Virtual Ground Circuits Made With Common Voltage Regulators
This circuit enables a two conductor DC power supply or DC wall adapter to function as a split supply with a three conductor output (i.e., positive, negative AND ground). It can also be used with portable gear running from a single battery, such as a 6V, 9V, or 12V, or any battery voltage from about 5V to 40V.
It is capable of supplying up to +/-18V at more than 1.5 amps on both the positive and negative outputs simultaneously. The TO-220 voltage regulators are each rated for 20W. However, they can handle a watt or more without heat sinks - for example an 18V DC Supply and a load of 75mA or less.
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 sonic performance may be that the voltage regulators create an "unbudgable ground" - holding the ground point in place more firmly than other circuits do, virtual or not.
The values for R1 and R2 shown in the chart above yield about 8mA of current through the LM336. The formula used to determine the values is:
R1 or R2 = (Vrr - 2.5) / .008 / 2
Example: If the rail-to-rail voltage is 24VDC, the formula gives 1343, so the closest 1% resistor value for R1, R2 is 1.33K.
If powering Circuit 1 with a battery, you may put less current through the LM336BZ-2.5V by choosing larger R1/R2 resistor values than shown in the chart. Also, you can reduce the size of C1, C4, and C5. Although this may result in a small ground point voltage offset, it is usually acceptable. An LM336BZ-2.5V can operate with as little as about 0.5mA, although they are typically rated for up to 10mA of forward current. The LM317/LM337s require about 1.5 to 6mA of load current to maintain regulation. They will continue to regulate with an Input voltage as low as 3.7 volts.
If you are using the virtual ground to power an audio circuit, adding another voltage regulator in front of the rail splitter section can further improve sound quality. (Circuit 2 below) The LD1085V, a 3A LDO 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.
Additionally, 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. Circuit 2, a three regulator circuit, draws twice the current (or more) compared to Circuit 1, so it is not as well suited for battery use.
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. (essentially the circuit at the top of this page).
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 mainly just for interest, Circuit 3 and Circuit 4 below use fixed value voltage regulators to create a virtual ground. Replaced (obsoleted) by the LM317/LM337 adjustable voltage regulator circuits shown above, the following fixed value voltage regulator circuits 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].
When a complimentary pair of fixed value voltage regulators are used to create a virtual ground this way, their output voltages must each be 1/2 of the rail-to-rail voltage. Further, this requires that the rail-to-rail voltage itself remains at a set, unvarying voltage which is twice the value of each of the rail splitter regulators. If the rail-to-rail voltage went 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 likely overheat). Without the third regulator, the rail-to-rail voltage could go up or down as the AC line voltage went up or down, for instance. So here 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 LD1085 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 be 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| .
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 1.75A, (you're aiming for about 0.050A or 50mA) you can switch the ammeter to a lower scale, such as a 2A scale. Then use a voltmeter to see how well the ground point is centered (which shows how well U1 and U2 are matched).
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.
Edited by Sonic Wonder - 12/1/13 at 5:17am