AMB Laboratories DIY Audio

[gh0st_inthe_machine]

This continues where my chassis buzz thread left off- I'm pretty sure the issue is in the psu so this is probably the better place for it. While the b22 works fine, something is still bugging me about having so much AC on the amps ground/chassis. If nothing else I'm intrigued as to how it could happen, edukashun and all that

When I noticed before that the potential between AC earth and o22's ground rail was ~100VAC, I failed to spot that this same potential is the same between earth and the +/- Vout as well, and this figure of ~100VAC (it differs by a few volts between the two o22s) is the same as what measures between earth and the G pads of AC-input where the 30V secondaries are tied together. There's no AC between the rails (or rather, my DMM can't see any).

Is that as expected? Could the transformers be at fault?

[amb]

The AC ground is not connected to signal ground, and the two are not referenced to each other at all, so a potential difference between them is expected (and somewhat invalid, because the two are effectively in separate "universes"). Ditto the potential difference between AC ground and the transformer secondaries, because they are also isolated.

Try plugging the unit into various AC outlets around the house (not just in the same room) and see if you observe the same behavior. Even though the two potentials are not directly comparable, 100V across them seem excessive, especially if it causes a buzzing sensation when you touch the chassis.

By the way, are you using a peculiar AC line filter or "balanced power" transformer on the AC mains?

[xneme]

That actually sounds exactly what all my computers do when connected to a non-earthed wall socket. No idea if these are any way related, but here's a short article about it anyway: http://www.epanorama.net/documents/pc/u ... ed_pc.html

[amb]

That actually sounds exactly what all my computers do when connected to a non-earthed wall socket. No idea if these are any way related, but here's a short article about it anyway: http://www.epanorama.net/documents/pc/u ... ed_pc.html

Yup, that's what I think too. That's why I asked about line filters, and trying different AC outlets (to see if there may be a ground fault problem).

[gh0st_inthe_machine]

I've tried it with and without a basic power conditioner (tacima thing), but the result is the same. I've tested the outlets and IEC cables to check that earth it not lifted, although what I'm checking is for 240V to Live and 0v to Neutral, which I suppose doesn't differentiate the earth from neutral anyway. I can't think of a way to properly test the earth connections, other than to short it to live and see if the RCD in my consumer unit pops, although this may not be great for my health ;/

I'll try in different rooms, thanks for the feedback.

[amb]

There are ground fault testers that you could use to check the AC outlets -- they're available from your local hardware/electrical supply store and are inexpensive. If you're going to get one, be sure it's suitable for use on both standard outlets and GFCI (ground fault circuit interrupter) outlets. Many of these testers (usually those that have a pushbutton on it) only work on the latter. Here is a USA-style tester:

[gh0st_inthe_machine]

thanks for the tip amb, I'll pick one up tomorrow. I moved around the house but the result was the same.

I learned that earth and neutral are connected at some point in the system, but that's about it. My reading is inconclusive as to whether that's normal or not. I'll try the plug, and failing that I'll call a sparky.

[gh0st_inthe_machine]

three-light tester result came back fine, all three lights are on, although the tests are pretty basic of course.

I think I'll ask on an electrical forum, see if anyone has any ideas.

If not, would it be possible to remediate the situation in the build itself, for example by connecting the b22 chassis to earth via a loop breaker?

[amb]

If not, would it be possible to remediate the situation in the build itself, for example by connecting the b22 chassis to earth via a loop breaker?

Yes, that's worth a try.

[cobretti]

three-light tester result came back fine, all three lights are on, although the tests are pretty basic of course.

I think I'll ask on an electrical forum, see if anyone has any ideas.

If not, would it be possible to remediate the situation in the build itself, for example by connecting the b22 chassis to earth via a loop breaker?

Only two lights on the tester should usually come on for correct wiring. There is a small label on the tester giving you the right config.
There should be no voltage between ground and neutral. You can also measure resistance between ground and neutral. You should measure few ohms (1....5) depends on the length of branch circuit that runs back to main house panel where all neutrals and grounds are bonded, therefore you read continuity. If there is potential between ground and neutral, it's also a chance that some wire in house wiring may be loose somewhere.