What is "ground"?

If you read only a little about amplifier circuits, you will soon encounter the term "ground", or "earth".   "Ground" means a common reference point or potential voltage, assumed to be "zero volts".   Ground is relative. That is, you could choose any point in the circuit to be "ground" and reference all other voltages to that point.  For instance, if you decided to make the +400VDC supply "ground", then the connection at the other side of the power supply would be at -400V with respect to "ground".   Normally, ground is chosen as the common return point for all power supplies and circuits, and for the "shield" connection of the input and output jacks. There is also a "safety ground", which is the "third" or "green" wire of the AC mains (at least, in the United States, that is).  This wire is connected to the chassis for safety reasons. The circuit "ground" is also connected to the chassis, so the entire metal enclosure will be at "ground potential", which offers some shielding from radio-frequency and other electromagnetic interference.

Below are the commonly used schematic symbols for ground.  The first symbol is commonly called "earth ground" and is usually used only for the chassis or safety ground connection.  The second and third ground symbols are used interchangeably, although sometimes one is used for "analog" ground and the other for "digital" ground, particularly in circuits that have both analog and digital components that must have separate grounds.

You may also encounter the term "ground plane".  A ground plane is simply an area of metal under a circuit which is connected to the ground potential.  It usually refers to a copper pour area on a circuit board, but may also be a metal chassis area.  Ground planes are used to provide low-impedance ground reference points for lowest circuit noise and for shielding purposes.

What is "star grounding"?

One of the best amplifier power supply grounding schemes is a "star" ground system, where all the local grounds for each stage are connected together at a single point, and a wire is run from that point to a single ground point on the chassis, either back at the power supply ground point  or at the input jack ground.  However, a full star ground generally results in a messy rat's nest of ground wires, so unless the chassis and components are properly layed out for it, a full star ground is not always practical.  It usually works best for PC board designs where you can control the component placement and copper pours on the ground plane layers.

Better yet is a two-point star, where the main the power grounds (PT center tap, first filter cap ground) and output stage grounds (output tube cathodes for fixed bias, or cathode resistors for cathode biased, and output transformer secondary ground) are connected together at a single point, right at the ground of the first filter capacitor. The ground of the second filter capacitor, after the choke or filter resistor, is the star ground point for the preamp stage grounds. Use a local common point for each preamp stage ground, and run a wire from this common point back to the second star point. If two stages are out of phase with each other, they can share a common local ground, but don't use more than two stages per local common ground.  Connect the star ground points together with a wire, and  use only one connection to the chassis.  This concept can even be taken further, with multiple star points for various amplifier stages.  

In any case, the connection to the chassis should be made at a single point only.

Why is it used?

The idea is to keep heavy power supply and output stage ground currents from flowing in the ground return of the low-level input stages. These ground currents can modulate the ground of the sensitive, high-gain preamp stages, and can result in hum or noise injected into the signal path. In particular, a capacitor input power supply filter can draw heavy currents for very short periods of time to recharge the filter caps at the top of each AC cycle. These currents need to be kept out of the preamp stage grounds.

A good analogy is to think of an amplifier power supply distribution as a river. All the small currents from the preamp stage feed into a larger river, which has the heavier currents from the output stage, and the still heavier currents from the power supply. You want each successive stage farther "upstream" from the power supply, so the heavy currents don't influence the smaller ones. In the case of the input jack ground, it is the farthest point upstream from the power supply, so it should be connected directly to the ground point of the first cathode resistor. If you give it an alternate path to ground through the chassis, it can be influenced by ground currents in the chassis. Think of the first stage as amplifying the difference between the signal on the grid and the signal at the ground side of that stage's cathode resistor. If you have a long path back through the chassis to get from the input jack ground to the cathode resistor ground, it can pick up all sorts of stuff along the way. Keep it short, and use quality shielded cable, with the input jack isolated from the chassis, and the shield grounded at the ground side of the cathode resistor for the first stage. 

How are the input jacks isolated?

There are some good isolated jacks available from Cliff and Rean that most companies use. Stay away from the plastic-nosed units and get the better chrome ferrule jacks. These are now being supplied by Neutrik, who purchased Rean. They aren't as good a quality as Switchcraft jacks, but they are isolated. You can use shoulder washers to isolate the Switchcraft type jacks, if you can find one that will fit the hole.  Newark Electronics carries them, but they are not listed in the catalog.  The part numbers are: 94F8935 (Switchcraft S1028). and 94F8936 (S1029). These mount in a 7/16" hole - not a 1/2" like the old ones.

