Grounds, loops, and Lifts

[smallstonefan]

Hi all,

I've got a question regarding grounds. The 1964 Super Reverb I just bought has the original two-prong plug on it. I understand that for safety reasons I should have this replaced. However, I had someone propose an idea the other day that I want to run by you...

I plan on using a passive ABY splitter to choose my Dr. Z, 64 Super, or both. The Dr. Z is gounded. If I make the splitter share a common ground, then - as I understand it - the 64 Super will be grounded via the ground of the Dr. Z. It was even further suggested that I put a ground lift on my VooDoo Lab Pedal Power so that everything shared the Dr. Z ground and I'd get no ground loop.

I understand that I have to make sure the polarity of the 64 Super is correct before I do this.

This actually makes sense to me, but when it comes to electricity I don't like to take chances.

Will this work, and will it be safe? I don't really want to use a buffered splitter, as I'm trying to avoid buffers (everything is either TB or in a loop box).

Thanks,

[LyleCaldwell]

It will work for a while, but eventually it won't, due to a cable coming loose or a bad connection.  And when that happens, it will not be safe and you will be electrocuted.

Never use a signal ground for a safety ground.  Keep the amp grounded with a big thick heavy duty grounded power cord, and let the signal cables provide the millivolts of ground needed for the instrument and signal path.

If both amps are grounded by their power cords and you have a ground loop, lift the shield at one end of one cable that connects to both devices.  You can always safely lift a signal ground like that, but never lift a safety/chassis ground.

[Carlos]

Hi!

I play two amps simultaneously. I read from your post that you use pedals that means your guitar signal is buffered before it runs into both amps.

I would definitely ground your Fender amp properly to protect your health, but I would insert a line isolation box in the signal path to your Fender amp. Here's the one I use:

http://www.thomann.de/prodbilder/140375.jpg

Regards

Carlos

[smallstonefan]

If both amps are grounded by their power cords and you have a ground loop, lift the shield at one end of one cable that connects to both devices.  You can always safely lift a signal ground like that, but never lift a safety/chassis ground.

I see - I could actually do this in my passive ABY box then - lift the signal ground. Will that cause noise when I use that amp?

[smallstonefan]

Hi Carlos,

My buffered pedals are all in bypass loops, so the signal is only buffered when one of these few pedals are used (the rest have all been true bypassed). I'm reluctant to use a buffer, but if I have to I'll go that route. Is the unit you mentioned a buffer?

Thanks,

[Transmogrifox]

If I was going to build a line isolation box, I would just use a high quality audio transformer--and I suspect that's what Carlos' box is:  just a high quality audio isolation transformer in a box with each output referenced to its respective ground.

This method allows noise, as an AC difference between the two grounds, to enter the signal path, but this is generally minimal compared to the noise introduced by ground loops.  If both amps are well grounded and referenced to ground at the wall outlet, this difference in ground potential would in theory be 0, and there would be no worries here.  In practice, it is not, but usually a ground is good enough that it's minimal (unless your house or the building you're in has a poor grounding system).  Now I'm rambling.  If I keep on, I'll be writing a research report with no sources but my own BS.

Just build a box with a transformer in it and that will isolate your grounds

[niftydog]

I have no idea what a Dr. Z is.. but I think I can answer your question.

Install a proper three pin plug on the fender super reverb. That is a legal requirement and it may save someones life. Absolute must do!!

Now, as for ground loops - I assume you want to continue using your voodoo labs supply, so here's some options.

1/ inbetween the last effect in the chain and your amp(s) install a transformer isolator box. This won't completely aleviate ground loop potential, but it will help greatly.

OR

2/ at the same point in the circuit, disconnect the sheilding on the cable at the jack that is plugged into the amp(s). Same deal as before.

OR

3/ make special patch cables that only have the sheild connected at one end and use them for interconnecting your effects. This means that the only ground reference for each piece of equipment is via the voodoo labs power supply.

OR

4/ sell your voodoo lab power supply and build ye'self a Spyder then you'll absolutely get rid of ground loops forever.

Be aware that disconnecting shields may lead to RF interference problems, so it's not an ideal fix.

Remember, my rule of thumb - if any single device has two or more ground paths it has ground loop potential. Remove all but one ground from every piece of equipment and you'll be close to being ground loop free!

[smallstonefan]

Thanks for all the help guys! What I think I will do is use plastic jacks on my passive splitter. I'll have a toggle switch that will determine which amp the instrument cable will be grounded to - so the instrument will always have a ground but it will be isolated so that both amps won't create a ground loop.