Paramount Hum issue

[8452]

I purchased a pair of Paramount amplifiers in 2008 (RMAF pair) and began using them with my Rega Planet CD player, shielded interconnects, DIY horn speakers, and Mapletree Octal preamp. This combination produced clearly audible hum which I could not get rid of. I borrowed preamps from friends, including a Counterpoint SA5.1, AR (forgot model), and a CAT, and all were no better than the active Mapletree. I then switched to the passive section of the Mapletree and got almost dead quiet. Since the passive mode produced enough volume, I have been using it this way since 08. I recently built a passive preamp using an autoformer pot and it is quieter than the active preamps, but unacceptable hum is still present. I am trying to figure out what the passive Mapletree is doing that the others are not. Why does the Foreplay work so well (or so I

[Grainger49]

Sounds like...

The grounding between the active preamps and the Paramounts is wrong.  You should check the ground lug on the Paramounts.  That is measure resistance from the power cord ground to the chassis.  It should measure almost the same as touching your meter leads together.

But first, use a jumper to short the input to one Paramount.  Unplug the cable if you haven't already.  Turn it on and let it warm up for 5 minutes.  Attach your meter to the speaker output posts and set the meter to the lowest AC volt range it has.  Adjust the hum pot to the lowest value of AC, hopefully mV, that you can get. 

Do the same to the other amp and post your values. 

Also post what reading you get from the AC plug and the chassis.  These are just two very simple things that should be done first.  Your problem sounds like a ground loop hum.  That is why a passive helps, the passive has no ground on it at all.

[8452]

The ground wires measured .000 ohms. I was able to adjust AC at the speaker outputs to .000 volts. When I reconnected one amp and turned it on, I got no hum. I then connected the other amp and both speakers had hum. I switched interconnects, no change. I switched power chords, no change. I then disconnected the preamp-to-amp interconnect of one channel and the other channel was dead quiet. By the way, this is with the transformer based passive preamp in the system. Could there be a wiring problem in the pre (something I've checked a couple of times)? 

[Jim R.]

Are the RCAs on the autoformer pre grounded or floating?  Is there a single point ground?  Are the autoformers arranged 90 degrees to each other?  Is there an AC cord or other component's power transformer in close proximity to the autoformers?  Is the autoformer control in a shielded box?

These are just a few of the off the top of my head questions I'd ask with an autoformer volume control as they can sometimes be very sensitive to pickup of induced noise from external sources and grounding needs to be well thought out and executed.

Also, are you plugging both amps into the same outlet and are they wired correctly?

-- Jim

[8452]

I'm not sure how to answer your grounding question other than to describe the wiring. Positive and ground inputs are wired through a DACT selector and the selector outputs to the autoformers positive and ground inputs. The positive autoformer output is wired to the output jacks. There is one autofromer ground connection which I wired as an input from the selector. The ground of the input jacks is wired directly to the output jack ground. This is the part that didn't make sense to me but was told it was ok. I'm not sure whether this is floating, single point, etc. The female jacks use plastic washers intended to keep the jack ground off of the chassis.

The autoformer is soldered to one side of a circuit board, with switches on the other. So, the entire assembly is mounted to the front panel. They are not at 90 degree, but they are mounted approximately 8 inches apart. The pre sits 2 feet from the amps and 1.5 from a variac/power filter. The innards are surrounded by an enclosure made from 1/8th inch thick copper, so I wouldn't think that interference would be a problem.

The amps are plugged into the same outlet. The amps were one of the pairs used by Bottlehead at 08 RMAF so they are wired correctly. 

Thanks for your responses.

[Paul Birkeland]

There is one autofromer ground connection which I wired as an input from the selector.

The pre sits 2 feet from the amps and 1.5 from a variac/power filter. The innards are surrounded by an enclosure made from 1/8th inch thick copper, so I wouldn't think that interference would be a problem.

First, can you elaborate a little on your ground connection?  The one "ground" wire for the autoformer should be connected to the ground that runs between the input and output jacks in your passive preamp.

Have you tried running all this without the variac/power filter?  That could be part of the issue.

[8452]

The ground wiring was as follows: input jack ground to autoformer ground, input jack ground to output jack ground. Nothing is (unless something is touching that I am not aware of) grounded to the chassis. I tried your suggestion and moved the wire connecting the input jack ground to autoformer ground to the output jack ground with no change in hum. I plugged the amps direct to the wall sockets, each on a separate circuit, and got the same hum. Though, the hum disappears when one input jack is disconnected.

[Grainger49]

  .  .  .    I plugged the amps direct to the wall sockets, each on a separate circuit, and got the same hum. Though, the hum disappears when one input jack is disconnected.

I have been following and wondered, were the amps plugged into the same power source before?  The reason I ask is you seem to get a ground loop when both amps are attached to the system.

Possibly one of the amps has a bad grounding solder joint that creates the loop when both are in the system?

[Doc B.]

Are the grounds for the left and right channel in the preamp tied together? Since this is single ended wiring you should have the case of the preamp tied to the signal ground at one point. Otherwise the case is not part of the shield.

[8452]

I tied the L and R channel grounds together and the hum cannot be heard from my listening chair. In fact, I could barely hear hum with my head stuck inside my mid-bass horns. Thanks to all for your responses. I learned a lot.

[Doc B.]

Woo hoo! That's one for me on this forum and one on the Tape Project Forum today, where a guy was having trouble with balancing L&R recording levels and I suggested - again as the odd man out - that it was the tape and not the machine. What's that? Why yes, I think I deserve a second martini too!

[Jim R.]

Good going, Doc.  Hope you enjoyed the second martini :-)

008452, glad it all worked out.  Grounding can be a really tricky thing sometimes.

-- Jim

[Grainger49]

I tied the L and R channel grounds together and the hum cannot be heard from my listening chair. In fact, I could barely hear hum with my head stuck inside my mid-bass horns. Thanks to all for your responses. I learned a lot.

That is a ground loop.  Unless the commons are tied together there is a chance for this problem.  With some equipment a certainty, not especially Bottlehead equipment.

[8452]

Fixing ground problems seems like a black art to me. I sure am glad there are some good witch doctors out there. Doc, when I finally decide to upgrade my Seduction with an Eros, I'll settle up for the martini. Thanks again.

[Grainger49]

Fixing ground problems seems like a black art to me. I sure am glad there are some good witch doctors out there. Doc, when I finally decide to upgrade my Seduction with an Eros, I'll settle up for the martini. Thanks again.

Doc and PJ have said it hundreds of times, the sources of hum cannot be listed.  There are too many sources of hum.  You can only chase it down through a painful process.