Mono Block Ground Loop

[babadono]

The two locations are:

1. One level down, same side of house but opposite corner.

2. Opposite side of house and one level down.

There are no "clean" areas, but the other two location are pretty quiet; just a very slight hum in one channel.

I do have another VAC and the hum is barely audible (but still present).

Based on Carl's post, I bought a Furman PL-8C 15 Amp, Advanced Level Power Conditioning, unit. If it does not work, I can return it. Amazon is pretty good about that.

Ok, I think you have a very noisy environment. In most cases, there is a specific device of some sort causing the RFI.

Using a pocket AM transistor radio, tune the radio to say 800kHz and pull out the antenna. Turn the volume up. Walk around the room, and see if you can make the noise louder or softer by orienting the antenna. Signals are strongest at right angles to the whip antenna.

It really feels like you have some undiscovered source of noise in the home somewhere. Doorbell transformer? Smoke alarms? CO detector? Cell phone charger? Camera charger? Computers? Modems? A wall wart somewhere? I once went nuts tracking such a noise in my garage. I discovered after days of searching that I had an old, unused battery charger for a cordless drill sitting behind some junk with no battery in it, but it was plugged in. Made a HUGE RFI noise.

Cool troubleshooting technique. Now where did i put that 40 year old AM radio?

[eth2]

Using a pocket AM transistor radio, tune the radio to say 800kHz and pull out the antenna. Turn the volume up. Walk around the room, and see if you can make the noise louder or softer by orienting the antenna. Signals are strongest at right angles to the whip antenna.

With the radio set to 800KHz the only hum I get is from the power cords (to each amp). I have been all over the room, and adjoining rooms, but there is no hum generated.

So where we are currently:

1. The amps have a slight hum with no connections

2. The amps have NO hum when both amps are shorted out

3. When one amp is shorted, the one that is shorted is silent - the other still has a slight hmm.

4. When I connect the preamp - one connection makes no difference. When both channels are connected the hum is very loud.It does not matter whether the preamp is plugged in or not.

I am waiting for the Peach to arrive in the hopes that some of the interference is being generated by a 30 year old Phase Linear preamp.

I am also waiting for he Furman (which will replace the other line conditioner)

I am waiting for new interconnects.

If none of these things work, I will take the amps to a local tech who is very good with Macs and has a good reputation with other amps to see if he can diagnose the problem. I would prefer to not send them to VAC just yet as they have told me it is not a problem with their amps, but an external issue. While I do not think that any of us are convinced of VACs position, no sense in spending $600 for shipping until we have better data.

Once again, thank you all.

Edited April 11, 2014 by eth2

[NOSValves]

4. When I connect the preamp - one connection makes no difference. When both channels are connected the hum is very loud. It does not matter whether the preamp is plugged in or not.

Here is the key!!!!! I take it both amps have a 3 wire grounded power cord and possibly so does the preamplifier. Try this EXACTLY you will need 2 - 3 to 2 power cord cheater plugs for the power cables only 1 if the preamplifier does not have a grounded power cord. Leave one amplifier grounded (no cheater plug). Install a cheater plug on the power cord of the other amplifier and on the power cord of the preamplifier if it has one. Do not have any other source device connected to the preamplifier you can add those later once you have achieved success with just the amps and preamplifier.

I'll be shocked if the loud hum is not gone. All devices will still have a ground reference via the interconnect cables which is not the ultimate on safety since if a fault (short) happens it could smoke the interconnect cable but if it ever happens (unlikely) the system will hum like a beast and the best course of action would be cutting the power before you go messing with the system (common sense). If want the ultimate in safety solve the problem with the Jensen products or have a properly wired GFI circuit installed and lift the entire system from that nasty noisy earth grounding.

The preamp is completing the ground loop by ground connecting the two amplifier together. If this does not remedy the problem then there is some type of interface problem between the amplifiers and preamplifier. The three devices just do not play well together.

This is absolutely not because of the power coming from the wall. No type of filtering device is going to do a thing about this problem.

[Schu]

eth2, on 10 Apr 2014 - 06:59 AM, said:

Thanks, Joe.

I just got off the phone with VAC (Brent). He believes, as many of you have told me, that this is an RF noise problem, which could be carried in ion the AC line. The PA90 has a "high bandwidth" (whatever that means). However, he also told me that if shorting the amp out causes the problem to go away, and it does, it is not a problem with the amp.

Again, as many of you have told me, noise is a difficult thing to locate. He gave me two recommendations.

1. Buy decent quality interconnects. He said Monster may be an economical choice, but look for good shielding.

2. Buy a APC power filter. .First try it on the equipment leading to the amps. If that does not work, connect it to the amps I currently have a line filter, but will try an APC as suggested

post #2

Edited April 11, 2014 by Schu

[eth2]

OK - The Furman arrived but makes no difference.

As Craig suggested, I tried all combinations of the 3:2 configurations. The hum exists with all combinations. (The preamp is two pronged)

However, attaching the RCA cables makes the hum far worse, especially when the preamp is powered off.

