Ground loop hum with Speedball upgrade

[drumtalking]

Greeting--let me preface by stating that I have researched previous threads regarding ground-induced hum and that my problem seems to be a little bit different (though correct me if I'm wrong as my knowledge/skill level is modest at best).  I just finished and installed the Speedball upgrade after using my Crack for nearly one year.  Prior to the upgrade, my Crack was pretty quiet already, and the minimal noise was not of the 60 hz variety.  My unit is plugged into a multi-plug power line conditioner that is,in turn, plugged into an isolated, dedicated 20 amp line with nothing else on it.  With the Speedball installed, I experience a definite 60 hz hum upon tube warmup that does not vary with the volume control.  The hum level does vary from outlet to outlet,however, ranging from mildly objectionable (only one particular outlet) to extremely objectionable (everywhere else). In my experimentation, found that by using a cheater plug on the power cord, the hum disappears completely. I am wondering if this is safe, though, and welcome any input or advice regarding my issue. I should add that I did go back over my Speedball construction and re-soldered anything that looked the least bit shaky,to no avail. All of my voltage readings are consistently about 10% higher than as specified in the manual. I really love the sound of my Crack and even more so with the Speedball mod, using HD-600s, but can't tolerate the hum.  Please help!

[Doc B.]

If the hum character is changing at different outlets and a cheater corrects the problem then you probably have a ground loop issue that would be 120Hz rather than 60Hz. The best solution would be to address that. Even though the cheater plug may solve the hum problem, we can't - from a standpoint of liability - recommend that as a solution, because it defeats the safety ground in the amp. Have you tried skipping the conditioner and going directly into the wall outet?  If you disconnect the source interconnect cables does the hum go away? Does the source component use a three wire power cord and is it on the same circuit?

[drumtalking]

The character (pitch) of the hum remains constant under all conditions, only the level changes.  I have tried plugging in to the wall without PLC and into outlets not on the same line of my other equipment without success (all other outlets result in the elevated hum level, except for a specific one of the eight on my PLC (not all used). Everything else does use a three-prog plug, the hum is there with or without the inputs connected.

[drumtalking]

I just did some more experimentation, and found that the hum is now equally loud using any outlet, including the one I thought was quieter before...

[Doc B.]

Do you know of anything that is different about the one outlet that is more quiet? Generally a ground loop occurs between two components that have grounds at a different potential. For example if two pieces of gear are plugged into different circuits there can be a difference in ground potential because of the different lengths and thus resistance of the ground wires in the walls. I have seen cases where the cause of hum was tracked down to gear was plugged into two outlets that were side by side but actually two different circuits, and also situations where the phase was reversed between two different circuits. But it might also be a ground loop within the amp itself due to a shaky connection somewhere.

It's difficult from the information we have so far to make a more definitive guess as to what the problem is. My next step would probably be to go back and reflow and tighten any ground connections in the amp itself and see if that helps. Make sure wire from the power transformer mounting screw to the safety ground screw is showing good continuity too.

[drumtalking]

I am curious as to why the hum was not present before I added the Speedball circuit and if this can help narrow down the source?

[Doc B.]

There are two scenarios I can think of.

One is that a ground connection does not have adequate continuity in the amp. The most likely would be a connection that was made during the installation or one that was disturbed during the installation. The solution if that is the case is as I mentioned, go back over the solder joints and reflow them to be sure they have good continuity.

The other possibility is simply that the gain of the amp is increased slightly when the Speedball kit is installed and that this has brought the hum level up from being inaudible to audible. The gain increase is small and the hum increase would be very small too. So if you feel that the hum went from none at all to pretty intrusive there is more going on than that, and the issue is more likely to be a bad connection.

[drumtalking]

Thank you for your direction, I think that I have solved and fixed the issue.  I did go back and re-solder the connections in the original power supply--primarily transformer and terminal strips--and the hum has resolved to a near-inaudible minimum into all ac sockets.  I suspect that one of these was on the questionable side when originally constructed and that I jarred it further in the process of installing the Speedball circuitry.

[Doc B.]

Good job! And I get to keep the corner office for the rest of the month.