I built a ebmm 3band bass preamp in a pedal but I'm just barely getting unity gain with the volume at max - which is expected since this was originally an onboard preamp.
I would like to get more than unity gain from this circuit but I'm not that familiar with this topology. Would I add a feedback resistor between pins 1 and 2 to get more gain?
Thanks
(http://i64.tinypic.com/b8wo4g.gif)
The first op-amp is configured as a voltage follower. This has a gain of 1 and is often used as a buffer. Horowitz & Hill's "Art of Electronics" helpfully points out that a voltage follower is just a special case of the non-inverting amplifier, where the feedback resistor from output to - input is 0 and the connection of - input to ground is infinity - R2 and R1 in the first diagram at http://www.radio-electronics.com/info/circuits/opamp_non_inverting/op_amp_non-inverting.php (http://www.radio-electronics.com/info/circuits/opamp_non_inverting/op_amp_non-inverting.php) The gain of a non-inverting op-amp is 1 + R2/R1, so for a voltage follower it's 1.
The resistors and capacitors before the op-amp are doing some kind of frequency filtering, but let's not worry too much about the details. Because the signal input has been AC coupled through the capacitors, any DC voltage generated at the op-amp inputs needs a path to ground or Vbias. Otherwise the op-amp will helpfully amplify the DC offset and hit a rail. This path to ground/bias is a large value resistor from + input to ground or bias, R3 in the attached diagram lifted from the radio-electronics.com page, and the 560K resistor to Vbias in your schematic.
What you'd like to do is have that op-amp run at a modest gain - not too much so you don't overdrive the tone-stack circuit. My guess is that you can do it by reconfiguring the op-amp as a non-inverting amplifier with some gain. So for example, since gain = 1 + R2/R1, you could make a gain of 3 by adding a 100K resistor from the - input to Vbias, and a 200K resistor replacing the direct wire from output to - input.
(By the way, note that if the 130K resistor in front of the - input were removed, the circuit would look a lot more like the simple ac-coupled example attached. Because that resistor is a lot larger than the 10K on the other input, my guess is that if you removed it, the circuit would still work similarly, although it might alter the frequency response in some way.)
thanks. i'll give this a try and see what happens!
It worked! Thanks!