6G15 build ground loop hum question

[dai h.]

okay so what I'm seeing which looks problematic is higher current, noisier portions of the "common" are tied to the chassis, and (assumption: AC input is opposite end to the signal input and safety ground is connected to chassis near AC input to the chassis) safety ground being in a physical position where those higher current, noisier returns could make their way towards the input if you make a loop back that way. One amp, no loop. Two then a loop (safety ground to safety ground through chassis through signal cable) backwards towards the front.

there might be other points not being thought of also (by me and in the thread of what exactly is happening) such as when the amps are plugged together with the signal cable and then the signal is lifted from ground at the jack (assuming typical switching where signal is shorted to ground when the plug is removed).

also, when I say the cable shield from the gtr. to amp input is a return it is, and I keep thinking of it as a super low current return path but maybe that is only for when there is a signal present(unless some tiny hum i.e. a tiny AC voltage is there--which might be noise, i.e. something that is not wanted but it'd still be a signal).

[Enzo]

Break your neutral wire and add an AC current meter. Watch how much current flows. Now reconnect that and break the earth lead and measure AC current. Almost none - very small currents if even measurable. The fact that heavey currents flow within the chassis don;t make them come down the earth lead. The fact that these currents are tied to the chassis doesn't send them down the earth lead.

[dai h.]

Steve Conner
12-02-2008, 09:30 PM
That Tore-T mod won't have any effect on ground loops, since they're caused by external influences, not the amp's own circuit.

I've never found a guitar or hi-fi amp circuit where I couldn't get hum below the noise floor with careful layout and grounding. (And in the case of Champs, a huge choke in the power supply ) If you only remember three things:

Make sure that the ground path between input jack sleeve and AC cord green wire doesn't pass through other circuitry. (For instance connect the green wire straight to the sleeve lug of the input jack, this being in firm electrical contact with the chassis.)

Ditto for the ground path between the power transformer's HT winding center tap and the negative of the first filter cap. Keep this ground wire separate and don't ground other stuff through it.

Either use a humbucking pot on the heater winding, or center-tap it and elevate the center tap with some DC voltage.

If you do these things, any remaining hum is from your guitar or reverb tank

I also agree that an audio transformer is the best way of breaking ground loops. Just stick one in a box with a 9v battery and a JFET or op-amp buffer. (A guitar doesn't have enough juice to drive many audio transformers by itself.)

However, bear in mind that audio transformers themselves can pick up hum. In my recent Crown restoration, I decided to go with transformer balanced XLR inputs, and I got shielding cans for the transformers and spent a while finding the best place for them in the chassis.

Oh and R.G. nobody's stopping you from making an amp with a balanced front end Why not change to XLR guitar leads and 200 ohm pickups while you're at it?



http://music-electronics-forum.com/a...hp/t-9660.html

looks like he is saying the same thing I'm saying. So why hasn't it been tried already(from a couple months ago)? Surely Steve-O is a much more authoritative source than me.

[Enzo]

Yes, in his first paragraph, he states mods to the circuit will have no effect on ground loops.

Steve Conner
12-02-2008, 09:30 PM
That Tore-T mod won't have any effect on ground loops, since they're caused by external influences, not the amp's own circuit.

Ground loops occur due to chassis potential, NOT from currents in the circuit. The message I have been trying to send.

[tubeswell]

I also agree that an audio transformer is the best way of breaking ground loops. Just stick one in a box with a 9v battery and a JFET or op-amp buffer. (A guitar doesn't have enough juice to drive many audio transformers by itself.)

http://music-electronics-forum.com/archive/index.php/t-9660.html

looks like he is saying the same thing I'm saying. So why hasn't it been tried already(from a couple months ago)? Surely Steve-O is a much more authoritative source than me.

FWIW I asked around for a schematic along the lines of what he meant, but I didn't get a clear response. (Decent 1:1 audio transformers for a passive link are an import item for me, so I was trying to think of other solutions)

[Enzo]

Most inexpensive LoZ mics have transformers inside them. Spin the element off your SM57 and look at one. The tiny diaphragm on that mic element has no trouble driving that transformer to send its little signal back to the mic input on a mixer at the other end of a snake. Two of those back to back would leave you in the HiZ world. I wouldn;t suggest buying a couple SM57s for thiws purpose, but if you have any dead mics with still good transformers, try that.

And those little mic impedance adaptors like you would use to conect your SM57 to a Hi Z input somewhere are also nothing but a transformer inside. They sell here for $15-20. Take two of those apart and back to back their little trannies.

These things:



Wire two of those up LoZ to LoZ and make sure the grounds at either end are not connected, and you have a loop buster. Totally passive. I don;t know why it wouldn;t work for a guitar, it works for microphones.

[tubeswell]

Old mics! Thanks for the tip Enzo. (I know where there are a pile of clapped-out mics)

[Enzo]

Remember, must have XLR connector. And of course you have to wire it right. I am inerested in how well that all works.

[dai h.]

tubeswell
12-02-2008, 06:19 PM
Further to this theme, my thoughts have run back to this over the last 2 months, and today's discussion with Sir Cuitous here:

http://music-electronics-forum.com/s...ad.php?t=10460

- has prompted me to revisit this, and wonder whether an adaptation of the Tore-T mod displayed on the Hoffman site would eliminate/reduce hum.

