Very simple op amp

[Pbassred]

I have a 100W MOSFET power amp (the object was reliability not diy knowledge). Its to use as in a powered stage monitor. Having use these power sections before, I remember that the source input is sometimes insufficient to drive it to the max. Also no local control.

Obvious answer:- make a simple preamp with a gain variable between (say) -9db and +9db . An inverting amp where the gain is R2/R1. R1 = 10K R2 = 2K7 + 100K log pot. Would that work, or does it need to be more complicated?

[darkfenriz]

If you want to use op-amp in inverting mode remember it should work with gain higher than 1, furthermore it may be noisy to have a pot within feedback loop, if it fails gain jumps to huge value. If you want to still have option to attenuate or amplify better use pot at input creating voltage divider and then amplify using inverting opamp with constant (and high) gain.
best regards

[richie00boy]

darkfenriz is right about the noise and potential for overload under fault. However, what you propose I have seen in professional use in various bits of kit for over 15 years without problems. The way to do it is fixed-pot-fixed connected at one side to the input source and the other side to the output and the take off for the inverting input is the pot wiper. Thus the middle of the pot gives unity gain and sweeping either way gives you cut or boost.

[Pbassred]

er. just to get it straight. This is it?

[Leolabs]

Should insert a resistor in series with the input

[richie00boy]

Originally posted by Pbassred
er. just to get it straight. This is it?...

Originally posted by richie00boy
...The way to do it is fixed-pot-fixed connected at one side to the input source and the other side to the output and the take off for the inverting input is the pot wiper...

So that would be no As Leolabs said you need a resistor at the input side of the pot, but you also need one at the output side, too. If you made the resistors 10k and the pot 10k you would get +/-6dB adjustment.

[vAD]

Originally posted by Pbassred
er. just to get it straight. This is it?

If you will make so " will not pass also year without problems ", esteem darkfenriz, sooner or later you Pot will be dirty, in parallel can include to it(him) the resistor easier, Play resistors as you want, read darkfenriz.
Approximately so.

[Pbassred]

Actually what I was trying to describe in the initial post was like this but with R1 in the more obvious position.

vAD, doesn't this have huge gain (unless R4 does something I don't understand)? The purpose of the preamp is to adjust a line level input 0.5 - 0.775V not boost a mike level.

I was only using inverting design because i thought I would be simpler. Maybe not.

[richie00boy]

That schematic is now correct except that the dashed resistor is redundant, the input connects to the left resistor on the pot.

[vAD]

If you, so are sure, what for to you comments.