[starkiller]

So, bought a PrimaLuna Dialogue 4 amp.  Mighty nice with sound to match but I do have a hum from both speakers..here is what I have done to scope it out...

1.  Swapped power cables..

2.  Swapped from using power cable into PS Audio Duet to just the wall outlet

3.  Added the adapter to go from 3-prong to the 2-prong

Only thing that got rid of the hum was number 3....

Thoughts??

Thanks!

[Flyin_V]

I had the same symptoms. Even though there was no direct RCA interconnect connection between the cable box and audio system, there was a ground loop through the AC. Installed a $20 cable ground loop isolator between the incoming cable and the box. Silence.

[jea48]

So, bought a PrimaLuna Dialogue 4 amp.  Mighty nice with sound to match but I do have a hum from both speakers..here is what I have done to scope it out...

1.  Swapped power cables..

2.  Swapped from using power cable into PS Audio Duet to just the wall outlet

3.  Added the adapter to go from 3-prong to the 2-prong

Only thing that got rid of the hum was number 3....

Thoughts??

Thanks!

3.  Added the adapter to go from 3-prong to the 2-prong

That would indicate the hum is caused from a ground loop.

To help find the culprit, piece of equipment causing the ground loop, you need to start with only the power amp and preamp connected together by interconnects. All other source interconnects need to be disconnected from the inputs on the preamp.

Turn on preamp and power amp. Check for hum.
If you have the hum then the problem is either the preamp or the power amp.

If the hum is gone.

Next connect just one piece of source equipment to the preamp's inputs.
Check for hum. If you hear the hum this piece of equipment is causing the ground loop hum.

Repeat the above test connecting only one piece of source equipment to the preamp's inputs checking for ground loop hum.

If by chance you have a cable TV box connected to the audio system in any way start there first. Disconnect the incoming CATV coax cable from the cable box and then check for hum. If the hum is gone then the problem is more than likely the CATV coax cable shield is not earth grounded properly on the outside of your house. For lightning protection it should checked. For a band-aid to stop the ground loop hum heard through your audio system add a CATV coax cable isolator as the previous poster mentioned.   

[FullRangeMan]

This amp have hum pot control?

[avta]

I would suggest running the amp alone ( i.e no preamp connection ) with both speakers connected. If hum is gone connect the preamp but nothing else. If hum returns the preamp and or the interconnects are the problem. Some preamps are quite sensitive to interconnects. If you get hum with preamp connected but not with amp alone I would try different interconnects first.

[rotarius]

I would not use a cheater on a tube amp, not safe.  In my experience with tube amps, lifting the ground of the preamp and source does the trick.

[starkiller]

I am not familiar with the term "lifting the ground"...can you explain please?

[jea48]

I am not familiar with the term "lifting the ground"...can you explain please?

Break, open, the safety equipment grounding conductor found at the wall receptacle from the metal chassis of a piece of equipment.

That is what a 3 prong to 2 prong adapter plug, (commonly call a ground cheater), does.
It breaks the safety equipment ground connection at the outlet from the power cord ground pin of the plug from the piece of equipment it is connected to. It lifts the ground.

If you must use a ground cheater, if you are concerned about your or a family member's safety from receiving an electrical shock in the event of a ground fault/failure of the piece of equipment, you can change out the existing wall receptacle to a GFCI (Ground Fault Circuit Interrupter) receptacle.

[rotarius]

Good explanation by jea48.  Let us know if the amp hums by itself with nothing connected or if it only occurs when another component is connected to it.  If it's the latter, you can try using the Ebtech Hum X on the preamp and source.  It does not break the ground, so safe but is only rated for 6A.

FYI, my tube amp is very quiet by itself but when I connect my AES AE-3 tube preamp, noticeable hum.  When I connect my NAD preamp which has a 2-prong plug, no issue.

[raindance]

You should note that a ground loop does not indicate there is anything wrong with the amp. You have a source component that is grounded and that is the likely cause. Putting a cheater plug on that source component will cure the problem.

Note also that using RCA connections means the chassis of each piece of gear is tied together through the audio cables, so as long as they are plugged in, you have a safety ground in place. This applies to most gear that does NOT use tubes. Properly designed tube gear has it's RCA inputs isolated from the chassis. In most cases they do tie back to the chassis, but the only way to be sure is to measure with a multimeter between connector shield and chassis.

Note also that back when tubes ruled we had the so called AC/DC radios that had neutral on the chassis in normal use and live on the chassis if an outlet was wired wrong or you inserted the plug the wrong way I think we were all tougher in those days... I come from a 240 volt country and I can testify that it kicks a lot harder than 120 volts over here .

