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



## Sonic Wonder

​ *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.
              ​ *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  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_​


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## KimLaroux

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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## Sonic Wonder

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?


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## tangent

Quote: 





kimlaroux said:


> 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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## Sonic Wonder

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.


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## wakibaki

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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## KimLaroux

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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## Sonic Wonder

​  ​ 
  ​


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## wakibaki

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

   
  w


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## Sonic Wonder

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


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## KimLaroux

Quote: 





sonic wonder said:


> 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: 





wakibaki said:


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


 
   
  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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## wakibaki

Quote: 





sonic wonder said:


> 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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## KimLaroux

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.


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## wakibaki

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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## Sonic Wonder

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.


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## Sonic Wonder

.


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## KimLaroux

Quote: 





sonic wonder said:


> 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
> ...


 
   
  This is what I explained in my last post. It's relevant to using fixed regulators. My design doesn't work the same way, so this is not a problem. With my design, the ground will _always_ be halfway between the rails.
   
  The sink and source difference is a valid point. An LM317 probably isn't designed to sink much current. Where can it even sink it to anyways? The only return path I see is the adjust pin... It's not like it can sink an Amp trough there. This would seem to dictate the use of both complementary regulators. The LM337 would "source" the current used by the V+ rail, and the LM317 "source" the current needed by the V- rail.


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## KimLaroux

Quote: 





sonic wonder said:


> Have I misread you design at first? (I hope so)
> 
> Are you saying that the LM317, for instance, will output +1/2 of the rail-to rail voltage (not zero volts in reference to the positive rail)
> and that the LM337 will output -1/2 of the rail-to-rail voltage (not zero volts in reference to the negative rail)?
> ...


 
   
  The LM317 outputs half of the difference between V+ and V-.
  The LM337 outputs half of the difference between V- and V+.
   
  Those two outputs are, in theory, at the same point : half of the rail-to-rail voltage.
   
  It, in theory, works with any supply voltage between 4 and 40 V.
   
  I would have breadboarded it already if I had the parts.


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## wakibaki

Quote: 





sonic wonder said:


> 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
> ...


 
   
  No I don't see. It doesn't sound at all likely to me. You obviously did not monitor the ground-to-opposite rail voltages when you did the tests. If the ground point moves toward the positive rail, the negative rail _must_ move toward the positive rail by double the amount, because the 2 resistors and the zener diode keep the regulator adj. pin at 1.25V negative of the centre point, and its output therefore _at_ the centre point.
   
  As Kim said, the 2 outputs should be at the same voltage. This is regardless of loading, otherwise the circuit is not working. Therefore only one of them is needed. Think about it.
   
  You built the original circuit with symmetrical regulators because your gut instinct told you to do it. Very often, however, in electronics our gut instincts betray us.
   
  It's simply untrue that there are substantial advantages to virtual grounds at all times, as you will find if you research the subject more fully.


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## Sonic Wonder

Um, you need to bench test what you're talking about. I did bench test it, and I'm telling you, you cannot use only ONE regulator (positive OR negative) to make a proper virtual ground. Will not workie right. I mean: TRY IT YOURSELF. YOU hook it up and load the top and/or the bottom respectively. You will see what we are saying is true. There is nothing wrong with virtual grounds if designed correctly! (Really) Have a nice day. Wishing you luck.


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## KimLaroux

I think he didn't test your circuit, but his with only one regulator - a fixed regulator. In which case his results confirm what I theorized in my post before his. You can't do this with a single fixed regulator.
   
  Wakibaki, there's a thing I don't understand about using only the LM317:
   
  In this design, the LM317 sources current to gnd and the current flow back trough V-. This is what the LM317 was designed for.
   
  But if you have no load on the V- rail, and a load on the V+ rail, then the current flows out of V+, back into Gnd... and where? The LM317 has no way to sink the current back to V-.  So how would it work?


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## Sonic Wonder

.


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## wakibaki

Good question Kim, I should have paid more attention.
   
  We're not dealing with DC here, we're dealing with AC. As long as the the net current is zero there will be no problem, there are some big caps there. What I _should_ have thought of, and which is why he is getting the result he does, is what happens at DC. If there is a net DC bias that exceeds the leakage through the caps and the adj. pin there _could_ be a problem. 
   
  Sorry goldpoint, your point is well taken, and probably the simplest solution is to use 2 regulators. You are mistaken about virtual grounds in general, however.
   
  w


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## KimLaroux

Quote: 





sonic wonder said:


> WOW! SUCCESS! This does sort of work you guys. The only problem so far is that even with 10 ohm output resistors, the circuit still draws quite a bit of juice (54mA), making it not ready for prime time battery use yet.
> 
> ​
> So this was a quick test circuit. (circuit above)  You can see the output voltages, relative to the virtual ground, are not exactly equal. However, they are "close enough for government" work, like we used to say... It's alive!  A rail splitter virtual ground using two, common, inexpensive adjustable regulators! When we get the _quiescent_ current down to nearly zero - then we will BE there!


 
   
  I've been thinking about all these problems.
   
  It's not designed for portable use that's for sure. I don't think it would be reasonable to even try to use such a device for portable use. And just to be clear, I'm of the opinion that for desktop use, there's very little reasons not to use a proper dual supply with a center-tapped transformer. 
   
  Since it's unregulated, you'd have to regulate each rails separately after the rail splitter. This would take care of unequal voltages between the rails and noise introduced by the zener (if any).
   
  Have you tried the circuit in real, or simulated it? In real, it would be impossible that both regs sit at 0 V. If they sit at different voltages, then they'll fight each others. Current sourced by one will be sourced by the other, introducing a quiescent current proportional to the difference in voltage between the two outputs.
   
  I'm guessing they don't sit at 0 V because the ground doesn't sit right between the rails. It would also explain the 54 mA quiescent current. Have you measured the voltages directly at the outputs of the regs?
   
  Many things could cause this: unmatched resistors, zener not exactly 2.5 V, and the variance in Vref of each regs. Datasheet says the Vref can vary between 1.2 and 1.4 V. This is enough to mess up the whole circuit.


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## KimLaroux

Quote: 





wakibaki said:


> Good question Kim, I should have paid more attention.
> 
> *We're not dealing with DC here, we're dealing with AC. As long as the the net current is zero there will be no problem, there are some big caps there. What I should have thought of, and which is why he is getting the result he does, is what happens at DC. If there is a net DC bias that exceeds the leakage through the caps and the adj. pin there could be a problem. *
> 
> ...


 
   
  Hhhmmm, yeah... I hadn't thought of it this way. I was actually only thinking in terms of DC. But I guess you're right. If this PSU is used to power a well designed audio amplifier, then there would be no current flowing trough the ground. But the moment there is, though, then the whole thing just collapses on itself.
   
  I can't decide which solution is cleaner, considering the latest results...
   
  But hey, a virtual ground with a 1.5 A rating, know any other that come even close? 
	

	
	
		
		

		
		
	


	



   
  I think this is what Nikongod calls "brute force", which is everything but an elegant solution.


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## Sonic Wonder

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## wakibaki

A simple (not so simple) way round this is to use a single regulator and make sure that it is on the correct side to pass any DC bias in the returned current from the amplifier. Unless you want to match each regulator to each amplifier this presents its own problems.
   
  You might find that with a suitable opamp, the opamp/transistor circuit I drew can actually be built to reliably outperform the regulator circuit in terms of wasted power. It can certainly be built to pass greater currents. I'll do a spice sim tomorrow.
   
  w


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## KimLaroux

This is great! Would you mind sharing a picture? 
	

	
	
		
		

		
		
	


	



   
  I'd also like it if you could give us voltages at different points. More precisely at those green dots, with reference to Ground:
   
   

  It would help understand what's going on with the 54 mA current. It'll also give us an idea of how well everything works together.


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## Sonic Wonder

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## Sonic Wonder

Here's that data:
 ​


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## KT88

A very cool implementation of a virtual-GND circuit.
   