See here for Doug Hoffman's reference to the origi-mod on the revibe

http://www.el34world.com/Hoffman/ima...maticToreT.gif

I have prepared this (crudely plagiarized yet somewhat original - if you can call any old idea original) schematic kinda figuring how I would go about it.

Basically it is turning the CF stage into a VA stage and padding down the output of that stage before blending it back into the mixer. Will this work to reduce ground loop hum? (or did they only do that Tore-T mod to the revibe because they added the vibrato circuit to it?)? The voltages are guestimates based on hunches.

If anyone has some comment, critique or whatever remark, please shoot me down.

Cheers

Steve Conner
12-02-2008, 09:30 PM
That Tore-T mod won't have any effect on ground loops, since they're caused by external influences, not the amp's own circuit.

Yes, in his first paragraph, he states mods to the circuit will have no effect on ground loops.

1. he is referring specifically to the Tore-T mod. He doesn't say "mods", or that anything done will have no influence. It makes no sense to say that just after I quoted him suggesting alterations to the grounding scheme.

2.

not the amp's own circuit.

okay, but let me add what I think should be added in the context that the statement is made:

not the amp's own circuit.

because the amp by itself when used by itself (since it doesn't hum) is already correctly grounded

Ground loops occur due to chassis potential, NOT from currents in the circuit.

1.not sure that makes sense. Current flow over a resistance causes a difference in chassis potential.

2. you can have internal ground loops

[dai h.]

FWIW I asked around for a schematic along the lines of what he meant, but I didn't get a clear response. (Decent 1:1 audio transformers for a passive link are an import item for me, so I was trying to think of other solutions)

so the hookup suggested hasn't actually been tried yet.

wasn't there at least one thread also to do with ground loops and using a transformer splitter? One project at GEO IIRC. Never tried it myself but my understanding was that you can get a cheap 600:600ohm and stick a buffer in front of it. Maybe a series R to raise it back up as well. And also that higher impedance ones were expensive

[tubeswell]

I tried a hook up with an el-cheapo passive transformer

http://www.jaycar.co.nz/productView....T&SUBCATID=378

But I'm not exactly sure whether I wired it right. Basically I did the thing is the attached schematic (i.e. without using the centre tap on the secondary)

I got a hum free signal alright, but the frequencies were all highly distorted and the signal was very, very weak with the reverb unit mix control dimed over to the reverb recovery stage. The signal was a lot louder and clearer with the mix control attuned to the CF bypass stage, but the frequencies were still all wrong (and I deduced that this was to do with the lower output impedance of the CF stage).

So I am still lost about how to wire it up. (But Enzo's suggestion about acquiring a couple of hi-lo Z trannies from old microphones and sticking them back-to-back seems to have some merit).

[austinb]

Have you ohmed out the power cable 3rd wire ground prong to chassis for amp and reverb unit? If that is good open chassis of amp and use voltmeter to measure volts between reverb unit chassis and amp input jack ground and then reverb unit and point to witch ac third wire ground point on chassis. If there is potential between input ground but not 3rd wire ground point then run a hefty wire between the input jack ground and 3rd wire ground point. This should either fix problem or cause amp to hum all the time.

[dai h.]

I think you need to buffer the reverb out before the transformer(lower driving impedance improves the freq. response). Plus correctly terminate the secondary(get weird peaks if not correct). Something like that. I've read a bit about how to use transformers but not much has sunk in yet.

anyway, here is R.G.'s splitter thing. Bet this would work. One dual opamp, one transformer, a power supply.

http://www.geofex.com/FX_images/humfree2.gif

commercial version:

http://www.generalguitargadgets.com/...d=16&Itemid=26

[Enzo]

Dai, I have too much respect for you to just disappear in the night, but I have taken our aspect of this debate as far as I care to. I make this response and then no more from me. I will be happy to continue on the transformer thing.

Steve Conner
12-02-2008, 09:30 PM
That Tore-T mod won't have any effect on ground loops, since they're caused by external influences, not the amp's own circuit.

What I see there is not specifically Tore-T, I am looking at where I added the emphasis - that ground loops are caused by external influences.

A lot of people use the term ground loop improperly. SO they add the words "ground loop" whenever they speak of hum. Much of what we discuss as "ground loop hum" isn;t. Hum yes, ground loop no.

Quote:
Ground loops occur due to chassis potential, NOT from currents in the circuit.

1.not sure that makes sense. Current flow over a resistance causes a difference in chassis potential.

Yes, of course, but WHERE is that current coming FROM? NOT THE CIRCUIT. Some small leakage potential - voltage - is appearing on the chassis, EVEN WHEN THE POWER SWITH IS OFF AND ALL INTERNAL CIRCUITS ARE UNPOWERED. The resistance of the earth lead allows a voltage difference between chassis and true earth.

[Enzo]

Hi austinb, welcome to the forum.

It is the nature of ground loops that they can only occur when all the ground connections are intact. Without intact ground connections there can be no "loop."

Woven through this discussion has been the solution, which is to break the ground connection at some point. We prefer to do that in the signal cord so there is nothing in the way of safety in the power cords.

It is also part of the ground loop equation that there is really nothing wrong with any of the equipment. Ground loops can occur most anywhere.

Yes, we probably could strap a short hunk of #0 welding cable between the reverb and amp chassis and brute force our way out of this loop. Then tomorrow we are at a different gig and the loop is between some other pieces of gear.

That is why various companies make loop busters.