When using isolated jacks, be sure to connect a 0.01uF capacitor from the ground lug of the jack to the chassis with as short a lead as possible, to prevent radio-frequency interference (RFI).

What about the output jacks?

The current in the secondary winding of the output transformer can be very large.  For example, in a 100W amp, the secondary current into a 16 ohm load is 2.5A.  It is even higher into a 4 ohm load, at 5A. This means that you need to pay special attention to the grounding of the output jacks and the output transformer.  It is important not to use the chassis for this return path. 

The output transformer secondary has a common wire and one or more speaker taps, usually at 4, 8, and 16 ohms.  The speaker taps usually go to an impedance selector switch, and then a single wire goes to the output jack tip connection.  The common wire should never be connected to the chassis right at the output transformer.  It should be run all the way to the output jack and connected to the sleeve  connection of the jack.  This accomplishes two things.  First, it maintains the continuity of the connection in the event the output jack becomes loose.  Second, it keeps the heavy secondary ground return currents from flowing in the chassis. 

Note that there still must be a ground return path to the rest of the circuit if the amp uses global negative feedback.  This should be in the form of a wire from the sleeve connection of the output jack to the preamp ground point where the phase inverter common connections are grounded, or to the ground of the portion of the circuit where the global feedback from the tip connection of the output jack is connected.  Note that there will be no heavy currents in this wire.  The speaker output jacks can be either isolated or non-isolated if you follow this plan, but it is usually best to isolate them to maintain control of the return current path for the global negative feedback, to insure it doesn't flow through a part of the chassis that may contain power supply ground currents. 

Sometimes it helps to ground the common (sleeve) side of the output jack to the chassis even when no global negative feedback is used.  Occasionally, an amplifier will have a high-pitched oscillation noise, or other type noise that will go away if you ground the output transformer common wire at the speaker jack sleeve terminal.   In addition, there may be a potential for a  small AC current to flow between the floating sleeve and chassis if the secondary is not grounded.  This current is due to  capacitive coupling  in the output transformer, and may cause a mild shock if the speaker plug sleeve and chassis (or guitar strings) are touched while running a signal through the amplifier.  Even though the potential for dangerous currents is low due to the galvanic isolation of the output transformer, the shock can still be annoying.  For this reason, it is best to always ground the common side of the secondary even when no global feedback is used.  

What about volume and tone control grounds?

The ground connections for the volume and tone controls should not be connected to the potentiometer case, for two reasons. First, it destroys the star ground scheme and can contribute to ground loops. Second, when the nut that holds the pot in place becomes loose (and it will, eventually), you will get a bad ground connection and noise or intermittent operation. You should always solder a wire from the grounded pot connections back to the common local ground of the stage the pot is used in. For example, the grounded pin of the volume pot, if it is located at the grid of the second tube section, should go to the local common point for that second tube section's cathode resistor and bypass cap. Don't use the Fender style brass plate and connect the grounds there. The pot cases will be grounded to the chassis via the mounting nut, so they will have the benefit of shielding, but you don't want the circuit connection to go to that point.

Where should the "star points" go?

The first and second star ground points are also important. It is best to lay out the chassis so the first power supply cap is closest to the power transformer, and this capacitor's ground should be the first star point. If you have an amplifier design with a "doghouse" like a Fender, where the caps are on top, under a shield, this may not be practical, in which case you can use the transformer HT center tap chassis ground point as the first star point. Note that the filament center tap (pot or resistor string) should also go to this point, unless you are using an "elevated" filament reference voltage for hum reduction purposes. There aren't any ground currents flowing in this center tap point, so it doesn't really matter that much, but it keeps the filament wiring short.  The second star point should be "upstream" from the first, or closer to the preamp sections, so that any stray chassis currents due to transformer magnetic field induction, etc., won't flow through that connection.  Both star points are then connected together with a wire.

If more than two star points are used, the additional points should be physically located between the output stage star point and the first preamp star point, and should be located on the ground terminal of the power supply capacitor used for decoupling of that particular stage.  Once again, the idea is to keep the later stage ground currents from flowing in the earlier stage grounds, so the physical placement of the grounds on the chassis is important.   For example, if you were to use three star ground points, the first would be the power supply and output stage ground, and it would be located nearest to one edge of the chassis, at the first power supply capacitor ground point.  The second ground point might be used for the phase inverter circuitry, and should be located at the ground point of the second filter capacitor (or the one that feeds the phase inverter), and this capacitor ground connection should be physically located away further "upstream" from the first star ground point.  The third ground point would then be used for the remaining preamp circuitry, and should be physically located upstream from the second star ground point.  In this manner, the ground currents of the later stages cannot flow thorough the chassis ground points of the earlier stages, so there won't be any ground loops created.  This requirement will dictate the physical positioning of the capacitors on the chassis during the design stage.  A straight-line flow arrangement of the capacitors is best, and should be used unless chassis space is a problem. If the capacitors must be arranged in a group of four, for example, the one on the edge closest to the power transformer should be used for the first star point, and the one farthest from this edge should be used for the first stage preamp star ground, with the other star grounds in between.  The ground lugs for the capacitors can then be connected to the chassis in such a manner as to keep the later stage ground currents from interacting with earlier stages.

