by Steve Anderson » Sat May 25, 2013 9:11 pm
OK, seems a bit of confusion has crept in here. I assumed that the source for this would be AC-coupled, if it's audio gear it is 99.9% certain it will be.
The LM358/324 etc are designed as single-supply op-amps, the input can go all the way to ground, feedback loop allowing, in fact to around 200mV negative. So with AC-coupling into the 'contrast' pot parts of the signal will be well below -200mV, so it needs 'lifting up' via said voltage divider to keep the input signal within the linear range of the op-amp.
Those bias resistors also provide a path to ground for the input bias current (Typ. 20nA) otherwise with the 1uF cap they have nowhere to go and the output will simply sit at one rail or the other depending on configuration., The input to these op-amps is a PNP transistor, so current actually comes out of the input terminals...even though it's called an Input Bias Current, it's given a -ve value, in this case typically -20nA. There is no need for the 10k resistor on pin 5.
An op-amp tries to perform the gymnastics required to keep the +ve and -ve inputs as equal as possible, if, as here, the +ve input goes slightly positive the dog will wag its tail to try and get the -ve input the same...this is how you get gain from the ratios of the two feedback resistors.
Now without the suggested 10uF cap the op-amp has a gain of around 6, even at DC, it will attempt to amplify the +6V bias by 6, but 36V isn't available! It will sit at the positive rail. The 10uF reduces the gain at DC to unity, 6V in, 6V out. The value is chosen for a -3db point of around 1Hz.
Many trip up on bias currents, even when they would appear insignificant, some op-amps are measured in fA (femto-amp), one thousandth of a pico-amp, but even so, that current needs to go somewhere.
The input impedance of this stage is 10k (the pot) two 220k in parallel across 10K drops it fractionally, the op-amp is megs...
Steve A.
You don't have to use a single-supply op-amp in this configuration, even a grotty old 741 will do the job, no circuit changes required...but I don't suggest using a 741...please, something better.