DC on the output arises from the input bias current flowing from the input pins of any opamp-chip. If there is an input resistor to ground, the current which flows out of the pin produces a voltage V = IR on the input pin. You have to do a bit of a balancing act between getting a small enough R value for low input offset, and a high enough R value so that the input pin is somewhat decoupled from the ground at high frequencies.
hi sl_1800,
Have you listened to your amp yet? Try some old speakers if you aren't happy with the offset.
It sounds to me like you have a good amp to me. 47mV/49mV is nothing to worry about in my opinion. There are hunderds of these BGT NIGC with varying offset. There are a few threads about this topic.
If you haven't listened, do it, get used to it, then play with the offset. See if YOU can notice a difference.
Have you listened to your amp yet? Try some old speakers if you aren't happy with the offset.
It sounds to me like you have a good amp to me. 47mV/49mV is nothing to worry about in my opinion. There are hunderds of these BGT NIGC with varying offset. There are a few threads about this topic.
If you haven't listened, do it, get used to it, then play with the offset. See if YOU can notice a difference.
I agree. 47/49 mV is not all that much.
But try a 620 ohm resistor instead of the 220 ohm resistor.
But try a 620 ohm resistor instead of the 220 ohm resistor.
Bo,
The 220 ohm resister you speak of is on the non inverting input. It is just the resister that is in series with the input signal, I don't see how that could make any difference. The resistor between the inverting input and ground is a 680 ohm.
The 220 ohm resister you speak of is on the non inverting input. It is just the resister that is in series with the input signal, I don't see how that could make any difference. The resistor between the inverting input and ground is a 680 ohm.
As jacknnj and others tried to point out, there flows a very small current into both the - and the + input. Normally they are of the same amount and they will generate a small voltage over all the resistors on the inputs. When these voltages is not of the same amount, there will be a voltage difference on the + and the - input that is amplified to the output - as you have already noticed.
So a good trick to minimize this effect is to try and make the two small voltages the same, and one way of doing it is to put in a resistor - the 220 ohm - exactly the way BrianGT has done it. It can be proven that its value must be:
R = (R-fb1 * R-fb2) / (R-fb1 + R-fb2)
In your case:
R = (680 * 20000) / (680 + 20000) = 658 ohms
So - try it 😉
So a good trick to minimize this effect is to try and make the two small voltages the same, and one way of doing it is to put in a resistor - the 220 ohm - exactly the way BrianGT has done it. It can be proven that its value must be:
R = (R-fb1 * R-fb2) / (R-fb1 + R-fb2)
In your case:
R = (680 * 20000) / (680 + 20000) = 658 ohms
So - try it 😉
Bo,
Thanks for the explination, I'll give it a try. Your explanation is now starting to make sense. Some of us have to be hit a little harder to see the light. 😉
Thanks for the explination, I'll give it a try. Your explanation is now starting to make sense. Some of us have to be hit a little harder to see the light. 😉
Good thread, especially for me (totally new at this, but wanting to try/learn). Not to take this OT, but how do you measure the offset voltage? I know a bit about what to do if the voltage isn't nearly zero, but I don't really understand how it is measured. I haven't put my kit together yet (any day now 
), and appreciate the pointers.
), and appreciate the pointers.
sbolin: You need a voltmeter or a multimeter. And you simply measure the voltage (DC) on the output (to the loudspeakers).
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