Chassis ground vs signal ground...
- Thread starter Ths364
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Ths -- The old classic equipment had no distinction between chassis ground and signal ground. Ground was ground, and that was it. In modern times, with the advent of 3 terminal/green wire grounded AC line cords, a distinction was then made, since connecting circuit (or signal) ground to the AC ground usually results in increased noise -- particularly if other equipment used in the system were built the same way. If so, a ground loop would then exist because of the dual ground paths available (through the interconnects and through the AC ground at the plug).
With the advent of the green wire grounding system then -- if it were applied to the old type of equipment -- the green wire ground would connect directly to the chassis, but the circuit ground, including all input and output connections, would have their own isolated ground run throughout the inside of the chassis, and never come into contact with the chassis itself. That way, the two grounds would be separate.
The problem with the older equipment, is that some components -- such as the power supply can capacitors -- are mounted and soldered directly to the chassis, making it very hard to separate circuit ground and AC ground. It could be done, but it would take a fair amount of work.
Since the older equipment used the chassis for signal ground, they then typically connected one side of the AC line through a cap to the chassis, so that the chassis could then reference true ground for minimum noise.
I hope this helps!
Dave
With the advent of the green wire grounding system then -- if it were applied to the old type of equipment -- the green wire ground would connect directly to the chassis, but the circuit ground, including all input and output connections, would have their own isolated ground run throughout the inside of the chassis, and never come into contact with the chassis itself. That way, the two grounds would be separate.
The problem with the older equipment, is that some components -- such as the power supply can capacitors -- are mounted and soldered directly to the chassis, making it very hard to separate circuit ground and AC ground. It could be done, but it would take a fair amount of work.
Since the older equipment used the chassis for signal ground, they then typically connected one side of the AC line through a cap to the chassis, so that the chassis could then reference true ground for minimum noise.
I hope this helps!
Dave
Bluelobster
Well-Known Member
Mr Gillespie gives a good explanation. really though, the long and short is, chassis ground is earth ground where the signal is isolated and "floats" above the earth ground.
Ths364
Super Member
Excellent! Thanks, guys. I recently picked up an Altec 353a tube integrated amplifier and after replacing the main filters and all coupling caps, I've still got a low level hum coming through at the speakers. I'm still using the 2 prong plug and have oriented the plug for the lowest AC on the chassis with respect to ground. I'm thinking that lead dress may be the key to eliminating the hum further, unless you think I would benefit by three-prong power and establishing a separate buss-bar style signal ground.
jaymanaa
RIP 1961-2018
What I do on these older pieces is totally isolate the signal ground from the chassis. It's really no big deal, they make insulators for everything, and you are in there re-capping anyhow. To check that I haven't missed something, I will do a continuity test from the chassis to my new signal ground and make sure I have infinity. For the circuit ground itself, I like a star ground, meaning all the ground wires come back to a single spot where the main filter ground is. There are some other tricks you can do with phono grounds to isolate them from trash, but this should give you an idea as to your original question. All the third (green) ground wire is there for is to make sure that the safety devices (IE fuses) have a clear path to ground so they can act quickly. If they had to go through you before they blew it would suck. I blew a 15 amp Edison fuse right across my chest once. I'm fairly convinced had it not blown I wouldn't be here harassing you good people.
Jay! Good to know your tolerance level!!!
No doubt that if you are going to rebuild a unit from the ground up, then making the circuit ground isolated from the chassis is easy, and a smart way to go.
Dave
No doubt that if you are going to rebuild a unit from the ground up, then making the circuit ground isolated from the chassis is easy, and a smart way to go.
Dave
jaymanaa
RIP 1961-2018
Yes, but I was about 25 years younger and in just a little better shape. I went to work on one of those freezers that you get bags of ice from, and someone had set it on wooden pallets inside a metal (Morton Type) building. I pulled the top cover off and grabbed the suction line of the compressor to see if it was cold and pumping. My other hand was firmly grasping the metal header of the building, which was apparently well grounded, unlike the ice merchandiser. The compressor motor windings were shorted to ground, which was me as soon as I grabbed the copper line that's brazed to it. I remember hanging there with a death grip on both ends, trying to call out to the people that were nearby, but my mouth wouldn't move or make any noise. It seems like minutes, but I'm sure it was maybe 5 or 6 seconds. When the fuse went I collapsed, and started spitting out pennies.
