Creating the 'virtual ground' in a single supply preamp/amp circuit??? - diyAudio
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Old 16th May 2002, 11:48 AM   #1
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Question Creating the 'virtual ground' in a single supply preamp/amp circuit???

I am building a single supply (24V) preamp and amplifier circuit that uses about 8 op amps for active filters and 4 LM3886 amplifier IC's (High and Low Bridged Outputs).

What is the best way to create a steady mid-rail voltage (VCC/2) for a reference voltage (also known as virtual ground) for the opamps and LM3886 IC's?

At the moment, I am trying an op amp follower circuit similar to the one shown on page 2 of this document (i originally didnt have the small resistor on the output and got oscillations from the op amp. the small 47R resistor fixed this)

I am using an LM324 op amp for the follower. (have a spare one available on the board). Can I also add a large electrolytic cap (68uF) to the follower output for filtering/regulation?
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Old 16th May 2002, 03:28 PM   #2
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Default Virtual PS ground

You can use an LM317/LM337 pair (positive and negative regulators). The input to the positive and negative rails go to the appropriate regulator input. The grounds are tied together -- and this is your virtual ground. In fact, for the best results I have used two LM317's and two LM337's -- allowing for the overhead in each. (As described in Audio Amateur sometime in the early 1990's). Make sure to bypass the regulators as shown in the National Semi applications notes and use the appropriate resistors to set the output voltage. I use this technique (beefed up with some power transistors) to derive +/0/- from an old Heathkit regulated power supply and it works fine.
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Old 16th May 2002, 06:41 PM   #3
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I have always had good luck with a simple pair of
resistors, equal value, set up as a divider to form
a voltage halfway between v+ and ground. Then
put a nice big capacitor from the center point (+)
to ground (-).

Typical resistor values would be a few K and capacitor
maybe 1000 uF.
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Old 9th April 2011, 01:36 AM   #4
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Hi Nelson - is this as simple as it sounds? One cap tied between the virtual ground and the real ground? (Just want to make sure I understand your comment correctly.)

Other schematics I've seen typically seem to show two caps - from virtual ground to B+ and ground respectively.
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Old 9th April 2011, 06:33 AM   #5
lineup is offline lineup  Sweden
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Nelson is right.
Two resistors and a cap wll do.
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Old 9th April 2011, 06:43 AM   #6
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Another very simple solution is TI's TLE2426 rail splitter. Available in a TO-92 case which takes up less space than a pair of resistors and a big cap.

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Old 9th April 2011, 07:09 AM   #7
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The TLE 2426 is awesome for when voltages are very low... for instance a virtual split rail with resistors from a 9V battery into a common cmoy, will often act up long before the battery is flat, simply because the voltage drop on each rail pulls it out of the operating zone, and suddenly you have two diffirent voltages, despite useing a resistor divider....
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Old 9th April 2011, 07:38 AM   #8
alayn91 is offline alayn91  France
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Some ideas on virtual ground:
- Virtual Ground Circuits

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Old 9th April 2011, 04:54 PM   #9
Bigun is offline Bigun  Canada
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One way to think of the virtual earth is that of a bridge circuit. The load is 'driven' from both ends, one by the amplifier playing music, the other end by an amplifier with a dc output. The reason I mention this is that you realize that the quality of the signal through the load depends on what is connected to both ends of the load. You might have a fancy amplifier on one end and a 'cheap' i.c. making your virtual earth on the other end - which doesn't make a lot of sense.

From this perspective the simple two (good quality) caps, two resistors approach might be preferred. If you size the resistors so that they can't flow enough current to damage the speaker you have a kind of simple speaker dc protection.

Of course, you could just make two amplifiers and bridge them.
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.
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Old 9th April 2011, 05:19 PM   #10
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Thanks for the feedback.

What might good starting points be for resistor values for a 2050 T-amp? I've seen 4k7 used in several schematics? It seems that 220uf Caps are popular in this position?
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