  As for the latest circuit in goldpoint's post, I see a couple of issues with it:
  A. The zener current is only about 0.8mA, and it probably need at least a couple of mA to operate with the nominal voltage across it.
  B. The zener voltage can be in the +-5% range, and the internal reference of the LM317 and LM337 can be 1.2V-1.3V which is a +-4% compared to the 1.25V nominal value. That means there could be a difference of up to 225mV between the zener voltage and the reference of the LM317+LM337. So that's up to 225mV across the resistors at the output which yields a current of <=225/(2*R)mA (assuming there's sufficient current in the zener to keep its voltage in specs - no enough current will force a higher voltage across these resistors and therefore more wasted current). The biggest issue is what would happen if the zener voltage is high, and both regulators have a low reference so that Vzener>(Vref317+|Vref337|) - this would make the LM317 output voltage be lower than that of the LM337, and the circuit won't work as its supposed to work. So the designer/builder must make sure this doesn't happen.
  Some current at the output is actually a good thing, since the LM317 requires up to 12mA of current at the output to maintain regulation (this is the maximum value, nominal value is about 3.5mA). However since the current will depend on the exact value of the reference voltages it can't be trusted.
  C. A different way to look at this circuit is as a push-pull op-amp capable of sourcing/sinking >1.5A (at least that one of ways I see it ). The LM317 sources current, while the LM337 sinks current. As long as you maintain a minimum current at the output of both regulators its basically a class-A/AB buffer with the zener diode trying to compensate for the offset of ~1.25V from the A to O pins of the regulators (1.25V for each regulator). So perhaps instead of trying to use voltage regulators and compensate for the voltage variations which yields great variations in the DC current, it would be best to implement a class-A or AB (probably better) follower with the input generated using a simple resistive voltage divider between the rails as its usually done. This is what most op-amp circuits do, so using an op-amp can work fine, its just a matter of finding one with sufficient current drive capability, and low cross-over distortion - most op-amps now-days actually keep the output transistors operating with a small current at all times, but this current is usually very small to minimize power consumption, and therefore some cross-over distortion can still be noticed.
  D. Another point I want to mention is that I think the virtual-GND circuit should be tailored to the amplifier in use. Some amplifiers have a very different PSRR between the two supplies, so in some cases it might be better to implement a "simple" virtual-GND circuit that regulates the virtual-GND with respect to one supply rail or the other (which one would depend on the amplifier used).


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## KimLaroux

Thanks for the inputs KT88.
   
  I think you have a point with the zener needing more current to operate properly. It's obvious the zener isn't working properly in the last test because it only has 1.5 V across it while it should be 2.5 V. The voltage divider has less than 1 mA going trough it with a 18 V input voltage. Goldpoint, could you lower the value of the 10K resistors and see what happens? Or just raise the input voltage?
   
  I also agree that the quiescent current has it's advantage : It takes care of the regulator's minimum load current to maintain regulation. 10 mA should be enough, though.
   
  As for everything else... I am not marketing this design as a proper solution for a virtual ground. For me it's nothing more than a novelty hack, an entertaining exercise. 
	

	
	
		
		

		
			





 Anyone who's doomed to use a virtual ground should look for more elegant, properly designed solutions.
   
  Oh and yeah, my math _was_ right last night : You really have a volt between each outputs :
   
  U = RI = 20 * 54E-3 = 1.08 V
  0.505 + 0.505 = 1.01 V
   
  Close enough.
   
  More math tells us there's room for improvement :
   
  0.505 - (-0.777) = 1.282 V
  -0.505 - 0.766 = 1.271
   
  Which are the Vref for each regulators, within their published range. So even here, the difference between the adjust pin and the output is equal to Vref. Fixing the problem with the zener not operating properly should prove interesting.


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## Sonic Wonder

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 ​  ​


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## Sonic Wonder

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## Sonic Wonder

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## Sonic Wonder

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## KimLaroux

Looking at the datasheet for the zener you use, it says it's forward voltage is 2.5 V at 20 mA. This seems like a lot of current. I didn't know low voltage zeners needed such high current to maintain their rated voltage.
   
  Just looked at the other offerings on Digikey, and they all need 20 mA. How inconvenient.
   
  One cheap fix would be to use a higher voltage zener, like 2.7, 2.8 or 3.0 V.
   
  The problem I now see with my design can be explained using this drawing from the datasheet:
   

   
  As you can see, the forward voltage is dependent on forward current. In other words, it's not a fixed value. This is verified by your latest series of tests. When I thought about using a zener for this application, I took for granted that it would be 2.5 V regardless of the current flowing through it. The current flowing through the zener is the current flowing through the voltage divider, which is dependent on the input voltage. Not good. The point of the design was to make it work with a wide range of input voltages. It turns out that using a zener inside a voltage divider is not a solution.


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## Sonic Wonder

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## KimLaroux

You could probably do it with a pair of Op-Amp. But if you're gonna use op-amps, you may as well use discreet transistors like in Wakibaki's design. You would not need regulators anymore, since the op-amps will do the job. And you'll free yourself of the annoying reference voltage.
   
  And it'll be so much more elegant. 
	

	
	
		
		

		
		
	


	



   
  I have a question regarding your original design using fixed regs. Do you power an amplifier directly from it, or do you add regulators after it to regulate each rails separately?


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## wakibaki

I had one of the opamps upside down in the original drawing...
   

   
  You should be able to see (I think), if you expand the picture, that the legend in the bottom left hand of the screen shows 498.301nA. This is the current in the ballast resistor. The offset voltage (from midpoint) is ~4mV. This takes no account of opamp input offset voltage, however, _but_ the error due to variation in regulator voltage references and consequent quiescent current is likely to be considerably higher than the opamp circuit error. 
   
  Of course the _total_ quiescent current is higher than this. There is the current down through the resistor divider to account for, and there is the current drawn by the opamps.
   
  The quiescent current can be minimised by picking a low-power opamp such as the LM2904 (0.7mA) and increasing the divider resistors to (say) 100k. The LM2904 will put out 40mA, so if you pick transistors with a gain of 100, we're talking 4A source/sink capability.
   
  All-in-all it can be made to outperform the regulator circuit by a considerable factor. It's probably cheaper too.
   
  w


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## Sonic Wonder

Kim,
   
  The headphone amplifier is powered directly from it. Some of the things I really like about the circuit I use (we're speaking of the 3 regulator "Regulated Virtual Ground" circuit at the beginning of page 1 of this thread) is that the TO-220 devices do not need any heat sinks. The circuit is rugged, dependable, inexpensive, and even fairly simple. But mainly, the ground is SOLID = unbudgeable. Using that circuit will wow you when you hear the lower/lowest octaves.


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## KT88

The current solution of using a zener + regulators has the very big problem of being affected by variations (I work mainly on IC circuit design, so variations is always in the back of my head ). The current design requires a manual adjustment of the zener reference voltage to fit the reference of the generator. As I've stated earlier, there's a 50% chance the circuit won't work at all as expected (if Vzener>Vref317+Vref337). KilLaroux, because of that its impossible to use a higher voltage zener without modifications to the circuit. Using higher resistors will minimize the maximum current, but will affect output impedance at lower frequencies + wont solve the issues involved with the variations such as minimum load current required and maintaining Vzener low enough. A possible solution is to use parts with tighter tolerances. The zener could be replaced with a precision band-gap IC (the additional noise that band-gap has over buried-zeners is not of much importance due to the internal band-gap of the regulators which adds lots of noise already). These IC's also need less current than a zener diode. The regulators could be replaced with a more modern equivalents with tighter tolerance as well (and lower minimum load current while were at it), this way it is possible to have a lower offset voltage between the output of both regulators which will allow lowering the value of the resistors and minimizing current flow at the same time.

 Keep in mind there's also the issue of the tempco of the parts. The band-gap has a very low tempco (the exact value will depend on the order and quality of the band-gap used in the regulators), while the zeners have a very significant tempco (and of course they aren't at the same temperature which is an issue as well).

 If you really want to stick to a zener diode I think some modifications should be made, I would suggest replacing it with a higher voltage zener (~5-6V zeners usually have the lowest tempco due to the combination of two different breakdown mechanisms at work at these voltages), this will also allow using a somewhat lower current in the zener. To handle the issue of increased voltage compared to the reference of the regulators (which is a big issue as I've posted earlier) it is possible to put a resistive voltage divider in parallel to the zener to drop its voltage from ~5-6V to ~2.5V (1K-1.5K-1K will work for ~6V). This voltage divider could have a potentiometer in it to allow adjustment for variations in the zener voltage and the reference voltage of the regulators (IMO this is actually a good thing to have if using a zener):
  

  The trimmer allows the user to set the voltage difference between the two regulators (a capacitor could be added in parallel to the trimmer for improvef noise performance - adding a cap in parallel to the zener is a waste of capacitance due to the low incremental resistance of the zener that requires a huge cap), and as long as the tempco of the parts used is low enough it'll work with minimal change in the DC current in the resistors.
   
  That's just my 2 cents


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## Sonic Wonder

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## KT88

Are you sure the diodes are in the correct polarity? Looks like they are connected backward.
   
