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TGRANT 12th September 2006 12:26 AM

Beginner Ground Connection Question
 
Iím new to this, so --- I hope this isnít a silly question. Iím putting together an amp with a regulated power supply (the typical transformer, bridge rectifier, regulator IC and caps bit). Iím uncertain about the grounding concepts for the star ground verus earth ground. I have the earth ground attached to the metal case (bomb proof of course). I have created a ground point to hook up the grounds for the preamp, tone control and amp circuits (? star ground). I was planning on attaching this to the chassis, but am afraid Iíll create a ground loop. From a forum search, it seems like just connecting it to the chassis is a bad idea. After looking at several reference books, Iím amazed that I canít find exactly how to do this to avoid hum.

How does one go about separating the earth ground and circuit grounds, but still connect them? Iíve seen some resistor/diode circuits on the forum, but not sure how to put them in. Any suggestions? Many thanks for the help.

SY 12th September 2006 12:44 AM

There's a few ways. In "Building Valve Amplifiers," Morgan Jones suggests connecting signal ground to earth ground (which is, as you say, anchored to the chassis in a bombproof way) via a pluggable shorted link. If there's no hum with the link in place, fine and good. Otherwise, the link can be pulled. This causes the signal ground to rely on a path through an external component, breaking up the loop. The same function can be built in with a ground-lift switch, and is often done in pro equipment.

In case of hum with signal ground and chassis/safety ground connected together, I would plug a small resistor (10R-47R, say) in place of the shorted link to knock down the loop currents while maintaining a DC path. Other people have reported success with paralleled reversed diodes, which clamp any voltage differential to +/-0.7V. Either way, try to pick components which will tend to fail short rather than open.

TGRANT 12th September 2006 04:09 AM

Thanks. That was helpful. It gives me a starting place.

AndrewT 16th September 2006 10:12 AM

Hi,
go and search
andrewt (user name)
+
ground (subject)

and you will find 378 posts (a few are irrelevant to this enquiry) defining my way of doing it.

Be careful to recognise the difference between a "safety earth" and an "audio ground"

You could repeat the exercise for "safety" if you need more.

AndrewT 16th September 2006 10:20 AM

Hi Sy,
I disagree on the safety of what you suggest.

Consider the senario of a mains fault putting Live onto the audio somewhere.
The fuse will only blow if the Live can escape. either by way of neutral or by way of earth (through the third pin).

If the fault prevents escape via neutral then the escape route to earth MUST be HIGH current capable.
Power resistor and/or power diodes should be installed that will carry the fault current without failing open until the fuse blows.
A small resistor or a low current diode will not achieve this requirement.

The shorting link is an example of how to mis connect the safety earth to audio ground. If connecting it shows up hum or buzz then it indicates that a grounding fault exists either in another connected unit or in the amplifier. Find the grounding fault and make good. Leave the shorting link open during all normal use.

TGRANT 16th September 2006 07:54 PM

follow up
 
Andrew T,

Thanks for the reply.

Let me see if I have it. (Iíll try to make myself as clear as I can). Itís a biamped system so I have a number of subcircuits that make up the amp - preamp, tone control circuit, active crossover and two separate power amps. I connect the incoming wire from the wall (green in the USA) and connect this to the metal chassis with a bombproof connection, this becoming the safety earth. The remaining power leads (black and white) connect to the transformer (with a fuse on black). Itís a dual polarity supply, so the center tap of the transformer then becomes the system ground and goes to the power supply circuitry and becomes the 0 volt potential of the rest of the amp. This is connected to a common connection point of some sort. I attach all the circuit commonís from the various sub circuits to this common ground point individually. I guess that would be called the Ďstar groundí (it makes sense). Then I connect this common ground point to the chassis with the parallel network of the resistor, diode and capacitor in series (this is described a few times in the forum). This way the circuit common from the ampís sub circuits are isolated from the chassis safety ground by the resistor, cap and diode network, theoretically defeating any loops and subsequent buzz. If there is a failure of some kind and high voltage from the mains gets into the amp somehow, the high voltage will make it to the safety earth either through the chassis or the star ground, thereby saving me or some other unlucky character from a shocking experience.

Does that sound right? It makes sense to me, but lots of things make sense to meÖ Anyway, I just wasnít sure the best way to isolate the system ground from the mainís neutral. Excuse me if I was longwinded, but if this description is correct, it might help someone else.

Again, thanks for your help

Regards,

TGrant

SY 16th September 2006 08:27 PM

Quote:

f the fault prevents escape via neutral then the escape route to earth MUST be HIGH current capable.
That's why I specifically said "fail short." Diodes are very good for this.

AndrewT 16th September 2006 09:54 PM

Hi Tgrant,
that sounds right.
It is important you understand the safety earth.

I don't care if it's long winded, more importantly you seem to follow the logic.

TGRANT 17th September 2006 03:46 AM

Many thanks. You both have been a great help.

Greetings and salutations


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