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schematic

simulate this circuit – Schematic created using CircuitLab

I am supplying an audio circuit with only a positive voltage of 12V. I have created a mid point (aka virtual ground, aka bias...) of VCC/2 like this in order to feed multiple OA. Now howewer I am facing confusion on how to modify the filter schematic to accomodate the single supply.

How do I do it properly?

After the buffer my signal is already centered at 6v, do I have to reference the second order filter to the VCC/2 point somehow?

I am confused

Status Re
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3 Answers3

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You want to short the bias voltage sources and replace all of your signal grounds with the virtual ground.

So remove the two bias supplies and short where they were, then connect all of the ground points to virtual ground (+6 V) except for the opamp negative supply pins, these stay connected to 0 V.

GodJihyo
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  • I don’t quite understand your answer. The input signal is also referenced to ground. – Status Re May 04 '23 at 15:55
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    @StatusRe Basically you're making a dual supply, +6V, -6V and common. The opamps get +6V and -6V to their supply pins, common is ground. In this case +6V and -6V are the connections to the 12V supply, common is the virtual ground. Unless there's a DC connection between the 12V supply and the input signal you should be able to connect the signal ground to the virtual ground. I'll try to add a schematic when I get time. – GodJihyo May 04 '23 at 16:21
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I would take C3 to the virtual ground to improve PSRR, but the unity gain Sallen and Key filter will work as is.

Note: I am not at all sure about the stability of the 5532/081 combo in the Baxandall volume control, a small cap across R5 will help here. You probably want a cap and bleeder resistor at R3 wiper to prevent the wiper on the pot carrying the 5532 bias current (Or use a jfet opamp instead).

Note that the 07x series opamps all have a lovely trap if you exceed the common mode range in that they can undergo 'phase reversal' where the inverting and non inverting inputs flip, the resulting clipping is NASTY.

Dan Mills
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  • Sorry Dan I forgot to change the name of the TL081, it should be another ne5532. I will change it – Status Re May 04 '23 at 13:51
  • Are c4 and r8 in the schematic what you meant? – Status Re May 04 '23 at 14:05
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    @StatusRe C4 is good, but R8 does nothing. Basically the bias current in the buffer amp on the pot wiper flows thru the wiper and risks making crackles as the pot wears. A cap in series with the wiper and a resistor to provide the bias current will minimise this. – Dan Mills May 04 '23 at 15:03
  • Ok thanks, I think I got it right now – Status Re May 04 '23 at 15:16
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I am facing confusion on how to modify the filter schematic to accommodate the single supply

Your Sallen Key low-pass filter (the last stage) is already suitable for single supplies right now.

After the buffer my signal is already centered at 6v, do I have to reference the second order filter to the VCC/2 point somehow?

No, it will work fine. The output of the filter will be centred at 6 volts.

Andy aka
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