[sdiy] removing dc offset

[Bob Weigel]

Hmm.  The 'offset' in a circuit normally isn't due to it being run by a 
single battery.  I mean let's say we're talking op amp circuits here.  
For example you configure as an inverting summing amp.  Three 10K 
resistors come from different voltages EACH with an inline capacitor 
also which is big enough to pass the lowest frequencies you need...but 
not DC.  Thus they will prevent (unless they are bad and leak..) any dc 
offset from transferring from the previous devices doing whatever they 
were doing. ok?
   MEANWHILE on the NON-inverting in..you have a pair of resistors which 
divide the voltage precisely in half.  You don't need massive precision; 
just enough to keep that input roughly half the total supply voltage.  
Now you should have an inverting summing amp that has relatively low DC 
offset on the output (unless you crank the feedback resistor so high for 
so much gain that it amplifies intput error.  All op amps have limits in 
this area.  See their spec sheets).  NOW...finally place a capactor in 
line with the output so that this amp won't transfer any DC to the next 
stuff you are doing in the circuit.  You may also have to put a small 
(22pf or so) cap to short HF stuff between output and inverting in.  
Otherwise it might oscillate. But there you have an example of a circuit 
that should work with single supply even if that particular op amp isnt' 
internally set up to do so without the extra parts. -Bob

gregory zifcak wrote:

> hi,
> i am modifying a toy that has very rich bass, but due to the fact that 
> it is run from a single battery supply, there is a dc offset. i've 
> tried filtering this with caps of various sizes, but i can't seem to 
> get rid of the offset without getting rid of the bass. is there a 
> simple way to do this?
> thanks for any suggestions,
> greg
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