Loud humming from pre-amp. . . ?? Ground?

Hantra · « **on:** 19 Mar 2003, 08:28 pm »

All:

Got this pre-amp off the 'Gon', and it works great, but it has a loud humming that starts when it gets warm. It is a triode pre-amp, and I think it may be a grounding issue. When I touch the source knob on front of the chassis, it stops as long as I have my hand there. I tried another power cord, and another outlet which is on a different circuit.

Should I get a cheater plug? Isn't there anything else I can do about this?

Thanks!

B

byteme · « **Reply #1 on:** 19 Mar 2003, 08:32 pm »

I'd try a cheater and see if that helps it, but wouldn't use it long term. It might help narrow it down to what it is.

I had a Foreplay preamp that got me started down the road to tubes, it sounded great but hummed like a mother - could never narrow it down to what it was. The prior owner didn't have any issues with it. I ended up having to sell it because it drove me nuts. The next buyer had no issues with it! Go figure. And I've got dedicated lines with Vans Evers (for the digital) and PS Audio UO (for the analog side) filtering.

doug s. · « **Reply #2 on:** 19 Mar 2003, 09:10 pm »

there's absolutely nothing wrong w/long-term use of a cheater. i have done this in the past. you likely have a ground-loop somewhere. w/a cheater, the equipment will still be grounded thru the interconnects. also, may wanna try the cheater on other components - it may be the combination of the pre w/something else, that's causing the ground-loop.

also, since touching the unit causes the hum to go away, ya may wanna try running a wire from the pre's chassis to a known ground in your abode - perhaps a water pipe, (no, not the kind for the illicit stuff!!!

), or the center screw that holds the coverplate to one of your electrical outlets, or???

hth,

doug s.

nathanm · « **Reply #3 on:** 19 Mar 2003, 09:31 pm »

Hmmm...sounds familiar. I recently got a preamp with triode tubes in it and the thing naturally hums. Nature of the beast, so I am told. There's little pots that adjust the hum level so you can minimize it as best as possible. I don't know if this is the same scenario or not. Something about the heater being inline with the rest of the audio or some such.

Mine also makes a loud hum when it warms up, as well as some other fun noises. I haven't had a whole lot of tube amp experience, but from what I've read this seems to be SOP.

Hantra · « **Reply #4 on:** 19 Mar 2003, 10:09 pm »

This is not a normal triode hum. It's extremely loud, and relative to the loudness of the music. I will try some of the grounding things, and see if it goes away.

Thanks!

B

mgalusha · « **Reply #5 on:** 20 Mar 2003, 12:49 pm »

Hantra,

To help determine what is causing the ground loop disconnect all the sources from the pre and see if that helps. Even sources that are powered off can cause ground problems.

Obviously if it still hums there is a ground potential difference between the pre and the amp.

There is a very good article on finding and fixing ground loops on the Jensen Transformers site - http://www.jensentransformers.com/apps_wp.html - file number AN004 and their troubleshooting guide. Of course they sell isolation transformers and suggest those as the best cure but their application notes are very good and explain the causes and how to find the culprits.

I have to disagree with Nathan in that triode preamps don't all hum. The 12au7/12ax7/12at7 tubes are all dual triodes and used in many preamps. Some have more hum and noise than others but most of them are very quiet. However, his mention of the heater being inline with the audio may indicate a directly heated triode. I don't own anything with DHT's, but reportedly they are much more prone to hum.

No question that a tube pre can produce some foul noises during warmup. I built a timed muting circuit into mine so they don't reach my amps and potentially nuke my woofers.

Mike