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Joined 8/2011
Location: Montr&al, Canada
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.
Joined 12/2010
Location: Sunnyvale, California
Yes, the resistors on the outputs of the two fixed value voltage regulators DO raise the output impedance, but not by very much. They serve to "take up the slack" of minimal output voltage differences between the two complimentary fixed regulators. But the regulators do not fight each other. (The minimal amount they "disagree" is swamped by the use of the output resistors.) I have had no problems at all using this ground. This virtual ground sounds great - because it is regulated.
The adjustable pre-regulator from the original circuit (beginning of thread) could possibly be replaced by a single fixed value regulator, so no potentiometer adjusting would be necessary. In that case, all three regulators should be pre-tested and selected so that U2's output voltage is close to the absolute value of U3's output voltage and U1 is close to U2 + |U3|. If necessary, R2 and R3 could be increased from 1 ohm to 2 or 3 ohms.
But you are right - I would really like a way to use adjustable regulators where they "self adjust" to 1/2 of whatever the rail-to-rail voltage is. Anyone want to design that?
Edited by Sonic Wonder - 3/10/13 at 3:19pm
5,898 Posts.
Joined 9/2001
Location: I come from a galaxy far, far away on the ? axis
Originally Posted by KimLaroux As for performance, I'm wondering about the influence of the resistors at the output of the regulators. Would not that raise the overall "output impedance" of the amplifier, and lower overall performances?
That's what those monster 10 mF caps are doing on the output: reducing the virtual ground impedance to near-zero.
You can think of the 78xx and 79xx in this circuit as doing nothing more than providing the tiny trickle currents to the big rail capacitors needed to maintain the virtual ground voltage.
A particularly nice feature of this circuit is that it effectively hides the dropout voltage of the 78xx and 79xx in the half-supply drops they are there to provide anyway. I suspect this circuit wouldn't work with the +/-3.3V versions of these regulators for the same reason: there isn't enough voltage room for the dropouts to hide in.
The 2V drop across the preregulator hurts enough that I don't think I'd want to use this in a battery powered circuit. The inability to get below a 12V supply also argues against using this in battery-powered amplifiers. That means either a wasteful 12-cell pack at minimum, since that lets you drain the pack to 1V per cell, getting maybe 80-90% of the energy out of the battery. If you want to get to 0.8V per cell to fully use the energy in the battery, you need 15 cells at minimum.
I don't mean to dismiss this circuit. This idea of using 78xx regulators to provide the midpoint of a vground supply has been kicking around as long as I've been involved in audio DIY — about a decade now! — and it's probably older than that. This is the first implementation I've seen that actually makes sense.
With your permission, goldpoint, I may include this in an update to .
Joined 12/2010
Location: Sunnyvale, California
Hello Tangent,
The TO-220 78xx/79xx devices actually heat up if you are drawing much more then 50mA to 100mA. They DO pass current. They actively hold that ground point in place under load, and solidly. I believe it sounds so good with my headphone amp circuit mainly because the ground is a fairly STIFF = a regulated virtual ground. It does sound better, by far, than any other virtual ground I've tried. But yes, although the big output caps are not essential, I believe that they do make this virtual ground sound better. Actually, in my headphone amp circuit they're each 11,700uF (3 x 3,900uF, 16V Panasonic low ESR FM series caps).
I bet the circuit would work just fine with any complimentary set of Pos/Neg regulators - you would only need to adjust the rail-to-rail voltage appropriately via an adjustable regulator in front. For +/-3.3V regulators, the pre-regulator would be set to about 6.6V of course - that should work just fine. (I am guessing that you could even use non-complimentary regulators. For instance, if you used a +5V with a -12V, you would adjust the pre-regulator to 17V. In that case the virtual ground would not be in the middle of the rails and you would have a +5/-12 power supply.)
Yes, this circuit was not originally intended for battery use. If someone could figure out a way to get complimentary adjustable regulators to "self adjust" their outputs to 1/2 of whatever the rail-to-rail voltage is - well then it would make a very fine virtual ground for use with a battery . Perhaps some opamps could be used to do that. (Such a circuit could be integrated into a single TO-220 or TO-247 package by one of the custom linear I/C companies. That would be so slick! It would be a 3-terminal "power rail splitter virtual ground" - and be so handy. Like a beefed up Texas Instruments TLE2426. One other thing I would like about such a device is that its internal regulators would not draw much battery current in quiescent mode [little or no load current] - unlike many other virtual ground circuits.)
I have seen your article many times when searching for virtual ground ideas - and I say "yes" - you should include this one there too.
By the way, I tried a similar back-to-back complementary regulators circuit work a few years ago - but could not get it to work. The secret to why this circuit DOES work is the pre-regulator to adjust the rail-to-rail voltage and use of those 1 ohm output resistors on the fixed voltage regulators. The 1 ohm value can be lowered or increased as needed.
Edited by Sonic Wonder - 3/8/13 at 10:00pm
1,072 Posts.
Joined 5/2011
Location: Devon, UK
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:-
1,082 Posts.
Joined 8/2011
Location: Montr&al, Canada
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 100uA, 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.
Joined 12/2010
Location: Sunnyvale, California
Edited by Sonic Wonder - 3/16/13 at 8:44pm
1,072 Posts.
Joined 5/2011
Location: Devon, UK
Looks good to me Kim, but maybe some of it is unnecessary...
Joined 12/2010
Location: Sunnyvale, California
Actually that does not work. (Only 1/2 of the ground is regulated, believe it or not...)
1,082 Posts.
Joined 8/2011
Location: Montr&al, Canada
Originally Posted by goldpoint Oh my Gawd! Did you do it?
Oops - nope - I think you must have the output of the LM317 be 1/2 of the rail-to-rail voltage
and the LM337 also be 1/2 - but negative!
Can you explain? I fail to see how it's different than your circuit in the 1st post.
The LM337 is just a mirror to what the LM317 does. It's all a question of what you use as a reference. If you decide to use the virtual ground as a reference, then V- is half of the rail-to-rail voltage, but negative. The virtual ground is then 0 V. The LM337 is configured to "regulate" a negative voltage using V- as an input. Here I just configure it to output 0 V. If I didn't have the zener in there, and just connected the adjust pins to the middle of the resistor divider, then the LM317 would output 1.25 V and the LM337 -1.25 V, referred to the virtual ground.
Originally Posted by wakibaki 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?
1,072 Posts.
Joined 5/2011
Location: Devon, UK
Originally Posted by goldpoint Actually that does not work. (Only 1/2 of the ground is regulated, believe it or not...)
There is only one ground. How can half of it be regulated?
1,082 Posts.
Joined 8/2011
Location: Montr&al, Canada
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.
1,072 Posts.
Joined 5/2011
Location: Devon, UK
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
Joined 12/2010
Location: Sunnyvale, California
I actually tried that circuit. It seems to work when you have a very small load.
If your load is connected to the virtual ground and the negative rail, it will work correctly. In that case it works just like a regular positive voltage regulator circuit - up to the current (amperage) limits of the LM317 regulator.
But if you connect a load from the positive rail to the virtual ground, the ground point will actually move - it will be pulled upwards towards the positive rail as the load increases. see?
You need both regulators there to "hold the virtual ground point steady" - to keep it from "moving". (One does it in one direction and one in the other direction, so to speak.)
But there are substantial advantages to using virtual grounds in certain situations - and no reason not to use them if they are properly designed.
Edited by Sonic Wonder - 3/10/13 at 10:58pm
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