  Also, it will probably be best not to use the pot in series with the diodes. This means the voltage divider is (3*rd+Rpot)/(3*rd+Rpot+20K). If the pot is at ~1K this means there's about 5% transfer from the supply voltage to the voltage between the two adjust pins. This will make the circuit behave very differently with changes in the power supply. Putting the voltage divider in parallel to the low resistance part (diodes or zeners) as in the picture in my last post will eliminate this issue.


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## Sonic Wonder

Hello KT88,
   
  <>  What we're trying to do is create a simple, inexpensive "self adjusting" virtual ground which has very low quiescent current - for use with batteries.
   
  <>  Self adjusting - so that a wide range of battery voltages could be used with it.
   
  <>  The virtual ground can be perhaps 0.1V or so above or below the exact "1/2 rail-to-rail voltage".
   
  <>  It seems using the readily available, inexpensive LM317/337 complimentary regulators is one good place to start from - high precision is not a goal on this...


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## KimLaroux

I have thought about using normal diodes, but figured a Zener would be more precise. Turns out I was wrong about the Zener, in the end.
   
  Goldpoint, your last design would not work with a varying input voltages. The pot creates an asymmetrical divider.
   
  I've been thinking about the last idea by KT88. It makes sense, but it's not really a solution. Datasheets indicate the same 20 mA current requirement up to 12 volt zeners. Also, using a higher zener voltage means you'll limit the voltage range at which the circuit will work. Using a 25 volts zener is simply out of the question, as you'd need 30 V min at the input for it to work properly. 
   
  Or did you mean to use an arbitrary voltage zener in a low current voltage divider (<20mA), which will give you a voltage lower than it's rated one, and then adjust that down to the voltage we need using another diver across the zener?
   
  It's a step forward, but it's still dependent on input voltage. The next step would be to add a constant current source inside the voltage divider to make sure the zener will always have the same voltage across it, at any given input voltages. I just can't see how you could add a CSS in the divider while keeping it symmetrical.


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## KT88

Hello goldpoint,
  I'm aware what you are saying, this is why I've stated that if we are going to use simpler solutions, a trimmer must be added. It's not a luxury, its a must. I merely suggested using tighter tolerance parts so the user won't have to adjust it manually, and since more modern regulators don't require a minimum load of up to 12mA, which is a lot of power wasted. IMO if we don't mind the manual adjustment, the minimum load current required by these regulators is a big issue when using batteries - it can cut battery life in half for some amplifier. For battery use, placing the trimmer in series isn't a good idea because of what I've explained in my last post. For example, if we'd use two 7-cell batteries (for a nominal voltage of 16.8V), and each cell drops by just 100mV we'll have 0.05*0.1*14=70mV of extra voltage across the 1ohm resistors, and that's 35mA of current. Usually the voltage of the batteries will change significantly more than that.
   
  KimLaroux, 20mA is a very significant current for a zener. If you'll have a look at the datasheet figures of I/V curves (I'm using the 1N4734A datasheet at the moment just as an example), you'll see a 5.6V zener can work great with just 5mA across it or maybe even less. Its incremental resistance will be a bit higher, but that's probably still about 10-20ohm or so. Its ~50X less than the resistive voltage divider goldpoint posted, so the transfer from the supply voltage to this point will be ~0.001 and even if the batteries drop significantly the change in voltage across it will be very low. This is actually low enough to elevate the need for a constant current biasing scheme in most cases (and probably in our case as well)
  
  BTW, using regular diodes like goldpoint did can work (just put them in the opposite polarity). They usually require a much lower current. We must remember there's a lower limit on the current as well, since the LM317/337 have a small current in the adjust pin, so we must make sure its smaller than the current in the resistors in series with the diodes, or the diodes won't operate with sufficient current.


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## Sonic Wonder

.
   
 ​


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## tangent

That voltage imbalance_ might_ be due to not providing enough current to the regulators' adjustment pins. They really want 5-10 mA, and that won't happen with those 10k resistors in the way.
   
  Is there a good reason to avoid adding a second 22 uF cap for the positive side of the circuit?


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## KT88

How can you change the voltage if the diodes will have a different voltage drop in real life compared to the simulations? Or perhaps the reference of the regulators will vary (it can vary by up to +-50mV). A trimmer is really a must in this case. Even of if the diode could be trusted completely (and we shouldn't do that), just the variance in the regulators internal reference is sufficient to make this circuit waste an extra 10's of mA's of current (the exact value will depend the circuit parameters). And if the reference of the regulators will go in the other directions (towards 1.2 instead of 1.3) the voltage might be too low to conduct appreciable (and required by minimum load of the regulators) current through the diodes.
   
  Other than that keep in mind the diodes have an incremental resistance of Vthermal/Idiode, so if we have about 10mA of DC current in the they will be about 3ohms each, so two diodes + 1ohm resistor will be equal to a 7ohm resistor at the output in terms of output impedance. To go to even lower current the resistance will rise (at 3mA it will be about 1+2X9=~20ohm). So in terms of output impedance the diodes should really be kept at the input.
  
  tangent, are you sure about the 5-10mA? The datasheet only states 100uA of maximum current for both regulators.


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## Sonic Wonder

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## tangent

Quote: 





kt88 said:


> tangent, are you sure about the 5-10mA? The datasheet only states 100uA of maximum current for both regulators.


 
   
  I'm really talking about the minimum load current. In the standard use of an LM317, you get this with ~120-240 ohms between IN and ADJ. In actual use, this circuit might level out on its own, because there is always current being sunk to ground. But in the simulation, the circuit is idle, thus my guess at the reason for the imbalance.
   
  So another way to go at it is to put some dummy resistors across the 470 uF caps, just for purposes of simulation. If it levels out, that's probably why.


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## KT88

Yes, the minimum load current can be as high as 12mA to maintain regulation. In the simulation he does have 6mV across the 1ohm resistors so there's 6mA across each of these resistors (going from the LM317 to the LM337).


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## Sonic Wonder

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## wakibaki

As shown simulated including worst case LT1097 input offset -
   
  1.4mA total current
   
  6 nanovolts output error
   
  0R1 ballasts
   
  With cheap LM2904 and 1R buffers, 50mV error, 3.4mA
   
  Current flows *out* of an LM317 adj, btw.
   
   





   
  w


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## Sonic Wonder

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## tangent

I was just hoping the offset was essentially a simulation error, the sort where you've lied to the simulator by not giving it all the real-world conditions, so it lies back at you in revenge. I don't think the 0.13 V of error is worth fixing, if it costs a trim pot. Those can be more expensive than the ICs!
   
  I don't see why Iq went down when the divider resistance went down. You had 6.4 mA Iq with 20k across the rails last night, and now you've got 5.2 mA Iq with ~4k across the rails. I see other circuit changes, but I don't see why they'd change Iq.


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## KT88

The trimpot is a must in such a circuit so the second option would be best. However, this circuit still fails to address one very important issue, and that's thermal stability. A silicone diode has about -2mV/C tempco, so a 10C rise in the temperature (and this can easily be the difference between running the amp at noon or at the evening) will lower the drop across the diodes by 4*2*10=80mV. Than you have an extra 40mA of DC current at the output. If the temperature drops down instead of going up, the circuit will simply stop working at all.
   
  I still think that if diodes are what we want to use a better approach would be using a 5-6V zener diode which has a very low tempco (this is the sweet spot of zener voltage for low tempco), and adding the trimmer in parallel to allow initial adjustment. I've posted it in the last page:

  It uses an extra 2 resistors, but it only has a single zener instead of 4 diodes - so in terms of parts count its about the same. The tempco of the regulators is much much lower than that of the diodes and therefore isn't the limiting factor in this case. The zener does require a few extra mA's, but its worth it since it can save 10 times as much at the output under different operating conditions.
   
  tangent, regarding the current. Now the resistors (1ohm) only have 2mV across them so that's 2mA, in the previous simulation they had 6mV so 6mA. That's the cause for the difference between the simulations.


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## Sonic Wonder

Don't you like the simplicity of this circuit, it's low cost and the common jellybean parts though? I am going to hook it up to my headphone amp this afternoon for a real-world listening test. Hmmm - maybe I _should_ run more current through the voltage divider...
   
​


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## KimLaroux

Quote: 





sonic wonder said:


> Don't you like the simplicity of this circuit, it's low cost and the common jellybean parts though? I am going to hook it up to my headphone amp this afternoon for a real-world listening test. Hmmm - maybe I _should_ run more current through the voltage divider...
> 
> ​


 
   
  How's this even working? It's not supposed to work. Are you simulating these or testing them using real components?


