Testing Unbalanced Devices on AP Portable One

Lucky guess on my part, but back in the 80's I got bad memories from working with prosumer 2 wire line cord gear where they steal a chassis ground through a cap to "ASSumed" neutral...... you know what they say about ASSume.. 

JR

Gold said:

Ding! Ding! Ding! We have a winner, John R.  Live and Neutral reversed at the AC receptacle. I reversed Live and Neutral in the extension cable, and put a hard to miss label on the cable. All is right with the world now.  The DUT is behaving as expected now. Thanks for everyone's help.

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By 'receptacle' do you mean the 'wall socket' - I'm in the UK and we use some different terminology.
If it is the socket outlet shouldn't you be rectifying that ? How are your other sockets wired ?

JohnRoberts said:

FWIW 2 wire unbalanced consumer gear can't have safety grounded chassis, and proper balanced interfaces so audio ground (0V) is a bit of a trapeze act.

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Not quite sure what is meant by 'proper balanced interfaces' wrt 2/3 wire mains ?
I may be misunderstanding the words...

Newmarket said:

Not quite sure what is meant by 'proper balanced interfaces' wrt 2/3 wire mains ?
I may be misunderstanding the words...

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Well I didn't literally say WRT 2/3 wire mains.

Professional balanced or differential audio interfaces use 3 leads, audio+, audio- and shield. Most unbalanced consumer or prosumer gear use 2 wire audio interfaces involving an Audio + and audio 0V (with 0V also serving as shield).

Some such gear with 2 wire mains leads (No safety ground) will bootleg a capacitor between chassis ground and presumed neutral (mains power 0V), for beneficial noise shielding.  Inadvertent swaps of line and neutral in faulty outlet wiring with such bootleg or stinger chassis ground caps, can dump mains current into the chassis. This current is limited to lower levels than human hazard by size of the small caps used, but can still corrupt the noise floor.

JR

Newmarket said:

By 'receptacle' do you mean the 'wall socket' - I'm in the UK and we use some different terminology.
If it is the socket outlet shouldn't you be rectifying that ? How are your other sockets wired ?

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Yes I mean wall socket. I had separate power run for the studio with a home run ground to a cold water pipe . The power in the shop is what came with the space. The reason I don’t address that is I have a strict policy of never asking the landlord for anything. That’s why he hasn’t doubled my rent when the lease is up.

I guess the thing to do would be to get an isolation transformer since the problem power feed is a shared circuit with other spaces. Maybe I’ll do that.

JohnRoberts said:

Well I didn't literally say WRT 2/3 wire mains.

Professional balanced or differential audio interfaces use 3 leads, audio+, audio- and shield. Most unbalanced consumer or prosumer gear use 2 wire audio interfaces involving an Audio + and audio 0V (with 0V also serving as shield).

Some such gear with 2 wire mains leads (No safety ground) will bootleg a capacitor between chassis ground and presumed neutral (mains power 0V), for beneficial noise shielding.  Inadvertent swaps of line and neutral in faulty outlet wiring with such bootleg or stinger chassis ground caps, can dump mains current into the chassis. This current is limited to lower levels than human hazard by size of the small caps used, but can still corrupt the noise floor.

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Understood. I wasn't considering the 'stinger' cap scenario. I thought that only really happened in old school guitar amps ! Cheers.

Gold said:

Yes I mean wall socket. I had separate power run for the studio with a home run ground to a cold water pipe . The power in the shop is what came with the space. The reason I don’t address that is I have a strict policy of never asking the landlord for anything. That’s why he hasn’t doubled my rent when the lease is up.

I guess the thing to do would be to get an isolation transformer since the problem power feed is a shared circuit with other spaces. Maybe I’ll do that.

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Ah I hadn't thought about the landlord / lease thing.
Is it legal though ? Not familiar with how the legal side of that works over the pond...

Newmarket said:

Understood. I wasn't considering the 'stinger' cap scenario. I thought that only really happened in old school guitar amps ! Cheers.

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Me too but I learned the hard way trying to interface consumer VCRs into low end SMPTE sync'd audio lock up (back in the 80s). I had to take apart the consumer VCRs to figure out why they interfaced so poorly with studio gear. Luckily for me I just had to read/write SMPTE time code signal but even that could be compromised.

JR

Newmarket said:

Ah I hadn't thought about the landlord / lease thing.
Is it legal though ? Not familiar with how the legal side of that works over the pond...

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It is certainly against code. I have insurance for my space and the landlord must have insurance. I’m not calling in a code violation. Next time I see the electricians I might point it out.