The AC mains input ground (the third, or green, wire) should not be connected to either star point. It should be connected to the chassis right at the point where the AC comes in, with a short wire. It should also be well bonded to the chassis, preferably soldered, so there is no chance of it coming loose.

Summary

The important thing to consider is the order in which the grounds are connected.  The basic idea is to keep the ground currents from the  high current stages out of the grounds of the low current stages, and to keep the ground currents from later stages out of the grounds of earlier stages.  People don't normally think of "ground" as carrying currents, and think all grounds are equal, but they aren't.

The highest currents flow in the output stage, so they must be kept well away from the preamp stages.  When laying out a chassis, first attention should be paid to the output stage, including the secondary of the output transformer and the cathodes of the power tubes.   Next, the power supply and preamp stages should be carefully planned.

The output transformer: 

The amount of current flowing in the secondary is huge compared to the signal currents in the rest of the amp, so the utmost care should be taken here.  The secondary current goes only to the speaker; it isn't used anywhere else in the amp, unless there is a negative feedback loop, in which case a small portion of the secondary voltage is fed back, usually through a large resistor, so the current in that path is small.   In either case, feedback or not, the secondary should be wired directly to the output jack.  Do not ground the output transformer common to the chassis and then ground the output jack to the chassis.  This will create a heavy ground current path through the chassis, which may run through a preamp section, depending upon the location of the output jacks and the output transformer.  Do not ground the output jacks to the chassis at all, they should be isolated from the chassis.  Also, do not route these output transformer secondary wires anywhere near the preamp stages, they should be routed as far away as possible, around the edges of the chassis to the output jacks.

If the amplifier  uses global negative feedback, there will have to be a ground path from the secondary common back to the phase inverter tail ground (or wherever the feedback is returned to).  This should be done via a wire from the ground of the output jack back to the ground of the stage the feedback is applied to.  The idea is for the feedback amp to amplify the difference between the feedback "hot" lead and the feedback ground lead, but nothing else.   Note that there is very little current flowing in this ground wire. The total secondary current only flows in a loop around the secondary and the speaker, not back through this wire, so it is a voltage sense wire only.  Even if global negative feedback is not used, it is sometimes necessary to ground the secondary common side to prevent noise or oscillations, and to prevent any chance of mild shocks when touching the jack sleeve and ground while running a signal through the amp.

The power supply:

There are usually several filter capacitors in a supply, each with a choke or resistor between it and the previous one.  The output transformer primary center-tap usually goes to the first cap, and the output of the choke and the screens usually go to the second cap, and the various preamp stages then go to the other caps.

If you look at the schematic for an amplifier, you will see the capacitors are arranged in a line or series configuration, or sometimes in a parallel or "branch" configuration.  Usually the series connection is used, because it provides better filtering as you go down the line.   The ground connections of these caps are the star points in multiple-star systems. The first cap ground is the first star point (or the only star in a single-star ground system).  It should be physically located closest to the power transformer center-tap.  The power transformer center-tap wire should be soldered directly to the ground lug of this cap, and a very short, heavy wire should run from there to the chassis ground connection (if there is one).  Do not connect the power transformer center-tap to the chassis and the first cap ground to the chassis at a different spot; this will cause heavy ground current flow in the chassis. Also, do not tie the AC mains safety ground to this point; it should be connected to the chassis with a very short length of wire right at the chassis entry point.

The output tube cathode grounds should be run to this first star point.  Do not connect them directly to the chassis ground. The phase inverter/output transformer secondary ground point may need to be connected to this first star for lowest hum, but usually to the second star, if a multi-star system is used.  Try it both ways and use the one which produces the lowest hum level.

