Should I directly connect my signal ground to chassis ground?

[Bricolo]

1:I'm wondering if I can connect the signal ground of my amp directly to the chassis (to earth)

I've seen things with diode bridges, thernistors...
Is this necessary?


2: The amp I'm building has 4 stereo inputs (RCAs) and a 2 channel input selector swictch.

Can I connect all input's grounds to analog grouns, or have I to go to a 4 channel input selector, and so only connect the selected input's ground to the amp's ground?


3: I've got a 2*18V transformer, with 4 secondary leads, what is better, 1 diode bridge or 2?


Thats "all"

[Steve Eddy]

Originally posted by Bricolo
1:I'm wondering if I can connect the signal ground of my amp directly to the chassis (to earth)

I've seen things with diode bridges, thernistors...
Is this necessary?

Not necessary per se.

The reason it's done like that is because typically the chassis is also connected to the AC safety ground. Tying your signal/power supply ground directly to the chassis may result in ground loop problems.

Can't hurt to try it both ways. For the semi-isolated chassis, use a 50 ohm resistor in series with a 0.01uF capacitor.

2: The amp I'm building has 4 stereo inputs (RCAs) and a 2 channel input selector swictch.

Can I connect all input's grounds to analog grouns, or have I to go to a 4 channel input selector, and so only connect the selected input's ground to the amp's ground?

Sure, you can connect all the input grounds directly to your main ground and just switch the input line.

3: I've got a 2*18V transformer, with 4 secondary leads, what is better, 1 diode bridge or 2?

Neither. All AC power supplies are just plain evil and should be avoided in the first place.

Seriously (though I wasn't entirely joking above), the double bridges would be a bit better.

se

[Bricolo]

Originally posted by Steve Eddy


Not necessary per se.

The reason it's done like that is because typically the chassis is also connected to the AC safety ground. Tying your signal/power supply ground directly to the chassis may result in ground loop problems.

Can't hurt to try it both ways. For the semi-isolated chassis, use a 50 ohm resistor in series with a 0.01uF capacitor.

se

So, if I don't have hum with the signal gound connected to AC safety ground, I can leave it as it


What are the pros and cons between a resistor and a diodes, for semi isolated ground and earth?

[Steve Eddy]

Originally posted by Bricolo
So, if I don't have hum with the signal gound connected to AC safety ground, I can leave it as it

Sure.

What are the pros and cons between a resistor and a diodes, for semi isolated ground and earth?

Actually the diodes don't give you semi-isolation, they give you complete isolation. So your signal grounds have no connection to the safety ground but by the same token, they have no connection to the chassis either, in which case the chassis no longer functions as an electrostatic shield.

The diodes are really only used for safety purposes when one doesn't care about shielding. A diode won't conduct until the voltage across it meets or exceeds the diode's forward voltage (typically around a volt or so for silicon diodes). So the diodes keep the signal ground isolated from the chassis under normal conditions, but provide a low resistance path to the safety ground in the event of a failure.

A resistor simply reduces the interchassis current a bit and helps reduce ground loop noise.

The reason the capacitor is used is to provide a much higher impedance between signal ground and chassis at power line frequencies reducing interchassis currents even further, but provides a much lower impedance at RF frequencies so that the chassis can be effective at shielding.

se