
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    \$\begingroup\$ Not exactly. You also need smaller ceramic capacitors, in the 0.1 to 10 uF range, mounted very close to each chip, to provide a low-impedance path for the higher frequencies. \$\endgroup\$
    – Dave Tweed
    Feb 22, 2015 at 20:34
  • \$\begingroup\$ Very close to each chip going VCC to GND, or VCC to the VCC pin on the chip? \$\endgroup\$
    – JoeBob_OH
    Feb 22, 2015 at 21:02
  • \$\begingroup\$ Vcc to Gnd on each chip. \$\endgroup\$
    – Dave Tweed
    Feb 22, 2015 at 21:20

2 Answers 2

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Why not run the JFET opamp on a proper split rail because its easy to get a neg rail off your existing psu set up Then use Zeners on the supply rails of the opamp to ground of 15volt 1 watt and feed the zenered opamp rails with 1k 1watt resistors Now if you decouple the opamp you can use 1microfarad ceramic caps If you still have problems then place 470 ohm resistors in series with each output pin of the opamp

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You are using the TL082 out of specifications, \$Input~voltage~range ~~\pm15V\$, in your application is almost \$17V\$ also the power supply is very near to the maximum supply voltage, you should check the voltage on your capacitors.

You could try to solve the problen choosing a common mode voltage most suitable to the application.

Probably, the opamps are out or order and you should replace it, violating \$V\tiny{ICR}\$ is a bad idea.

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  • \$\begingroup\$ 24VAC secondary is before rectification. After full wave rectification and smoothing, VCC is only 18VDC. The TL082 is only seeing +,- 9VDC. \$\endgroup\$
    – JoeBob_OH
    Feb 22, 2015 at 22:32
  • \$\begingroup\$ Did you actually measure the voltage? AC voltages are usually RMS values, so 24VAC would mean a peak DC voltage of almost 34V. \$\endgroup\$
    – Adam Haun
    Jul 20, 2015 at 15:32

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