The other filter cap grounds may or may not be connected to the chassis.  If not, they should have a wire back to the main star point ground, but the individual local grounds should be connected to the cap ground lug for the cap that supplies their B+ line, not back to the main star point. Only the cap grounds should fan back to the main star for a single-star system.  This will keep the cap charging currents from modulating the stage supply relative to it's local ground.  If the caps are connected to the chassis, care must be taken in the physical arrangement of the caps to properly route the ground currents.  Remember, the important thing is to keep the ground currents of later stages from flowing in the ground path of earlier stages.  The caps should best be arranged in a straight line, with the main supply cap, or first star point, closest to the chassis edge, and the caps arranged in order as they flow on the schematic, so the ground currents through the chassis from later stages don't flow through earlier stage ground paths.  The individual star grounds should still be connected to the capacitor grounds.

The preamp sections:

The individual tube preamp stages have their own "local" grounds, where the cathode resistor and capacitor ground sides are connected.  Each stage "local" ground should be run back to the star point on a separate wire.  Alternately, if two consecutive stages are out of phase, sometimes they can be connected together at the second stage local ground, and a single wire can be run from there back to the main star, if the stage currents are nearly balanced.

Do not use the pot casings as ground points.  Any pot terminals that are grounded should go to that stage local ground point.  Do not ground the input jacks, and be sure to use isolated input jacks.  Run the input jack ground to the local ground of the first stage, which then goes to the star point.

Important note: While the star ground is excellent for eliminating ground loop hum, it is not always the best scheme for preventing radio-frequency interference (RFI).  Fortunately, there is a simple addition to the star ground scheme that will make for a very quiet amplifier with no RFI.  Simply add a 0.01uF capacitor from the chassis to the ground lug of the isolated input jack using very short leads.  This will shunt all of the RF "riding" on the shield of the cord straight to the chassis ground before it can get into the amplifier and cause problems.   Any kind of ground lug can be used for the chassis connection of the capacitor.  You may be able to find a solder lug that slips over the shaft of the isolated input jack for a convenient ground lug.  Use of an internal-toothed lockwasher is recommended for these types of connections to insure a good "bite" into the chassis for a good ground.

You can experiment with variations on this system.  All the above suggestions are not always necessary, especially if you are willing to put up with some residual hum.  Star grounding is not always necessary, and some very quiet amplifiers have been built using buss grounds or other grounding schemes. A little planning in the early stages can save you a lot of aggravation in trying to eliminate the hum after the amp is built.

Buss grounding
 
A properly implemented buss ground can be every bit as quiet as a star ground, and is usually neater looking.   By the same token, an improperly implemented buss ground can be a humming, oscillating nightmare (just as an improperly implemented star ground can cause problems).

Okay, so what is a buss ground? A buss ground is simply a buss bar (or wire) that goes from one end of the circuit to the other, and is grounded to the chassis at one end.  If you use a buss ground, you must make sure your circuit layout is in order - if you have the output tubes connected to the buss somewhere in the middle, and preamp or other stage grounds connected elsewhere, they can oscillate.

Typically, the best approach for a buss ground is to run the power transformer secondary center-tap directly to the ground of the first filter cap (do NOT connect it somewhere else to the buss or you'll get 120Hz hum).  This keeps all the high-current charging pulses in a closed loop from the transformer secondary to the first filter cap and back, so they don't get into the sensitive preamp grounds.   You also should connect the cathodes of the output tubes to this point (sort of a "mini-star ground" for the power and high-current output stage).

The remaining filter caps should be located at the stages where they are used.   For example, the first preamp's filter cap should be physically located next to the first tube's components.  All other filter caps are located near the circuits they are decoupling.  It is not a good idea to lump all the filter caps together in one spot, like Fender did under the "doghouse".  While this will work most of the time, it is far better to locally decouple each stage with a cap at that location.  

Run a buss wire (heavy ground wire) from the first filter cap on down the line, to the ground of each filter cap in the line up to the last one at the first preamp stage.  Each stage's "local" ground (composed of all the parts that connect to ground for that stage, such as the cathode resistor and the cathode bypass cap) should be connected to the buss with the shortest possible wire.  Note that it is best to have each stage's components connected together in a "mini-star" that then goes to the buss, unless the buss wire is physically located on the board or near the components.  This minimizes the number of wires needed to go to the buss - you can simply connect together adjacent turrets or eyelets and run a single ground wire from each stage to the buss.

The final thing to be considered is the chassis ground.  You only want your buss to connect to chassis ground at one point, either at the power supply first filter cap or at the other end of the buss at the input jack.  

If you connect the buss to ground at the power supply, you will need to use an isolated input jack (run the shield connection over to the ground point of the cathode components of the first stage).  Then you will need to add a 0.01uF capacitor (ceramic disk caps are good for this, as are film caps) from the shield terminal of the input jack directly to the chassis with the shortest possible wires.   This will keep radio frequencies from getting into the amp.  Do not leave this capacitor out, or you will find yourself playing along with a radio station on your most important gig!

