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(Do-It-Yourself) Discussions › Virtual Ground
(regulated!) - and Rail Splitter Circuits!

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Virtual Ground (regulated!) - and Rail Splitter Circuits!

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3/7/13 at 5:53pm

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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 supplying 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 @ 75mA.

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.

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)

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)  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

__________________________________________________________________________

Development Credits:

Arn Roatcap: (Founder of Goldpoint Level Controls  http://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  http://www.glass-ware.com/  and  Tube CAD  http://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: (http://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: (http://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
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).

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 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).

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.

__________________________________________________________________________

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

Edited by Sonic Wonder - 2/21/15 at 5:06pm

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#2 of 120

3/7/13 at 9:23pm

KimLaroux

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Interesting concept. It's weird to see two regulators connected this way,
kinda makes your head spin.

As for performance, I'm wondering about the influence of the resistors at the
output of the regulators. Would not that raise the overall "output impedance" of
the amplifier, and lower overall performances?

I'm also concerned about the two regulators fighting each others. As you
said, it is very important that the input voltage be exactly the sum of the
absolute voltage output of each linear regs.  It just seems unsafe.. and
open to all sort of troubles if the load and source is variable. I bet it would
be possible to use adjustable regulators in a circuit that would compensate for
the varying loads and input voltages. That way the virtual ground would always
be halfway between the rails, no matter what voltage the rails are. It's one of
the reasons why the rail splitter circuit is so popular: it works regardless of
the voltage between the rails, making it usable with batteries.

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#3 of 120

3/8/13 at 8:58am

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Sonic Wonder

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Yes, the resistors on the outputs of the two fixed value voltage regulators
DO raise the output impedance, but not by very much. They serve to "take up the
slack" of minimal output voltage differences between the two complimentary fixed
regulators. But the regulators do not fight each other. (The minimal amount they
"disagree" is swamped by the use of the output resistors.) I have had no
problems at all using this ground. This virtual ground sounds great - because it
is regulated.

The adjustable pre-regulator from the original circuit (beginning of thread)
could possibly be replaced by a single fixed value regulator, so no
potentiometer adjusting would be necessary. In that case, all three regulators
should be pre-tested and selected so that U2's output voltage is close to the
absolute value of U3's output voltage and U1 is close to U2 + |U3|. If
necessary, R2 and R3 could be increased from 1 ohm to 2 or 3 ohms.

But you are right - I would really like a way to use adjustable regulators
where they "self adjust" to 1/2 of whatever the rail-to-rail voltage is. Anyone
want to design that?

Edited by Sonic Wonder -
3/10/13 at 3:19pm

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#4 of 120

3/8/13 at 1:48pm

tangent

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Quote:

Originally Posted by KimLaroux

As for performance, I'm wondering about the influence of the resistors at the
output of the regulators. Would not that raise the overall "output impedance" of
the amplifier, and lower overall performances?

That's what those monster 10 mF caps are doing on the output: reducing the
virtual ground impedance to near-zero.

You can think of the 78xx and 79xx in this circuit as doing nothing more than
providing the tiny trickle currents to the big rail capacitors needed to
maintain the virtual ground voltage.

A particularly nice feature of this circuit is that it effectively hides the
dropout voltage of the 78xx and 79xx in the half-supply drops they are there to
provide anyway. I suspect this circuit wouldn't work with the +/-3.3V versions
of these regulators for the same reason: there isn't enough voltage room for the
dropouts to hide in.

The 2V drop across the preregulator hurts enough that I don't think I'd
want to use this in a battery powered circuit. The inability to get below a 12V
supply also argues against using this in battery-powered amplifiers. That means
either a wasteful 12-cell pack at minimum, since that lets you drain the pack to
1V per cell, getting maybe 80-90% of the energy out of the battery. If you want
to get to 0.8V per cell to fully use the energy in the battery, you need 15
cells at minimum.

I don't mean to dismiss this circuit. This idea of using 78xx regulators to
provide the midpoint of a vground supply has been kicking around as long as I've
been involved in audio DIY — about a decade now! — and it's probably older than
that. This is the first implementation I've seen that actually makes sense.

With your permission, goldpoint, I may include this in an update to my vgrounds article.

