More Grounding Questions

Hifihvn, yes, AC ground (not AC neutral) is wired to chassis.

Heyraz, there was a ground loop between the preamp and the secondary power amp. (To be clear, I only use one power amp at a time). So it hummed. Then I isolated the signal/circuit ground from the chassis while keeping the AC ground wired to the chassis. The hum stopped because no ground loop is between preamp and the secondary power amp.

I opened up the secondary power amp to disconnect the wiring. I did check the polarity and it was fine. The manufacturer wouldn't give out schematic. But it is a simple KT88 PP amp and is easy to trace.

When I use the main power amp, there is still a ground loop between the preamp and this main power amp. But it does not hum.

Maybe I should re-phrase my questions:
1. Will ground loops degrade sound quality even though it does not hum?
2. When you "float" your power amp, do you float the signal/circuit ground from the chassis or you float AC ground from the chassis?

Thanks all.

Will ground loops degrade sound quality even though it does not hum?

In addition to producing hum, ground loops can also result in high frequency noise and buzz, by coupling harmonic distortion components and/or noise that is typically present on the ac lines into signal return paths. However, if you don't hear that when no music is playing, or if any low level buzz or noise that you do hear is the same whether or not the equipment is temporarily floated with a cheater plug, I doubt that it would have any effect on sound quality. I suppose it is slightly conceivable that inaudible ultrasonic noise that is introduced by a ground loop could somehow intermodulate with the music and have audible effects, but it's easy enough to rule out that possibility -- just compare the sonics with and without the ac safety ground temporarily lifted via a cheater plug.

When you "float" your power amp, do you float the signal/circuit ground from the chassis or you float AC ground from the chassis?

I would never isolate circuit ground from chassis, assuming the equipment was designed with them common. And I'm not sure how it would even be practical to do that, unless the design had them connected together at only one point, and the rca jacks were the type that has isolated ground sleeves.

Assuming that circuit ground and chassis are connected at more than one point, isolating those connections would most likely result in significant impedances, especially at high frequencies, between circuit points that should be common. In other words, grounds at different points in the circuit would no longer be truly common. Also, stray capacitances between various circuit points and ground would be different than those which were present when the initial design and development was performed, in effect changing the design in unpredictable ways. Also, instead of the chassis functioning as a shield, it would very conceivably function as a path for emi/rfi to couple into internal circuit points.

Of course, as you probably realize floating the ac safety ground is in principle not a good idea either, due to the safety risk it creates. Although a lot of people do it anyway. As far as I am aware the only way to break a problematic ground loop that is both effective and completely safe is with something like a Jensen Isomax.

This Jensen paper will probably be of interest, if you haven't seen it already.

Best regards,
-- Al

Al,

Most amps I know use "star ground" or some variation of it for grounding various common points. In short, these various common points are wired together first and only goes to the chassis at one point. A good article can be found from:

http://aikenamps.com/

Click "Tech Info", then "Advanced (Aiken)", scroll down a little, and you can see "Star ground".

Adding Jensen transformers in the signal path may get rid of the ground loops. But I hesitate adding more components in the signal path.

So for chassis to act like a shield, do you have to wire the chassis to the signal ground or AC ground?

Hey Al,
Very nice Jensen paper. I gave it to my son who is studying to be an EE. Even though the paper starts out a little technical, the conclusions and tips section at the end make recommendations to minimize ground loops pertinent to this thread.
Vet93- From a purely academic standpoint, that's a good question, "has the signal been degraded even if you can't hear it?" Unless you can get your equipment into a lab, you'll probably never know. Your ears are the ultimate instrument, maybe you should invest in The Isobar.

I cannot hear noises when the system is idle. But I am not sure it (ground loop) does not hurt sound quality when the music is playing. As an example, I couldn't hear any hum, buzz, or noises before I put a Shunyata PLC for the digital and preamp gears. But the Shunyata PLC does improve sound quality, IMO. So this is not an academic question. Rather, it is more like trial and error.

This is why I want to solicit opinions from others who have experiences in this area.

The Jensen paper may be technically correct. But it may not describe all real life scenarios. Take Figure 1 in that paper as an example. It tried to explain the hum voltage potential at the power amp and suggested that it would cause hum. This may well be correct. But both my power amps had the same grounding scheme before the mod. However, only one of them hummed. So his theory cannot be true at all time.

From a personal experience, the theory one learned at the regular University EE program has very limited areas of application in real life. In the MS and PhD programs, one would probably learn that there are other ways of looking at the same problem, and a theory/equation is only valid when certain assumptions are true. One challenge in real life is finding out which assumptions are true. :-)