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previous: lion Thank for the replies Dave and Davi... -- 1/6/2000 1:15 PM

Re:Still confused – maybe Randall Aiken?

1/7/2000 6:54 AM
Randall Aiken	Re:Still confused – maybe Randall Aiken? There are many schools of thought on the subject of grounding. Some people swear by the star ground method, some swear by the buss ground method, some just swear. I prefer the star ground method. Your mileage may vary. 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. 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 OT 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 amp 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. 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 PT 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 PT 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/OT 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. 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 a little hum. 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. Randall Aiken

1/7/2000 6:54 AM

Randall Aiken

Re:Still confused – maybe Randall Aiken?

There are many schools of thought on the subject of grounding. Some people swear by the star ground method, some swear by the buss ground method, some just swear. I prefer the star ground method. Your mileage may vary.

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.

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 OT 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 amp 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.

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 PT 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 PT 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/OT 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.

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 a little hum. 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.

Randall Aiken

Replies:

Bruce Excellent!You must have bee... -- 1/7/2000 3:55 PM
lion Re:Thanks RG and Randall! -- 1/8/2000 4:24 PM
Benjamin Fargen Correct wire gauge for ground wires vs. noise? -- 1/9/2000 6:04 PM