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## Sonic Wonder

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## KimLaroux

Because those 4 diodes are supposed to have 4 volts across them, not 2.69.
   
  So you've been using a simulator for those? I seem to remember you telling me you did not.. but whatever. The thing is, I don't trust a simulator for this application. We're using components in ways they were not meant to be used.


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## DingoSmuggler

Quote: 





kimlaroux said:


> Because those 4 diodes are supposed to have 4 volts across them, not 2.69.


 
  Check the data sheet at the same current he is using here, just under 4mA.
   
   



sonic wonder said:


> Hmmm - maybe I _should_ run more current through the voltage divider...


 

  The adjustment pins should only use up to 0.1mA each, and you already have 4mA - What advantage does more current give you?
  I though low power was one of the design goals, but i do realise a lot of ideas are being thrown around.
   
  I think testing it will give you some idea, but testing will probably just deliver the best result when at a given current/temp/etc the diode voltages match up best with the internal v-ref of the regs. But then in variable load conditions, and variable ambient temps, how good is this at maintaining good performance, and efficiency?
   
  I've found this thread very interesting, but haven't had the time yet to sot down and go through any of the circuits in any level of depth. (not that i go all that deep lol)


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## Sonic Wonder

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## Sonic Wonder

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## KimLaroux

Just looked at the datasheet. Turns out even diode's forward voltage is dependent on current. So it's still not a solution.
   
  I think you're just chasing fairies here. When you consider everything, there's no way it's gonna work in real world. You'll need an active control system. At which point there's no reason to use regulators anymore.
   
  Wikibaki's design is just so much better, and not much more complicated. What's wrong with it?


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## Sonic Wonder

Well the current is not going to change enough to really make much of a difference in the forward voltage drop, likely. KT88's point about temperature affecting current is true too. Um, the LM317/LM337 rail splitter virtual ground looks quite usable for what I'm doing with it. It's not super precise, sort of a brute force approach, but also interesting, to say the least. However, in my case, the proof is in the actual testing.
   
  The virtual ground introduced at the beginning of this thread is a great fallback. As I claimed, it really sounds better with my headphone amp circuit than anything else I tried (other than running it directly from batteries, actually). The big deal is that main devices are husky voltage regulators. The ground point is held in place, but solidly! "I suspect that the reason for it's unusually good sound quality is this virtual ground is not made by "driving the ground point", but rather by "aggressively holding it at one potential" via the two complimentary voltage regulators."
   
  Try it!


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## KT88

goldpoint, regarding what you wrote before editing the post, I can't build it because I don't have a zener with that nominal voltage, and placing an international order from mouser/digikey for a zener diode will cost too much 
   
  However, there's really not much to it. Its a simple circuit, components variations are taken care of by the trimmer, temperature variations are of less importance due to the low tempco, and all that's left it variation in the current (due to variations in battery voltage). This is indeed the only real problem at the moment and could cause the voltage across the zener (or the 4 diodes in your circuit) to change by 10's of mV's. The simplest solution to this is to use a reference IC instead of the zener in the same circuit. It uses orders of magnitude less current for proper voltage generation, and being an active device with self biasing its much less sensitive to variations in the supply voltage.
   
  If you are completely against using a reference IC we can try and think of a way to regulate the current in the zener, but using a JFET (CRD) isn't a good option because it has a minimum voltage drop across it to maintain regulation which will add to the zener voltage, and it will not give a good regulation in terms of the outputs being close to the supply/2. That's because it regulates the current and that current will still go through a resistor to the second supply line and it'll only be a constant offset above (or below, depends on how you connect it) that supply. I guess we could think of some more sophisticated way to regulate the current through the zener, but it'll probably hurt the simplicity of the circuit quite significantly so the question is if we even want to do this.


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## vixr

This thread is amazing...thank you all for sharing this.


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## DingoSmuggler

Had a bit of a think about this circuit tonight and have come up with a different approach, still using LM317 & LM337, but using CSSes on the outputs to bias it.
  Advantages? Tolerant of variable supply voltages due to CCS design.
  Should be pretty good thermal wise, as the performance doesn't depend on the temp-co of the transistors relative to the v-reg, only each to its own complement which should stay pretty close. The CCS current will change with temp, but i don't think that will matter too much for the operation of the circuit.
  Resistor values are just pulled from a hat, certainly not optimised, but should give a good idea of the concept.
  It seems pretty nifty, and works in a sim but not totally convinced it will go as well in the real world. Maybe with a few tweaks and add in some trimming capability.
   
https://www.circuitlab.com/circuit/tr4a5y/virtual-ground/


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## Sonic Wonder

Hello KT88,
  
 Good idea - the voltage reference. Um, I am not "against" that at all. Draw your preferred schematic for that and post it, please? However, it appears that this circuit, with or without a trimmer pot, works well for my application. The *voltage regulators* hold the ground point at one potential better, it seems, better than other circuits which "drive" the ground point with discreet transistors with or without opamps - my guess as to why the original posted circuit sounds so good too...
  

  ​


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## Sonic Wonder

We certainly have been having fun here!


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## KimLaroux

Quote: 





dingosmuggler said:


> Had a bit of a think about this circuit tonight and have come up with a different approach, still using LM317 & LM337, but using CSSes on the outputs to bias it.
> Advantages? Tolerant of variable supply voltages due to CCS design.
> Should be pretty good thermal wise, as the performance doesn't depend on the temp-co of the transistors relative to the v-reg, only each to its own complement which should stay pretty close. The CCS current will change with temp, but i don't think that will matter too much for the operation of the circuit.
> Resistor values are just pulled from a hat, certainly not optimised, but should give a good idea of the concept.
> ...


 
   
  Interesting idea. Care to explain the theory behind it? I don't see what's keeping the output voltages at half the supply voltage.
   
  Yeah I'm a newd, I know. Trying to learn.


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## wakibaki

Quote: 





dingosmuggler said:


> Had a bit of a think about this circuit tonight and have come up with a different approach, still using LM317 & LM337, but using CSSes on the outputs to bias it.
> Advantages? Tolerant of variable supply voltages due to CCS design.
> Should be pretty good thermal wise, as the performance doesn't depend on the temp-co of the transistors relative to the v-reg, only each to its own complement which should stay pretty close. The CCS current will change with temp, but i don't think that will matter too much for the operation of the circuit.
> Resistor values are just pulled from a hat, certainly not optimised, but should give a good idea of the concept.
> ...


 
   
  Quote: 





kimlaroux said:


> Interesting idea. Care to explain the theory behind it? I don't see what's keeping the output voltages at half the supply voltage.
> 
> Yeah I'm a newd, I know. Trying to learn.


 
   
  I think the idea is that if you connect the output to ground, then it will be at 0V. 
   





   
  w


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## DingoSmuggler

Quote: 





kimlaroux said:


> Interesting idea. Care to explain the theory behind it?


 
  I tried all sort of things last night, and ended up with this circuit, but it's not ready yet, but the basic idea is there. It was past 1am in the morning when i posted, I just wanted to get it up there 
   
  Each Vreg has a 1.25V reference between ADJ & OUT. R1/Q1, and R2/Q2 use this Vref to set up CCS. The voltage across the resistor R1 will be the Vref - BE diode drop, so about 0.6V. Solving ohms law will give the CCS current. So with the values provided its roughly 1mA, but might work better with a higher current, not sure yet.
   
  Now the idea is, both current sources are joined through the 1-ohm resistors (probably could go smaller there), so with 1mA flowing through 2-ohms, the 2 outputs are drawn together so that they are 2mV apart. BUT... 
	

	
	
		
		

		
		
	


	




 The LM317 will be able to source the current for the CCS attached to it, and likewise the LM337 will be able to sink the current for its CCS. I'm hoping this doesn't upset the whole design. But really we should just need some current to flow through the resistors, not all the current. Maybe we just need to reduce the 1 ohm resistors to 0.1ohm so that the impedance is similar to the output impedances of the Vregs.
   
   


> I don't see what's keeping the output voltages at half the supply voltage.


 
  Indeed!
  I realised there was a bit of an issue here, but haven't decided how best to tackle it, but I think there should be a simple solution.
  I think it works in the sim though, as each transistor is turning on enough to push the output into the middle. I don't think real world transistors would be as kind.
  Certainly more work needs to be done.
   
   


> Yeah I'm a newd, I know. Trying to learn.


