Grounding Problems

Hi

I have a bit of a mess with bad hum. I have a new ST-70, a completely rebuilt PAS 3 (new boards, transformer, tubes, etc) and a rebuilt SE-10 EQ. By themselves, they do not give me any hum (tested using a pair of headphones on the outputs of the EQ and preamp) but when they are all connected, I get a nasty hum and buzz. I have tried eliminating the EQ and just connecting the preamp to the amp and I still get the nasty buzz. I have tried ground loop isolators between the equipment with no improvement.

Do I need to run a ground lead to all three pieces of equipment or just ground one of them to the house ground or is there something else that I have completely missed?

Any help is appreciated.

Chris

Hi Chris,

A few questions first:

(1) Which components have grounded 3 wire power cords and which do not?

(2) Are all of the power cords plugged into the same power strip?

(3) Do the input selections on the preamp or equalizer change the hum?

(4) have you tried other rca cables?

Regards,
Dan

Dan

None of the components have the 3 wire grounded cords. I was thinking about converting everything to see if that made a difference but I have read in other posts that adding a 3 wire grounded cord can actually introduce more hum.

All of the cords are plugged into the same power strip. I know plugging different components into different outlets can introduce ground loops which can cause problems.

I'm not sure what you are getting at with question 3. When I connect the ST70 to either the EQ or preamp, or if I have all three components connected (preamp to EQ to amp), I tend to get a loud buzz and a hum. The volume of the buzz and hum does not change when I mess with any of the controls on the preamp or EQ. I am able to play music just fine without any distortion and at levels that will mask the buzz.

I have tried different RCA cables. I have noticed that wiggling the cables where they plug in can change the buzz but will not make it go away. I had thought that the RCA jacks could be bad or not making good contact so I have actually put just a bit of an oval on the jack to make the connections for the center pin and the ground fit tight for an experiment. I will probably replace the jacks that I ovaled a tad at a later date.

Chris

Hi Chris,

OK, those answers helped. Some more:

(1) Have you tried running a battery powered device like an Ipod or CD player into the ST-70 inputs to look for the buzz?

(2) Do you have any fluorescent lights, motors, etc., on the same circuit as the stereo? If so, try disconnecting these.

(3) Have you double checked the input wiring on your new ST-70 to make sure that the grounds are attached correctly?

Regards,
Dan

You wrote:

"None of the components have the 3 wire grounded cords. I was thinking about converting everything to see if that made a difference but I have read in other posts that adding a 3 wire grounded cord can actually introduce more hum."

Two things to try here. First, if your two-prong AC cords permit it, reverse one plug at a time into the AC socket. Second, run a ground wire from ONE of the chassis, and one only, to the third, ground hole on the AC socket. I'd run it from the Stereo 70. The other chassis should be grounded via the shields on the interconnecting cables.

"When I connect the ST70 to either the EQ or preamp, or if I have all three components connected (preamp to EQ to amp), I tend to get a loud buzz and a hum. The volume of the buzz and hum does not change when I mess with any of the controls on the preamp or EQ. I am able to play music just fine without any distortion and at levels that will mask the buzz. "

This would tend to suggest a ground loop in the Stereo 70.

"I have tried different RCA cables. I have noticed that wiggling the cables where they plug in can change the buzz but will not make it go away. I had thought that the RCA jacks could be bad or not making good contact so I have actually put just a bit of an oval on the jack to make the connections for the center pin and the ground fit tight for an experiment. I will probably replace the jacks that I ovaled a tad at a later date."

This is a clue, too. Try pulling out one of the two RCA plugs so its ground shield does NOT make contact...that is, open that ground. In a properly constructed Stereo 70, this should not cause EXCESSIVE hum but could make your buzz go away, or at least get better. If you find this happening, you need to look inside the Stereo 70 for a ground loop.

When you say "new Stereo 70", do you mean an unaltered, new old stock factory-built Dynaco unit, a new old stock Dynaco kit that you just built, a Dynaco Stereo 70 with someone else's circuit board and other modifications, or a Panor "ersatz-Dynaco"? I can only speak for the original Dynaco Stereo 70: I've seen modifications that have been done without regard to ground loops. Some people will install "upgrade RCA jacks" on a metal plate, replacing the original phenolic-mounted ones. Doing that improperly will create a ground loop; each individual jack needs to be insulated from any metal mounting plate, with the shield connected to the correct eyelet on the main circuit board as the original Dynaco design called for; there are 10Ω resistors on the circuit board that lift the shields of the input jacks above chassis ground to break a possible ground loop. The circuit board is to be grounded from the specified eyelet to the specified ground lug on the chassis (for good measure, check the integrity of that ground). NO OTHER GROUND WIRES should be installed to the chassis from the circuit board.

Hi

My ST70 is a new kit I purchased from Bob Latino. I have done what he has told me to check for hum by shorting the RCA cable from center pin to the ground. All the buzz and hum goes away when I do what he says for testing. I have a pair of headphones that I have attached RCA plugs and with that setup, I am able to connect to the output of the PAS preamp and EQ to listen for hum or buzz and have not heard anything coming from the EQ or preamp.

I have taken an automotive ground loop elimiator and have connected it between the preamp and the st70 and have not had any improvement with either the buzz or the hum. I am at a loss for figuring out where this noise is coming from. With my luck, the problem is probably something very simple and I just haven't found it yet. I don't think the problem is with my ST70 but you never know. I am at present working on my PAS 3 that I have completely rebuilt to see if there is something that I have missed there.

Would it cause more problems if I was to add 3 prong ground power cords to all the equipment or am I better off leaving things as they are?

Chris

When I began building my latest system, I was plagued with ground loop problems. I went so far as to purchase an isolation transformer for power and also one for line level in-out before discovering that an ungrounded CATV splitter was the culprit. After finding that, I went outside and grounded my CATV line to the grounding system I use for radio antennas and I have ZERO noise. I'm running old amps with original trannys too.