Need help with ground loop hum.

[TLS Guy]

Just did a Google search... the Outlaw 975 and 60Hz hum is the subject of many threads on several forums. Ugh. Of course, I also did a search on Marantz pre-pros, and got some similar results, though fewer. Hmmm.

Not surprising as professional practice is to only ground the preamp and never a power amp. That is why Peter Walker had IEC cords especially made. The grounding pins were dummies and not connected as supplied with his power amps. If people lost the original cord, they were perplexed as to why they now had hum!.

With what you have just said, make a chassis ground to the preamp and lift the ground to the power amp. Chances are that will solve your problem. I have not grounded a power amp in years as it invariably leads to trouble.

 

[KEW]

Unfortunately, DJ, your guess wasn't correct. Completely disconnecting the Xfinity box didn't change the hum at all.

Now I have to do the unplug-one-input-at-a-time method, which will be a PITA. My suspicion is that the problem is rooted in the Outlaw 975 pre-pro.

In that case, you may want to disconnect everything and plug one thing back in at a time!

 

[Irvrobinson]

With what you have just said, make a chassis ground to the preamp and lift the ground to the power amp. Chances are that will solve your problem. I have not grounded a power amp in years as it invariably leads to trouble.

Good suggestion. I'll try that soon.

 

[djreef]

Unfortunately, DJ, your guess wasn't correct. Completely disconnecting the Xfinity box didn't change the hum at all.

Now I have to do the unplug-one-input-at-a-time method, which will be a PITA. My suspicion is that the problem is rooted in the Outlaw 975 pre-pro.

DOH!!!

DJ

 

[djreef]

In that case, you may want to disconnect everything and plug one thing back in at a time!

Yep, because oddly enough it could be a problem with a single connection port on the Outlaw, or it could even be a bad cable. Design Engineers sometimes have a bad day, too.

 

[Speedskater]

Not surprising as professional practice is to only ground the preamp and never a power amp. That is why Peter Walker had IEC cords especially made. The grounding pins were dummies and not connected as supplied with his power amps. If people lost the original cord, they were perplexed as to why they now had hum!.

With what you have just said, make a chassis ground to the preamp and lift the ground to the power amp. Chances are that will solve your problem. I have not grounded a power amp in years as it invariably leads to trouble.

It is both dangerous and illegal to remove a Safety Ground/Protective Earth connection.

 

[TLS Guy]

It is both dangerous and illegal to remove a Safety Ground/Protective Earth connection.

If you use good cables between pre and power amps, then all grounds are properly bonded by the shields. Many pro amps include switched to lift the ground.

Many time this is the only way tp stop the hum. If you buy a three pin pre and three pin power amp, then there is pretty much always misery. Best practice is to have the pre grounded and the power amp grounded through the interconnecting cables.

 

[highfigh]

Yep, because oddly enough it could be a problem with a single connection port on the Outlaw, or it could even be a bad cable. Design Engineers sometimes have a bad day, too.

They should be able to design an amp properly in their sleep and I would bet that they have a test rig that allows them to alter the quality of the ground path, but that could just be optimism on my part.

 

[TLS Guy]

They should be able to design an amp properly in their sleep and I would bet that they have a test rig that allows them to alter the quality of the ground path, but that could just be optimism on my part.

Its not that simple. The problem does not lie within the amp. A power amp has both voltage and power amplification, so is very sensitive indeed to even very small resistances between grounds. So much so that the resistance difference between the interconnecting cables and the house ground will nearly always cause some or a lot of trouble.

I and others have found that the lowest hum from ground loops comes from grounding at the preamp and grounding the power amp back though the shields. With decent quality cables the shields have very low resistance indeed.

That is why Peter Walker made it look as if his amps were grounded with the power cable, when they were not. Hum problems pretty much solved and everyone happy.

This is one advantage of integrated amps and receivers, as preamp and power maps share the same SHORT ground plane.

 

[KEW]

It is both dangerous and illegal to remove a Safety Ground/Protective Earth connection.

Agreed, but what may not be clear is that Peter Walker is the designer of the Quad Power Amps, so I do believe the manufacturer can choose whether to design as a grounded or non-grounded component!
I'm not sure why he didn't simply use a non-grounded IEC cable instead of having custom cables that appear grounded made, but suspect it has something to do with European plugs or maybe when he did this.

 

[TLS Guy]

Agreed, but what may not be clear is that Peter Walker is the designer of the Quad Power Amps, so I do believe the manufacturer can choose whether to design as a grounded or non-grounded component!
I'm not sure why he didn't simply use a non-grounded IEC cable instead of having custom cables that appear grounded made, but suspect it has something to do with European plugs or maybe when he did this.

Well his amps had 3 pin IEC plugs and the ground pin on the amp side was connected. I suspect that was something to go with EEC rules. However there was no ground in the cable for North American market anyway, even though the AC plug was a grounded plug by external appearance. However it was not a grounded cable. I still have my cables.

Now the UK uses MK plugs, and they have to have a grounding pin even if that pin is non conductive plastic. In the latter case the pin is non conductive, but has a metal tip for strength. This is because the longer ground pin opens up the live and neutral sockets as it is inserted. All MK plugs are child proof from inception without adding protectors. This is a very elegant, safe and fool proof system.

No two pin plug can be inserted into a UK electric outlet.

 

[slipperybidness]

Its not that simple. The problem does not lie within the amp. A power amp has both voltage and power amplification, so is very sensitive indeed to even very small resistances between grounds. So much so that the resistance difference between the interconnecting cables and the house ground will nearly always cause some or a lot of trouble.

