circuits enable a two conductor DC power supply, DC wall adapter, or a
battery to function as a split supply with a three conductor
(i.e., positive, negative AND ground). This sort of circuit is called a
"Virtual Ground" and/or a "Rail Splitter".
|Adjustable Voltage Regulators
inexpensive LM317/LM337 circuits below are capable of delivering up to
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
regulators are each rated for 20W. However, they can handle a watt or
without heatsinks - example: Output = +/- 9VDC @ 60mA.
the LM317/LM337 Basic and VG1 Circuits below
quiescent current of only 4 or 5 milliamps - great for battery use!
Circuit with Adjustable Voltage Regulators
it works: The LM317 (positive) and LM337 (negative)
voltage regulators operate in parallel with their outputs tied together
through small resistors to create a virtual ground. The LM336BZ-2.5V
reference compensates for the LM317's (+1.25V) internal reference and
LM337's (-1.25V) internal reference. So when the LM317/LM337 adjust
are connected inside the R1/R2 voltage divider as shown, each voltage
output voltage becomes 1/2 of whatever the rail-to-rail voltage happens
to be. Thus, together, the voltage regulators "split the rails",
a "rock solid" virtual ground.
a simple and inexpensive virtual ground solution, some audio designs
sound better when using it. For example, when powering a headphone
with this circuit the bass notes may sound considerably clearer and
life like. The reason for this unusually good performance may be that
voltage regulators create an "unbudgable ground" - holding the ground
in place very firmly compared to other circuits, virtual or not.
|VG1 PC Board (Top)
| (Bottom) 1.4" x 1.8"
||similar earlier version
VG1 PC Boards are available for $5 each. P/N = VG1 Above, at right, an assembled earlier version.
Mounting holes are for 4-40 screws, spaced 1.0" x 1.5".
|All parts are redily available and easy to find - and can be ordered from Mouser.com
are not critical - you can substitute near or alternate values.
VG1 Parts List:
R2 - **see VG1 schematic chart
dependent on DC Supply
R4 - 0.75 ohm to 1 ohm
C3 - 1000uF/25V**
C5 - 22uF/50V
|D1 - D4 - 1N4002 (or
||for SMD: MMRA4003T3
package Digi-Key, etc.
package Digi-Key, etc.
R1 and R2 shown in the chart above yield about 2mA of current through
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
- 2.5) / (R1+R2).
The adjust pin
on the LM336 voltage reference is not used, so leave it
the "+" and
component count: When using a battery for the DC source, such as a 9V
battery in a low current application, you can skip
C1 - C5 and D1 - D4 altogether and simply use the Basic
as shown at the top of this page. However, when
powering low-noise audio circuitry and if your DC power supply is
plugged into an AC source, you
should install all of the
capacitors. C1 - C3 can be 2,200uF - 10,000uF more. This necessitates
installing all of the diodes to protect the voltage regulators from the
large discharging capacitors at turn-off.
|4) A test of the VG1 Circuit was done with an Eveready Gold 9V alkaline battery
as the DC supply, the R1/R2 resistors were each 1.62K, and there was no load on the output of the circuit. The 9V battery itself actually measured 9.3V. Results: The ground remained perfectly centered (+/- 4.65V), while the
total current being drawn was only about 4.5mA. This shows that with
through the voltage divider section, the rest of the circuit was
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
would last about 12 to 20 hours or more of continuous use.
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
resistors). A small ground point voltage offset, if it happens, is
acceptable. An LM336BZ-2.5V can operate with 0.5mA to 10mA of forward
require about 1.5 to 6mA of load current to maintain regulation - and
will continue to regulate with an Input voltage as low as 3.7
size of C1, C2 and C3 can be sonically advantageous. They can be 220uF
to 12,000uF, (or as much as you can afford or have room for.)
electrolytic capacitor rated voltages should be at least 30 percent
than whatever their power supply voltage is.
you are using the virtual ground with an audio circuit and your DC
supply has an AC source, adding another voltage regulator in front of
rail splitter section can further improve sound quality. An LD1085V,
3A LDO (Low Dropout Voltage) voltage regulator sounds better for this
than others I've compared by listening tests. When using this
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)
to the VG1 Circuit, so it may not be as well suited for
|The following circuit makes a good phono preamp power supply (for use with a high-quality opamp):
for Low Noise Audio Applications
(Founder of Goldpoint Level Controls www.goldpt.com
) - Prior to the LM317/LM337 circuits, built virtual grounds
fixed value voltage regulators (see circuits below). Integrated
ideas, constructed all of the prototypes and performed extensive
Broskie: (GlassWare www.glass-ware.com
and Tube CAD www.tubecad.com
) - Suggested many virtual ground circuit ideas from 2006 to 2013.
the use of 1 ohm output resistors on the rail splitter voltage
forums) - Had the ingenious idea to offset the
internal voltage references by using a single 2.5V zener diode.
forums) - Contributed the key idea to use a LM336 voltage reference,
of a zener diode, to compensate the LM317/LM337 internal voltage
|Fixed Voltage Regulators
here because of their simplicity, the following two circuits use fixed
value voltage regulators to split the rails. They MUST have
a third voltage regulator (U3) to keep the U1/U2 rail-to-rail voltage
going up or down. Some possible fixed value U3/U1/U2 voltage
[+12V, +6V, -6V], [+18V, +9V, -9V], [+24V,
Circuit with Fixed Value Voltage Regulators
a "complimentary pair" of fixed value voltage regulators are used to
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
You therefore must use the third voltage regulator (U3).
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..
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
vary by as much as 5% from their rated values, buying extra ones and
them to find their actual output voltages lets you select them to meet
the desired U3 = U1 + |U2| .
U3 consumes twice as much power compared to U1 or U2, a good choice for
it is an adjustable voltage
regulator such as a 3 amp LD1085V or a 5 amp LD1084V. This also
gives the advantage
of allowing the use of any fixed value voltage regulators
for U1 and U2. With an adjustable regulator for U3, the virtual ground
point does not have to be centered between the rails. For example, you can make a
+5V/-12V power supply
by setting a variable voltage regulator U3 to 17V, selecting U1 as a
(+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.
Ground With Fixed
Value Voltage Regulators for Rail Splitter Section Only
|An alternate way of setting P1
above to the correct voltage is as follows:
1) Set the ammeter on a high scale, such as the 10A scale.
2) Insert the ammeter in-line between the DC supply and the +V input.
3) Turn on the DC power supply.
4) Quickly adjust P1 to give the lowest quiescent current. If it
reads below 2A, change 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.
you can replace P1 above with a fixed resistor (R2). This is even
recommended - if you already
know the exact voltages of U1 and U2. U3 = U1 + |U2| The U3 output voltage = (R2/R1 +1) x 1.25.
12V fixed value regulator
could operate as low as 11.5V or as high as 12.5V.
Inexpensive ($1), adjustable, (1.25V to 28.5V), 3A positive, Low
(fixed) and LM317
(adjustable): Inexpensive (about $0.25) and commonly available.
A Power Op
Amp Virtual Ground Circuit
is a rail splitter virtual ground circuit which "works", but is a
or third choice sonically. While it does center the ground point
it requires a constant current source (the LD1085V) hung on its output
to sound any good when powering audio circuits. Furthermore, both the
and the LD1085V require heat sinks, so this circuit is not good for
use (too much wasted power).
L165 comes in a five lead TO-220 package, and is rated for up to 3 Amps
Roatcap - Goldpoint Level Controls - (first
20 August, 2012)