Shocked!!

[thomasdj]

Hi,

I've got two homebuild amps in an A/B setup and yesterday I got shocked. Each amp sounds great and is completely noise/hum free when hooked up individualy (guitar directly to the in of the amp) but if I plug the amps to the 'fulltone' passive A/B box I get a nasty hum. Both amps are connected to earth via IEC plug so I thought this could be ground loops caused by the earth in the IEC plug. I disconnected the earth connection from one of the amps and got shocked by the shield of the guitar cable!

Could this be me..wired the L and N in reverse? The amps are in the rehersal and didn't checked that issue.

Thanks

Thomas

[tubeswell]

What do you mean "dissconnected the earth in one of the amps"? If you lifted the earth in one of the amps, then my hypothesis would be that you may have caused the chassis to have a different voltage potential in that amp to what it was in the amp that was earthed, due to having both the chasses being connected through the leads, and via the AB splitter box (esp if it had a mixer pot, which could possibly set up a voltage?), and when you went to unplug it, you, who were also probably at the same ground potential as the earthed amp, experienced an equalizing (dis)charge. Hope it wasn't a nasty experience. (I've had one of those a while back). I suspect there are probably safer ways of lifting ground loops.

Please correct me if I've got this all wrong

[thomasdj]

What do you mean "dissconnected the earth in one of the amps"? If you lifted the earth in one of the amps, then my hypothesis would be that you may have caused the chassis to have a different voltage potential in that amp to what it was in the amp that was earthed, due to having both the chasses being connected through the leads, and via the AB splitter box (esp if it had a mixer pot, which could possibly set up a voltage?), and when you went to unplug it, you, who were also probably at the same ground potential as the earthed amp, experienced an equalizing (dis)charge. Hope it wasn't a nasty experience. (I've had one of those a while back). I suspect there are probably safer ways of lifting ground loops.

Please correct me if I've got this all wrong

Hi tubeswell, thanks for your reply!

The discharge approx sounds right to my ears, I've tried this before with my old rack-series marshall power amp so guess that I am about to get used to it However, The A/B box has no pots, only 1M resistors from tip to sleeve on the in/out jacks to avoid pops.

what to do about it? It is realy a nasty loud hum and I realy need to be able to select between the two amps..

Best regards

Thomas

btw..All jacks are insolated from chassis on both amps and the A/B box

[MWJB]

Find an XLR ground lift adaptor for a mic, wire up a pair of 1.4" to XLR leads and run the ground lift adaptor between the A/B box and one amp.

[thomasdj]

Hi MWJB,

Thanks for your reply, I actually have seen one of those little blue boxes I also googled the topic and found the 'ground potential' issue..

RG has a transformer isolated signal splitter on his homepage..is this what the ground lift adaptor is doing??

Thanks

Thomas

[Steve Conner]

It's what any ground lift adaptor that would actually work in this application is doing. But bear in mind that the transformer in an adaptor designed for mic-level signals may be pretty poor for guitar signals. RG is a smart guy, so the Mouser transformers he recommends are presumably OK.

You may also reduce the hum by revising your grounding setup.

Plug both amps into the same outlet, and keep their mains leads and guitar leads close together, to reduce the loop area for induced ground currents.

Rewire the guts of the amps so that the ground wire from the mains lead connects to the chassis right at the input jack, which will minimize the effect of these ground currents on the amp's guts. The worst thing is to have all the ground current flowing through your preamp's ground bus.

[thomasdj]

Hi Steve, thank you for your reply!

I've talked to RG and the problem with his version of the splitter is the transformers. his circuit has a frequency limit right below the low E-string. I play only seven string guitars so can't use his circuit (with those transformers).

However, yesterday I bought this one:
http://www.behringer.com/HD400/index.cfm?lang=ENG

I know "behringer" don't sounds good to the ears..but this little box do the job! It has two tiny transformers that perfectly transfer the signal. It is very very transperent..actually I can't tell the difference. I tested it yesterday with my whammy pedal and the frequency response is perfect for my purpose (two octaves up and one octave down - with a seven string!). I would like to hear the story from a Bass player using this box.

so, 20$ for a hum-free setup!

One thing I noticed was that when I "decoupled" the high gain amp from ground (by running through the splitter/HD400) it actually did hum..since I wasn't connected to ground anymore.. you know, if you touch the strings the amp is silence but if you don't touch the strings the amp humms/hiss'

Do you know what to do about that?

however, I just connected on of the amps to the HD400 and now I don't have any hum in that whole pedal chain..impressed!

Best regards

Thomas

[Steve Conner]

Wow, that is pretty good for $20.

I guess the hum pickup happens because the shield of your guitar lead, and hence your guitar strings and your body, aren't connected to ground any more. You could solve that by connecting the guitar (A/B box?) directly to one amp, and using the hum destroyer only in the cable to the other amp.

This will still eliminate the hum. It's caused by a ground loop, so inserting one channel of the HD400 anywhere in the loop will break it. You don't need both channels.

[TD_Madden]

Steve,

I used the EbTech version of this as well, but it needed a higher signal level than a guitar (or AB/Y output) as input to it. Works great if another pedal of some sort (I used a delay) is run into it.

For ground loops between amps, I now just plug one of their power-cords into a HumX. THAT is a pretty "invisible" solution.

[thomasdj]

Hi steve,

That was what i did!

I wrote; "...on of the amps to the HD400.."
should have been; "...one of the amps to the HD400"

But now I understand why! ..actually I use a low-cost cable between the A/B-box and one of the amps. However, It can also be all the wall-warts, extra AC connecters and all the other crap that is lying around in our little garage ...I'll tell the guys to clean up before doing anything else!

You have helped me more than once steve, thanks again!

Thomas