Help me diagnose a nasty ground loop issue

I have an older set of Hafler 945 Pre-Tuner and a 9180 amp. I recently had the amp gone over and cleaned/calibrated to get rid of a hum.

After getting it home and hooking thins up with a CD player, I get a real nasty buzz in the speakers. So now I figure I have a problem with the Pre. I took it to the shop and they bench tested it on the spot and it appeared to be fine by itself however he hooked it up to the oscilloscope so I took it home and hooked it up. With just the pre and amp plugged in and connected, all is fine but after cabling the CD player and plugging it in, the buzz is there.

Below are the steps I took:

If the Pre and Power are connected only, different outlets, same outlet, or with amp plugged into pre, Tuner (within pre) is just fine with no buzz.

As soon as I connect any other component to the pre (any input) and plug that in-not even turned on, same or different outlet, that is when the buzz happens.
I also get the buzz if using a 3 to 2 prong adapter no matter where the amp is plugged in.

I have used a DVD player and VHS to rule out the DVD.
I used different ICs to rule out bad interconnect.

The only thing I have no tried is different IEC cords. Do you think that will matter or do can I still have a grounding problem within the pre?


Thanks for any input.

Try a cheater plug to see if that breaks the loop.

Sorry, I see where you already tried this. Are you sure you don't have a bad/leaking power supply?

Wait, did you use a cheater on the pre?

See if the hum goes away if you just have one IC connected between CD and pre. Might be a grounding problem on one of the inputs, so you develop a ground loop in the IC returns since they are at slightly different potentials. Or just take your ohmmeter and verify both input grounds are solid, and that they are physically tied closely together. Sometimes the input returns are also isolated from the chassis ground with a resistor to break potential ground loops.

Quote:

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Originally Posted by Davey

See if the hum goes away if you just have one IC connected between CD and pre. Might be a grounding problem on one of the inputs, so you develop a ground loop in the IC returns since they are at slightly different potentials. Or just take your ohmmeter and verify both input grounds are solid, and that they are physically tied closely together. Sometimes the input returns are also isolated from the chassis ground with a resistor to break potential ground loops.

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It happens with any input and any component I plug in.

I'm not sure how to meter the inputs for ground, but thanks for the reply.

Quote:

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Originally Posted by Hyfi

It happens with any input and any component I plug in.

I'm not sure how to meter the inputs for ground, but thanks for the reply.

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Might not have been very clear, but by "one IC", I meant either left or right channel, not both. With both left and right connected, there is a loop established if the two input grounds are at slightly different voltages, so if it still occurs with just the one channel connected then we can rule out that potential issue.

Wow, I was just poking around doing a Google search for my Hafler 945 to see what it's worth at this point and cam across an old thread from here where I was complaining about the same problem. I guess I totally forgot about it's issue and put it away for years until I tried using it again last summer when I started this thread.

From the Archives I found this from 2002, 10 years ago....

"You don't know what you have till it's gone! HYFI
Jul 15, 2002 9:11 AM
I am just wining here about my system. After I waited 8 weeks for my Stratos amp I was in audio heaven. But only for 3 days. On the fourth morning I fired up the rig and my pre-amp started blowing it's fuse. Transformer or power supply shot. So I hook up my old Hafler pre-tuner and get nasty hum out the speaks with the Stratos. Pluged my cd player direct to amp to make sure amp was ok and it was. I can't run that way since my cd player starts it's volume at max everytime you power up. I can see the speakers smoking in my head as the wife hits the play button. So now I am back to the system i replaced. Hafler 945 pre tuner and 9180 amp. Seems the Hafler pre has a grounding problem so all i can do is use the Hafler amp and a cd player. If I plug anything else into the pre I get the same nasty hum through the speaks. So now I am in audio hell until my real pre gets fixed. So now I can really appreciate the system I built after being forced to listen to my old one for now. I almost don't even want to turn it on. I guess I am spoiled. Oh well!

Bruford is coming soon.
Brian"

Well it's finally been diagnosed and I will pick it up on Saturday. It did have the following issues so there was not much I could do about it.

The pre-amp has 2 bad capacitors and the output connectors are corroded and the in line 2 resistors changed value.

They have to remove everything to repair it but I will be glad to have it back in the system and in case I decide to sell both the Pre and Amp as a set and buy some Chinese Dime Store Gear to play with.

Got my Hafler 945 back yesterday all repaired and ready to go. I asked if I was a fool to bring it back to life for $150 and all three guys at Quest For Sound chimed in simultaneously with a "NO, that is a great pre amp".

There were several things wrong including resistors gone bad, all output contacts corroded, and output caps were bad and replaced with much better ones.

After leaving it on for 8 hours, they told me to let the caps burn in some, I dropped this little pre into my main rig and was pleasantly surprised at what it could do.

Of course being so used to an all tube pre, there was the immediate differences between tubes and SS in what it did to the music. A little more punchy and forward but you could hear the congestion on heavy passages where the VAC just keeps every instrument separate and fully detailed.

Anyway, glad to have it back. After a few days in the main rig, it will be re-united with it's old pal, the 9180 amp that it grew up with since I bought them in 1990.