Member of the Trade: Goldpoint Level Controls
- Dec 6, 2010
- Reaction score
Member of the Trade: Goldpoint Level Controls
- Dec 6, 2010
Virtual Ground Circuits from Voltage Regulators
These circuits enable a two 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 inexpensive LM317/LM337 circuits below are capable of delivering up to +/-18V at more than 1.5 amps, 75 times the current of a TLE2426 rail splitter chip. 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 @ 50mA.
Both the LM317/LM337 Basic and VG1 Circuits below draw quiescent current of only 4 or 5 milliamps - great for battery use!
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.
VG1 Virtual Ground Circuit
All parts for these various circuits are easy to find - and can be ordered from Mouser.com or Digikey.com.
Resistor and capacitor values are not critical - you can substitute near or alternate values.
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)
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) The adjust pin on the LM336 voltage reference is not used, so leave it unconnected;
only connect the "+" and "-" pins.
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) 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.
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.
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.)
Enhanced Virtual Ground for Low Noise Audio Applications
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].
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 are each 1/2 of the rail-to-rail voltage. And the rail-to-rail voltage must remain at a set, unvarying voltage which is the sum of the absolute values of both of the rail splitter regulators output voltages. You therefore must use the third voltage regulator (U3).
Without U3, the rail-to-rail voltage could go up or down with load changes, battery drain, as the AC line voltage went up or down, etc. And if the rail-to-rail voltage went up or down, the two fixed value regulators would begin to compete with each other to establish different ground points, one or both constantly wasting current (and possibly overheating or burning up). So U3 is essential to ensure that fixed value regulators 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 voltage regulator such as 3 amp LD1085V or a 5 amp LD1084V. This also gives the advantage of allowing the use of any value fixed voltage regulators for U1 and U2. With an adjustable output voltage regulator for U3, the virtual ground does not have to be centered between the rails. For example, you could make a +5V/-12V split supply by setting the adjustable voltage regulator U3 to 17V, selecting U1 as a 7805 (+5V), and U2 as a 7912 (-12V).
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| before powering up.
Fixed Value Voltage Regulators for Rail Splitter Section Only
An alternate way of setting P1 above to the correct voltage is as follows:
1) Insert an ammeter between the +V or -V input and the DC power supply.
2) Set the ammeter on a high scale, such as the 10A scale.
3) Turn on the DC power supply.
4) 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.
5) Then use a voltmeter to test the output voltages relative to the ground point.
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.
Power Opamp Rail Splitter