Anyway, just do the cheater thing on the source device/s and you'll be good.

[JerryM]

It could also be DC offset on the AC line. This is a well known problem that can be quite maddening. Search for the term here on AC and you'll find some good solutions. 

Have fun,

Jerry

[raindance]

Jerry M - that will make the transformer hum but will not come out of the speakers.

[glynnw]

I recently bought an amp that had a low level of hum through my 95 db speakers- hum level did not change with change in volume.  So I sold it to a friend with Maggies - problem solved. Any friend with inefficient speakers will do.  He has no hum at all when in his system. 

[DaveC113]

You should note that a ground loop does not indicate there is anything wrong with the amp. You have a source component that is grounded and that is the likely cause. Putting a cheater plug on that source component will cure the problem.

Note also that using RCA connections means the chassis of each piece of gear is tied together through the audio cables, so as long as they are plugged in, you have a safety ground in place. This applies to most gear that does NOT use tubes. Properly designed tube gear has it's RCA inputs isolated from the chassis. In most cases they do tie back to the chassis, but the only way to be sure is to measure with a multimeter between connector shield and chassis.

Note also that back when tubes ruled we had the so called AC/DC radios that had neutral on the chassis in normal use and live on the chassis if an outlet was wired wrong or you inserted the plug the wrong way I think we were all tougher in those days... I come from a 240 volt country and I can testify that it kicks a lot harder than 120 volts over here .

Anyway, just do the cheater thing on the source device/s and you'll be good.

IC cables and associated wiring/isolation/etc. in chassis is extremely unlikely to be of sufficient gauge to serve as a safety ground, it's not to code and is potentially dangerous.

If you choose to use a "cheater" plug you should be aware of your potential liabilities including insurance not paying for damages due to a fire and you being liable for any injury or death caused by inadequate safety grounding.

[raindance]

Seriously? Doom and gloom much?

[starkiller]

Talked with the folks at PrimaLuna(Upscale) and they said a cheater plug is fine as the amp is grounded through the preamp...

[rotarius]

Yes, in theory the components need only one ground because they are connected by cables but what others are saying is the interconnect ground wire or shielding is not sufficient in gauge to act as a safety ground (risk of catching fire!). 
So you tried putting the cheater on the preamp instead?  That should reduce the hum as well.  I ask because if a cheater on the preamp works, you can replace the cheater with the HumX and maintain the safety ground.

[sfox7076]

Cheater plug masks a problem, it does not solve it.  I am in favor of grounding...  I am a fan of figuring out the issue.  I would unplug the power cord of everything except the amp and preamp.  Disconnect the coax for cable if that is in the mix.  Turn the amp and preamp on. Is there hum?  If there is, unplug the power to the preamp.  I would switch interconnects and then try again.  Then plug each other item in one by one.  When the hum returns you have your culprit.  Then check the grounding of that unit and how it is plugged in and connected to your system.  Come back when it is isolated. 

[jea48]

Talked with the folks at PrimaLuna(Upscale) and they said a cheater plug is fine as the amp is grounded through the preamp...

LOL, I am sure they did. Ask them to send you an email saying it is ok. Report back what they say.

For what its worth a dead short from the hot to the chassis should cause the AC line safety fuse for the power amp to blow before damaging the ICs or possibly the jacks at the amp and or preamp. The inrush current is super fast though. Even though the small signal ground wire in the ICs will work like a resistor, causing a voltage drop across the ICs, the current passing through the ICs could still easily be 50 or 60 amps for a split second.

Just curious, has any one reading this thread ever used a cheater on a tube power amp that developed a hot to chassis ground fault were the ICs saved the day long enough for the equipment AC line safety fuse to blow? Of course the fuse would only blow if the ground fault was on the load side of the fuse. If the ground fault happened on the AC line ahead of the fuse the poor little signal ground wires in the ICs would carry the full ground fault current on the safety equipment grounding conductor back to the electrical panel service neutral conductor. In that case the ICs and jacks on the audio equipment will surely lose the battle.

OP,

If you must use a cheater plug on the amp.
Change out the wall receptacle feeding the power amp to a GFCI duplex receptacle. If there is ever a ground fault within the amp the GFCI will trip open if 5 to 6 ma travels on the equipment grounding conductor, saving the ICs.

[starkiller]

Here is the reply from Upscale(Primaluna)..

"As long as at least one component in your system is grounded (i.e. your preamp), then you're fine.  The power amp is sharing the ground with your preamp, via the interconnects. "