Earth Connection

[Sherman]

I am building a gainclone in some cigar boxes which are of course wood. In order to provide some shielding I've covered the inside of the boxes with aluminum tape. It is fairly thick, is conductive and it adheres well to the wood boxes.

I've completed the ps but have a question before going further. I am using a three prong plug (I'm in the US, 110 voltage). I plan to put a screw through the back of the cabinet and attach the ground (green) wire to the screw inside with a nut and washer. This will effectively ground the chassis.

Should I run a wire in my ps harness from the ps box to the amp box and connect it to the chassis in the same way grounding that chassis as well? I have been planning to use five position Molex connectors to carry V+, V+ return, V-, V- return and earth. I would simply terminate the earth wire with a screw, washer and nut on the inside of the amp case just like the ps.

Thoughts?

Sherman
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Don't touch that! Its hot!

[Thoru]

Yes, you should also ground the chassis of the amp. This way you create a low impedance path to ground for any interference current that might occur. Don't forget to also connect the ground of the signal inputs to the amp chassis, preferably in a coaxial way.

Remco Poelstra

[jneutron]

The purpose of a safety ground is to prevent a hot voltage in the event a short to ground occurs.

The purpose of the breaker in the main house panel is to clear the line of all voltage in the event a short to ground occurs.

The "chassis" must be capable of causing the breaker to trip if a short to chassis does occur.

If one does occur, will your tape be capable of causing the main panel breaker to clear the line?

If not, you are making a very hazardous condition.

Cheers, John

[Sherman]

Originally posted by Thoru
Yes, you should also ground the chassis of the amp... also connect the ground of the signal inputs to the amp chassis...

Remco Poelstra

Thanks for the feedback. I had planned to use star grounding, one for signal and one for power and then connect the star points together. What I will do now I think is use star grounding and connect both stars to the earth or safety ground.

Originally posted by jneutron
... The "chassis" must be capable of causing the breaker to trip if a short to chassis does occur.

If one does occur, will your tape be capable of causing the main panel breaker to clear the line?

Cheers, John

Thanks for that bit of reality John. I want this to be different and maybe a little goofy, therefore the cigar boxes, but not dangerous.

From hard experience I know the tape will be sufficiently conductive to trip the main panel breaker. The tape didn't burn through before the breaker tripped so that's a good thing.

Sherman
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Don't touch that! Its hot!

[Ropie]

Hi Sherman,

Similar to you I have a wooden case lined on the inside with thick kitchen foil. One of my power ground stars is connected to the foil via a 100R resistor and 0u22 film cap in parallel. The foil is then connected to earth.

The cap and resistor will help prevent any noise from the RFI that your foil picks up making its way into the signal.

[Thatch_Ear]

That is called floating your ground. The chassis is ground to the device so you must float the earth ground to keep it from becoming the ground for the device. Basically it just separates things with a filter to keep bad noise out. A good practice.

[Ropie]

Thanks for the explanation, David. I wasn't sure if my reasoning was quite right, though I know it's good practice to use the cap/resistor.

[Sherman]

It looks like I should do the same, adding a cap and resistor to the earth or safety ground.

Do I need the cap/resistor only in the ps box where the earth ground enters from the mains cord or should I put this in both boxes?


Sherman
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Don't touch that! Its hot!

[Ropie]

The cap/resistor only keeps noise out of the signal, therefore it needs to go in the amp box, between the power star ground and the connection to the metal of the case.

This connection to the case is then hooked up to the earth connection of the mains, so I imagine you would take a wire from your amp into the psu where it should be connected to earth. This wire could run with the other wires of your 'umbilical' cable between the psu and amp boxes.

Remember, the cap/resistor is only used as a buffer between the power star ground and the metal of the amp case. In practice you probably won't need to foil-line your psu case (as the transformer(s) are not affected by RFI) so you won't need to earth this at all if it is wooden.

[Sherman]

Got it! That makes sense to me. Thanks for the help. I've never built an amp with a separate ps before but I guess if I just imagine the ps being in the same box I would essentially do the same things.

This amp will look interesting when it is finished to say the least. The PS is on the left of course and the amp on the right. I don't have the umbilical built yet. The mains switch on the PS is DPST and disconnects both AC lines. The indicator light is a 120V neon which I've used in tube amps before without causing noise. I hope it works as well here but since it is in a separate shielded case I think it will be fine.

The left knob on the amp is volume and the right is the input selector. It will have four inputs- CD, Tuner, Tape and Phono. The phono preamp is a separate box as well.

Thanks again for you help!

Sherman