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#5 of 120

3/8/13 at 6:21pm

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Hello Tangent,

The TO-220 78xx/79xx devices actually heat up if you are drawing much more
then 50mA to 100mA. They DO pass current. They
actively hold that ground point in place under load, and
solidly. I believe it sounds so good with my headphone
amp circuit mainly because the ground is a fairly STIFF = a
regulated virtual ground. It does sound better, by far, than
any other virtual ground I've tried. But yes, although the big output caps are
not essential, I believe that they do make this virtual ground sound better.
Actually, in my headphone amp circuit they're each 11,700uF (3 x 3,900uF, 16V
Panasonic low ESR FM series caps).

I bet the circuit would work just fine with any complimentary set of Pos/Neg
regulators - you would only need to adjust the rail-to-rail voltage
appropriately via an adjustable regulator in front. For +/-3.3V regulators, the
pre-regulator would be set to about 6.6V of course - that should work just fine.
(I am guessing that you could even use non-complimentary regulators. For
instance, if you used a +5V with a -12V, you would adjust the pre-regulator to
17V. In that case the virtual ground would not be in the middle of the rails and
you would have a +5/-12 power supply.)

Yes, this circuit was not originally intended for battery use. If someone
could figure out a way to get complimentary adjustable regulators to "self
adjust" their outputs to 1/2 of whatever the rail-to-rail voltage is - well then
it would make a very fine virtual ground for use with a battery . Perhaps some
opamps could be used to do that. (Such a circuit could be integrated into a
single TO-220 or TO-247 package by one of the custom linear I/C companies. That
would be so slick! It would be a 3-terminal "power rail splitter virtual ground"
- and be so handy. Like a beefed up Texas Instruments TLE2426. One other thing I
would like about such a device is that its internal regulators would not draw
much battery current in quiescent mode [little or no load current] - unlike many
other virtual ground circuits.)

I have seen your article many times when searching for virtual ground ideas -
and I say "yes" - you should include this one there too.

By the way, I tried a similar back-to-back complementary regulators circuit
work a few years ago - but could not get it to work. The secret to why this
circuit DOES work is the pre-regulator to adjust the rail-to-rail voltage and
use of those 1 ohm output resistors on the fixed voltage regulators. The 1 ohm
value can be lowered or increased as needed.

Edited
by Sonic Wonder - 3/8/13 at 10:00pm

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#6 of 120

3/8/13 at 8:39pm

wakibaki

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Somebody else look at this and tell me if it works, it's 4:25 am here:-

Maybe you could even get away with this:-

w

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#7 of 120

3/9/13 at 10:57am

KimLaroux

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This schematic came to me in the middle of the night. I suppose insomnia is
good for something after all...

The idea is simple:

You can adjust an LM317 simply by driving a voltage to its adjust pin. The
output is always 1.25 V higher than the voltage at the adjust pin. If you ground
the adjust pin, you have 1.25 V out. If you drive -1.25 V into the adjust pin,
you get 0 V out. This is because the regulator has an internal reference voltage
of 1.25 V that shows up between Adjust and Out. Same for an LM337, just
reverse the signs.

R1 and R2 are your voltage divider that sets your 0V virtual ground. Since
each adjust pins needs a max of 100µA, we can assume the voltage divider is
pretty much unaffected by the loads on the PSU.

The 2.5 V zener is there to create the 1.25 V and -1.25 V needed to
compensate for the regulators' internal Vref. The zener sits across the 0 V
reference, creating a 1.25 V at the cathode and a -1.25 V at the anode, with
reference to the virtual ground. Connecting the Adj pin of the LM317 to the
-1.25 V will have it output 0 V. Connecting the LM337's Adj pin to the 1.25 V
will have it output 0 V.

My last concern is that each reg has a minimum load requirement to maintain
regulation: 3mA for the LM337 and 4mA for the LM317. It's something to take into
account, which has not be dealt with in this schematic.

Now I really wish I had a 2.5 V zener and a lm337 to test it out.

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#8 of 120

3/9/13 at 12:40pm

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Edited
by Sonic Wonder - 3/16/13 at 8:44pm

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#9 of 120

3/9/13 at 1:47pm

wakibaki

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Looks good to me Kim, but maybe some of it is unnecessary...

w

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#10 of 120

3/9/13 at 2:01pm

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Actually that does not work. (Only 1/2 of the ground is regulated, believe it
or not...)

