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small R to deal with summing error?

1/8/2006 3:42 PM
Dai H.
small R to deal with summing error?
saw this in a thread on a Japanese hi-fi diy site bbs, basically saying some old hi-fi amps had a small R on the ground side of volumes (dual L/R pot) to deal with the summing error (when the pot is low or something like that). What is that about and would it be something useful, say for a PPIMV using a dual pot?
1/8/2006 8:10 PM

SOmetimes when you have stereo, or just the L and R busses on a mixer, you find that all the way down is off and all the way up is balanced, but in the middle the two controls don't track. This is inconsistency in the linearity of the pots themaselves. You could add your trimmer in there somewhere, but that assumes the error is constant and needs exactly that amount of offset throughout the range of the control. Unlikely.  
Now if your controls are reasonably similar and you have level differences betwen the channels you could probably do that to a certain extent to match levels.  
Are the errors in your MV that large that they need the correction? How closely matched are the two signals going into the MV control in the first place?  
The best thing to do is find a source of dual controls that are actually pretty close to each other. Like the "laser trimmed" ones for demanding applications.
1/8/2006 9:38 PM
Dai H.

thanks Enzo. I actually don't have a problem with a MV or anything, I was just curious what that was supposed to do, the idea behind it. As far as L/R side differences in a dual pot, I think you can try tapering Rs from wiper to either side (there is a page where a guy does it on a cheap dual pot).
1/9/2006 5:55 PM

Sure, I just put a resistor across a pot yesterday to create something I needed. But whether you tack it across or at the end, it still only solves the problem if the entire range of the control is offset somehow from the other. If it simply has a different linearity, there is no simple cure.  
The equivalent would be trying to use two differnt PA cabs and balancing the system with a one band EQ.
1/10/2006 2:56 PM
Dai H.

sorry if I am miscommunicating but do you have any idea about the single low value R on the ground side? I was wanting to understand the idea behind doing that.
1/10/2006 6:11 PM

Adding a bit to the botom is the same thing as moving the control upwards a bit - you are adding resistance under the wiper. When you have a dual pot, you can't individually adjust the sections, so this way it is trimmed. Keeping in mind the other issues I mentioned above.
1/10/2006 9:08 PM
Dai H.

sry to reiterate once again, but I wanted to be clear that I'm talking about a single R on the ground side for BOTH pots and not just one. If it was just one side, then this:  
"Adding a bit to the botom is the same thing as moving the control upwards a bit - you are adding resistance under the wiper."
makes sense, but I still don't understand why a single very low ohm R for both sides? Oh wait, maybe to phase cancel when the volume is at its lowest to ensure the signal cuts out when the volume is turned to zero (since as above, there may be a difference in the R in both sides)? Would that make sense?
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