 
  As am I, a newb, trying to learn.
  I find the best way to learn is by doing, and this thread just happened to capture my attention as such.
  Now whether this circuit has potential to succeed, or is going to fail, I'm not sure yet, but certainly I will learn a few things in the process.


----------



## wakibaki

Quote: 





dingosmuggler said:


> I think it works in the sim though, as each transistor is turning on enough to push the output into the middle.


 
   
  No it's not. It's fubar.
   
  Split the voltage source into 2, connect the midpoint to ground (like in my sim) and try again. If you connect the output to ground like you did, the simulator just forces it to 0V and you get a nonsense result...
   

   
  w


----------



## DingoSmuggler

Quote: 





wakibaki said:


> No it's not. It's fubar


 





   
   


> Split the voltage source into 2, connect the midpoint to ground (like in my sim) and try again. If you connect the output to ground like you did, the simulator just forces it to 0V and you get a nonsense result...


 
  That may explain a few anomalies i got with the sim lol.
  Back to the drawing board.




   
   
  Thanks w. I've been learning so much from your posts recently - Wakipedia.


----------



## wakibaki

OK, D-S
   
  Glad you didn't take my somewhat blunt response amiss. I don't mean any offence, and I certainly don't mean to discourage you. Stick at it.
   
  w


----------



## KT88

Quote: 





sonic wonder said:


> Hello KT88,
> 
> Good idea - the voltage reference. Um, I am not "against" that at all. Draw your preferred schematic for that and post it, please? However, it appears that this circuit, with or without a trimmer pot, works well for my application. The *voltage regulators* hold the ground point at one potential better, it seems, better than other circuits which "drive" the ground point with discreet transistors with or without opamps - my guess as to why the original posted circuit sounds so good too...
> 
> ...


 
  Hello goldpoint,
   
  sorry for the very late response, didn't really have time to open my laptop until now.
  The idea is exactly what I've posted earlier with the zener, but instead it can be replaced with a reference IC (the exact model still has to be determined, there are many options, both 2 and 3 terminals).
   
  So the LM317/337 and everything after that is the same as in your schematic, but the voltage divider with the diodes is replaced with this structure:

  and the zener is actually a voltage reference IC. We must use a reference voltage of about 2.6V so it'll allow trimming for worst possible variations of the LM317/337. As I've stated before, the use of the resistive divider in parallel with the reference also allows the use of a relatively small capacitor for filtering in parallel with the trimmer (since the incremental resistance of the zener/diodes/reference is very small and would require large caps in parallel with it). This will help lower the noise significantly.
   
  Edit: when I think about it a little more, the circuit can actually be simplified further. Since we can live with a small error between the 2 supply's, we can use a reference of 3V and than use just the trimmer in parallel without any resistors in series with it:

  Now there are 2 resistors less which makes the circuit even nicer, and it still allows trimming for every possible value of the internal reference of the regulators. There could be an error between the supplies, which we should be able to live with (up to ~500mV between the 2 supplies). There is one other drawback with this circuit compared to the one with the 2 extra resistors, and that's the resolution of the adjustment the trimmer allows which is now not as good since any change in the trimmer directly translates to a similar part of the reference voltage. With a 10-turn trimmer this shouldn't be a problem.
   
  There's also the option to use an IC that lets you choose between 2.5V or 3V (the AD780 is the first option I though of, but its very expensive so we won't use it, I guess there are others as well), so as long as the reference of the regulators combined is 2.5V or below (50% of times or maybe even more) you set the reference IC to 2.5V and only get a few mV's of imbalance, and if the regulators references combined are above 2.5V you set the reference IC to 3V and than you have to pay a little extra in the imbalance. The problem using the 2.5V state is the it'll be very close to the reference of the regulators, so the trimmer will be set very close to the limit. This means there's very little resistance between the reference IC (which has low incremental resistance) and the output of the trimmer, and therefore if we'd like to add a filtering cap in parallel to the output of the trimmer we'll have to use a large cap.


----------



## Sonic Wonder

KT88,
   
  Thank you again. Do you like the TL431 series from ST Microelectronics?  see:  http://www.st.com/web/catalog/sense_power/FM1963/SC1771/SS1408/PF65361?s_searchtype=partnumber
  Could I burden you to draw the exact schematic you are describing? (Or at least the voltage divider section. My power supplies are either 15V or 18V.) Include the recommended resistor and capacitor values...


----------



## KT88

I want to start off by apologizing for the very long post 
   
  I'm not sure if the TL431 is the best options, its tempco isn't as low as some of the other IC's available in the market. However, the schematic will be similar with all of these. Here's a something basic:

   
  R=(Vsupply_min-2.7)/0.003 - this will allow at least 1.5mA (a little less because of the resistors in parallel) of current through the reference at all times, which ensures it doesn't stop working correctly.
  The reference is of a different model, but I've only used it for its symbol, the TL431 could be used instead.
  The 1K-10K will ensure the output voltage is >2.684+2u*1K=2.686V which is larger than the worst case reference of the regulators. The trimmer should be of a large value (5K or more) so it doesn't eat too much of the current in this branch of the circuit.
  For an fc of 20Hz the cap should be about 22uF - but its optional.
  With worst case variations in temperature (assuming we work with a temperature range of about 10-50C in the case of the amplifier) the reference will change by about +-10mV. There are other factors such as the tempco of Iref, the tempco if Iadj (of the regulators), and the tempco of the internal reference of the regulators (which only shows nominal curves in the datasheet). The variations due to current tempco are quite small in this case, so the references are the main issue here. assuming the reference of the regulators has an opposite tempco (which is possible due to variations) we can get as much as 10mA of extra current in the output 1R resistors. The even bigger problem is what will happen if the tempco's are opposite so that the reference IC goes higher and the regulators go lower, which will make the circuit stop working unless sufficient DC current was set with the trimmer.
   
  However, this is indeed a worst case scenario. For 99.9% of times the zero tempco temperature of the reference will actually be in the vicinity of 25C so we should only see a change of 1-2mA at the output with a change of temperature of +-20C. I still think it would be better to replace the regulators for something with a lower minimum load current just to save power, but since that is very easy to do when building the circuit, its of less importance at the moment.
   
  BTW, it is possible to change the circuit so it'll have less resistors and will look like this:

   
  I would advice against this option for our use as it can't generate a reference voltage lower than the IC's internal reference (which could be as high as 2.55V, depending on the exact model of the IC).
   
  Its also possible to implement the voltage divider using a fixed resistor and a trimmer in series. This option is also not as good as it both requires more parts, and uses a divider with parts of different tempco (a constant resistors and the trimmer).
   
  I have another question about this circuit. What happens if the amplifier it drives has a different DC current from each of the supplies? At DC the output caps do nothing, so if there's a constant DC current to/from the virtual GND the circuit might not work. Let assume you have a difference of 10mV's between the 317 and the 337 so a current of 5mA in the 1ohm resistors. Now we add some current (lets say 10mA) from the amplifier to the virtual GND (because there's a DC offset at the amplifiers output, or because someone used a LED returned to the virtual GND, or whatever). Now these 10mA go to the 337 through a 1ohm resistor, dropping 10mV across it. So now the 1ohm resistor at the output of the 317 has 0V across is, so no current is coming out of the 317, and its not operating as it should. This is a big issue with these output resistors since they make the output resistance of this structure very high at low frequencies.
   
*A different direction:*
  The way we are trying to do this stabilization is actually not the smartest. If you'll think about it, we are trying to regulate the current using an open loop. By open loop I mean we have no feedback of what the actual current flowing through the resistors is. The best option to do this would be by setting the voltage of one regulator at ~Vin/2 using a simpler method such as resistive divider, and than regulating the adjustment pin of the second regulator to to keep a constant DC current at the output. This must only work on frequencies below the audio range.
   
