Meridian Unplugged - Powered by MrTechGuy.co.uk Forums Discussion Meridian - Systems G68 v 568.2mm hum / ground questions

After recently acquiring a G68J to replace my 568.2mm I now have a very slight low level hum in my analogue system.

Unfortunately, I don't still have the 568.2mm to do a direct comparison but as far as I remember when I tried George's low level resolution test files there was no hum (and I practically had my ear touching the driver).

To make comparisons even more complicated the arrival of the G68J coincided with a channel failing in my Bryston 9B-ST power amp and while it was away getting repaired I opted to have it upgraded to 9B-SST status. When the channel failed I had a suspicion that the interconnect from the G68 for that channel had become faulty and had allowed ground to momentarily fail (the RCA connector relied on being screwed together tightly to form a ground clamp and over the years it had become loose). When I powered the Bryston on the LED for that channel briefly showed overload before going out. So I decided to replace the G68 to 9B interconnects as well.

I didn't notice the hum before the Bryston was upgraded and I changed the interconnects but I didn't really have much time with that combination and what time I did have was during the day when background noise could easily have masked the hum.

The Bryston only came back on Tuesday and last night was the first time I had tried playing it when the house was really quiet.

Suspecting a ground loop I disconnected everything apart from the speakers and when the Bryston was switched on I could just detect the very faintest of hiss with my ear almost touching the tweeters - but no hum. I then powered off the Bryston and started to connect the G68 and that's when I got a shock...

When I plugged the first interconnect from the G68 into the 9B I was doing it by touch and I managed to get my finger in between the connector and the Bryston and received a small electric shock. I think my mistake was to have had the G68 in standby when I was connecting it to the 9B. I suspect the G68 power supply has a floating ground and I temporarily became a way of bridging that to the mains ground in the Bryston.

Does anyone know if the G68 does have a floating ground and would it be feasible for that to give me the electric shock I felt?

Also, has anyone else had similar hum problems with a G68 and managed to overcome them?

Thanks.

-Ronnie

Apologies to everyone - including Meridian and Bryston.

After trying another round of interconnect swapping and hearing some more alarming hum I dug out my multimeter with the intention of testing the earth connection of all the mains cables.

That's when I found out that the plug on the mains cable to the G68 had managed to edge out of the mains distribution strip just enough so that live and neutral were still connected but earth was not. The distribution strip was hidden behind my subwoofer but in hindsight it should have been one of the first things I checked instead of the last. I had always assumed that the UK mains plug design where the earth pin is longer then the others would prevent that from happening.

I guess I'll never know for sure whether that was partly responsible (in conjunction with the loose RCA connector ground clamp) for the channel failing on the Bryston.

My only consolation is that the Bryston upgrade to SST status has worked out well :-)

-Ronnie

P.S. My worry about the floating ground on the G68 was originally sparked by looking at one the pictures in the PSU upgrade thread where it shows that the AC wire coming into the PSU is only two conductors. After looking for more internal pictures of the G68 I found another one which shows the IEC mains input socket and a separate earth lead coming from that to the G68 chassis. So definitely NOT a floating ground unless you do something stupid like I did.

That's difficult to picture!

Is the earth wire inside that plug connected properly, or has it come adrift? It should be physically impossible for the earth to be out before the other two, as you rightly say.

Cheers

George

Hi George,

The mains cable was the original moulded one with no access to the internal conductors. My multimeter seems to show that there are no continuity problems even when flexing the cable.

Space is tight behind the subwoofer so the distribution strip normally rests on its edge so that the cables coming out of the plugs point vertically upwards. I think the cable was managing to put just enough force on the plug and at just the right angle to keep live and neutral in and make earth marginal.

As you say I always thought it was physically impossible for a UK plug to make a power connection without earth - now I'm not so sure. As soon as I pushed the plug properly home the hum disappeared.

-Ronnie

Sounds like the dist. block isn't to BS spec.?

There should be no way the earth can disconnect before the live and neutral do.

When I first read your post I had to check your location; I couldn't see a UK plug doing what you said.

All the best,

Steve.