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Old 4th December 2006, 07:15 PM   #1
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Default Ground questions

I have a couple of Robert/Akai mono block amps that I'm slowly converting into mic pres (see my earlier thread).
I have a question concerning grounding in tube amps. The existing power cord is only 2 wires...so no ground. How do i add a ground, is the metal chassis "groundable" without affecting the amps operation? When I come to add the balanced XLR sockets and input and output tannies what constitutues the required grounding for those items under these circumstances?
I don't want to risk electrocuting a singer in my studio. Also since the items will be racked, the chassis via the face plate will be directly in contact with the rack mounting rails and hence other equipment...any problems forseen?

BTW, I don't have the amp model numbers or any schematics but the units were very well prepared in their present state by a retired audio engineer.
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Old 4th December 2006, 09:13 PM   #2
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I see conflicting advice when i do a search on this subject, some say a main ground close to the power transformer plus a second "star" ground terminal as far away from the main ground as possible with indiviual ground wires going to it from the variuos componenets that require grounding. The others say ONLY one ground!
HELP!!!
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Old 5th December 2006, 01:15 AM   #3
Sherman is offline Sherman  United States
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Here is what I do- I usually use an IEC inlet (like the power cord connector on the back of a computer). Solder a relatively heavy gauge wire to the 3rd connector and then attach it to the chassis near the power inlet. This is 'safety ground'. If for any reason your chassis becomes 'hot' this ground gives a lower resistance path to earth than your body.

I run the AC 'hot' lead from the inlet to an appropriately sized fuse and then from the fuse to the on/off switch. The AC 'neutral' can go directly to the PTX from the inlet, though some people prefer to switch using a DPDT and switch both the hot and neutral.

Make a separate ground pointnot connected to the chassis for your power ground. Then make a 3rd separate ground point also not attached to the chassis for your signal ground. Lastly run a single heavy gauge copper wire from each of these two (power and signal) grounds to the safety ground.

It works for me.
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Old 5th December 2006, 03:12 AM   #4
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"Make a separate ground pointnot connected to the chassis for your power ground. Then make a 3rd separate ground point also not attached to the chassis for your signal ground. Lastly run a single heavy gauge copper wire from each of these two (power and signal) grounds to the safety ground."


Thank you for responding.
OK, treat me as a really dumb individual and expand on this statement with a lot of extra detail. When you talk power ground what exactly do you mean? If the power ground and signal ground are not immediately grounded to the chassis, where do you put their ground terminals? Do you use isolated star terminals (isolated from the chassis) and then link them with heavy duty copper wire to the safety ground?
I'm a little lost I'm afraid.
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Old 5th December 2006, 11:49 AM   #5
SY is offline SY  United States
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The circuit's ground and safety ground are two separate things. In theory, the circuit ground doesn't even have to connect to the safety ground. The use of the term "ground" for both functions is very confusing to many people.

First, let's start with what Sherman said. His recommendation of tying the IEC ground directly to chassis at the input is precisely what you need to do for safety. And his recommendation to have circuit grounds isolated from the chassis is also exactly what you want. Now, the circuit ground can and in many cases should be eventually connected to the chassis, but this should happen at one and only one point, usually at the input. If a ground loop occurs with other equipment, the circuit ground can be lifted from the chassis (safety) ground. Under no circumstances should the safety ground be lifted!

Once this is established, there are several possible grounding schemes (e.g., star, bus, star + bus, ground plane...) from which to choose.

There's a very nice explanation with pictures and some practical suggestions in the excellent book "Building Valve Amplifiers." The author shows a neat trick for ground lifting using a telephone plug and some illuminating photos. I would also highly recommend the white papers on grounding and isolation posted at the Jensen Transformers website, and the articles and examples at Aiken Amplification's site.
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Old 5th December 2006, 02:01 PM   #6
Sherman is offline Sherman  United States
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Quote:
Originally posted by SY
]The circuit's ground and safety ground are two separate things. In theory, the circuit ground doesn't even have to connect to the safety ground. The use of the term "ground" for both functions is very confusing to many people.

...

Good point. Maybe we should use the term "earth" like the Brits for the "safety ground" and just use "ground" for circuit grounds. (As if we will change after a hundred years or so. )
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Old 5th December 2006, 04:54 PM   #7
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So is the star ground terminal or the buss ground insulated from the chassis? ie. if I use a section of copper buss do i mount it with insulated fasteners, run all the individual component copper ground wires to it and then one main ground wire over to the main power ground terminal ?

The existing grounds on the Roberts/Akai mono blocks for say the 1/4" unbalanced jacks are prsently just to the chassis, I assume that I would re-route them to the buss? What about the main power transformer ground?
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Old 5th December 2006, 05:04 PM   #8
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Actually, there are some standards. Being an engineer, standards are extremely important, and are there for a reason. Typically standards are adopted more for personal safety, compliance, and specification than for design, but nonetheless we can see how a lack of standards here adds to confusion.

In my experiences, the attachment shows a proper symbolic nomenclature for all generic electronic equipment.

The third prong of your IEC connector is 'earth ground', or simply 'earth'. The implication is that at some point this connection is directly bonded to earth.

