# Need help with DC bias of opamp

Hello .I want to make a simple headphone amp with active mixer ,use single power supply.
The first schematic is combine of simple opamp mixer and cmoy headphone amp,and its work fine.But can i want to reduce the board size.so can i remove a resistor(schematic2) and a capacitor (schematic3)  ?

5 years ago
I think for both of those op-amps, the DC level is going to try to follow the level seen at its non-inverting input.   I mean if if you imagine that the capacitors look like open circuits to DC, then after doing that each one looks like a follower ( i.e just an op-amp with its output tied to its inverting input ).

I would tend to leave alone that 10 uF capacitor and 10 Meg resistor.  So that it is VB that sets the DC level for both op-amps.

Otherwise it would be the DC level of the output of the first op-amp that sets the DC level of the second one.  Although that ?might? work too, assuming the DC level of the first output of the first op-amp is doing a good job staying put, near 1/2 the supply voltage.

If you are looking for components to remove, I think you could take out both of those 1K resistors on the non-inverting input of each op-amp.  I mean the input impedance looking into an op-amp input is supposedly infinite, or at least very large.  I don't really see what those 1K resistors are doing there.  To me that seems like just adding 1K to some number that is much larger than that.
gmoon5 years ago
No offense to J.A.L.,
I've made some guitar effects and was surprised that DC coupling actually worked fine between opamps. So you might indeed try eliminating the AC coupling RC filter (10uF /10M).  How that works without clipping could depend on the input, but if you're capacitively coupled for the input and output, it should work. You could adjust the gain of the first stage to compensate...

If you DC couple, you cannot eliminate the resistor between the two opamps--without it the gain is "always infinite" due to real world concerns like parasitic capacitance, etc. I doubt you'll hear a signal without it.

In fact, something closer to a 4.7K resistor might work even better. The input impedance of a non inverting op amp is so high, adding a few K ohms won't attenuate the signal in any noticeable way.