Announcement

Collapse
No announcement yet.

Ground hum with two amps

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Ground hum with two amps

    I have a JTM45 clone head and a '73 Bandmaster Reverb head which I use with an A/B switch. The BR was stock when I got it, I added a 3-prong grounded power cord to it. Whenever the two amps are connected via an A/B switch, there is a good bit of hum. I've had this problem in the past as well, with other amps - usually a Marshall-type amp and Fender-type. It seems to happen to some degree regardless of the venue wiring. Is there any suggestion other than getting a transformer-isolated buffer box?

  • #2
    Just use a ground lift adapter on one of the amps at the wall supply.What you are getting is ground loop hum.When the two amps have the ground on the mains you basically have a big "loop antenna".One amp is connected to the ground at the outlet,the chassis are connected together via the ground in the A/B switch and then the other amp is connected to the ground at the outlet creating a loop.By using a simple 3 to 2 prong adapter on one you will break the loop,problem solved.Just be sure, for safety reasons to connect the grounded amp first,then the cords to and from the A/B switch and last,plug in the "ungrounded" amp,and when breaking down do it in reverse.Some people will say that it is not safe because you have an ungrounded amp here,but in fact the amp is still grounded via the cables and the outlet ground on amp #1.

    Comment


    • #3
      safer to break the ground on ONE of the amp-to-switch cables, and plug the amps into the power properly.

      Comment


      • #4
        Really couldn't hurt to try a dedicated grounding wire from one chassis to the other - while this could produce another "loop" my fear would be that someone who doesn't understand the set up would decide to plug into the ground "lifted" amp and then find them self with that nice little sharp bite on their lips when they got too close to a grounded mic (and I'm sure all of the other old farts remember having encountered this on stage at sometime). While cosmetically funky I'll admit - if the wire were connected to a push connector mounted as close to the input cord as possible on each unit and of the same length/resistance as the connecting cord the likelihood of a loop would minimized (but I'd try back panel to back panel first - and use a green wire just to increase the likelihood that someone would know why it was there).

        Rob

        Comment


        • #5
          Plugging into the amp with the lifted ground wont cause the shock from the mike as you say because the whole thing is still grounded thru the amp that isnt lifted.No matter if you break the ground at one of the amp to switch cables or the adapter you get the same result.A ground lift adapter is about 59 cents,cheaper than ruining a cable,or installing a switch.

          Comment


          • #6
            Originally posted by TD_Madden View Post
            safer to break the ground on ONE of the amp-to-switch cables, and plug the amps into the power properly.
            I tried this, didn't work.

            Comment


            • #7
              using the ground lift adaptor on one amp should work in terms of solving the audio problem but that isn't the safest way since in that state the safety ground connection (via the signal cable) can easily be disconnected even while the AC is connected to the ground lifted amp. Besides Rob's suggestion (I've thought about trying this maybe using a thick wire on the Marshall chassis bolts), buying one of those ground loop eliminators (sry the brand name escapes me at the moment) which apparently consist of a low ohm R, diodes, and maybe a cap in the AC line to resistively isolate the ground a bit would seem safer and more practical esp. with a vintage amp where you don't want to disturb anything. The resistive isolation might be attempted on the signal line also (low ohm R--typically seems low like 10-15ohms--lifting the ground plus maybe a cap across it so the return AC signal bypasses this resistance. I'm not sure what this would be though. Around 0.22uF?? Guessing). Again, the AC ground lift will probably solve the hum/buzz audio problem but it isn't safe in those "what if" situations as Rob infers to (someone coming along, un-plugging the cord thereby disconnecting the safety ground and plugging in his gtr. thereby connecting himself to an AC-powered piece of equip. which has no safety ground) and creates a potential safety hazard (may be illegal also). Also, two amps should be powered from the same AC outlet (i.e. same safety ground so the grounds are at the same potential).

              Comment


              • #8
                Plug one of the amps power-cord into a EbTech HumX, and into the wall.

                Works perfectly, worth the cost.

                Comment


                • #9
                  I have been using a ground lift adapter for 30+ yrs without a problem.The chance of someone coming along and disconnecting something creating an unsafe condition are slim if you get in the habit of not letting people come along and mess with your gear.There is nothing "unsafe" about it.As long as you keep everything connected you have a safe ground connection thru the amp that is connected to the outlet ground.If you let people "mess" with your gear you have as much of a chance of something going south even if you are using just one amp without any ground issues.Imagine someone coming along and disconnecting your speaker,you start your next set and before you realize what happened you fry your OT.Dont let people mess with your gear and you wont have any problem.

                  Comment


                  • #10
                    I agree in that for the most part, nothing will happen when someone uses a ground lift adaptor. I see it more of insurance for something that could happen, rather than something that necessarily will. Sort of..."Are you going to be struck by lightning?" "Probably not." "Can it happen?" "Yes."

                    Comment


                    • #11
                      Yep...sort of like the surge-protector this computer is plugged into.
                      Cheap insurance.

                      Comment


                      • #12
                        While I whole heartedly agree that you shouldn't let folks mess with your gear that sometimes isn't that easy. Over the past few years most of my playing on stage has been at festivals - I only play out perhaps a dozen times with the band but almost every time it's a venue with many bands, sometime multiple stages, changing every 45 minutes to 1.5 hours. And things happen like the staggering promoter coming up between acts to let his son/son in law/cousin play a couple of tunes using whatever amp is handy on stage - and if you/your band is playing a couple of sets 3 hours apart you often leave your rig on stage. Or the stage manager notices your rig "doesn't look right" and "helps" you by reinserting a cord "where it's supposed to be" cuz your amp "didn't look right." And one time I as I started on stage to set up my already on stage amp I noticed that one of the guitarists with the previous band was walking off with it. But after questioning, and bit of his embarassment, we both noted that he also had a black face Vibrolux Reverb and he'd simply mistaken my amp for his as one of the roadies had moved his amp close to mine.

                        There's a saying I "stole" years ago: "You can never make something "idiot proof" enough to stop a sufficiently determined idiot." so I like floor drains, seat belts, and lotsa safety grounds.

                        Rob

                        Comment


                        • #13
                          Originally posted by TD_Madden View Post
                          Plug one of the amps power-cord into a EbTech HumX, and into the wall.

                          Works perfectly, worth the cost.
                          I wondered about that, also the EbTech Hum Eliminator which works at line level, what are the pros & cons of each? The line level device has two channels, which I like as I eventually plan a 3 amp set up. I'll probably get one or the other of these and report back.

                          Comment


                          • #14
                            The "hum eliminator" is passive and needs something like a pedal (even in bypass-mode) to provide enough signal for it to work properly....I think there is also a change in tone. It DOES work, though.

                            I preferred the HumX because it's a simpler solution.....just plug the amp into it and then into the outlet. No extra cables or pedals to fool around with.

                            Comment

                            Working...
                            X