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The circuit I designed is a buffer amplifier type design intended to take two channels of audio input from a passive monitor controller, send to either the resistor circuit or low-pass filter (as seen in the schematic), and then to the second op-amp (which has a trimpot to control DC offset for each channel), and then send as an audio output at unity gain (or near unity gain).

The circuit is powered by +/-15 VDC, taking a 36 VDC power supply and sent through a virtual ground type divider and regulated with 7815 and 7915 voltage regulators (although I’ve tried disconnecting the power section and using a bench power supply setup to provide + and – voltages with the same problems).

I designed the circuit using the Falstad applet and I’ll attach the text for the circuit so you can see for yourself how I imagined it would work, and I’m attaching images of my schematic, screenshots of my oscilloscope, and images of my soldered perfboard circuit.

When I try to input a 120 Hz, +/- 2V sine wave with my function generator, all I get from the output is a mostly flat voltage. It wiggles and shows occasional noise and whatnot when I try tuning the trimpot, but what you see in my image is basically the gist of it. In addition, the opamps (OPA2604APs) get a little warm, which I don’t think is a huge problem, but the 7815 gets pretty hot.

https://i.sstatic.net/H2Qcu.jpg

Schematic: Schematic

Regulated Virtual Ground Circuit: Regulated virtual ground circuit

Top View: Top View. Power circuit/regulated virtual ground with +/-15 VDC output.

Channel 1 and Channel 2, Buffer amplifier/filter: Channel 1 and Channel 2, Buffer amplifier/filter

One channel buffer/filter: One channel buffer/filter.

(The question editor is now telling me that I need more "reputation" to post more links. You can find all the images at the imgur URL that I posted at the end of my description.)

Falstad circuit simulation

The following are URLs for the relevant datasheets:

https://www.vishay.com/docs/28193/mkt373.pdf

https://globalsources.com/gsol/I/Slide-switch/p/sm/1142899729.htm#1142899729

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  • \$\begingroup\$ Please upload the images using the image button. That way all the information is in the one place. \$\endgroup\$
    – Transistor
    Jul 6, 2019 at 9:39
  • \$\begingroup\$ Use a DVM, and measure from Gnd to +15, and then from Gnd to -15. \$\endgroup\$ Jul 6, 2019 at 9:43
  • \$\begingroup\$ Paste Falstad URL without short link which failed, tinyURL's work in comments however \$\endgroup\$ Jul 6, 2019 at 13:56
  • \$\begingroup\$ You have, I hope, tried to trace the signal through the chain? What does the output of the left-hand op amp look like? \$\endgroup\$ Jul 6, 2019 at 14:55
  • \$\begingroup\$ Use insert link icon to paste \$\endgroup\$ Jul 6, 2019 at 15:22

3 Answers 3

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Your question lacks details of the Virtual ground in the schematic and BOM.

In combination with a Capacitive load ( which you did not shown unless you follow the datasheet recommendations, ) it could be oscillating.

  • include full URL with keyword using Link icon.
  • comments can use tinyurl.com not sure about yours but it fails.
  • always include full schematic with BOM ( bill of material) and URL links to pertinent parts.

  • I haven't seen a BB part in 30 yrs and I wonder, did they get divorced from TI? Or did you have a cache of old parts? or is it just the BB laser print on the IC? https://media.digikey.com/pdf/Data%20Sheets/Texas%20Instruments%20PDFs/OPA2604.pdf

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  • \$\begingroup\$ @analogsystemsrf I measured the + and - voltage from ground, and for the positive voltage, I'm getting between 13.69VDC to 15.12VDC depending on how I tune the trimpots on the buffer circuits. For the negative voltage, I'm getting -15.06VDC. \$\endgroup\$
    – fas11030
    Jul 7, 2019 at 0:11
  • \$\begingroup\$ Sunnyskyguy EE75 I updated the falstad link, added more datasheets for the devices I'm using, and added the schematic for the virtual ground circuit. Could you please explain what you mean by the capacitive load? I see, now that you mention it, that there is a section in the datasheet which describes this, although I don't quite understand how it applies to this circuit. By the way, the resistors to ground on the output of the circuit are just an approximation of the load resistance and are not actually part of the circuit. \$\endgroup\$
    – fas11030
    Jul 7, 2019 at 0:21
  • \$\begingroup\$ Your Falstad link is too long due to content \$\endgroup\$ Jul 7, 2019 at 12:29
  • \$\begingroup\$ all you need is an Op Amp for Vdd/2 and not dual LDO’s \$\endgroup\$ Jul 7, 2019 at 12:49
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Your 15V regulator is not regulating probably because the input battery voltage is not 15V + 2V + 15V + 2V= at least 34V. Each 15V regulator needs an input of at least 17V.

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    \$\begingroup\$ Like I said in the description, I'm using a 36V power supply. \$\endgroup\$
    – fas11030
    Jul 7, 2019 at 1:22
  • \$\begingroup\$ @fas11030 * Generating +/-15V from a single DC voltage with 2 complementary voltage regulators doesn't work. As the op-amps only need a modest current, use an additional op-amp to to make an artificial ground. \$\endgroup\$
    – Raonoke
    Apr 27 at 19:44
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Your virtural ground does not work. Please make a normal schematic like this: opamps

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