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Old 11th May 2006, 07:53 PM   #1
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Default Start Grounding Questions

I have read many posts about ground loops and grounding practice. However, none seem to address my questions. I have built a preamp with separate phono and line stages. There is a separate power supply for each channel and each module has voltage regulation and filtering on the module. I have read that a star grounding system is the best approach and there should be a separate gound point for signal grounds and power supply grounds. Here are my questions:

1. Should there be separate star points for signal and power supply grounds or can they go to a single point?
2. The power supply grounds enter through a Neutric XLS connector. Should the grounds go to a star point and then to the power in on each module or should the grounds go to the module then to the star ground?
3. I have tied one signal ground from each module to a star ground point. However since the signal ground and power ground are tied together on each module, doesn't this result in ground loops if the power ground also is tied to a star point?
4. I have read that the DC supply grounds should be tied to earth ground to prevent AC getting to the case if the transformer shorts. When I did this it doubled the hum out of the phono preamp from 3mV to 6 mV. Would the fuse in the power supply (2 amp) protect against the transformer shorting?
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Old 11th May 2006, 08:16 PM   #2
VEC7OR is offline VEC7OR  Lithuania
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Default Re: Start Grounding Questions

1. Should there be separate star points for signal and power supply grounds or can they go to a single point?
2. The power supply grounds enter through a Neutric XLS connector. Should the grounds go to a star point and then to the power in on each module or should the grounds go to the module then to the star ground?
3. I have tied one signal ground from each module to a star ground point. However since the signal ground and power ground are tied together on each module, doesn't this result in ground loops if the power ground also is tied to a star point?
4. I have read that the DC supply grounds should be tied to earth ground to prevent AC getting to the case if the transformer shorts. When I did this it doubled the hum out of the phono preamp from 3mV to 6 mV. Would the fuse in the power supply (2 amp) protect against the transformer shorting?

1. You generally connect both at star point but run those along the system separated.
2. I thing its better to connect power to regulator board and then connect regulators ground to star ground.
3. Cant comment this one, I'm also intereste what other people will say
4. Some one (Nelson Pass?) uses a diodes to connect earthing to power ground, this way until potential is 0.7V nothing get in or out.
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Old 11th May 2006, 08:40 PM   #3
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"2. I thing its better to connect power to regulator board and then connect regulators ground to star ground."

That's just the problem though. All the grounds on a single channel on a single module are tied together. It seems that more than one wire to ground would form a loop.
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Old 12th May 2006, 07:41 AM   #4
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Just from my experience, and for safety: ...

2 Types of ground:
1) Earth ground - Earthing
2) Audio Ground - Grounding

Starting with (#2) power coming in...

2) Is it 120V AC comiing through the connector? Is it rated for it? (concerned), as from what I'm thinking XLS is a mini-XLR. Preferably a modular IEC socket is best (like the type on computers) to allow for easy and safe connection, even when accidentaly hot.

1) Out of the IEC socket, the earth wire should be immediately grounded to the chasis, and no where else. Remember to use 1 fuse per line that is hot to the transformer (post-switch). The other leg(s) can be connected directly to the socket. OR you can connect a single fuse directly from the socket, making it easy to replace externally. I prefer the 1 fuse per tranformer rule, so that you can use lower ratings per transformer for safety.

3) Out of the transformer(s), the ground(s) should go to the single star point; and this being the only audio star ground.
The AC lines out of the transformer (pre-bridge) would also be best fused on both rails. Typically only for high-current, but you can never skimp on safety. When building a dual-mono design, it also allow you to disable a channel for easy testing. You can also fuse post-smoothing (after the regulators), but really only necessary if you think your audio board might have the chance to short.

4) Audio ground connected to AC earth ground is nothing but trouble. I haven't seen one piece of equipment yet that has ever been able to do this with success. As long as the chasis is properly earthed and your audio circuits are completely isolated from the chasis, there should be no sign of ground loop (other than from other external components).

Always test for any crazy current/voltage from your audio ins/outs. Since it's a preamp, things are probably very low power anyway (SS). Tubes are a world I was never born into... so can't help you there.
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Old 12th May 2006, 03:33 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
Quote:
. Would the fuse in the power supply (2 amp) protect against the transformer shorting?
no!!! It might kill you first.

Let's just get the safety part out of the way.

All mains powered equipment must be built to operate safely.
All exposed conductive (metal & others) chassis MUST have a safety ground connected back to the electricity suppliers earthing point. Some countries do not insist on this, If you live in one of these then emigrate to a safer country.

There is an exception:- Double Insulated equipment with the double square symbol does not need a safety ground.

Let's for a moment look at your fuse idea.
Imagine you have a metal chassis without an effective safety ground. Now imagine you have a faulty connection inside that allows mains voltage to touch the chassis. Now touch the chassis.
I will let your relatives explain the consequences.

In case your still working this one out, your body has just blown the fuse, drawing an instantaneous peak current from the mains that could approach fifty (50) times the fuse rating.

The RCBOs (RCDs) installed to provide additional protection have a trigger level of 10mA, 30mA and 100mA. The 30mA is normally chosen for domestic use because there is evidence that currents of about 100mA from the mains can incapacitate and/or kill.

ALWAYS keep the safety ground connected even when maintaining the equipment.
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Old 12th May 2006, 04:31 PM   #6
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OK my explanation wasn't so good. I have a separate DC supply which has the chassis connected to earth ground. The PS output is +-32 VDC and there are two supplies in one chassis. When connecting the two DC grounds to the PS chassis and hence earth ground, the hum from the phono stage increases. The XLR connector into the preamp is rated well above the voltage and current being supplied to the preamp.

Since the DC grounds are connected to the signal grounds in each of the modules, I would think only one connection to ground per module per channel would be required. But I have read that the DC grounds and signal grounds should be connected separately. Maybe that is for fully balanced circuits where the DC ground and Signal ground are not connected in the module?
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