Grounding Issues

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Hi Im sure this has been exhausted before, but I have searched and havent been able to find any topics on this.

Anbyway I'll explain my setup. I've had my chip amp sat on a piece of MDF board for a little while and decided to put it into an aluminium box. Im not using a star grounding system, as I didnt on the MDF board and never had any problems. Im using the PCB's from Chipamp.com, and am using one trafo and rectifier PCB for two amplifier boards. The trafo (0-25, 0-25 rectifies to +-36v) has two seperate windings so Im using 8 diodes and have the V+, PG+, V- and PG - connections going from the rectifier to each Amp board. Also each input and ouput ground is connected to the PCB at the correct place. The only electrical connection to my alumunium chassis is earth from the mains socket.

The problem I have is that there is a voltage from any of the grounds on the PCB to the chassis which is earthed. The strange thing is that it isnt 36v, but 12v. This is fine when im using a source which isnt referenced to earth (laptop etc) but If i try to use my PC for a source the soundcard's ground is earthed and so I get 12v running from my amp back to earth through the PCB, making the cable very hot!

I know you may just say "do a star ground" but I dont see how this would solve the problem. If someone could explain what Im doing wrong It would be much appreciated!
 
Hi,
you can read up on the subject of earthing here,
http://sound.westhost.com/earthing.htm
Although you´ll find 1000x as much here in the forums,
it´s just not so nicely summed up.

but If i try to use my PC for a source the soundcard's ground is earthed and so I get 12v running from my amp back to earth through the PCB, making the cable very hot!
But the 12V as you say are only on the chassis and earth from the main socket. So how can that cause any current flow and particularly go through your PCB?:confused:

greets
 
Well my RCA grounds are 12v DC above the earth in the mains, whereas the grounds on the output of my soundcard are at 0v with reference to earth, so there is 12v between the inputs to my amp and the ouputs of the soundcard, and when i connect them up with a phono cable there is 12v with about 0.1 ohm of resistance in the cable = bad :)

I just tried another amp I have, and its RCA connectors are at ground, just like my PC.

Thanks for the link though, I'll have a read!
 
I looked at that site, but I cant see how exactly that will help me. As far as I know I dont have a ground loop problem. In fact maybe I should change the title of this thread.

Any other suggestions before I rip this amp apart and put it back on the piece of MDF?
 
I think the DC he is seeing, is actually alot of AC... probably due to asymmetry in the AC input and the capacitance fron pri to sec, OR the phasing (polarity) of the secondaries.

It's probably nothing... but the 12 volts DC to earth is cause for question.

:confused:
 
Hello again. I've just made a bit of a strange discovery. The box the amplifier is in is very small so I have to take it apart somewhat in an attempt to get the voltages you asked for. Well I found that when the amp was apart the voltage from earth to PG/SG/OG was gone completely, and the amp is now working fine.

After reassembling the case I was expecting the problem to return, but it didnt. Looking at how tightly everything is crammed in Im thinking that maybe the top of one of the rectifying diodes could have been touching the bare metal on the top of a capacitor. Would this have caused problems?

Anyway, I've decided that when I get time, I will probbaly redo the whole grounding as suggested in the other thread and perhaps use the whole chassis as a star ground that is also earthed. The one thing Im worried about now is the offset reappearing in use and destroying something.

Anyway, some pics:

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.
 
Ex-Moderator
Joined 2003
And you plugged that into the mains?

Welcome to the real world where things are a bit more complicated than on paper or in computer simulations. As you've noticed, your case is a bit small. We normally like to enclose all dangerous items like toroidal mains transformers to avoid the risk of shock to the user or casual passer-by. Inputs near outputs? You're very lucky your amplifier didn't oscillate and destroy itself due to HF instability the moment you applied power.

Seriously, look upon this project as a valuable lesson - make the case big enough. Don't cram things in until you have twenty years experience. Invest in a new box that's big enough to put the mains transformer inside and put it at one end of the box. Put the amplifier at the other end. Mount the heatsinks so that their fins are vertical (allows the cooling air to flow past them). Keep at least 50mm between inputs and outputs. Bond mains earth from the IEC to the case with a solder tag secured by a firmly tightened M6 screw with a serrated washer between the solder tag and the case (ensures a gas-tight long-term reliable joint).
 
Yes I'm plugging this into the mains, even as we speak actually.