  If we try and solve both these issues (DC current causing the circuit to stop working, and keeping the current from rising significantly with changes in temperature) we should make some changes. This is the first idea I have of how to do this (its more complex than the circuit we were discussing, and I'm sure there are issues with it, but its an idea ):

   
  Keep in mind I didn't test/simulate this, this is just an idea. There's always the chance I've made some stupid mistake and the entire circuit won't work. Also don't pay attention to the part numbers, I just took some parts with the right symbols and didn't pay attention to the exact part number.
  The idea of the circuit is quite simple actually:
  R1-D1-R5 is a simple divider to create Vsupply/2-1.25V. This is used as the Adj voltage of the 337 to make the output Vin/2. D1 is a 2.5V zener (or 4 diodes), its exact voltage isn't very important, it'll just shift the virtual-GND around the mid-point a bit, it won't affect the current at the output.
  Then the 317 is connected with its output directly to this point - but now there's a closed loop used to keep its DC current constant.
  R2 is used as a current-shunt to create a voltage proportional to the current of the 317 regulator
  R3-C1 is used as a LPF to make sure we only regulate the DC current (very slow changes like that of temperature), and don't affect AC dynamic of the circuit.
  Q1-trimmer are used to generate a DC voltage referenced to the positive supply. The jfet is used as a CCS, the trimmer allows setting the DC voltage to the input of the op-amp.
  The op-amp should be able to work with its inputs very close to the positive supply (perhaps something similar to the tl072 which is very cheap).
  C2-R4 are used for compensation. The low frequency path is through IC4-R2-LPF, and so for stability we must allow a different feedback path at higher frequencies which is accomplished using C2. R4 is there to add a significant resistance between C2 and the supply in case the trimmer is of low resistance, or the amplifier will see a very low impedance load at high frequencies. It has to go the the inverting input while the low frequency feedback goes to the non-inverting because of the phase inversion from the regulator+R2 path (higher voltage at the output of the op-amp generates lower voltage at input of the op-amp).
  The resistors R6-R7 can be much smaller now (in theory even omitted altogether), as we directly regulate the DC current of the 317, and the 337 will simply sink any current from the 317+the load.
  To solve the issue of DC current from the amplifier all that has to be done is set the trimmer value accordingly. If the current is coming from the amplifier into the circuit, than just set a few mA in the 317, and the 337 will sink the current of the 317+the current coming from the amplifier. If current is flowing from the circuit to the amplifier the trimmer must be set so that the 317 has a few mA more than the current the amplifier draws, this will ensure the 337 is on and sinking current (or to sum it up, set the trimmer so both regulators have at least a couple of mA flowing in them).
   
  The feedback factor B of the closed loop must be considered for stability. Assuming the regulator has a constant voltage between the adjust and the out pins (gain of ~1) we get:
  B=1*R2/[(R6+R7)*(1+sC1R3)] at lower frequencies. This means we might indeed have an issue with stability unless we use a very low frequency at the LPF (the lower R6+R7, the lower the BW must be). C2 helps us with stability since B=~1 at higher frequencies, maintaining sufficient phase margin.
  The current value is set by the trimmer, and it'll be (Vtrimmer/R2=Ijfet*Rtrimmer/R2) where Vtrimmer is the voltage between the two pins of the trimmer. The exact value of the jfet current isn't critical since we can set the trimmer to compensate for that.
   
  I want to repeat it once more, this is just an idea I've had, and I've only made a very quick stability analysis which at least to me seems to make sense. If anyone could go over it and verify it'll be great, a fresh point of view is always welcome. If anyone would like to prototype it/simulate it, I'll be more than happy to help with calculation of the components values.
   
  I apologize once more for the long post 
   
  Edit:
  there's a mistake in the schematic, the Adj of the 337 should connect above the zener and not below it.


----------



## wakibaki

I see you are a member of the trade goldpoint. Why haven't you registered the fact with Head-Fi moderators?
   
http://www.goldpt.com/index.html
   
*No Members of the Trade can use his/her business name, product name, brand name, postal address, e-mail address, telephone number or URL as part of his/her Head-Fi username.* All Members of the Trade must contact the forum administrator at jude@head-fi.org to notify him of an interest in posting before making any posts. If a forum member who was not previously a Member of the Trade becomes a Member of the Trade, he must notify the forum administrator of the change in status. After notification of the "Member of the Trade" status, the forum administrator will then add the appropriate tag (Audio Dealer, Manufacturer, Distributor, etc.) to the appropriate profile(s).
   
  w


----------



## Sonic Wonder

Long post is OK - lots to say


----------



## Sonic Wonder

Oops! At least I can see who blew the whistle! Obviously, we should read the Service Agreements BEFORE we post! Thank you for your continual help WakiBaki. I guess that you will be seeing me as a different name soon. I am hoping all your simulations work out well. Sincerely, the guy.


----------



## KimLaroux

Goldpoint KT88, there's a major flaw in your latest design : R2 is in the path of V+.
   
  So that any current flowing out through V+ and being sunk back trough the LM337 is also affecting the voltage across R2.
   
  I doubt that's an efficient way of using an op-amp anyways. And honestly, the moment you introduce an op-amp in the circuit is the moment the regulators become redundant. There is no reason not to use a simple circuit like wakibaki posted on page 4. And it's just so, _so, *so*_ much prettier. And guess what? His design has a significantly lower part counts. Does this not make his design easier and cheaper to build?
   
  edit: the post was so long I forgot who wrote it.


----------



## Sonic Wonder

Hi Kim,
   
  Um, that's not MY design, it's KT88's design you know.  But I like MY design because the voltage regulators HOLD the ground point in place, (either the iteration with two adjustable regulators or the original posting iteration with two fixed regulators and a pre-regulator). Even though the zener approach did not work out, I hope to implement KT88's possible voltage reference approach.


----------



## wakibaki

Quote: 





kt88 said:


> To solve the issue of DC current from the amplifier all that has to be done is set the trimmer value accordingly


 
   
  Ignoring for the moment the fact that your circuit doesn't work, KT88, anything with a trimmer is just nonsense. Something that needs trimmed to accomodate the amplifier is double nonsense.
   
  We're looking for self adjusting.
   



sonic wonder said:


> Hello KT88,
> 
> <>  What we're trying to do is create a simple, inexpensive "self adjusting" virtual ground which has very low quiescent current - for use with batteries.
> 
> ...


 

   


> Originally Posted by *goldpoint* /img/forum/go_quote.gif
> 
> I like MY design because the voltage regulators HOLD the ground point in place


 

   
  Yeah, that's what you *say*, but anybody who reads the whole of the thread will know that _if *you're* building an amplifier_, the circuit *you'll* use will be *mine*, because it will _*SOUND BETTER, COST LESS*_ and have _*BETTER BATTERY LIFE*_.
   
  w


----------



## KT88

Quote: 





kimlaroux said:


> Goldpoint KT88, there's a major flaw in your latest design : R2 is in the path of V+.


 

 Of course R2 is in the way, its used as a current shunt 
  It can be in the order of a few ohms which is low enough to have no effect for something like a headphone amplifier.
   
   
  Quote: 





kimlaroux said:


> I doubt that's an efficient way of using an op-amp anyways. And honestly, the moment you introduce an op-amp in the circuit is the moment the regulators become redundant. There is no reason not to use a simple circuit like wakibaki posted on page 4. And it's just so, _so, *so*_ much prettier. And guess what? His design has a significantly lower part counts. Does this not make his design easier and cheaper to build?


 
  A. That's what I've said earlier in this thread, there are other simpler ways that do it well. The entire thread as far as I see it is mainly having some fun, to see what other not-standard ways to do this we can come up with.
  B. Yes, the 317 here is just used as a follower, a transistor will do the exact same job in this case.
  C. In the same circuit you've linked to, I think it will be even better to have a single op-amp instead of two, and have the feedback coming from the output instead of the emitters (obviously it would be best to add a couple of diodes between the bases of the transistors to bias it in a class-AB form). This will include the resistors within the loop and will drop the output resistance even lower. This will be similar to using an op-amp with large current sourcing/sinking capability as a follower or a simple buffer IC, which is basically the easiest way to accomplish that.
  D. The circuit wakibaki posted, as it is right now, also has an issue with a constant DC current at the output. Lets say we have 10mA from the vGND to the negative supply. This means that the vGND voltage is 0-10m*R3=-1mV. So now the voltage across R4 is -1mV and current should flow from Q2 to R4. But it can't be done since its a PNP and it can only sink current, so now U2 will see 1mV across its inputs and will saturate the output of the op-amp to the positive supply. This will lead to a turn-on delay when Q2 will have to sink AC current from the load since the op-amp will have to slew half of the entire supply voltage to turn Q2 on. In fact, there's a much more serious problem. If the supply voltage is high enough (>10V), the VBE voltage will be higher than the maximum allowable voltage of the transistor and it'll fry, so this circuit has a very serious problem. It might even happen without any load at the output, just replace the polarities of the offset voltages of the 2 op-amps (so the voltage at Q2 will try and be a little higher than at Q1 - which can't be done because of the transistor polarities), and the offset might be high enough to fry both transistors. So the circuit as it is actually doesn't work at all.
   
