6G15 build ground loop hum question

[dai h.]

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.



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.



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.

roger that. I think my explanation could be wrong (maybe REALLY wrong, lol), but I still think that the way suggested is the correct way to try to attempt to solve the problem (make all the grounds the reverb and amp see in re: to ea. other reference grounds).

[tubeswell]

FWIW out of interest, I just happened to find myself (quite by accident) chatting to (none other than) Simcha Delft yesterday (in person here in Wellington),

http://archive.ampage.org/articles/6...cha_Delft.html

and she said that if I put a 100R - 1K resistor paralleled with a .01uF to 1nF cap all in series with the signal cable shield, that might be enough to suppress the hum. Another way she suggested was elevating the 6G15 returns from the amp chassis with a '63 Ri style setup (back to back 6A diodes in parallel with a 100R resistor and a large cap) but having another smaller ceramic disc cap going from the return path to the output socket ground.

[Enzo]

Well, it is two small parts and a cable. DOn;t tell us what someone else thought would work, tell us the results from trying it.

[Sir Cuitous]

tubeswell, here's another transformer you could try if you're still considering a transformer.

[Sir Cuitous]

Here's another source

[tubeswell]

Well, it is two small parts and a cable. DOn;t tell us what someone else thought would work, tell us the results from trying it.

Well I tried the resistor and cap in parallel tonight (because I had some spare time) - thought it couldn't do any harm. I tried 150R and then 1k5, both in parallel with a .01uF cap, everything in series with the shield (and all inside a shielded project box with one of the phono socket grounds lifted off the box ground). On both amps this just made the hum progressively noisier (because I guess the voltage difference was merely being exaggerated in each respective case. So okay - that was a lesson in futility, well learned. :-)

I have still to get my hands on those old mics yet.

[tubeswell]

Hey thanks Sir Cuitous - I didn't see that one before. Cheers

[Five_E35]

I don't know what type of power or codes NZ has, but you do have lots of boats & saltwater. If all else fails, you might want to check out what types of Galvanic Isolators are available or allowed. They're commonly used on boats moored in salt water and connected to shore power to stop corrosion while protecting swimmers from getting zapped on outdrives. The marine versions here are expensive and typically good for 30 or 60 amps working current. The newest versions also have to provide a self-test indication that they're still working.

http://www.boatus.com/seaworthy/galvanic/default.asp

Someone on the net did some non-destructive tests on a HumX and found it's basically the same thing. Maybe smaller Marine versions are available in NZ??

[ahamay79]

I use one of those 95 cent ground lifts from the hardware store.

[TD_Madden]

[Five_E35]

I use one of those 95 cent ground lifts from the hardware store.

I used to do that, but I don't trust Chinese transformers. I added my own version of a HumX to a Vintage Voltage Adapter I had previously built in to a 4 outlet Isobar. I 'reconfigured' a 35 amp 600V bridge rectifier(~$5) to do the job, bolted and heatsinked into the Isobar case and connected with 10 gauge wire and a UL cap. It conducts at right around 1V in each direction and I tested it by running a small ceramic heater through it hot to ground and it survived. The Isobar aleady got a 5 amp breaker to protect the Vintage Voltage transformer, and it trips as it should. Beyond that, I can't say it's 'safe', but it works for me and 5 amps is enough to run 2 medium sized amps and a pedal board.

[Enzo]

Yes, you can disconnect one piece of equipment from the electrical system earth and eliminate the ground loop, but then your equipment is no longer grounded. The reason we are discussing the alternative ways to solve the problem is to come up with solutions that don;t reduce the safety.


I am not sure how a common supply with 5A breaker solves ground loops though.

[Five_E35]

With 2 amps, one is grounded in the normal fashion, the second is grounded to the first through the guitar cable. As long as the chassis differential is less than 1V, the diodes won't conduct and there's no ground loop. I can't claim that it's safe, only that it works and in my mind it's safer than the $.95 ground lift adaptor. There may be much better ways to do it such as with diodes that fail short circuited when hit by a massive surge. The 5 amp breaker is there to protect the 5 amp transformer used in the Vintage Voltage adapter, but in any case, panel breakers are meant to protect the wiring in the wall, not what's plugged into it.

Just to clarify, really old Tripplite Isobars had oval shaped outlets with grounds isolated from the metal box. Internally they had inductors in the ground line between the outlets. They used nylon washers under the outlet mounting screws. Somewhere along the line, they eliminated the inductors and started using steel washers and a heavy conductor inside, but these are easily converted. The newest ones with square outlets would probably be harder to modify. FWIW, I have one of the really old ones with the inductors and while it doesn't fix ground loops, it does do a reasonable job cleaning up ground contamination from dimmers, SCRs, etc. I use that one for my mixer, monitors, etc.

[ahamay79]

Am I not correct that all the chassis are connected together by the shields of the patch cables? And, that if one of the amps is grounded at the wall socket that all the connected components are thus grounded?

[Sir Cuitous]

Yes, but both amps are grounded creating a loop, which is what this thread is about.

I am halfway through building one myself with an audio isolation transformer at the output. Will it work? I'll know soon and post a thread. Guitar work for my living first - then my own projects.

The advantage would be that it won't matter what amp I connect it to, so I don't have to know what the ground potential of any given amp will be. The disadvantage might be how it affects the tone, as Tubeswell found out.