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Old 22nd April 2013, 11:34 PM   #1
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Default Virtual Ground (A Regulated Rail Splitter)


Virtual Ground Circuits from Voltage Regulators



These circuits enable a twp conductor DC power supply, DC wall adapter, or a battery to function as a split supply with a three conductor output (i.e., positive, negative AND ground). This sort of circuit is called a "Virtual Ground" and/or a "Rail Splitter".

The LM317/LM337 circuits below are capable of supplying up to +/-18V at more than 1.5 amps.
The DC Supply Input can be from 7.5VDC to 40VDC. The TO-220 voltage regulators are each rated for 20W. However, they can handle a watt or more without heatsinks - example: Output = +/- 9VDC @ 75mA or less.

Both the LM317/LM337 Basic and VG1 Circuits below draw quiescent current of only 4 or 5 milliamps - great for battery use!


Click the image to open in full size.

Basic Circuit with Adjustable Voltage Regulators


How it works: The LM317 (positive) and LM337 (negative) adjustable voltage regulators operate in parallel with their outputs tied together through small resistors to create a virtual ground. The LM336BZ-2.5V voltage reference compensates for the LM317's (+1.25V) internal reference and the LM337's (-1.25V) internal reference. So when the LM317/LM337 adjust pins are connected inside the R1/R2 voltage divider as shown, each voltage regulator output voltage becomes 1/2 of whatever the rail-to-rail voltage happens to be. Thus, together, the voltage regulators "split the rails", creating a "rock solid" virtual ground.

Although a simple and inexpensive virtual ground solution, some audio designs will sound better when using it. For example, when powering a headphone amplifier with this circuit the bass notes may sound considerably clearer and more life like. The reason for this unusually good performance may be that the voltage regulators create an "unbudgable ground" - holding the ground point in place very firmly compared to other circuits, virtual or not.

Click the image to open in full size.

VG1 Virtual Ground Circuit

NOTES:
1) The values for R1 and R2 shown in the chart above yield about 2mA of current through the LM336BZ-2.5V.
The formula used to determine the values is: R1 or R2 = (Vrr - 2.5) / .002 / 2 For example with a 12V power supply:
(12 - 2.5) /.002 / 2 = 2375. So use a 2.37K resistor for R1 & R2. Also: I = (DC supply - 2.5) / (R1+R2).

2) A "no load test" of the VG1 Circuit with an Eveready Gold 9V alkaline battery (it actually measured 9.3V) and 1.62K R1/R2 resistors yielded the following results: The ground remained perfectly centered (+/- 4.65V), while the total current being drawn was only about 4.5mA. This shows that with 2mA through the voltage divider section, the rest of the circuit was consuming only an additional 2.5mA. And that says if we add a 20mA load to the output, and if the 9V battery could supply 350mAH to 550mAH, the battery would last about 12 to 20 hours or more of continuous use.

3) When using an AC powered DC supply, "proper power supply design" recommends C1, C2, C3, C4, C5, D1, and D2 be installed. But when using a 9V battery for a low current application you can skip installing C1, C2, C3, C4, C5, D1, and D2 altogether - and simply use the Basic Circuit as shown at the top of the page!

4) The adjust pin on the LM336 voltage reference is not used, so leave it unconnected; only connect the "+" and "-" pins.

5) You may be able to reduce the size of the 1 ohm output resistors to 0.75 ohm or less by minimizing the current through the LM336BZ-2.5 (by using larger value R1/R2 resistors). A small ground point voltage offset, if it happens, is usually acceptable. An LM336BZ-2.5V can operate with 0.5mA to 10mA of forward current.

6) The LM317/LM337s require about 1.5 to 6mA of load current to maintain regulation - and they will continue to regulate with an Input voltage as low as 3.7 volts.

7) Increasing the size of C1, C4 and C5 can be sonically advantageous. They can be 220uF to 12,000uF, (or as much as you can afford or have room for.) Generally, electrolytic capacitor rated voltages should be at least 30 percent higher than whatever their power supply voltage is.

8) Printed circuit boards for testing/prototyping the VG1 Circuit are available from goldpointsales@gmail.com for $5 each. The actual size of this VG1 PC Board is: 1.4" x 1.8" (35.5mm x 45.7mm). The three larger radial mounted capacitors are 12.5mm (1/2") diameter and the two small ones are 5mm (1/4"). The recommended electrolytic capacitor type is the Panasonic FM series. All parts are easy to find, or can be ordered from from Mouser.com or Digikey.com.

Click the image to open in full size.__________ Click the image to open in full size.