Firstly, this is being used in a room accessed only by me, or in some cases me and some other people. There is no access to passers by, so that is not a problem, Even if it was, the only way someone could get a shock from the outside of this amp is by sticking a sharp object into the toroid, and yes that is a posibility, but I think its very unlikely to happen.

For the heatsinks I had originally planned to use them with fans (They are computer CPU heatsinks as you may have noticed) but realised that this wasnt really necessary at the moment since I only listen at very low levels. If I ever run it hard enough to get hot I'll reattach the fans. Cooling isnt an issue at the moment, the amp is barely warm.

On the front of HF oscillation, I've never heard of this actually destroying an amp, especially with relatively good heatsinking which is what I have now. I would notice if it started to get overly hot.

The IEC socket im using is actually a filter and so is metal backed, with the metal case being earthed. This is screwed firmly into a cut hole in the case of the amp. Although this maybe isnt as good as a serrated washer, I see it as a good connection that is unlikely to fail.

I appreciate your concern, and although I may not have as much experience as you or other members here, but I do know what I'm doing, and have been around mains and higher voltages enough in my life to know not how to kill myself and others.
 
Ex-Moderator
Joined 2003
Ultimately, what you do is up to you, but I suggest you ask yourself why you haven't seen any commercial amplifiers with exposed mains toroids.

The reason for using a serrated washer versus a metal IEC filter in contact with the case is that the case is aluminium. Aluminium oxidises, and as you know, aluminium oxide is an insulator. It may take a few years, but there will come a time when your earth bond is no longer zero ohms and no longer able to protect you.

HF oscillation is one of the more popular causes of amplifier destruction - usually of prototypes. I'm afraid that the amount of heatsinking you have is neither here nor there - when an amplifier goes into oscillation, everything happens so fast and the overload is so great that the heatsink often doesn't even change its temperature before the devices melt. Output transistors are really small - it doesn't take much to melt them. The picture is of a 115W 15A transistor (2N3055) - look how small the actual transistor is and how thin its wires are...
 

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You are probably right... the voltages I was looking for would have shown you an AC component on top of the DC you were measuring.

Now, if you're worried about that returning... THAT IS WHY YOU EARTH THE CHASSIS. SAFETY, not just for you... but also your other components.

Connect the "PG's" from the amp board to the chassis at the same point as the earth wire and you are good to go. Make sure your inputs and outputs ground to the amp board and not the chassis.

:D
 
Hi,
I disagree.
You already have an isolated audio circuit that works. Keep it isolated.

Connect a safety earth from mains input to aluminium chassis.

I do not recommend you modify your isolated audio grounding system by connecting to the safety earth.
These two systems do not need to be connected, they are doing different jobs.

Shame about the diode shorting but it does raise the problem of small packaging to the fore.
Can you ensure that the central bolt through a toroid NEVER touches the chassis. Insulate it and that big plate washer. Similarly insulate all the mains connections, just in case you need to do maintenance with the circuits live.
 
Hey, At the moment the bolt through the toroid IS touching the earthed chassis, thats correct right? However the toroid is "padded" from the chassis and the washer by rubber sheets which stop the windings being crushed into the alumium and shorting out.

I just tried quickly shorting the RCA ground to the earthed chassis with my screwdriver and there was no audible difference. Im also concerned that the DC offset may reappear for some reason, which would melt my phono cable and maybe start a fire if I wasnt here when it happens. For these reasons I would feel better doing as poobah said and connecting the PG to the chassis.
 
Hey all,

Ground loops suck... but they don't ALWAYS cause audible problems. It is the price we pay for doing so called "high-end" audio with single-ended RCA's (unbalanced) signals.

Earthing your chassis is a no brainer. U.L. would demand that your PG's be earthed as well to protect the ground on the RCA's from becoming hot.

The bonus is that this protects other equipment.

Make a solid connection... if you have a hum problem, "lift" the amp board with 100 Ohm connection instead of a solid wire.

There is also capacitance between the pri and sec in the power trans... these can cause weird voltages and noises too. Earthing the common provides a path for these currnents.

As :king:SY says: "death is permanent"... be safe first.



;)

Edit: Your bolt is fine... just don't complete an entire loop of metal through the hole and around the outside of the toroid... the toroid thinks that is a "turn" of wire and yout don't want to "short" that turn.
 
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