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Old 11th August 2004, 06:16 PM   #1
Sci is offline Sci  Sweden
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Default Ground connections

Hi

I'm sorry to ask a question that has probably been asked many a times before, but I have not been able to find an answer.

I'm currently in the planning stages of building my first amp, based on LM3875. The thing I have not been able to figure out yet is what to do with all the ground connections.

From what I can think of now, I will have:
AC ground
Positive rail DC ground
Negative rail DC ground
Input ground
Output ground
(Input shield)

From what I currently understand, I should connect pos. and neg. DC grounds together in my PSU. Tie AC ground to the chassis. Then connect everything together in a star, possibly with a 10 to 100 ohm resistor between AC and DC ground and the input and output ground.

That still leaves the shielding, should I connect those to the star ground as well? Seems to me like it could cause a ground loop with the source. Just leave it unconnected?

Erik
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Old 11th August 2004, 10:32 PM   #2
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Try this.
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Old 12th August 2004, 10:54 AM   #3
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This might seem a dumb question , but here we go:

I am building a GC amp with separate PSU. Obviously the umbilical will carry the +ve and -ve for each channel separately; but is a ground connection wire also required between the PSU chassis and the amp chassis?
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Old 12th August 2004, 12:44 PM   #4
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Quote:
I am building a GC amp with separate PSU. Obviously the umbilical will carry the +ve and -ve for each channel separately; but is a ground connection wire also required between the PSU chassis and the amp chassis?

Your umbilical will have positive voltage, negative voltage and zero voltage wires (or two zero voltage wires if you are using two bridges).

If you have a metal case for your amplifier, it should be connected to ground, through a wire in the umbilical to the earth wire in your mains lead. The metal case (or shielding) should then be connected to one of the power ground stars via a cap and resistor.

More details HERE .
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Old 12th August 2004, 07:53 PM   #5
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Quote:
Originally posted by Nuuk
Your umbilical will have positive voltage, negative voltage and zero voltage wires (or two zero voltage wires if you are using two bridges).

Thanks Nuuk.
In the case of Joe's VBIGC, where I have 3 transformers (one for the amps, two for the valves), may I simply connect all 3 zero volt wires for each amp together at the PSU, and then run a single ground wire per umbilical from there to each amp?
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Old 12th August 2004, 09:03 PM   #6
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Quote:
In the case of Joe's VBIGC, where I have 3 transformers (one for the amps, two for the valves), may I simply connect all 3 zero volt wires for each amp together at the PSU, and then run a single ground wire per umbilical from there to each amp?
No, I would keep each supply separate for as far as possible. Take a look at how I did my VBIGC .
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Old 13th August 2004, 09:29 AM   #7
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Hmm... okay, so all the 0v ground leads from the PSU should remain separate until they are bolted to the amp chassis.

I have 8-pin plugs and sockets for umbilicals. That leaves me 1 ground lead short (i.e. each bridge, of which there are 3 per channel, has 3 leads out, giving a total of 9). Would it be permissible to squeeze the valve and filament grounds into one, leaving the amp with its own ground lead? And (another stupid question but I'm open to possibilities) would making that one combined ground lead heavier make any difference?

Sorry to harp on about it; but it just seems to me unlikely to make much of a difference if the 0v ground is terminated at the PSU or at the amps chassis...
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Old 13th August 2004, 01:38 PM   #8
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Quote:
Hmm... okay, so all the 0v ground leads from the PSU should remain separate until they are bolted to the amp chassis.
Before you go any further, please understand the difference between zero volt and ground as they are two different things.

Your zero volt rails ( from the PSU) will come together in the power star point for each channel although sometimes it is necessary to join those together too.

The signal zero volt (cold, return or what you will) wires come together in the signal ground stars. Signal ground stars and power ground stars are connectred to each other (in each channel).

The earth wire is connected to the case or shielding and then to one of the power ground stars via a cap and resistor.

I don't want to sound like a school teacher but study all the many circuit diagrams until you understand all this, then and only then, build a GC.
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Old 14th August 2004, 08:43 PM   #9
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Seems like I have hijacked this thread, but it also seems that the power supply may be the single most confusing thing when building an amp...

It has been a few years since I was building radios with my grandad (long since departed), and these days I am working in the field of taximeters (~auto electronics) and GPS. I do know the importance of a really good ground. Nevertheless I am a newbie with regards to audio, even if spending hundreds of hours studying circuit diagrams, surfing for audio info and lurking on this forum...

Anyway, thanks for your schoolmasterly ways, Nuuk . I understand completely the concept of signal ground and power ground. My confusion, it may seem, has come from 2 points:

a) I assumed that the power ground and chassis ground would be the same thing. (Signal ground obviously different.)

b) I was originally intending to power both amps from individual transformers, but decided finally to use one Tx for both, since I hear that it may sound better. Now this latter has caused me some confusion because I assumed that each secondary, each with a bridge, would power one amp each. I guess that this must be completely wrong - feel free to berate me if you wish.

As I am a visual sort of person, I have attached a rough sketch of what I had intended with 2 individual transformers (sketch A) and what it seems like I have to do with just 1 Tx and 2 bridges (sketch B). Please tell me if this latter schematic is correct for 1 Tx with 2 amps.

Now, 2 questions re. sketch B:
a) Can those 2 x 0v wires be joined before the umbilical?

b) If not, how am I to connect the buffering caps after the bridges, now that I have 2 x 0v rails?

Thanks for the leg up!
Attached Images
File Type: jpg psuschem.jpg (31.8 KB, 141 views)
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Old 14th August 2004, 08:49 PM   #10
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The two zero volt wires are joined at the power ground star which is the point between the two decoupling caps.
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