  Quote: 





wakibaki said:


> Ignoring for the moment the fact that your circuit doesn't work, KT88, anything with a trimmer is just nonsense. Something that needs trimmed to accomodate the amplifier is double nonsense.
> 
> We're looking for self adjusting.
> 
> ...


 
   
  Lets not ignore the fact it doesn't work. While I don't really mind since I'm not going to build it, I would like to see what I've missed that will make it not work.
   
  BTW, I've made a small mistake in the schematic, obviously the regulator should actually connect with its Adj above the zener and not below it or the voltage will not be ~Vin/2. The lower regulator is the 337, and the one in the feedback loop is the 317 in case it wasn't clear.


----------



## KT88

Here's a different and simplified approach at the same thing:

   

 The zener is a 2.5V as before, and it is used to set the adjust pin on the 337 (only this time I didn't make the same mistake, and the connection is above the zener so Vout=Vin/2).
  R9 has a DC voltage of 1.25V across it which is used to set the current the 317 at DC (for 10mA it should be ~120ohms).
  C3 is there for bypass at higher frequencies - 220uF should give a cut-off frequency of about 5Hz in this case (with a 120R resistor for R9). A low -ESR cap should be used here.
   
  The thermal stability issue of the DC current is solved since the 317 has feedback of this current using R9. This circuit doesn't solve the issue of having a constant DC current flowing out of the vGND into the load unless this current is lower than the current in R9 (it does handle the opposite case where the current is coming from the load into the circuit), so that issue still has to be solved somehow.


----------



## Sonic Wonder

Wow, fully untrue Wakibaki. I prefer my own tried and tested circuit - for reasons already stated repeatedly.
   
  This whole virtual ground thread I started sure took off like wildfire, eh? Don't you also "just hate" my original post which started the thread? To each his own, child.


----------



## Sonic Wonder

RESULTS:
  
 I got lazy and simplified the circuit again (practical considerations).
 The Voltage reference is an* LM336Z*. (KT88's suggestion)
  
 You can see results here similar to those obtained earlier using 4ea 1N4148 diodes in series to create a 2.69V voltage drop. The main difference is that the voltage divider is programmed for just about 1mA of current with the 8K resistors.
  
 a)  With no adjustment trimpot:
  
 ​  
  
 b)  With a trimpot to center the virtual ground closely:
  
 ​  
 ​ Wondering if we would get better results with more current in the voltage divider, I upped it to about 4mA of current with 2.21K resistors. All that did was to use up more current, increasing the voltage drop across the output resistors slightly.
  
 I suspect that the different voltages measured at the adjust pins of the regulators are coming from tolerance differences amongst the positive & negative regulators themselves, so using different ones should yield different voltage measurements at the adjust pins.
  
(a) above is the quick and dirty solution. 
  
(b) above is if you need/want a closely centered virtual ground point.  [Reduce the value of the associated resistor connected to the trimpot so that it, added to the midpoint value of the trimpot, is equal to the resistance of the fixed resistor at the other end of the voltage divider stack, of course.]
  
 Either way, this circuit should make a good battery powered virtual ground, drawing only a few milliamps (2 or 3mA) of quiescent current - while also able to handle 50mA (without heatsinks) - or up to 1.5A per rail with heatsinking the two TO-220 devices.
  
_This note is being added a few weeks after posting the above: Further experiments with the two "self-adjusting" virtual ground circuits above show various instabilities. Therefore, use instead the circuits shown in the first article of this thread (on page one of this thread). If anyone works out a good way to use three terminal  fixed or adjustable voltage regulators to make a "self-adjusting" "rail-splitter virtual ground, I would like to know about it. Thanks!_
  
_UPDATE to the comment above. The self-adjusting circuit now included in the first writing of this whole thread on page 1 works just fine! I love it! I had a wiring error in my prototype where I had connected the Adjust pin of the LM336 to its + pin. After removing THAT error, all is well. (So see first page - updated 15 April, 2013 - it's now showing the corrected circuit.) Also shown here:_
  
​


----------



## wakibaki

I'm happy to leave it to readers to judge.
   
  wakibaki versus goldpoint, same capacitance, ideal voltage source in goldpoint...
   
   

   
   
  w


----------



## KT88

You are basically measuring output impedance, the 1R resistors in goldpoint's circuit will make this value very high compared to the 0.1R in the other circuit.
   
  BTW, can you measure the VBE of the transistors in the same simulation?


----------



## wakibaki

Quote: 





kt88 said:


> You are basically measuring output impedance, the 1R resistors in goldpoint's circuit will make this value very high compared to the 0.1R in the other circuit.


 
  Exactly.
   
  w


----------



## KT88

Since you didn't simulate it, I ran a similar simulation of your circuit in ORCAD (I don't use LTspice). I've used a different BJT since I didn't have the model for the 2n3906, but its quite similar.
   
  Here's the output waveform:

  which is very similar to what you get, so the simulation is fine.
   
  Now here is the base-emitter voltage of the transistor (the npn in this case):

  It gets to -6V, which is too much for most BJT's.
  And this is at 1KHz with a cap at the output for help. If you change the amplitude of the sin-wave source, or use a smaller cap/lower frequency it gets even worse.
   
  Here's the same circuit exactly at 100Hz:

  It clips to the supply rail, no BJT can survive 20V of revere voltage on its Base-Emitter junction.
   
  So in effect, the circuit destroys itself.
  And again, the same thing will happen without any load if you just change the polarity of the offset voltage of the op-amps.


----------



## wakibaki

What are you saying, the regulator circuit doesn't have output transistors?
   
  w


----------



## KT88

I'm saying the circuit you've posted will destroy itself every single time.


----------



## wakibaki

And I'm saying you're categorically wrong when you say that -6V will destroy a 2N3904
   

   
   
  w


----------



## KT88

VEBO Emitter-Base Breakdown Voltage 6.0 V 
   
  So yes, it'll destroy it. And again, that's at 1KHz, at 100Hz it'll die with the full supply voltage across it.


----------



## wakibaki

But not now. Thanks...
   
   
   
   

   
   

   
  w


----------



## KT88

Exactly, its a very simple fix, but one the must be made before the circuit can really work.
   
  Now we can simply move these diodes to the bases, get rid of one of the op-amps, and wrap it all in the feedback of the single op-amp. This will be in effect an op-amp with a discrete class-AB buffer after that. It'll have much better performance than at the moment (the output resistance will much lower than it is now since the 0.1R resistors will be within the loop). Its the straight-forward way to do this. Some people (like tangent in the Pimeta) have used an IC buffer at the output of the op-amp instead of the discrete transistors.


----------



## KimLaroux

Quote: 





sonic wonder said:


> RESULTS:
> 
> I got lazy and simplified the circuit again (practical considerations).
> The Voltage reference is an* LM336Z*. (KT88's suggestion)
> ...


 
   
  That's interesting. I didn't know such devices even existed. They seem to have a wide range of applications. There's even an example in the datasheet where they use one instead of resistors to adjust an LM317. I'll have to remember those chips...
   
  But I think you forgot that the regulators need 3.5 mA to stay stable. They only have 1.2 mA flowing trough them in this test.


----------



## wakibaki

Thanks, KT88, but what took you so long? We could have been here about 60 posts back...
   

   

   
  w


----------



## Sonic Wonder

Sour grapes - porque so sour? How very childish some of these comments are! We apparently allow unsupervised, spoiled kids to post on this site, but something should be done about that...
   
  The load itself (not shown here) demands more than enough quiescent current for the regulators to be happy..
   
  Um, I like testing for sonic qualities - actually *listening* to circuits - not just saving a few pennies with minimal parts counts or whipping out a theoretical "best circuit". Sketches and "computer simulations" only don't cut it completely. Know what I mean Sparkie and Wobblie?


----------



## wakibaki

Quote: 





sonic wonder said:


> The load itself (not shown here) demands more than enough quiescent current for the regulators to be happy..


----------



## KT88

Quote: 





wakibaki said:


> Thanks, KT88, but what took you so long? We could have been here about 60 posts back...
> 
> 
> 
> ...


 

 This isn't exactly what I meant. I meant really build it like a class-AB buffer. You should add a resistor from D3 to the positive supply and another one from D4 to the negative supply. This will keep the diodes slightly conducting at all times. It can also be done with BJTs instead of diodes:

  But in this case its not really important since what's driving this point is an op-amp so no need for the extra current gain of the first transistors.
   
  You can now also get rid of D1-D2. The bases of the output transistors only have 2 (forward biased) diode drops between them, so the B-E voltages will be well within the allowed voltage range.
   
