OpAmp Design Question

[Khas Evets]

What purpose does a series resistor in the signal path serve? I know that's a broad question, so let's refer to R3 and R9 in the MXR Microamp at GGG.

[Paul Perry (Frostwave)]

In this particular unit, my guess is that R3 is just providing some protection for the op amp in case there is a huge transient input to the effect.
And R10 is part of a (fairly gentle) treble boost on the output (in conjunction with C5 and R10).

[BJF]

Hi,

R3 protects the input from currentsurge and a value of 1K Ohm should be used if input cap has equal or higher capacitance than 100nF. During powerremoval the input cap vcan discharge through the input
of OP and cause failiure. Also R3 protects against zener-emitter breakdown in bipolar inputs, while this is a fet input and thus it is sensitive to static charge R3 protects current surge here.
It is a common problem that unprotected OP amp input break down
and this most easily if power is removed. The first series of Marshall Valvestate had massive failure due to this, especially since many users continued playing while power was removed- and the solution was to change OP and add a current limiter resistor. This typoe of failiure is common in pedals too and is something a manufactuer want to protect against.

R9 serves similar function at output,both prtecting against shortcircuit and also protecting output in case of reverse voltage as for instance when the in and outs are mistaken.
The value of 470 Ohm is a matter of perference and minimum for reasonable protection is 100 Ohms, while values up to 10K can be used but can then react with the capacitance of the output cables and next stage even to the extent that signal level is lowered and generally a value about 100 times smaller than the nominal impedance circuit is to drive is choosen and then leads to losses of 1% due to voltage division and one could use up to 1tenth of nominal load at 10% loss which is less of an object if circuit is to run reliably and this loss can be made up for by virtue of the gain of the circuit. However voltage is what is transfered in circuits like this.
Finally R10 keeps output at groundpotential and as can be seen here voltage division is created and likely the intended load impedance
en calculated to be 10 K ohm's at the lowest

It is good practice to protect coupling to the outside world

Have fun
BJ

[Eric H]

It is good practice to protect coupling to the outside world

BJ

It certainly is.
Great post, and welcome to the forum

-Eric

[BJF]

Thank you,

At your service
Bjorn Juhl
BJF Electronics

[Sam]

Is it equally important to protect a discrete circuit in the same manner?

[BJF]

Hi,

Yes, you never know what happens in the outside world and musicians always tend to use things in a nonintended way.
Some failiure mechanisms are particurlar to OP amps and intergrated circuits in general but the general rule is to protect any part of circuit that can be reached from the outside regardless of circuit type.

Regards
BJ

[BJF]

Hi,

Yes, you never know what happens in the outside world and musicians always tend to use things in a nonintended way.
Some failiure mechanisms are particurlar to OP amps and intergrated circuits in general but the general rule is to protect any part of circuit that can be reached from the outside regardless of circuit type.

Regards
BJ