OA bias res. - differences inTE, DS nobels ODR ?

[Phorhas]

Hello...

I there a sonic differance when using 10k bais resistor for an OA (after a buffer - like the TS series, DS1, and nobels OEDs for example) than using a 100k resistor?

I know it changes the currents, but what's the reason fo doing so?
what's the differance between a 0.1uf cap with 100k res. from a 1uf to a 10k resistor - the lo freq. roll-off is the same...

And - while on OAs... does aseries resistor in the AO's input act like a grid stopper in tubes?

[niftydog]

can't say for sure the reasons for using a 10k over a 100k... but I can tell you that it will affect the impedance that the op amp is seeing.

Same goes for the RC filter combos. Also, tolerance is usually measured in percentages... thus, a 0.1µF cap will drift less than a 1µF cap. The trade off is that larger value resistors create more noise.

I know nothing about tubes, but usually the series resistor on the input is designed to limit the input voltage so as not to go over the opamps maximum input voltage specification.

[Phorhas]

Hey, thanx for replying

Same goes for the RC filter combos. Also, tolerance is usually measured in percentages... thus, a 0.1µF cap will drift less than a 1µF cap. The trade off is that larger value resistors create more noise.

Yeap... a smaller cap will drift less, and very low tolorance resistors are way cheaper than very low tolorance caps... so from this angle it's more accurate to use large resistor with a smaller cap.

can't say for sure the reasons for using a 10k over a 100k... but I can tell you that it will affect the impedance that the op amp is seeing.

I know it affects the impadence th OA will see, but lets say it follow a very low impadence stage, such as a BJT buffer - so the greater input impadence of the OA is less critical - is there another affect on the OA's operation?

[niftydog]

not sure I follow your last question, but here goes;

the series resistor on the output will not greatly affect the op amps INPUT impedance. It will set the OAs OUTPUT impedance at the value of the resistor itself.

Thus, a BJT buffer followed by an OA with a resistor in series with the output will see virtually zero effect from the series resistor.

If you swap the two around, have the OA feeding the BJT buffer, then it will have a significant effect (depending on the value of the resistor and the input impedance of the BJT).

It really all depends on the following stages input impedance. But essentially, I think the resistors are meerly to set the output impedance. It could therefore be used to attenuate signals by creating a voltage divider between the resistor and the input impedance of the next stage. Perhaps some might even use it to limit the output voltage of the op amp (because an op amp can only supply a finite amount of current).

[Phorhas]

thanx for the further expenation but I knew that...

I guess that my question is, if I need to sharpen it more, is the current limiting made by using bias resistors changes the sound

[niftydog]

is the current limiting made by using bias resistors changes the sound

no. It will only affect the levels... assuming it's of an appropriate value and there are no capacitances involved.

[Phorhas]

so basically it noise vs. cost and accuracy?