  Here's the output for this exact circuit with 2.2K for the resistors to the left, and 0.1R resistors at the output. The op-amp used for this simulation is the good old opa134, it can only work with a +-18V supply, so I've used that value and not a +-20V like in the previous sim. Everything is wrapped in feedback of the opamp. Capacitors are just 1uF (due to the very low output impedance there very little need in the capacitance, even lower value could be used) and load amplitude is exactly the same as before:

  The feedback drops the output resistance much lower than before.
  You can also try that with the LT1097, but it doesn't perform as good in this case (yet still much better than the previous circuit did under the same conditions).
  In this case since the feedback to the op-amp is coming from the output directly, using huge caps might hurt the stability. The output resistance of the circuit and the load capacitance add another pole in the loop. It must be kept high enough to maintain stability. This turns out not to be an issue since the very low output resistance is so low there's no real need for the capacitance anyway (and the results back-this up), so just use a small cap and you're fine. It's possible to have large capacitance at the output but than the feedback must be slightly modified to have an alternative path at high frequencies (using a small cap from op-amp out to inverting input + resistor between the inverting input and the vGND node), but it will actually only hurt performance in most cases and requires extra parts/larger caps.
   
  BTW, the reason I didn't post this circuit before is very simple, this is what everybody else do. tangent has an IC buffer in the pimeta, and a discrete (and better performing) buffer in the PPA. So this is simply a simpler implementation of the same thing that everybody else use. The reason I liked the way this thread started is that the circuit didn't use the straight forward way, it had a very strange look to it. Finding new and different ways to do things is always more fun than doing it the "right" way


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## wakibaki

Quote: 





sonic wonder said:


> Sour grapes - porque so sour? How very childish some of these comments are! We apparently allow unsupervised, spoiled kids to post on this site, but something should be done about that...
> 
> The load itself (not shown here) demands more than enough quiescent current for the regulators to be happy..
> 
> Um, I like testing for sonic qualities - actually *listening* to circuits - not just saving a few pennies with minimal parts counts or whipping out a theoretical "best circuit". Sketches and "computer simulations" only don't cut it completely. Know what I mean Sparkie and Wobblie?


 
   
  At least some of us have the good grace not to edit our posts retrospectively to remove our mistakes in an effort to make ourselves look better.
   
  If you don't like the site you have the option of posting elsewhere.
   
  w


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## wakibaki

Quote: 





kt88 said:


> tangent has an IC buffer in the pimeta, and a discrete (and better performing) buffer in the PPA. So this is simply a simpler implementation of the same thing that everybody else use.


 
   
  Yes, but both these options will cost more in quiescent current, and simpler is not without its advantages. I've never been an advocate of virtual grounds and I drew up this circuit in the middle of the night, and have had little opportunity to think about improving it, having been obliged to defend it versus a _much_ worse performing circuit in part due to your interventions.
   
  w


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## KT88

As you said already, lets leave it at that. I'm sure whoever is reading this thread will able to decide for themselves.


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## Sonic Wonder

Wakibaki,
   
  I removed only what might have been confusing. This thread overall has had a lot of activity while we were working out the "self adjusting" virtual ground using the LM317s/LM337s. Furthermore, as you may have noticed, I integrated the final results into the first post of the thread, again for the sake of new readers of the thread - not to "cover up" anything about my posts - jeeze!
   
  There is plenty of room for multiple circuits by many people who have worked with virtual grounds - or who wish to develop new circuits. That's OK with you, right?
   
  Live and let live, especially recommended in public forums.


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## curiousmuffin

so where am i suppose to connect the adjust pin of lm336?


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## KimLaroux

Quote: 





curiousmuffin said:


> so where am i suppose to connect the adjust pin of lm336?


 
   
  You don't. We don't need to use this pin in this design. The ADJ pin is there for an external circuit that would compensate for a change in current. Seeing the large margins of error in this design, the 3 mV per 1 mA change falls way bellow the acceptable margin.


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## curiousmuffin

so my build works, and works well. i was first drawn into this design for the simplicity of parts, powering my desktop cmoy. it first ran a passive divider which sounded horrible. but goldpoint's circuit is good enough for me to keep the amp on the desk, thanks goldpoint!


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## Sonic Wonder

Thanks curiousmuffin!
   
  Yep, I concur. This circuit works well enough - and SOUNDS better than anything else I've tried, well almost...
   
  For the "ultimate in sound quality", I do like putting another regulator out in front. The LD1085 3A adjustable regulators seem to sound really good in audio circuits wherein I've compared them to other voltage regulators. Here's that - what I actually use in my headphone amps at this time: The very BEST sounding virtual ground as far as I can find or develop.
   
​   
  Note that using this extra voltage regulator "in front" is likely beneficial for me because I power it all from a DC adapter. The LD1085 regulates my incoming DC and must be "cleaning that up".  (But if you are using a battery to power the virtual ground, I'm not sure this extra voltage regulator would really do anything for you - in that case it might be only a waste of power.)


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## gastro54

Is this circuit really appropriate for battery-powered use?  There are going to be huge I-V losses through the linear regulators when the ground is sourcing current.


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## wakibaki

Don't mention the I-V losses. I did, _but I think I got away with it._
  
 w


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## KimLaroux

From my point of view, this whole design was nothing more than a mental exercise. When considering efficiency, price and footprint, there really is no reason to use this circuit over an op-amp-based circuit.


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## lrtsenar

Hi,
  
 Thank you for your thread.
My source is 2 x 2 Lipo = 14.8 V (16.8 V when full charged). 
  
1/ As I don't want to have a very accurate output voltage, my question is can I replace the 2.5 V voltage reference by 1 2 ou 4 diodes (basic or Schottky) ?
  
2/ Is it possible to use transistor like 2N3055 or FET to handle more current like 5 Amperes ?
  
 Thanks in advance.


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## jcx

by 5 A current most people become worried about efficiency - the linear techniques in this thread essentially double your "ground" current drain
  
 usually it becomes cost effective to use a switching circuit for ~ 80-90% efficiency
  
 and if you have 2 batteries why not just use the middle point
  
 if current draw is heavily unequal then again switching converters could balance the battery load


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## lrtsenar

Hello,
  
 And what about this :
 http://electronicdesign.com/power/hex-buffer-mosfets-build-high-power-lossless-virtual-ground


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## Ahimgeon

Hello, I lift the old topic. I would prefer to assemble this Rail Splitter because ""rock solid" virtual ground". However, there is a little offset of zero potential, -17.8 vs 18.2.  Unfortunately under load(300-500ma) i have a no little  skewness of voltage, like -17.3 vs 18.7. Vin 36v, r1-r2 6.8k or perfect 8 - no matter.  What can be wrong? I have attached a diagram, but the error did not find it ...
http://imgur.com/EeqOYR1


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## Ahimgeon

Hello, I lift the old topic. I would prefer to assemble this Rail Splitter because ""rock solid" virtual ground". However, there is a little offset of zero potential, -17.8 vs 18.2.  Unfortunately under load(300-500ma) i have a no little  skewness of voltage, like -17.3 vs 18.7. Vin 36v, r1-r2 6.8k or perfect 8 - no matter.  What can be wrong? I have attached a diagram, but the error did not find it ...


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## buggalugs

You can trim the voltage up or down by introducing a trimpot in either R1 or R2 (of course reducing the value of the appropriate resistor accordingly).
 This circuit does work but nowhere near as elegantly as Wakibaki's contribution to this old thread. His circuit is cleaner and rather better sonically.


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## FritzS

Please take a look to
 http://www.diyaudio.com/forums/headphone-systems/304015-better-railsplitter-virtual-ground.html
  
 Two options about railspiltter
 .

  
 .
 What are the better way?


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## Rroff

ahimgeon said:


> Hello, I lift the old topic. I would prefer to assemble this Rail Splitter because ""rock solid" virtual ground". However, there is a little offset of zero potential, -17.8 vs 18.2.  Unfortunately under load(300-500ma) i have a no little  skewness of voltage, like -17.3 vs 18.7. Vin 36v, r1-r2 6.8k or perfect 8 - no matter.  What can be wrong? I have attached a diagram, but the error did not find it ...
> 
> 
> http://imgur.com/EeqOYR1



 


Double check you've got the LM336 the right way around.

I've built the full version with the 1085 - held the rails very tightly with less than 1/10th of a volt difference IIRC - seemed to work well but since I've either used a TLE2426 with op amp ground buffer or DCP DC-DC POL (dual output) w/ LM317/337 regs.


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