Why a resistor in series with the input cap?

[BAARON]

What purpose does R1 serve, other than to attenuate the input signal a tiny bit?  I see it at the input of lots of commercial pedals before the input buffer, whether that buffer is transistor or op-amp - Boss and Ibanez in particular seem to use it a lot.  Lots of DIY designs seem to omit this resistor.
Btw, the value of C1 is a typo.  I meant to type 0.047µF.

[John Lyons]

I beleive it affects input impedance to some extent as well.
In conjunction with R2.

[alanlan]

I believe it's there to limit current should you apply a very large AC input which goes above the rails of the op-amp.  Otherwise, you could fry the protection diodes (which most op-amps have these days).  As you say, it doesn't really have much of an impact on the low cut-off frequency.

[R.G.]

I believe it's there to limit current should you apply a very large AC input which goes above the rails of the op-amp.  Otherwise, you could fry the protection diodes (which most op-amps have these days).  As you say, it doesn't really have much of an impact on the low cut-off frequency.

Hold your calls, please. We have a winner.

[BAARON]

Well, that certainly makes sense.  I considered the "current limiting" option before I asked, but I wasn't sure why people would Need to limit the current because I obviously didn't know much about the guts of the IC.  Thanks for filling me in!

[Rob Strand]

While they limit current I believe the main reason is to prevent parasitic oscillations and RF by forming a low pass filter with the input capacitance.

In the old tube days these were called grid-stopper resistors, but are often just called stopper resistors.

(They do limit current as well but I don't think that was the original intention.)

[BAARON]

While they limit current I believe the main reason is to prevent parasitic oscillations and RF by forming a low pass filter with the input capacitance.

In the old tube days these were called grid-stopper resistors, but are often just called stopper resistors.

(They do limit current as well but I don't think that was the original intention.)

I'm aware of grid stoppers and Miller capacitance.  You're suggesting that R1 is meant to form a low pass filter with the input capacitance of the input opamp/transistor, to eliminate possible high-frequency oscillation/RF interference?

[alanlan]

That's probably a more likely reason than the one I gave

[R.G.]

While they limit current I believe the main reason is to prevent parasitic oscillations and RF by forming a low pass filter with the input capacitance.
In the old tube days these were called grid-stopper resistors, but are often just called stopper resistors.
(They do limit current as well but I don't think that was the original intention.)

That was indeed the reason for them in the old days - and the new days with MOSFETs. Both tubes and MOSFETS have such high input impedances that they are very prone to self oscillation, particularly in follower applications. There may even have been that carry over from tubes into tube-based opamps like the historical Philbricks.

But modern opamps are not nearly as susceptible to this as tubes and bare MOSFETs. In addition, all opamps have either internal or external compensation to dramatically lower their gain at high frequencies. This must be done to keep them stable at all with feedback. In fact, with the early opamps, it was a real challenge to get them to have flat responses to the upper end of audio. They're better now, but they still have a dramatic falloff in response somewhere in the partly-ultrasonic, and this makes them much less prone to the kind of thing a stopper stops.

While they could prevent parasitic oscillations under some circumstances, the primary reason to use them is current limiting so that transients on the input do not blow out the fairly delicate protection diodes or bonding wires on the inputs. As another aside, reverse breaking of the input base-emitter junctions on an opamp is one mechanism of decay in opmps, and it's common for opamps to have internal antiparallel diodes to prevent the inputs being broken by differential voltages. But that just makes the inputs conduits for input surges which then break internal junctions and can fuse either input or other bonding wires. A series resistor limits those currents down to a level where the bonding wires can live.

Note that any resistance on the input adds noise.

[arawn]

doesn't that also create a voltage divider between r1and r2 lowering the input as well as acting as a stopper?

[BAARON]

doesn't that also create a voltage divider between r1and r2 lowering the input as well as acting as a stopper?

A little bit.  The input capacitor already acts as a frequency-dependent AC voltage divider (which is all an RC filter really is).  If R1 is 10k and R2 is 1M, though, it's only 1% attenuation, which is essentially inaudible.

[Rob Strand]

> the primary reason to use them is current limiting so that transients on the input do not blow out the fairly delicate protection diodes or bonding wires on the inputs.

I'm not disputing your comments (they are all well and good!).   It's one of things where it serves multiple  functions.   Perhaps leaning toward one or the other in particular instances.   The *original* reason being lost in (or blurred by) history.   My personal opinion they probably do more good as input protection than as a filter but sometimes they can save the day for oscillations.   However, people were using the input resistors long before the current ESD paranoia era.   It could very well be people just copied tube circuits and used the resistor without real purpose other than mimicry, then realize it had other positive effects (other than noise degradation if overdone).