The top plate/chassis of your amplifier is 'frame ground', or better yet, simply 'frame' (IEEE frowns on the use of 'frame ground' due to its misuse). While not technically necessary, it is required by code that your 'frame' be tied to 'earth'. The exception to this code requirement is when a piece of UL listed equipment (none of which we DIY-ers make) is double-insulated. Therefore, you must tie 'frame' to 'earth', no exceptions. Frame should also include your transformers' chassis as well.

The last entity is circuit common. It is incorrect and tragic to call the negative rail of your B+ 'ground', for there is absolutely no requirement that it be 'earthed'. The shield of your input RCA or XLR is almost universally tied to circuit common. Ii is not necessary that this be tied to either 'frame' or 'earth', but there are reasons to do so. We should note that in dealing with a B+ supply of a few hundred volts, it is quite feasible for our circuit common to float to voltages we as humans will not like. So some coupling of circuit common to earth is wise. This could be through a capacitor, resistor or jumper.

This produces some difficulties, however. Typically we have a CD player, turntable, preamp, receiver, etc in the audio system. Without interstage transformers, there is the chance of having multiple 'earths' of our 'circuit common' through the various components. This increases our chances of ground loops and noise injection. It is my recommendation to float circuit common on all devices but one, and if you feel the need to bond any additional, do it with both/either a ceramic cap and Meg sized resistor.

Note that most CD players bond their circuit common (shield of RCA) to the frame. The cord is typically 2 wire (no earth connection). But there is some coupling between the case and your furniture/house/other components to earth. The whole problem continues to exacerbate. If I ever get around to building my own DAC w/ tube output, I will eliminate this bond, and keep my circuit common floating except at the amp.

Moral of the story is that transformers are beautiful things. Not that you should use an interstage transformer only for isolation, but it does indeed provide galvanic isolation very well.
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File Type: pdf grounding.pdf (8.2 KB, 149 views)
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Old 5th December 2006, 05:30 PM   #9
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Hello Limey22,

A floating ground is isolated from the chassis. Buss grounding and star grounding are merely ground topology strategies which supposedly each have influence on the sound of an amplifier. In either star or buss grounding, the ground can be connected to the chassis or not.

BTW, I have heard many superior sounding amps that use the chassis for all their grounding, neither buss nor star.

At any rate, most amps use the chassis as part of the signal ground. You mention having a 1/4" phone jack. Check to see if the phone jack's ground is connected to the chassis. If it is not, then you have what is known as a floating signal ground. Make sure not to connect the chassis and signal ground if separate. You might introduce ground loop hum (whatever that means) or at worst a short circuit which may damage the amp.

With two wire AC cord, the manufacturer is hoping that the electrician conected the neutral to ground at the fuse/breaker box. (it is done in most homes. It is in both homes I have lived in). So there is a ground. (I am not sure if it is the bigger of the two polarized AC plug prongs).

As far as safety is concerned, as SY said you will want to make sure, if you have three wire AC, to connect the green wire (earth, true ground) to the chassis.

In addition to what SY has said about the confusing "ground", there is also a signal ground. So one can have three grounds. The power supply circuit ground, the signal ground, and the safety ground, which is the chassis and as Sherman said should be called "earth". In fact, the schematic symbols for circuit ground and safety ground or earth are actually different.

The difference between signal ground and circuit ground is that signal ground is isolated from the DC of the circuit by either a transformer or capacitors.

Signal/circuit ground is this:

|
---|---
-----
--

and "earth" is this:

|
-----|-------
/ / / / / /


I hope these come out. If not, the upper one is a vertical line birsecting a horizontal line, with two other lines spaced below the to line, but each being shorter than the previous one.

The bottom one is a vertical line bisecting a horizontal line with diagonal lines across the bottom.

Gabe
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Old 5th December 2006, 06:38 PM   #10
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I would like to clarify some things, with all due respect. Please do not get offended.

Quote:
Originally posted by Gabevee
A floating ground is isolated from the chassis.
There is no such thing as a floating ground, unless it is a wire INTENDED to be grounded, but there is an unintentional open i.e. there is a malfunction that causes what SHOULD be grounded to float. Circuit common may float, but ground does not. This is the very point SY and I were making, there is too much improper naming out there, confusion is added. I would clarify your statement by saying "circuit common is floating, or isolated, from the chassis"

Quote:
If it is not, then you have what is known as a floating signal ground.
Again, this is inaccurate naming. 'Floating shield' or 'floating circuit common', maybe, but not a 'floating signal ground'. Any floating ground is a bad thing, period. If you design it that way, it's clearly not a ground, it's common.

Quote:
With two wire AC cord, the manufacturer is hoping that the electrician connected the neutral to ground at the fuse/breaker box. So there is a ground.
I don't know if I understand you here, but if a piece of equipment is only provided with a 2-wire cord, you cannot connect the neutral to your chassis, even if it is bonded to earth back at your service (which is required by Code). This is very important !! You can never connect the neutral in this manner, it must be isolated from personal contact. The only exception to this is you are permitted to bond the neutral of an electric dryer or electric stove to its chassis if a separate grounding conductor is not provided. This is an exception, however, and not the norm. In North America, you absolutely cannot ground (earth, bond) the neutral anywhere downstream of your service panel, especially to the chassis of electronic equipment.

I don't completely agree with your circuit symbols, but oh well. There will clearly not be a consensus until our generation is extinct. Maybe generation Z will figure it out.

No offenses, I hope
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