ampage
Tube Amps / Music Electronics		 For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Search for:  Mode:  

 

Ground loops on multiple amps


 :
6/17/1999 8:36 PM
Keith Manlove
 Ground loops on multiple amps
I have'nt paid enough attention, I guess. I see people using more than 1 instrument amp at once (stereo effects outputs, for example),but all I've gotten was a nasty A.C. hum. I'm not big on lifting grounds. How is it done? I just wanna know. Thanks for any helpful returns.  
 
Keith
  
6/18/1999 1:05 PM
R.G.
 Re: Ground loops on multiple amps (R.G.)
The hum comes from slight differences in the "ground" voltage level of the two amps being interpreted by the inputs of the amps  
 
as signal. Guitar amp inputs have a common mode rejection of zero!  
 
 
 
There are three ways to fix this:  
 
1. lift a ground on one amp; The easiest way is with a ground lift  
 
adapter on one amp. This leaves you **completely unsafe** and a fault in the AC power section of one amp could put line voltage on your signal ground, with possible electrocution resulting. Note that on amps with two prong plugs, this is no help at all, as they already don't connect to safety ground.  
 
2. Use a transformer isolator. There is a suitable circuit for an active  
 
transformer isolated splitter at GEO (http://www.eden.com/~keen) in the "new effects schematics" section.  
 
3. Use a differential amplifer setup to reject the common mode voltage and supply a "clean" signal to one amp. The schematics are (or were!) linked in the archives here at ampage. This only works for the cases where the hum voltages are low enough to be within the common mode range of the opamps used, probably 3-4V for most opamps and a 9V battery supply.  
 
  

  Page 1 of 1