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#11 of 120

3/9/13 at 2:12pm

KimLaroux

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Quote:

Originally Posted by goldpoint

Oh my Gawd! Did you do it?

Oops - nope - I think you must have the output of the LM317 be 1/2 of the
rail-to-rail voltage

and the LM337 also be 1/2 - but negative!

Can you explain? I fail to see how it's different than your circuit in the
1st post.

The LM337 is just a mirror to what the LM317 does. It's all a question of
what you use as a reference. If you decide to use the virtual ground as a
reference, then V- is half of the rail-to-rail voltage, but negative. The
virtual ground is then 0 V. The LM337 is configured to "regulate" a negative
voltage using V- as an input. Here I just configure it to output 0 V. If I
didn't have the zener in there, and just connected the adjust pins to the middle
of the resistor divider, then the LM317 would output 1.25 V and the LM337 -1.25
V, referred to the virtual ground.

Quote:

Originally Posted by wakibaki

Looks good to me Kim, but maybe some of it is unnecessary...

w

It was my first thought when I read the 1st post. I've been thinking about it
ever since, and I'm not sure if using two complementary regulators is necessary,
even in the original design.

If it works using only the LM317, then the original design should work
without the 79XX, no?

Goldpoint, can you explain why you used two regulators in your design instead
of a single one?

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#12 of 120

3/9/13 at 2:29pm

wakibaki

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Quote:

Originally Posted by goldpoint

Actually that does not work. (Only 1/2 of the ground is regulated, believe it
or not...)

There is only one ground. How can half of it be regulated?

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#13 of 120

3/9/13 at 2:47pm

KimLaroux

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I think I see what he means.

The 78XX regulates the difference between the ground and V-. It make sure
this voltage is stable.

The 79XX regulates the difference between ground and V+. It make sure this is
stable.

If you remove the 79XX, you have a regulated ground relative to V-, but
nothing regulates the difference between the ground and V+. Sure since he uses a
pre-regulator, then you'd take for granted that V+ be whatever is left over the
ground. But I think this creates an unbalanced PSU where one rail is more stable
than the other. If you suddenly load V+ creating a drop, the 78XX would not
care, and ground would stay the same voltage with reference to V-, while V+
would fall closer to ground.

My modification is different. I'm not regulating the difference between the
rails. The resistor voltage divider sets the ground reference point, and the
regulator just output a regulated voltage with reference to that. The ground
will always be halfway between the rails, even if their load changes. The
resistor voltage divider is unaffected by the change in loads between the rails.
If one rail is suddenly loaded, it'll created a voltage drop that also drops the
voltage divider. Since the voltage divider is connected across the rails, the
LM317 would simply adjust it's output so it stays halfway between the rails.

Beautiful.

Edited
by KimLaroux - 3/9/13 at 2:49pm

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3/9/13 at 3:26pm

wakibaki

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I think that with just 1 regulator the sinking impedance and the
sourcing impedance may not be symmetrical. This would have to be
tested, at least in simulation. The advantage is that there are no problems with
matching and no necessity for the ballast resistors with a consequent lower
output impedance and no possibility of power being wasted if the two regulators
end up producing slightly different voltages.

It just illustrates IMO, the reasons to stay away from virtual
grounds

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#15 of 120

3/9/13 at 3:34pm

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Sonic Wonder

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I actually tried that circuit. It seems to work when you have a very small
load.

If your load is connected to the virtual ground and the negative rail, it
will work correctly. In that case it works just like a regular positive voltage
regulator circuit - up to the current (amperage) limits of the LM317
regulator.

But if you connect a load from the positive rail to the virtual ground, the
ground point will actually move - it will be pulled upwards towards the positive
rail as the load increases. see?

You need both regulators there to "hold the virtual ground point steady" - to
keep it from "moving". (One does it in one direction and one in the other
direction, so to speak.)

But there are substantial advantages to using virtual grounds in certain
situations - and no reason not to use them if they are properly
designed.

Edited by Sonic Wonder - 3/10/13 at
10:58pm

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