One chassis ground or two?

[tony321owen]

I’ve been reading about grounding in merlins book and there’s one thing that’s confused me...

He states that In a bus ground system there is only one chassis ground connection which is to be close to the input yet in some of his diagrams it shows another chassis ground right after the first filter cap... maybe someone can help clear this up for me once and for all.. I’ve built amps both ways but wondering which is theoretically superior...

[silverfox]

Star Ground. You may have a buss, however, different ground points should be consolidated. What you are seeing is the association of grounds with the related power supply feeds. While the preamp stages  ground to the realted power supply caps, the output stage, B*, can ground through the chassis or by separate ground, but chassis can work.


Bottom line: You want each ground associated with a particular stage, to connect with that power supply capacitor. Then each buss, connects to a common ground. It's called Star.


Grounds can cause many problems since a small ground resistance will develop inter-stage noise to be amplified.


silverfox.

[Colas LeGrippa]

Hi !


First  you need an independent ground for the 110V cable


After, you want a ground connection for  trannies center taps, power supply caps, power tubes and phase inverter.


Then a ground connected to (and only to ) the input jack for all preamp gnds including pots and the supply caps. For this I hook up a no 16 or 14 ground wire taken from an electric wire just like the one used in your house.. Then you solder everuthing from the preamp to this buss
Just make sure the other end of the bare #14 wire is ONLY CONNECTED TO THE INPUT JACK.
For good connection always scrape off metal oxide from the chassis with a flat screwdrivetr or  whatever you like AND use a star washer.


Good work !

[Colas LeGrippa]

I ve already seen humfree amplifiers with all of the power caps : reservoir, power tubes and preamp connected together to the same point.


In my original builds, I find it more practical to place the preamp supply caps near the preamp tubes so I use the preamp buss to connect their neg side.


That has alwsys worked great for me. Even in a high gain amp the noise is reduced to minimum.


Colas

[PRR]

> grounding in merlins book

Is what you see in the Sample Chapter: Grounding?

If so, point.

If not, then I would scrupulously obey Merlin's Grounding Chapter (or any other fully thought-out grounding plan). Any holistic plan will work. Mix-n-match grounding makes trouble.

[tony321owen]

It was fig. 15.10 on page 271 that shows one chassis ground but then on 15.15b on page 275 shows 2 chassis grounds..

Some people have a separate bus bar for the preamp grounds with a chassis connection by the input and one chassis connection after the the first filter cap for the power amp.
 As far as I understand, in this method, reservoir cap, trannies center taps, powertube grounds, and phase inverter grounds, all share this power-amp chassis ground point.

Then in merlins method fig 15.10 he uses a method where there is one buss bar that goes from reservoir cap all the way to preamp grounds with one and only one connection by the input of the preamp.

Both methods also have a safety earth connected securely to the chassis by the PT. Which is the 3rd prong of the power cord.

[PRR]

> 15.15b on page 275 shows 2 chassis grounds..

15.15b is TWO Chassis and two power cords. "..a simplified diagram of two devices –in this case a separate preamp and power amp– connected together,.... Is that what you are doing?

[Leevi]

Then in merlins method fig 15.10 he uses a method where there is one buss bar that goes from reservoir cap all the way to preamp grounds with one and only one connection by the input of the preamp.

I have used that in many low power amps and it works perfectly.

Based on my own experiences high power amps need a separate grounding for power amp (incl. PI).
In the preamp the buss works always.


I recommend to plan the grounding schema in a way that it can be easily changed if problems exist.

/Leevi

[jjasilli]

There is no superior answer to the original question posed.  I.e., there is more than one acceptable grounding scheme which works: Hoffman's, Merlin's, etc.


If you're building a certain type of known amp, then follow that grounding scheme.  (Unless it's a bargain basement type amp which might benefit from a better than stock grounding scheme.)


If building a new concept, I guess I would consider a star scheme.


Note PRR's point about connecting separate chassis, such as standalone preamp > power amp.  This magnifies the ground loop issue - even the sacred power cord ground becomes part of the problem.

[d95err]

Safety ground must always be connected to a dedicated bolt on the chassis which is not shared with anything else (such as a bolt for a transformer). At least that’s how I’ve understood electrical safety regulations.

Safety ground should definately not be part of the star ground connection. 

[jjasilli]

It is true that the safety power ground should not be part of the amp's general grounding scheme.  But, safety power ground can be part of a ground loop problem, especially when standalone devices are connected.  One way to safely break power ground, at least for the end user who does not open the cover, is for the chassis of the device to be inside an insulated housing.

[shaun]

Great thread - I've been more confused by grounding than anything else, and this really clarified things.

From what I gather after looking at Merlin etc, is that in order to have a proper grounding plan, I need to insulate all jacks (except where grounding is required) and pots from the chassis. I can see why it makes sense, but is that the idea? I'm wondering why I haven't seen isolating gizmos on such items when I'm shopping for parts. Maybe I missed something. Is it simply a matter of using plastic washers?

Thank you, as always, for a great forum.

[sluckey]

The pot innards are isolated from chassis. Of course the case is grounded through the threaded bushing and nut. No need for special insulating bushings. Hoffman sells insulating shoulder washers for Switchcraft jacks if you feel a need to use them (I don't). And Cliff style jacks are insulated due to the plastic body.

[jjasilli]

@shaun: Yes.  To sum up, ground is not a black whole that electrons disappear into.


Earth or safety Ground is to put the chassis at the same el potential as the Earth; or a least the earth nearby.  If the chassis were at a different el potential (voltage) than the nearby earth, then a user would get shocked if he touched both the chassis and the earth, or a metal pipe or floor connected to the earth.


Inside the amp, the chassis serves as ground, meaning the return path for the various currents in the amp: HT power; filament power; signal, etc.  When each current goes to chassis ground, the electrons do not drain away to somewhere.  They are pulled back into the loop of the circuit they belong to, ideally.  Unfortunately, electrons going to chassis ground from one circuit might get pulled back into the wrong circuit.  E.g., electrons bearing 60 cycle filament hum might get pulled into the signal circuit.  Proper grounding schemes can eliminate this or at least reduce it to low enough levels. 


Also, if a circuit has more than one path to chassis ground (ground loop), then it might switch which path it takes to ground in random sequences.  Except that (in the US) we are surrounded by a lot of 60Hz ambient el energy.  This might induce itself upon the otherwise random selection of ground paths, causing a repetitive 60 cycle ground path selection.  As a result, 60 cycle hum appears seemingly out of nowhere.

[shaun]

Nice. Thanks.

[PRR]

...One way to safely break power ground, at least for the end user who does not open the cover, is for the chassis of the device to be inside an insulated housing.

Works for radio/TV. Ether waves in, light/sound waves out.

It does not apply to Guitar Amps where almost invariably the chassis links through the guitar cord to the user's sweaty hands.

[jjasilli]

Yes, it could only work with studio, or rack gear; probably custom builds with an educated user,