6G15 build ground loop hum question

[tubeswell]

I saw your schematic, but I wonder if a third connection to ground is just another ground loop. Would any impedance in the ground connection help to break the loop? At that point why not just break the ground connection between the two units?

I'm not sure if it would be another loop because it is isolated from either amp by buffering resistance. The purpose of the other lead would be earth reference for the voltage divider/mixer hence is it also buffered by the divider leg. I was thinking that since the voltage potential between the two amp chassis is an AC one, then one possible way to cure it might be with a voltage divider that was able to be tuned more one way or the other. But these are just hunches that I want to explore. I guess the next step is to try it and see?

(PS I'm really looking forward to seeing how your one turns out by the way)

[Enzo]

Andrew
I found a few a long time ago when I needed to make a few of these boxes. I do not remember where I got them. it would have been someplace common though, like Mouser or Allied, since i wasn't looking for anything cosmic

COnsidering the Nady product linked to earlier was selling for $40, that is $20 each for two transformers if you open up the product and steal the parts. I don;t know about that product where you are.

Sir-
My main mission here is to get everyone understanding WHERE this particular hum comes from. There is no free lunch solution. To break a ground loop is to break it. The transformer is simple and effective, as you point out. But i cannot stop at "just use a transformer" because they don;t learn anything from that.

TW-
I see in your drawing "non grounding sleeve" on the jacks. Unless your circuit plans to spend its life floating inside there, it will be connecting to its chassis at some point. SO whether that point is at the input jack or at some screw inside doesn;t matter to me. The existing chassis are only separated electrically now by the resistance of the signal cord ground. Not much. I don;t see how raising that resistance brings the chassis potentials closer together. If anything it might allow the difference to grow.

Both pieces already have their chassis connected to the mains earth. Their chassis did not end up at the same potential. I don't see how adding a wire with a resistor in it to mains earth from ANYWHERE would help. Your wire with resistor would essentially be in parallel with the existing mains earth lead or leads.

Imagine two blocks a meter apart, and a flat plank sitting atop them - a shelf. At least that is how I make a shelf. I'm no carpenter. If the blocks are the same size and the floor flat, your shelf will be level. Now imagine one block is 1cm taller than the other. We don;t know or care which one is off dimension. Maybe each is a half cm off. One of them is 1cm taller than the other. Now the shelf leans. it drops 1cm over that meter. A 1% slope. Set a marble on it, the marble rolls.

You could move the blocks closer or farther apart. You could add bracing in the center, either to the floor or to one of the blocks. The ends are still uneven. The only way to level the shelf is to not make both ends dependent on the two blocks' dimensions.

We would find a slim transformer, 1cm thin and set it under the shelf atop the shorter block.

We could consider shaving off one block. How? Where exactly is that chassis potential coming from in that unpowered circuit? My working theory is that most of it comes from the power transformer magnetically coupling and inducing a voltage. But that is a mere hypothesis on my part. Assuming it is correct, then what?

And what if we find the two are identical transformers and both chassis have the identical chassis potential except one is 180 degrees out of phase with the other?

That would be like our blocks sitting on subwoofer cones and then,,, well analogies break down after a while don;t they?

[Sir Cuitous]

My main mission here is to get everyone understanding WHERE this particular hum comes from. There is no free lunch solution. To break a ground loop is to break it. The transformer is simple and effective, as you point out. But i cannot stop at "just use a transformer" because they don;t learn anything from that.

Enzo, I think what I was trying to say was that placing any kind of potentiometer or resistor into the ground loop is something akin to partially breaking the loop - why not go all the way? It was more of a rhetorical question.

I do appreciate your efforts to explain. Your contributions on this forum have helped to advance my knowledge probably more than any other source - and in ways not found in books or online - thanks.

We could consider shaving off one block. How? Where exactly is that chassis potential coming from in that unpowered circuit? My working theory is that most of it comes from the power transformer magnetically coupling and inducing a voltage. But that is a mere hypothesis on my part. Assuming it is correct, then what?

An aluminum chassis would not couple magnetically with a transformer, right? My understanding is that even magnetic coupling between a steel chassis and a transformer can be greatly reduced by introducing a small gap between the transformer and the chassis.

[Enzo]

I am winging it in terms of the description. Maybe magnetically is the wrong term. Winding turns could be made with aluminum wire, couldn;t they? Maybe not. So you are suggesting then if we always used aluminum chassis we would not have at least this one kind of chassis leakage? MAybe so, I don;t know. Copper isn't magnetic, yet it works fine as transformer wiring. I think I am gonna stick with aluminum would also.

I remember being amazed at how effective a chassis could be come a winding turn the first time i discovered that a steel bolt through a toroid transformer could become a shorted turn when it touched both floor and lid in a rack chassis. (Don't do that , by the way.)

Inductively coupling perhaps is a better term. I haven't studied this stuff in many decades.

But I'd agree with you on the isolation.

[dai h.]

is there anyone that understands my point about moving the higher current returns off the chassis out of the way of the 2nd return created by the introduction of the safety ground so that these two paths will not share a common resistance? I don't think the presence of a loop necessarily creates a problem if currents are absent. My suggestion doesn't completely eliminate currents but moves the higher curret returns out of the way to remove their influence.

[dai h.]

oh and also, one amp works (almost well?) with the reverb (correct?)? What is different about those in the way the grounds are connected versus the problematic one? Seems like that is something to look into.

[Enzo]

Yes, did you understand my reply that said there ARE no heavier current returns when power is removed? The currents you are referring to are all contained within a given circuit, within a given chassis. In other words when you play a 100 watt amplifier loud, much greater current flows through the mains wire than when it idles. But not through the earth leads. Neutral yes, earth no. That is why the GFI outlet in your home can power a 1200 watt space heater, but still will trip when a tiny earth current flows.

