Ground loop cure?

[harlequin]

Hi,

I am trying to figure out what is going on with my valve based amplifier system.

I have a valve preamp connected to a very sensitive valve power amplifier. The preamp has a passive line stage (as well as a mm phono stage) and so doesn't need to be mains powered when listening to my CD player. All is good but when I then plug the preamp to the wall socket I get a vicious hum coming out of the speakers, irrespective of volume level. I figured that there must be a ground loop, which I have struggled to fix.

Note that I get this hum through the speakers even when the preamp is switched off but plugged into the wall socket. What is confusing me is that when I then connected the preamp to an integrated amplifier (solid state in this case), turned the preamp volume control to maximum and controlled volume with the integrated amp I get no hum whatsoever, irrespective of whether the preamp is plugged in or not. I also don't hear any hum when the CD player (that has a pair of variable output sockets) is connected directly to the power amplifier.

Regards
Harlequin

 

[azazello]

You can try to rotate plug of preamp. to the wall to 180 deg.

 

[popilin]

You have a ground loop between amp and preamp earth connection, put a loop braker in the amp earth.

Edit: I mean between audio ground and chassis ground on power amp.

 

[harlequin]

Thanks popilin,

Do you mean a low value resistor and capacitor in parallel? Where would be the best place to add this breaker?

 

[popilin]

Also two 3A diodes anti-paralleled should work.

Your amp must be one and only one ground connection to their chassis, cut this connection and put the loop breaker, from audio ground to chassis.

By safety you always must keep the earth connection to the wall socket.

 

[Junk Audio]

I've had this happen before and a jumper between the amp and pre chassis solved it. Take an alligator clip jumper and see if it works. If it does, you can make a permanent connection.

 

[harlequin]

I tried putting a loop breaker in the power amp but this didn't help and actually worsened the situation. I then placed a jumper between the preamp and power amp chassis. This did help to reduce the amount of hum, although it is still quite high.

 

[speakerfritz]

I had the same problem....if the preamp plug ground was used..hum even if preamp was off....if preamp ground prong not used...no hum.

 

[MelB]

I had a humm in my system due to a ground loop through my satellite receiver. Apparently quite common. One of these solved everything:
JENSEN TRANSFORMERS, INC. - ISO-MAX® CI-2RR Stereo Audio Input Isolator

 

[popilin]

I tried putting a loop breaker in the power amp but this didn't help and actually worsened the situation. I then placed a jumper between the preamp and power amp chassis. This did help to reduce the amount of hum, although it is still quite high.

Your problem is illustrated in the attachment, despite balanced or unbalanced connection.
BTW, if you don't use balanced connections, you can't cut the cable shield.

If loop breakers doesn't work, you can try to cut your chassis connection between audio ground and chassis gound, but never ever remove the chassis safety earth at power socket.

All chassis MUST be at safe earth potential.

A good, but more expensive option is that adviced by MelB, also suggested here

http://www.google.com.ar/url?sa=t&r...0wEami8Kv4fZG3w&bvm=bv.59930103,d.eW0&cad=rja

 

[Ken Newton]

For more info. on Safety Loop Breakers, see the linked page on Rod Elliott's site.

Earthing (Grounding) Your Hi-Fi - Tricks and Techniques

 

[Speedskater]

When all else fails - read the instructions!
See the troubleshooting section starting at page 101.

"An Overview of Audio System Grounding and Interfacing"
by
Bill Whitlock, President
Jensen Transformers, Inc.
Life Fellow, Audio Engineering Society
Life Senior Member, Institute of Electrical & Electronic Engineers

http://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf

*******************************************************

Or use the Audio Precision method.

 

[Wavebourn]

If you have an AC filter in your power amp remove it. Or, use line level transformer between your preamp and power amp.

 

[454Casull]

When all else fails - read the instructions!
See the troubleshooting section starting at page 101.

"An Overview of Audio System Grounding and Interfacing"
by
Bill Whitlock, President
Jensen Transformers, Inc.
Life Fellow, Audio Engineering Society
Life Senior Member, Institute of Electrical & Electronic Engineers

http://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf

*******************************************************

Or use the Audio Precision method.

Oh my God, are those adhesive-back cable tie mounts?

 

[kouiky]

If he breaks the audio ground connection to the chassis, how can the circuit complete? In other words, what will it ground to?

 

[Lavcat]

I had a humm in my system due to a ground loop through my satellite receiver. Apparently quite common. One of these solved everything:
JENSEN TRANSFORMERS, INC. - ISO-MAX® CI-2RR Stereo Audio Input Isolator

I second this recommendation.

 

[gni]

Thank-you!

I have to say that this thread and link to Bill Whitlock's document is the most important reading any audio engineer and DIYer can read. Thank-you.

When all else fails - read the instructions!
See the troubleshooting section starting at page 101.

"An Overview of Audio System Grounding and Interfacing"
by
Bill Whitlock, President
Jensen Transformers, Inc.
Life Fellow, Audio Engineering Society
Life Senior Member, Institute of Electrical & Electronic Engineers

http://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf

*******************************************************

Or use the Audio Precision method.

 

[Lavcat]

Thanks for reviving a favorite thread. I've been a good Jensen customer. Both with buying several ISO-MAX units and building Jensen transformers into my own equipment. Still there is a mystery and maybe someone could shed some light: for many years my computer speakers have been a Bose satellite/subwoofer system. I can feel the hate but the Bose has been ideal for the purpose. I feed the Bose from a tape out of my Threshold FET ten/hl. There is no trace of hum or any problem. Thing is if I put a Jensen ISO-MAX between the Threshold tape out and the Bose I get awful hum. I'd love to understand what is going on.

 

[TonyTecson]

there is a lot to learn looking inside of an ATX pc desktop psu....

 

[steveu]

Ground loops often happen as a result of sloppy ground wiring. A common mistake in tube equipment was to connect the safety ground to the power supply negative terminal instead of a separate screw on the chassis. The difference is that the capacitor negative terminal has a small AC voltage wrt the chassis, which then connects through the safety ground to the preamp output. Connected together does not mean zero voltage. "The nearest ground point" is a classic mistake, and using the chassis as a ground wiring.