Massive buzz when using CJ PV10A with NAD C372

I think I have a ground loop problem, but the magnitude of it is so off the scale I am not sure where to start. Help, please.

I have an NAD C372, Magnepan MG1.6, and a Vandersteen 2Wq sub. A friend loaned me his Conrad Johnson PV10A for the next month. I am very eager to hear what a good tube pre-amp stage will do.

The NAD pre-amp stage was completely disengaged. The Vandersteen sub was also removed from the configuration. The CJ's main outs are tied into the main in's of the NAD. CJ is turned on first and then the NAD.

When the NAD powers up, I get a massive buzz. I am talking about a cover-your-ears, jump-out-of-bed loud buzz.

I re-configured the system to use the NAD pre-amp stage with the CJ signal on the AUX input. I can now at least control the volume and hear the hum safely. It most definately sounds like a the buzz from a ground loop problem and it is independent of CJ level setting.

When I gradually increase the NAD volume level I notice a change in humm character as I get into the low-mid range. If I crank it a bit more and/or leave it on at that moderate level, the NAD detects something is not right and shuts down.

I've heard ground loops before, but this one is so dibilitating that I wonder if there is a fault in the CJ unit.

Any ideas? Thanks.
the plug on the pv10 is not polarized,so try flipping the plug.
Sounds like you have DC voltage on the PV10's output to me. Usually you can tell by looking at the speaker drivers from the side but with Maggies, I guess you can't do that. You could check the output jacks of the PV10 with a good multmeter and see what DC voltage you have. If it is anything more than 5mV, that is your problem and will definately make your NAD shut down. A lot of tube preamps have excessive DC on the outputs and SS amps hate that. We couldn't get a Cary SLP98 to work with a Classe CA200 for the very same reason. The Classe would go into shutdown immediately. To fix the problem, we got rid of the Cary.

The plug on this thing is three pronged; I cannot invert power. I did try moving it off the common power strip and plug it into the same wall socket directly. Frankly, I would have been surprised if it improved things. It didn't.

When the CJ is run as an AUX input, the humm is ever present, even when the CJ is not powered.

Unfortunately, I don't have an oscilloscope or a good multimeter that will go down into the millivolt ac and dc ranges. Nothing registers on my household meter but that doesn't say much. The NAD itself is silent ... very very black when on a channel with no input signal. Put it on AUX and I cannot get it above 2 on a scale of 1 to 10 before it wants to shutdown.

My father is quite fond of saying, "The right tool for the right job is invaluable." Now, I have a gift certificate for a local electronics store ... . "Hey, Honey, I need to talk with you."

Any more ideas?
Hmmm...the fact that it hums even when the CJ is not powered precludes the idea of DC being the problem. So it must be a ground issues, as you surmised.

Here is something you could try that has worked for me in the past: Take a wire, any old thin guage wire like bell wire or stranded lamp cord wire, strip an end and wrap it around one of the CJ's chassis screws and put the screw back in so that the wire is pinched in place. With the other end of the wire, do the same on the NAD. If the NAD has a phono ground post, that will work great.

Then see if that solves the problem. Connecting the two chassis together will make sure they are at the same ground potential. If this doesn't work, it might be the CJ's or the NAD's ground is isolated from the chassis but I doubt this is the case since you have a loud hum. If you are willing, give it a try and see what happens. Keep me posted.

PS, I too always say you have to have the right tool for the job. No doubt about it! It takes a quality meter to measure millivolts indeed.

While a mile or two above ground today travling across the country, I was struck by a thought. The CJ has a three pronged power cord. All of the other equipment is two pronged. I can well imagine my problem resulting from a slight difference in the potential from the safety grounmd and the negative line seen by the power supply. Tying the two chasis together as you suggest, Aball, sounds promising. Another quick fix to try would be simply to remove the safety ground from by using an adaptor. That's not a long term solution, though.

When I get back from business travel later this week I will try your suggestion.

BTW, know of any good sources for reasonably good but relatively inexpensive oscilloscopes? Something well under $200 would be nice. This is the type of problem that awakens long forgotten electrical engineering studies. I'd love to have had one to use for this problem.

No dice. I tried connecting the two chasis together. The noise is still there. I tried isolating the ground leg on the power cord. The noise grew which didn't surprise me.

Now I havea new symptom. I cannot get a signal from my CD through the CJ preamp to the AUX channel on the NAD. I can play the CD via either the NAD AUX and CD inputs. Inserting the CJ only gives me backround humm. I've toggled the source/tape switch without effect. The Source Selection is clearly set on CD and levels are well above minimum. Nothing but the humm.

I'm not sure what to do from here. My very limited tube knowledge comes basic electrical engineering classes 20 years ago. I wouldn't mind learning, I just need some pointers on where to start. (Truth be said, I have considered building a tube pre-amp as a project once I convinced myself that I could hear differences.)

I've wondered from time to time if the Vandersteen high-pass filter between the pre-amp and the amp was introducing humm, but if it were the case, I would expect it on all channels.

I now think that someting is actually broken in the CJ and that the nature of break was settled in in starges. A few days ago I had humm but could get a signal through the CJ to the NAD on its AUX channel. Now I cannot even do that.

Yeah, sounds like there is something wrong with the CJ. That it too bad but Lew Johnson is a swell guy and he can get it fixed, no problem.

Perhaps I spoke too soon.

Do these preamps require 20 seconds or so to warm up before processing a source signal? The painful loudness of my first experience hooking this up has caused a hypersensitivity to having it on if something seems wrong. I quickly turn it off if I don't hear what I think I should hear. I have since found if I leave it on for 20+ seconds I get a signal processed through the preamp.

I have also discovered that tying the ground post on the CJ to the negative of one of the main out RCA removes a component of the hum but not all of it. There is a lower frequency component that remains. I am not sure, but it is almost sounds like I remove a harmonic.

In case anyone wondered, I have also pulled the surge protected powerstrip out of the equation and instead used an unprotected connection back to house power. Having it in or out of the configuration makes no difference.

So, I'm back to having a signal but noise that I cannot remove by any means I know of. This time I am truly out of ideas. I sent an e-mail to Conrad Johnson last night and referenced this tread. Hopefully someone there will have some ideas.

Thanks for the help.
I had the CJ looked at. Lot's of DC noise present. It needs repair. I need to talk with my friend who loaned me the unit before doing anything else.

Thanks for the help.