If you instead ground the buss at the input jack, which is usually best for EMI/RFI you don't need to use an isolated jack and you don't need the capacitor.  However, you *must* solder the input side of the buss to the chassis right at the input jack.  Do not, under any circumstances, depend on the jack nut and lockwasher to provide the ground.    They will loosen or corrode over time and you'll get massive hum.  Do not connect both the input jack and the first power supply ground to the chassis (or any other point, for that matter) or you will get low-level ground loop hum.

Under some circumstances you might be able to get away with using the chassis as the buss ground (instead of an isolated, heavy-gauge buss wire), but  this almost always leads to low-level ground loop hum problems, and should be avoided.

One more point - the output transformer secondary "common" should be run directly to the shield connection of the output jack (preferably using an isolated jack), not to the buss.  Then run a second wire from the output jack shield connection over to the buss at the point where the global negative feedback is implemented (usually the phase inverter ground point).  If the amp is not using global negative feedback, just run the wire over to first filter cap ground.  This keeps the heavy output stage currents flowing in a loop from the secondary of the output transformer to the speaker and back, keeping them out of the sensitive preamp or buss ground circuits, and off of the chassis.  The wire back to the phase inverter carries no significant current, but provides the "reference" ground for the feedback loop to work properly.

Lastly, the AC mains "safety" ground should connect to the chassis with a short wire.   It should not be tied to the buss at any point.


Addendum for clarification

Many questions have come up since I originally published this article in 1999, so here is a summary for clarification:

(1) You can connect your ground buss or star to the chassis (and you should), but only at one end, either the power supply end (main star) or right at the input jack.

(2) This article assumes you are using isolated jacks and are connecting the ground at the power supply. If you do this, you *must* have a low impedance path for AC signals (mainly high frequency AC signals) at the ground side of the input jack, otherwise your amp will be susceptible to RF interference. The way to do this is to put a capacitor (typically a 0.01uF) directly from the shield side of the input jack (which must be isolated) to the chassis with as short leads as possible.

(3) If you are using non-isolated input jacks, you can instead connect their shield to chassis directly with a short wire (don't depend on the nut to make contact, because they corrode over time). If you do this, you should not also ground the main power supply star node to the chassis, or you will likely develop massive hum.  If you want a "safety ground" connection at the power supply in addtion to the input jack ground, use a pair of back-to-back high-current diodes bypassed with a 0.1uF capacitor as a "loop-breaker" to prevent ground loop hum.

For my amps, I tend to favor a combination of star and buss ground. I always star the main power supply - you have to run the center-tap of the power transformer B+ winding (or the bottom of the B+winding if you are using a bridge rectifier) directly to the first filter cap without going through the chassis or any other part of the ground buss, or you will get buzz noises from the high-current in the return path. Also run your output transformer common wire directly to the output jack shield connection, to keep high currents from flowing in the chassis.

If using a choke, I run the second filter cap close to the first one, connecting it's ground to the star point. This node can have some large ground currents as well (but not near as high-energy as the first filter cap's ground return), so we want to keep it from causing mischief in the circuit. Note that choke wires themselves can radiate a lot of noise, so keep them away from sensitive areas of the preamp.

The preamp grounds I will either fully buss, or I'll have little "islands" of ground for each stage, and then at time of layout, I'll decide how the islands connect back to the main star point. Sometimes I'll use an entire ground plane on top, or I'll cut the plane into several copper pours for individual "star-type" returns. You have to apply a knowledge of circuits and signal flow to figure out what is critical and do your layout accordingly, so there is no one way that will always be correct, because the circuit paths may be intertwined. Proper circuit layout can minimize the risk and make the grounding easier.

I also tend to put the preamp filter caps in the circuit where they are used ("local" bypassing). For example, the first preamp tube's filter cap will be physically placed in the area of the first preamp tube, with the ground and B+ connections made right to the bottom of the cathode resistor/cap and to the top of the preamp tube resistor. If I decide to instead group all the filter caps together, I'll be sure to also locally decouple each filter cap node with a smaller capacitor, typically a 0.1uF/400V) directly from the top B+ node of that stage to the ground node at the bottom of the cathode resistor. You'd be surprised how many brand new filter caps have very high reactances at frequencies within the range of a distorted guitar. Sometimes you can take a "bad" filter cap and bypass it with a 0.1uF cap and it will sound fine. It never hurts to have good high-frequency decoupling at all nodes.