VG1 Parts List:
R1, R2 - (see chart above) - ** dependent on DC Supply voltage
R3, R4 - 0.75 ohm to 1 ohm - 1/2W or 1W
C1 - 470uF/50V** - Panasonic P/N EEU-FM1H471
C2, C3 - 22uF/50V - Panasonic P/N EEU-FM1H220
C4, C5 - 1000uF/25V** - Panasonic P/N EEU-FM1E102
D1, D2 - 1N4002 - (or similar)
U1 - LM317TG - TO-220 package (On Semiconductor)
U2 - LM337TG - TO-220 package (On Semiconductor)
U3 - LM336Z25 - 2.5V voltage reference (Fairchild)

__________________________ __________________________

If you are using the virtual ground with an audio circuit and your DC power supply has an AC source, adding another voltage regulator in front of the rail splitter section can further improve sound quality. An LD1085V, 3A LDO (Low Dropout Voltage) voltage regulator sounds better for this purpose than others I've compared by listening tests. When using this additional voltage regulator (U4), be sure that your DC Supply (input voltage) is always 1.5V (or more) higher than your desired LM317/LM337 rail-to-rail voltage - because the LD1085V needs at least 1.3V across it to stay in regulation. note: The maximum DC Input Voltage for a LD1085V is 30VDC. (This three regulator circuit draws twice the current (or more) compared to the VG1 Circuit, so it may not be as well suited for battery use.)

Click the image to open in full size.

Enhanced Virtual Ground for Low Noise Audio Applications

__________________________ __________________________

Development Credits:
Arn Roatcap: (Founder of Goldpoint Level Controls www.goldpt.com) - Prior to the LM317/LM337 circuits, built virtual grounds using fixed value voltage regulators (see circuits below). Integrated new ideas, constructed all of the prototypes and performed extensive listening tests.
John Broskie:
(GlassWare www.glass-ware.com and Tube CAD www.tubecad.com) - Suggested many virtual ground circuit ideas from 2006 to 2013. Directed the use of 1 ohm output resistors on the rail splitter voltage regulators.

Kim Laroux:
(www.head-fi.org forums) - Had the ingenious idea to offset the LM317/LM337 internal voltage references by using a single 2.5V zener diode.
KT88:
(www.head-fi.org forums) - Contributed the key idea to use a LM336 voltage reference, instead of a zener diode, to compensate the LM317/LM337 internal voltage references.


__________________________ __________________________

Shown here because of their simplicity, the following two circuits use fixed value voltage regulators to create a virtual ground. They MUST have a third voltage regulator (U3) to keep the U1/U2 rail-to-rail voltage from going up or down. Some possible fixed value U3/U1/U2 voltage regulator combinations are: [+10V, +5V, -5V], [+12V, +6V, -6V], [+18V, +9V, -9V], [+24V, +12V, -12V].

Click the image to open in full size.

Basic Circuit with Fixed Value Voltage Regulators


When a "complimentary pair" of fixed value voltage regulators are used to create a virtual ground this way, the absolute values of their output voltages must each be 1/2 of the rail-to-rail voltage. Further, the rail-to-rail voltage itself must remain at a set, unvarying voltage which is the sum of the absolute values of both of the rail splitter regulator output voltages. You therefore must use the third voltage regulator (U3). Because if the rail-to-rail voltage went up or down, the two fixed value rail splitting regulators would begin to fight each other - each trying to establish a different virtual ground point - causing one or both to constantly waste current (and possibly overheat or burn up). Without this third regulator, the rail-to-rail voltage could go up or down as the AC line voltage went up or down, for instance. So in a circuit like this with fixed value regulators doing the rail splitting, U3 is essential, ensuring that U1 and U2 do not interact with each other.

The output of U3 needs to be close to the value of U1 added to the absolute value of U2. As the output voltages of common fixed value voltage regulators vary by as much as 5% from their rated values, buying extra ones and pre-testing them to find their actual output voltages lets you select them to meet the desired U3 = U1 + |U2| .

__________________________ __________________________


Because U3 consumes twice as much power compared to U1 or U2, a good choice for it is an adjustable LD1085V 3A voltage regulator. This also gives the advantage of allowing the use of any value complimentary fixed voltage regulators for U1 and U2 - and they would not necessarily have to be exactly matched. However, it is still a good idea to pre-test U1 and U2 to find their actual output voltages - then adjust the output voltage of U3 (via P1) to meet the the desired U3 = U1 + |U2|. The virtual ground does not have to be centered between the rails: You can make a +5V/-12V power supply by setting a variable voltage regulator U3 to 17V, selecting U1 as a 7805 (+5V), and U2 as a 7912 (-12V).


Click the image to open in full size.

