First GC Amp .... something wrong?

[jackinnj]

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.

[sl_1800]

I'm lost, are you saying I need to increase or decrease the value of that resistor?

[Greg Erskine]

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.

[boholm]

I agree. 47/49 mV is not all that much.

But try a 620 ohm resistor instead of the 220 ohm resistor.

[sl_1800]

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.

[boholm]

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

[sl_1800]

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.

[sbolin]

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.

[boholm]

sbolin: You need a voltmeter or a multimeter. And you simply measure the voltage (DC) on the output (to the loudspeakers).

[sbolin]

Bo-

Thanks, seems simple enough. I do have a multimeter, so should be able to carry this out once I assemble my kit.