Power Amplifier mains hum

[sussexchris]

Hi...I have an IMG Stageline mosfet power amp with a mains hum that I can't solve and would be grateful for any advice.

The hum increases with volume and is present even when no equipment (except speakers) is connected. The hum is removed if I disconnect the earth wire in the plug (I did this is safe conditions and realise this is not a safe solution). The amp has a gnd lift switch which makes no difference whatsoever!

Could it be poor design, or a fault?....is there a magic solution!?

I'm handy with a soldering iron!

Many thanks for any help,

Chris.

 

[Gizmologist]

Hum

Some amps have a 10-56 ohm resistor between the earth ground and the audio ground. One solution is to leave the electrical(safety) ground connected and add a .1 mfd (100v or higher) ceramic disc capacitor between the resistor tying audio ground and the chassis ground. This will provide sufficient grounding for the audio path but isolate the AC ground.

Look for a resistor tied to the audio and /or the power supply ground connection (the center tap on a =/- supply) then tied to the chassis ground. Lift one end of the resistor and put the cap in series.

 

[TLS Guy]

Hi...I have an IMG Stageline mosfet power amp with a mains hum that I can't solve and would be grateful for any advice.

The hum increases with volume and is present even when no equipment (except speakers) is connected. The hum is removed if I disconnect the earth wire in the plug (I did this is safe conditions and realise this is not a safe solution). The amp has a gnd lift switch which makes no difference whatsoever!

Could it be poor design, or a fault?....is there a magic solution!?

I'm handy with a soldering iron!

Many thanks for any help,

Chris.

Power amps are generally best left ungrounded. It will ground through the connecting cables.

My Quads come with three pin connectors, but there is no connection between the grounding pins.

I would leave the ground lifted.

 

[Gizmologist]

absolutely NOT!!!

"Power amps are generally best left ungrounded. It will ground through the connecting cables.

My Quads come with three pin connectors, but there is no connection between the grounding pins.

I would leave the ground lifted."

Serious and illegal mistake. Safety grounds must NOT be disabled and DEFINITELY not expect and audio ground path to function as a safety ground.

This can be extremely dangerous to people and equipment.

 

[TLS Guy]

"Power amps are generally best left ungrounded. It will ground through the connecting cables.

My Quads come with three pin connectors, but there is no connection between the grounding pins.

I would leave the ground lifted."

Serious and illegal mistake. Safety grounds must NOT be disabled and DEFINITELY not expect and audio ground path to function as a safety ground.

This can be extremely dangerous to people and equipment.

All I can tell you is that if you ground a preamp and an amp even with no 4 copper between them you usually ave a poorer S/N ratio. So I use the EEC connectors that came with my power amps that have no connection on the ground cable.

I have read many articles in pro journals that advise grounding only the mixer or preamp and not the power amp as well.

The OPs problem is not unusual in my experience.

 

[WaynePflughaupt]

I have read many articles in pro journals that advise grounding only the mixer or preamp and not the power amp as well.

Typically that only applies for portable touring systems that move from venue to venue. Often they don’t have total control over their power situation and they’re just trying to eliminate a noise problem anyway they can to get through a show. A permanent installation will typically include the proper electrical service – dedicated circuits and/or dedicated service - and won't have any ground loop problems (or at least shouldn't).

Bottom line, Gizmologist is right. The safety ground should not be bypassed. The OP should troubleshoot and try to eliminate the problem, or perhaps switch to home audio amps that don't use a grounded plug.

Regards,
Wayne A. Pflughaupt

 

[TLS Guy]

Typically that only applies for portable touring systems that move from venue to venue. Often they don’t have total control over their power situation and they’re just trying to eliminate a noise problem anyway they can to get through a show. A permanent installation will typically include the proper electrical service – dedicated circuits and/or dedicated service - and won't have any ground loop problems (or at least shouldn't).

Bottom line, Gizmologist is right. The safety ground should not be bypassed. The OP should troubleshoot and try to eliminate the problem, or perhaps switch to home audio amps that don't use a grounded plug.

Regards,
Wayne A. Pflughaupt

Quad power amps of various vintages have never been grounded. The 303 had a two lead mains lead so did the 405 in its various iterations. The 606 and 909 have what looks like a standard IEC cord, but the ground connection are empty. Quad preamps have grounded plugs.

If that was good enough for Peter Walker its good enough for me.

Even with my robust star cluster ground system using 4 gauge copper and grounded racks S/N is better the way Peter Walker set it up. I have used Quad amps exclusively for 42 years and have never had a problem.

Right now I have two 303s in use, five 405 2s and three 909s.

 

[WaynePflughaupt]

Quad power amps of various vintages have never been grounded. The 303 had a two lead mains lead so did the 405 in its various iterations. The 606 and 909 have what looks like a standard IEC cord, but the ground connection are empty.

That means the amps were double-insulated Class II designs and as such didn’t need an earth ground connection (even if its power cable had one for some strange reason). The Adcom amps I used to use were also Class II, as are many (if not most) consumer-grade power amps.

The OP’s amp is most likely a Class I appliance that requires an earth ground. It shouldn’t be bypassed. I haven’t had any “problems” either (assuming by that you mean getting shocked), but that doesn’t mean no one has. The issue is safety, not S/N.

Bottom line, the OP should do it right and take the necessary steps to fix the ground loop, not use a cheater plug.

Regards,
Wayne A. Pflughaupt