Fixed Value Voltage Regulators for Rail Splitter Section Only

Alternatively, this circuit can be set up without pre-testing/pre-measuring the actual output voltages of U1 and U2. You can do this by using an ammeter to monitor the total current the circuit draws from the input DC power supply. Initially, you may want to set the ammeter on a high scale, such as the 10A scale. Quickly adjust P1 to give the lowest quiescent current. If it is below 2A, switch to the 2A scale. If it is then seen to be below 200mA, (you're aiming for perhaps 5mA to 50mA), switch to the 200mA scale. Then use a voltmeter to test the output voltages relative to the ground point.


__________________________ __________________________


Click the image to open in full size.

A 3 terminal fixed value voltage regulator rated for 12V could operate as low as 11.5V or as high as 12.5V.


The LD1085V is an inexpensive ($1), adjustable (1.25V to 28.5V), 3A positive voltage regulator.

The 78xx/79xx, LM317/LM337 voltage regulators are all commonly available and inexpensive (about $0.25).


__________________________ __________________________


A Power Op Amp Virtual Ground Circuit

Here is a rail splitter virtual ground circuit which "works", but is a second or third choice sonically. While it does center the virtual ground point perfectly, it requires a constant current source (the LD1085V) hung on its output to sound any good when powering audio circuits. Furthermore, both the L165 and the LD1085V require heat sinks, so this circuit is not good for battery use (too much wasted power).

The L165 comes in a five lead TO-220 package, and is rated for up to 3 Amps at +/-18V.

Click the image to open in full size.

Power Opamp Rail Splitter

Note: In the case the schematic images are lost, this page is mirrored at:
A Regulated Virtual Ground Circuit and
Virtual Ground (regulated!) - and Rail Splitter Circuits!


Last edited by Sonic Wonder; 25th July 2014 at 01:05 PM. Reason: clearer schematics
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Old 23rd April 2013, 10:11 AM   #2
AndrewT is offline AndrewT  Scotland
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Could the usual high current (say 5Apk to 10Apk) modification, of adding a parallel transistor to each regulator, be applied to this circuit?
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Old 23rd April 2013, 01:55 PM   #3
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Thanks for posting the schematic. It looks cool, but I'm struggling
to find the fundamental difference to the usual, class AB amplifier-based approach.
While the LM317 tries to keep its output 1.25V above the reference on pin A, an NPN transistor tries to keep its output (=emitter) 0.7V below the reference pin (=base). The same analogy exists between LM337/PNP.
With the usual approach, you use two diodes to generate a bias voltage of 2x0.7V. In your design, you use the 2.5V reference and connect the adjust pins "crosswise" to generate -2.5V. So in the end, this topology is quite close to a class AB amp.
The only difference I see is that in your design, both regulator can source and sink current, while in class AB, the one transistor is doing the sourcing and the other one the sinking.
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Old 27th April 2013, 04:31 AM   #4
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Quote:
Originally Posted by AndrewT View Post
Could the usual high current (say 5Apk to 10Apk) modification, of adding a parallel transistor to each regulator, be applied to this circuit?
I'm not sure - probably!
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Old 27th April 2013, 04:38 AM   #5
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Quote:
Originally Posted by sebastian_s View Post
Thanks for posting the schematic. It looks cool, but I'm struggling
to find the fundamental difference to the usual, class AB amplifier-based approach.
While the LM317 tries to keep its output 1.25V above the reference on pin A, an NPN transistor tries to keep its output (=emitter) 0.7V below the reference pin (=base). The same analogy exists between LM337/PNP.
With the usual approach, you use two diodes to generate a bias voltage of 2x0.7V. In your design, you use the 2.5V reference and connect the adjust pins "crosswise" to generate -2.5V. So in the end, this topology is quite close to a class AB amp.
The only difference I see is that in your design, both regulator can source and sink current, while in class AB, the one transistor is doing the sourcing and the other one the sinking.
Well, small current (2-5mA) or large (1500mA), these voltage regulators will constantly output definite voltages 1/2 of the rail-to-rail voltage. That's a different way of operating compared to CL AB or CL B amps, which output ZERO volts unless demanded to DO something. It's really nice having so few components in the VG, the output devices already being 20W TO-220 packages, and all low cost parts.
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Old 27th April 2013, 06:02 AM   #6
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If we're talking 9v battery power... just use a TC1044 and you're done.
That kind of thing is useless for high power, but for low draw stuff, in the <20mA range, just go with cheap inverter chip like the '1044.
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Old 27th April 2013, 07:39 AM   #7
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My applications typically start a bit bit higher than 20mA. Also, sonically, I have not been very impressed with charge pump type negative rail generators.
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