2. amps-preamps
  3. 1176092929

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.
sonicrealism

Showing 3 responses by aball

aball
1,994 posts
 
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.

Arthur
aball
1,994 posts
 
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.

A
aball
1,994 posts
 
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.

Arthur
More to discover