I had to go home and take a nap on that one. I've been hit harder, but never longer than that one. :thumbsdn:
The schematic for my Sherwood makes a distinction between chassis ground and B- ground. I haven't opened it up to trace it and see how they physically did this, but I'm interested in what this really means. I can understand why they would be separate, but I just haven't noticed the grounds for the bias current stuff looking any different than anything else in the amp. That said, I wasn't really looking for this last time I had the covers off.
jaymanaa
RIP 1961-2018
Yes, the circuit ground symbol looks like a comb, while the chassis ground is the typical segmented arrow. It's been too long since I've been in a Sherwood to be much help.
wa2ise
Super Member
I usually try to arrange things so there is no currents flowing thru the chassis. No heater current, and have the cathode currents for output tubes flow from the filter cap's negative side, where the transformer centertap also connects. And the chassis connected to the B- near the amp's input side, often the input RCA jacks are riveted to the chassis. It's basically management of currents thru the ground, keep the heater currents separate, and pay attention to the filter cap and centertap connection (that current waveform, induced into a voltage by even a fraction of an ohm resistance in a chassis, will sound quite buzzy if it gets into the input).
The SCA35 amp does this scheme of grounds.
The SCA35 amp does this scheme of grounds.
I second the approach that wa2ise described. It eliminates ground loops, and has the chassis always referencing the lowest potential of the ground circuit to minimize any noise it might distribute throughout the amplifier.
Dave
Dave
Which Sherwood, and who's schematic? I ask only because my S7000 with the Sams schematic doesn't do this. There are sometimes differences between say a Sams and the manufacturer's schematic, and of course between different models. Entirely likely yours has a totally different setup than mine has. Mine doesn't use anything special or particular for grounding though. Terminal strips are soldered to chassis, and used as ground, along with the tube socket ears and filter can ears in spots.
Curiously, my Stromberg-Carlson ASR-433 is drawn to indicate its just using common chassis ground for everything. In reality, the whole works is a big star ground rig with only one point actually going to chassis, which I seem to recall is the main filter cap.
rave0035
Disassemble first, ask questions later.
Not familiar with this amp or the schematic, but although different schematics use different notation, I've seen these used most frequently:
Every schematic should have a key. It's crucial that you understand where the B- ends up so you don't inadvertently make yourself part of the loop. Also, if the unit has a chassis ground, there may be a capacitor between the line and the chassis before the xformer - make sure you replace this with a safety cap with a high voltage rating. It would be bad news if that cap opened up.
-Mike
jaymanaa
RIP 1961-2018
Everything that I know about grounding and chassis current, I learned from Tom Bavis. He put it into words that even I could understand. Now I look at electrons like ants running all over the chassis. There are small ants, and bigger ants, black ants, and red ants. It's easier to me to just keep ALL the current (AC and DC) out of the chassis period. That said, I have always pondered this; if you have a single piece of copper wire for a star connection with the PT CT at one end, do you want the smaller or larger currents to be closer to the CT? My thinking is the smaller ones closer, but I'm not sure why, and it makes my head hurt just a little.:scratch2:
Ground noise is basically created by virtue of the fact that the ground system has resistance to it. If you put the power supply on one end of your buss ground, and the high current ground connections at the other, then there will be a voltage drop across the ground buss that all the other stages -- connected in between the two ends -- will have injected into them, which they will amplify, and ultimately appears as noise in the output.
On the other hand, if you have the power supply at one end, and then as you proceed towards the other end from there, you next have all the high current (output stage) grounds, and then proceed from there with each preceding stage moving backwards in the audio chain, it means that at the other end of the ground buss is where the greatest sensitivity occurs within the amplifier, or usually the phono preamp circuits. With such an approach, the drop across the ground buss is greatly minimized then, with the drop at any point usually being inversely proportional to the sensitivity at that point in the circuit. Therefore, as circuit gain is increasing in the various stages along the buss, the drop (or noise) produced on the buss to be amplified is decreasing with such an approach.
Then, grounding the the buss to the chassis at the opposite end from where the power supply is connected to ensures that the chassis is representing the ground potential of the most sensitive circuits, for the most effective shielding, with the least amount of noise induced into them.
Dave
On the other hand, if you have the power supply at one end, and then as you proceed towards the other end from there, you next have all the high current (output stage) grounds, and then proceed from there with each preceding stage moving backwards in the audio chain, it means that at the other end of the ground buss is where the greatest sensitivity occurs within the amplifier, or usually the phono preamp circuits. With such an approach, the drop across the ground buss is greatly minimized then, with the drop at any point usually being inversely proportional to the sensitivity at that point in the circuit. Therefore, as circuit gain is increasing in the various stages along the buss, the drop (or noise) produced on the buss to be amplified is decreasing with such an approach.
Then, grounding the the buss to the chassis at the opposite end from where the power supply is connected to ensures that the chassis is representing the ground potential of the most sensitive circuits, for the most effective shielding, with the least amount of noise induced into them.
Dave
jaymanaa
RIP 1961-2018
I'm glad I finally asked. :thmbsp: Thanks