Using thicker 12' RCA cables increases the hum when the preamp is off.

Using cheap 3' RCA cables decreases the noise but it is still worse than with no cables or with only one cable plugged into the amp.. So it seems like the cables are acting as antennas BUT ONLY WHEN BOTH RCA CABLES ARE PLUGGED INTO THE AMPS. These would seem to confirm Mark's theory that the interference is airborne, The question still remains, why is it so much worse with the PA90 than the PA80?

[eth2]

New cables are arriving on Saturday and the new preamp on Monday. Last two possibilities.

[babadono]

eth2 and NOS,

I was thinking along the same lines. If both amps are quiet with no cheaters and the shorting plugs installed how about trying just one cheater on the preamp. I believe Eth2 said in an earlier post that the preamp has a 3 prong plug also.

[eth2]

Sorry, the preamp is a two prong.

I have tried one and two cheaters. Right now it makes no difference.

[babadono]

Eth2,

Have you tried this? Both amps hooked up with no cheaters. Both RCA input cables plugged into the amps but disconnected from the preamp. Now with alligator clips or similar hook each of the center pins of the RCA plugs at the preamp end to their respective shield contact. Effectively this is the same as the shorting plug on the input to the amp but with the RCA cable added to the equation. What happens? It should be quiet just like the shorting plugs on the inputs.

[eth2]

Eth2,

Have you tried this? Both amps hooked up with no cheaters. Both RCA input cables plugged into the amps but disconnected from the preamp. Now with alligator clips or similar hook each of the center pins of the RCA plugs at the preamp end to their respective shield contact. Effectively this is the same as the shorting plug on the input to the amp but with the RCA cable added to the equation. What happens? It should be quiet just like the shorting plugs on the inputs.

Thanks. I will get some alligator clips tomorrow.

[eth2]

New cables are arriving on Saturday and the new preamp on Monday. Last two possibilities.

I want to be sure to emphasize that the Peach preamp is not going to solve this problem. Adding more "stuff" to a circuit that is already noisy won't solve it. It will become worse, and harder to debug. Until the TWO amps are quiet on their own, you are stuck in no man's land.

I don't know how old these amps are, or what the input circuit looks like, but either they have been modded since manufacture, or busted, or there is something odd in the design. If you can ask VAC for a schematic I would happily take a look at the possibilities. Short of that, Can you take the bottom off and snap a couple pictures?

And if you have a local technician, that's the best possible place to start.

Thanks, Mark.

Now the PA80 is also humming, but the B&K is not. It is raining out and very humid. Perhaps the humidity is causing more tracking on the sub-transmission lines at the edge of my property, I will call the utility and ask them to check their lines.

Perhaps the only solution is to sell the tube amps and go back to ss amps. It just can't be anything in the house as we have taken the house down to one circuit and therefore shut down all equipment (refrigerators, pumps, security alarms, televisions, computers, lights, invisible fences, etc.). I even disconnected the battery back up from the fire and security alarms and disconnected the CATV line coming into the house.

[eth2]

As for VAC, they have made it pretty clear they do not want to be bothered again. I guess I understand as the amps are out of warranty. They did upgrade them in 2012 and the previous owner says he did not have any issues when they were shipped. The fact that the PA80 is now acting up tells me that, while the PA90 is exhibiting the problem to a greater extent, it probably is not the equipment. Nonetheless, next week I will take it to a local tech and ask him to check it out. This has taken to much of all of your time. I am very grateful.

[USNRET]

Just a thought, how about taking an amp and speaker to another house?

[eth2]

That is a really good idea and I will try that this weekend. I will need to take them all as when only one is hooked up there is no problem

[babadono]

Eth2,

Have you tried this? Both amps hooked up with no cheaters. Both RCA input cables plugged into the amps but disconnected from the preamp. Now with alligator clips or similar hook each of the center pins of the RCA plugs at the preamp end to their respective shield contact. Effectively this is the same as the shorting plug on the input to the amp but with the RCA cable added to the equation. What happens? It should be quiet just like the shorting plugs on the inputs.

BTW the "dummy plugs" would allow you to do this easily. They REALLY are a great troubleshooting tool to have around.

[eth2]

Bobadano,

I made dummy plugs as you suggested and used them. I have reported the results. So if the dummy plugs test for the same thing as the alligator clip test, I guess I should not bother?

[eth2]

Mark,

It is a single circuit at either 69kv or 138kv.

They are about 700 yards from the house, crossing our property at a diagonal to the house,. There is also a service line (12kv?) that comes to about 200 yards from the house and then goes to BUD.

Edited April 11, 2014 by eth2

[eth2]

Rain stopped, humidity going down, no RFI on the PA80.

Hmmm

Edited April 11, 2014 by eth2

[Tarheel]

They are pretty heavy amps......might make good door stops

[USNRET]

Rain stopped, humidity going down, no RFI on the PA80.

Hmmm

http://www.consumerreports.org/cro/dehumidifiers.htm

Holy cow, they are getting worse even starting to show up in your posts.

Edited April 11, 2014 by USNRET