earthing painted metal chassis (buzz + safety)

[lost eden]

I'm casing up my gainclone monoblocks into a 3U black-painted steel chassis (please see the photo, apologies for the messy kitchen) & I have questions about earthing the chassis.

Firstly there is a loud buzz when I connect speakers but no RCA input. This buzz was not present when I tested the monoblocks separate from the enclosure, with the input/output connected straight to the amp PCB, but is very pronounced now that I am using RCA sockets & binding posts mounted to the rear panel. Do I have to connect the RCA sockets & speaker binding posts to earth?

Secondly, to do with safety, obviously the metal chassis can conduct mains so is extremely dangerous if not earthed correctly. I have already connected the AC earth to the bare aluminium that the amps are built on but I am unsure how I should go about earthing the chassis panels as well. Should I go as far as to drill extra holes at all of the corners, scratching away the paint & screwing earth wires between all of the panels? The base is already earthed due to the bolts holding the amps to it, but I wouldn't trust the screws that hold the other panels to the base to have continuity to this earth.

I know there is a very thorough article here but I was wondering whether somebody could very briefly tell me what sort of work I'm looking at.

[scott17]

Check out Rod Elliott's article on grounding:
Earthing (Grounding) Your Hi-Fi - Tricks and Techniques

That should make it clear what you should do. Basically, the safety ground (mains earth ground) should attach to the chassis right at the point of mains power entry. All signal grounding, including the input/output jacks, should be isolated from the chassis. If the heat sinks are at signal ground, isolate them from the chassis. Use the "ground breaker" circuit to connect your signal ground to the chassis at one point only. Make sure all the panels of your enclosure are bonded together.

Hope this helps.

[lost eden]

Thanks, I've only made it halfway down that article but it looks extremely helpful. I think if I dismantle the chassis & use a grinding tool to remove all of the paint along the joints it will make secure continuity so that I only have to attach the AC earth to the chassis in one place, which will be less work than drilling earth lugs through every single piece. As it stands atm only the base has a decent earth connection which obviously isn't safe. I'll probably give it a go after work tomorrow.

[scott17]

Originally Posted by lost eden

I think if I dismantle the chassis & use a grinding tool to remove all of the paint along the joints...

I agree, you should be fine as long as you have clean metal-to-metal contact and a tight connection between the panels. For steel-to-steel connections, I don't think an anti-oxidant compound is required.

edit: Meant to say, yes, only one safety ground should be attached to the chassis, right at the mains power entry point. Cheers!

[marce]

standard procedure for getting rid of buzzes, try big thick bits of wire between various points, when the buzz goes away it a fix of sorts. The fun is trying to determine where the returns have gone wrong.
These are worth a read, from the EMC complience club site:
Designing for Interference-free Audio System Components

A Practical Interference Free Audio System (Part 2)

[lost eden]

I've had a chance to work on the amp some more & am now satisfied that it is 'safe'. I used a Dremel to remove the paint from around all of the screw holes so that there is bare metal to bare metal & in addition I reassembled it with spiky/toothed washers so that there is a really good connection between all of the panels with the washers biting into the panels. I checked it all over with a multimeter on continuity test mode & it's all linked up.

As far as buzz/noise goes the amp is utterly silent when the RCA connections are plugged in - connected to 86dB (IIRC, possibly higher) sensitivity speakers & with my ear right up against the tweeter I can't hear a thing. But if I turn the amp on without RCA connections plugged in there is a loud buzz, which disappears when I turn it off & it is running on the charge of the filter caps. I'm not worried about this because obviously I'm never going to use the amp without a signal, but was just wondering whether this is normal or whether it indicates something amiss?

[scott17]

Can you provide a schematic of the input circuit?

[lost eden]

What do you mean by input circuit? The amps are simply a LM3886 board from HK with a toroid & a piece of stripboard with 4x MUR diodes & 2x filter caps.

[scott17]

What I mean is what circuit do you have. Is this the one?

PA100 DIY 2x LM3886 in parallel gainclone audio amplifier

Or is this the one?:

http://www.shine7.com/audio/gainclone.htm

[lost eden]

I'm using these --> 2x LM3886 68W Power Amplifier Kit (Mono)_Solid-State Amp_Analog Metric - DIY Audio Kit