I and others have found that the lowest hum from ground loops comes from grounding at the preamp and grounding the power amp back though the shields. With decent quality cables the shields have very low resistance indeed.

That is why Peter Walker made it look as if his amps were grounded with the power cable, when they were not. Hum problems pretty much solved and everyone happy.

This is one advantage of integrated amps and receivers, as preamp and power maps share the same SHORT ground plane.

Most AVRs that I have seen tend to be 2-prong cords.

Of course your arguments still hold true, but I thought I would mention that.

 

[TLS Guy]

Most AVRs that I have seen tend to be 2-prong cords.

Of course your arguments still hold true, but I thought I would mention that.

They do but probably should not. They are so called double insulated. I believe all systems should have one good ground and just one ground. The best spot is the preamp, or if no preamp the integrated amp or receiver. The problem is that manufacturers don't want all the customer service calls from improperly grounded cable/satellite systems and phone lines. So often times there ends up being either no ground or a poor ground, which is not good I don't think.

 

[Speedskater]

When ever you have more than one piece of equipment with a three pin grounded plug you will always be prone to this issue, what ever you do.

Personally I would just break the ground on the sub with the 3 to 2 adapter. The sub will still be grounded. If you are worried about a shock, just make sure you unplug your sub before disconnecting the sub signal cable..............................

WARNING, WARNING, WARNING
It is extremely dangerous to remove the AC power Safety Ground/Protective Earth wire.

The task of the Safety Ground/Protective Earth wire is to trip the circuit breaker in the event of a Ground Fault (short circuit). An interconnect system Shield/Return is not designed for that task! If asked to trip the circuit breaker, it could fail (fuse) long before the breaker trips. So now you need to trouble-shoot the audio problem. When you disconnect the interconnect, there is full AC line voltage between the interconnect shield and the chassis.

 

[TLS Guy]

WARNING, WARNING, WARNING
It is extremely dangerous to remove the AC power Safety Ground/Protective Earth wire.

The task of the Safety Ground/Protective Earth wire is to trip the circuit breaker in the event of a Ground Fault (short circuit). An interconnect system Shield/Return is not designed for that task! If asked to trip the circuit breaker, it could fail (fuse) long before the breaker trips. So now you need to trouble-shoot the audio problem. When you disconnect the interconnect, there is full AC line voltage between the interconnect shield and the chassis.

Actually the screen of good cables has much more current capacity then the ground wire of most power cables. You just need to make sure the amp is off if the preamp is not connected. In practice though the chassis is NOT going to become live in any decent design. For a start the power transformer will float the unit above ground, so the live would have to touch the chassis ahead of the transformer and this would blow the AC line fuse immediately.

If the amps rail voltage gets connected to ground this will blow the amp line fuses immediately. It is not in any way dangerous to lift the ground of a properly designed power amp.

Output transistor failure is not uncommon. This always connects the collector rail voltage to ground and activates protection and or blows the fuse. I have never seen a chassis become live in over 60 years experience of amps, both tube and solid state.

 

[Alex2507]

This is an old thread, but relevant because I've noticed I have a very annoying 60Hz hum on my HT system. It's probably been there since Day 1 and I'm just recently detecting it, but you know how humans are, once trained the subtle becomes obvious.

Everything is plugged into a Tripplite surge protector, and only one component has a three-pronged power plug, the ATI amp. With the exception of the pre-pro outputs to the amp, which are single-ended BJC interconnects, the rest of the system is interconnected with HDMI. The system is moderately complicated, including an Xfinity cable box, a Sony BD player, Roku, Apple TV, cable modem, a Netgear wireless router, and a partridge in a pear tree. I suspect contributors to the issue are the hyper-sensitive Klipsch speakers, as when I swapped in a pair of inefficient JBL mini-monitors the hum was nearly inaudible.

I tried plugging the amp into an isolation transformer - no difference.
I put a Jensen transformer on the CATV coax - no difference. ($70 down the drain.)
I tested all interconnect connections - no difference.

The only real hints I found is that when I switch inputs on the Outlaw pre-pro it mutes the outputs, and the hum goes away. That probably means the hum is in the pre-pro or earlier in the signal path. When I did that test the BD player and the Xfinity box were turned off. Switching the input selection on the pre-pro makes no difference; the hum is present on all selections, including the internal FM tuner.

I'd pull my hair out, if I had any.

Okay, so the solution was ... ?

Selling it to me?

 

[Alex2507]

If you buy a three pin pre and three pin power amp, then there is pretty much always misery.

I'll see how it goes lifting the ground on my amp. Don't worry though, I'll have a bucket of water to throw on her if she start to smoke.

EDIT:usually I don't pay much attention to Irv's problems because usually I can't afford his problems.

 

[Irvrobinson]

Okay, so the solution was ... ?

Selling it to me?

Have you even taken it out of the box yet?

I forgot about this problem. I sold it to you months after I replaced it with the Marantz AVR. If you have similar hum problems when you do install it, I'll refund your purchase price.

 

[Alex2507]

If you have similar hum problems when you do install it, I'll refund your purchase price.

This is how people end up with free pre-pros.

You're right about it still being in the box.

I have a couple of Marantz MA700 monoblocks that as luck would have it, have a 2 prong cord. Plus they're THX certified so I think that means the gain is min 29 db and the input for rated power is just 1 volt.

I don't think I'll have an issue.

 

[Alex2507]

I forgot about this problem.

You can forget about it again. I finally hooked it up for a trial run with the Marantz MA700s. It's fine.

I figured you deserve to know that you're off the hook. I'm listening to Queen out of the tiniest Infinity Primus speakers they ever made. Sounds like a system costing 5X more.