Electric shock / Voltage leak from 12V Amplifier

[DTA]

Hi,
I have had a few of these cheap amplifiers which have been great for different little projects, but this recent one I got once plugged in gave me a minor shock from the metal case.
It is a Lvpin (also seen as lepai) LP-2020A+ as shown on this ebay link:
https://www.ebay.com.au/itm/Lvpin-Tripath-Class-T-Hi-Fi-Audio-Mini-Amplifier-With-12V-Power-Supply-LP-2020A-/281845921682

The ones I've had before have been great little units for the price, and put out quite a lot of sound for the package.
It runs of 12V DC so it doesn't have an earth pin, and something must be going wrong as it was a noticeable shock, though far from other electric shocks I'd received in my life.

I attached a wire to the case and to earth with my multimeter in series to take some readings and it is showing steady 84 volts AC, and 0.04 A.
I opened the case and tested a few points and got the same voltage, although some points such as the two tone potentiometers in the middle as shown in the attached photo were sitting at 15 volts AC.
Even the 3.5mm audio jack and the speaker outs were 84VAC! However the unit operates completely normally aside from this.

I'm a computer systems engineer so my electronics knowledge is fairly basic, but I'm trying to learn more. Any idea of what could be the cause of this, and if it is possible to fix, or is it not worth it?
From a basic visual glance nothing seems damaged or out of place.

Thanks,
Darren

[Gyro]

Welcome to the forum. 

Leakage current is common with non-earthed switchmode mains adapters. It is caused by so-called Y-Capacitor leakage - the capacitors that bridge from the primary to secondary side for interference suppression. This causes the output to float at around half mains voltage if this leakage has nowhere to go, this can cause a high impedance low current tingle.

What is NOT normal is your measurement of 40mA as the leakage current - this is way above the permitted safe level and means that the adapter is either faulty or badly designed. Under certain conditions, a 40mA flow through the body can be fatal - causing the heart to fibrillate.

Check that you have your decimal point in the right place in your current measurement, but if it is 40mA then it is unsafe to use! [EDIT: The mains adapter, that is].

[eeviking]

The problem is of course the cheap Chinese 12Vdc plugin power supply.
The amplifier itself is probably fine.

[Chris56000]

Hi!

I would look for a LINEAR 12V supply mains adaptor to suit your household plugs - a linear one will feel noticeably heavier than the cheap switch-mode adaptor supplied with your Amplifier and the leakage current will be less as well!

These Amplifiers, by the looks of the metalwork, look like they were originally designed for in-car 12V battery use, where the question of leakage currents from a.c. mains do not arise!

Another type of supply that would be useful is one of those metal-case "CB Rig" type linear supply units - these are transformer-isolated as well and are specifically intended for 12-14V car/mobile gear!

(Look for "13.8V Linear PSU" on eBay)

Chris Williams

[DTA]

Thanks for your prompt and informative replies fellas, you're legends.
I've been a fan of Dave's videos for a while, but hadn't realised there was such a populated and prolific forum here.

I would have never have guessed it was the power adapter. I had checked that and it was just showing the 12V DC, but I just tested the amplifier with a different power adapter (yes another cheap chinese one, that came with a LED strip) and there is zero voltage showing on the case with that.
I did mean to say that I was dubious about the 0.04 A reading (either false reading or my misreading) for the reasons you mentioned.
On my digital multimeter when on the ACA setting at 200m the display shows 00.0 mA, but turning it to the 20m/10A setting it shows 0.04 (without the mA) so was a bit confusing. Also, it shows the same with the other power adapter as well.

Chris, you are correct - these units are often listed as motorcycle/marine/car amplifiers. They do pump out some decent power - I wanted something small, and didn't have to be too high end for a home made jukebox I have sitting on my back verandah.  The amp would be enclosed in the box but wasn't that keen on getting a jolt if I ever adjusted the volume (or just touched it in general).. haha