Meridian Unplugged - Powered by MrTechGuy.co.uk Forums Discussion Meridian - Systems Sorting out Ground Loop problem with Modded Oppo

RE. the noise... anyone know if the Oppo shares its audio ground with its neutral?

Shawn

If you already tried that then it's unlikely switching to optical.

It sounds like a direct conflict between AMP and Oppo.

I think only Gary can answer that since he modded the power supply, from what I understand.

I don't believe any audio devices hard wire Neutral to audio ground because it's too easy to defeat the keying and make chasis hot. I've seen some devices have a circuit between neutral and chassis but you'd know better than I what that commonly is. Sometimes house ground is audio ground and sometimes it floats (usually floats on 99.9999% of DVD players). Me thinks that's where the conflict is, in switching from floating ground on Oppo to house ground causing a conflict with the amp. If adding the Oppo to the system causes a ground loop, it HAS to be half the culprit.

Mark,

just to make sure we all understand what's going on:
- When you connected just the Oppo to the amp were there any other cables connected to either the amp or the Oppo? It is vital to disconnect all cables from the two components. If you leave any connection say from the amp to your set top box you may have a ground loop going through the set top box (in through the cable from the amp and out into the antenna or through the power plug even when switched off, because the ground is NOT cut off even then).
- Also make sure the subwoofer is disconnected (not just turned off).
- This should leave you with: 2 power cables, 1 audio cable from Oppo to amp, several speaker cables. Nothing else!
- Are you using 2 power cables that both have a ground? Does the amp have a ground receptable for the power cord?
- Does your amp allow a "ground lift" (physical switch or maybe in the setup)? What model is it anyway?
- Try placing the Oppo as far away from the amp as possible. Maybe the amp induces a current in the Oppo.

Since you write that plugging more components into the amp reduces the noise I would strongly guess that there is NO induced current but a ground loop. Plugging in more components offers the current different paths and if those paths are easier (= less resistance) then the current will take this route. That's why connecting the 2 chassis (attach to any screw) with a thick cable should help (current flows through the thick cable that has less resistance thereby reducing the current in the audio connection).

Tell us how you go, cable guy

My advice would be to lift the ground (use an ungrounded power cord) on the Oppo.

1. By "amp" presuming you mean headphone amp as the Oppo would never go to the power amp directly. With only the 1 pair of analogue RCAs from Oppo to headphone amp, its all fine (when the power amp is on too). The STB issue was what seemed to occur in scenario 2. I might try disconnecting STB and antennae entirely though just to eliminate that possibility.
2. Yes, was disconnected
3. Are you suggesting trying with Oppo and power amp on, cable to headphone amp (but headphone amp turned off(, speaker cables and that's it? That's what I had last night + inputs from the Meridian to the power amp, which I had not disconnected. But it was this scenario that worked ok.
4. Yes as far as I know (3 pins, Aust plugs); 3 pins on the IEC end too. Ground receptable?
5. Do not know. Model is Redgum Audio RGH900 6 channel MOSFET power amp - an Australian company.
6. It's as far apart as it can be already. I'm still learning about this, but inducement doesn't seem like the answer.

I have to do some work tonight (spent the last 2 nights playing with the cables), so I will to complete all the various suggestions Friday/Saturday.

Thanks again to all.

So that would have just the 2 pins to the power point instead of the present 3?

Does that mean then because it is connected to the power amplifier, the Oppo would be grounded that way? I'm just checking that this wouldn't potentially eave the Oppo 'live' in certain cirumstances so that I (or the family) wouldn't get a nasty shock.

I thought you use a home theater amplifier = "amp". Are you using the headphone amp as a pre-amplifier that bundles the signals from CD, STB, LP-player, ...? If so then disconnect anything that is attached to the headphone amp except the Oppo and the power amp. Put all 3 devices into the same power board and turn them on. Is there any humming? If it's gone try adding the other components. Start with the STB. I bet that as soon as you connect it to the antenna you'll hear the hum again. If this is the case you need a ground filter for your antenna cable. You can get this for little money in any electronics shop (maybe 20-50$).

I meant: does the socket on the amp where you plug in the power cord have 3 pins also? If so then the chassis is grounded by the 3rd cable inside the power cord.

For now I wouldn't use a ground lift (it's not a real disconnection but rather a resistor & capacitor combo; more info here: http://www.epanorama.net/documents/groundloop/groundlift.html ). Try the antenna solution first even if it means spending some money, because that would actually remedy the problem!

Enjoy!

Nutfish,

I use the Meridian 568.2mm as the pre/pro into the 6 channel power amp. Headphone amplifier is standalone (powers headphones only). It uses analogue outs from the STB and Oppo as the Meridian has no line level analogue out.

Also, yes, both the Oppo and the power amp have 3 pins on the socket.

As to more testing I have eventually done tonight taking components out of the loop entirely:

1. I took the TV out of the loop (cables, power plugs etc)- it's not the TV.
2. Then I took out the STB and antennae - it's not the STB/antennae.
3. Then i took out the headphone amp - it's not the headphone amp.

All that was left was Oppo and power amp (both powered on) plus cabling via Meridian to the power amp.

I then realised the other constant was the Meridian as last night I had never disconnected the analgoue outs to the amplifier. Of course, take the Meridian out and then there is never a connection to the power amplifier so I'd never hear anything from the speakers.

So, I tried to think how to test to make sure it is not the Oppo/Meridian combo - I thought i'd double check via the headphone amplifer and headphones and leave the power amplifier out of it entirely.

So I now had in the loop the Oppo & STB both with analogue/digitial cables into the Headphone Amp/Meridian. There was no headphone noise at all. However, this same combination caused problems last night with the power amp in the loop.

* * * * * *

I think I have exhausted cables in out, component testing etc now.

The 2 components that are always constsant are the Oppo and power amplifier and presumably, I need to work out how to isolate one from the other or remove/null this grounding conflict.

He seems to be trying everything but the OBVIOUS !!!

I totally agree. I didn't want to say it directly because I DON'T know how safe it is (between Gary's mod, not knowing what an Oppo normally does and 220V single phase I'm not familiar with). But I can almost guarantee your right and that will remove it. Now as to a long term SAFE fix I don't know. It may be perfectly safe to lift the ground on the Oppo. And I certainly would try it to confirm the source of the problem anyway.

I moved this discussion on the ground loop here

I don't have an ungrounded cord to do this, plus safety aspect (as I noted a few posts up) is also a concern. Should this be where I go, I could probably find some external help to ensure I don't cause myself damage.

Appreciate all the help though.

I am also contacting my power amp manufacturer for their comments.

Probably also need Gary to advise what the power mod is.