Correct that a loop itself won;t make hum if currents are absent. But the currents are from chassis potential, NOT the circuits within the chassis. if the two chassis were at equal potentials from earth, then there would be no hum.

If we ran one side of the heaters to chassis, then there would be several amps of current flowing within the chassis. But those amps don;t leave the chassis. They don;t flow down the earth lead. They are not part of the chassis potential.

[dai h.]

I'm not referring to the amp with the power off. I'm referring to the one with the power still on in which (by connecting to the other amp) you've created an incorrect return path (due to the chassis being used for the higher current returns which I'm saying to take off to get them out of the way). It's like if you think of the powered off amp path as just a wire that you've made a (problematic) loop with in the powered on amp.

[tubeswell]

An aluminum chassis would not couple magnetically with a transformer, right? My understanding is that even magnetic coupling between a steel chassis and a transformer can be greatly reduced by introducing a small gap between the transformer and the chassis.

All the chassis are made from 16 Gauge aluminium sheet metal. However the PT on the 5E3 is bolted (Z-mount fashion) right up against the chassis, whereas the other two amps PTs have a small gap between the iron and the chassis. So I might re-do the PT mounting on the 5E3 and see it that makes a difference.

Thanks for that insight Sir Cuitous

[Enzo]

OK Dai, use your example. The non-powered reverb is an unplanned for alternate path to earth. If that were the case, then all we would have to do would be take a plain piece of wire, earth one end , then touch the chassis of the amp. Under your multiground theory, that should also make the amp hum. Is that consistent with your thoughts? But it won;t make the amp hum.

[dai h.]

what I'm saying is that if you see the path through the now unpowered reverb as a wire, since the two-point grounding using the chassis as a return for higher current portions of the ground line is there, it'd be like putting a wire across two points of the circuit ground that should not be there and result in a problematic ground loop.

not exactly sure what you mean but if you mean "earth" as in say stick a stake into the ground and touch the other end to the chassis, and that causing hum, this is not what I mean.

what I'm trying to says is make the situation so that the redundant connection is such that it's essentially equivalent to making a redundant connection across the same portion of the ground line.

also, in the little bit where this is discussed in his book, Dave Funk doesn't say it's impossible for this to work so give up and forget about it, (plus the tip with the courtesy AC outlet) so that seems to say it can be done

[Sir Cuitous]

tubeswell, if the chassis is aluminum I believe it does not conduct magnetic flux. So you should be ok just bolted to it.

Could the difference in the ground potential come from the difference in the resistance to ground of each unit? The 5E3 hums even when the power of the reverb unit is off. The 5E3 is seeing a ground loop through; it's own chassis to; the patch cable to; the chassis of the 6G15; to the power cord of the 6G15. That has got to be a higher resistance to ground than the 5E3's own path to ground. If the higher resistance is the cause of the imbalance in the potential, then the only way to balance it would be to raise the 5E3's resistance to gound (bad) or decrease the 6G15's resistance to ground. I speculate that this could be why a shorter patch cord produces less hum - shorter path to ground through the 6G15 - better balance of ground potential. You could check different lengths patch cords and also the lengths of the power cords, but I'd bet an isolation box is the real way to get rid of the hum totally.

[tubeswell]

Hi Sir Cuitous

Yep I had figured all as much out. And I had been wondering whether I could use a ground voltage balancing/loop buster thingy (described in my earlier posts) inserted in the signal cable path between the reverb unit and the amp, to strike a balance between the potentials of the two chassis that I was connecting. I thought that if I had a pot wired as a mixer/voltage divider, I could connect the ground leg of the divider (the resistor connected to the pot wiper) to the buster box chassis, and have the buster box grounded with its own separate connection to the mains ground. I had thought that the mixer would provide ability to adjust voltages slightly between return connections to bothe amp's chassis, and that the leg part of the divider would help run excess voltage to ground via the separate ground connection. (Kinda like in Enzo's tilted plank analogy, I thought I would be adding a rubber band to stop the marble sliding one way or the other off the surface).

But so far all bets are off. Looks like I'll have to splash out on a good 1:1 audio tranny. (Dang! I was hoping to get away with being a cheap-skate again. )

[Enzo]

Sir
You describe pretty much what I described for Dai. You described an external alternate path to earth, through the reverb signal and power cables. Putting a higher impedance path in parallel with a low impedance one won;t raise the lower one any. That results in a wire from earth to the input jack. But there is no chasssis potential there. This entire thing occurs because both chassis have some sort of voltage difference. COUld we measure this and then make some tiny variac and set it to the exact amount and insert that in the cord? probably so, but really... The voltage differences exist, and it is the connecting them together that causes these loop currents to flow.

If you took two absolutely identical amps - assuming it were possible - and stuck a power cord on one twice as long as the other, that might indeed result in a change in chassis potential. After all if the earth leads and signal grounds had zero resistance, we would not have this discussion. But that doesn;t address where the potential comes from in the first place.

[Enzo]

Dai
ABout "earth." I am trying to avoid using the word ground. Ground to some means chassis, when they really mean common. SO if I said connect something to ground, nobody would know where I mean - chassis? earth? input jack? Ground should mean the third wire in your power cord, referred to in many places as earth. Ultimately at your power service panel, it DOES go to a stake pounded into the ground. But here we refer to the third hole in the wall outlet.

SO if I mention an external lead to earth I simply mean a wire running down to the third prong in the wall outlet. it is that different voltage at the end of each earth wire that causes the hum. The whole concept of GROUND loop is that the two pieces of gear share a common earth connection. That point is where the two circuits diverge.