guitar-pedalimpedance
So it's me working on a pedal again. I would like to add some equalizer effects on the pedal and found a fascinating circuit "Smash Drive":
As far as I know, maximum power with lowest effect is attained with high impedance input and low impedance output. However, I see in the circuit using 500kOhm pots up to 1MOhm pots. Do they result in a high input impedance? I have numerous 10kOhm pots. Are they better alternatives, with suitable capacitance values calculated to have same RC?
An approach to high input impedance buffering with an op-amp is to create a non-inverting unity gain buffer, using a very high input impedance op-amp, such as the Intersil CA3140 (1.5 Tera Ohms), or the Texas Instruments OPA2107 (10 Tera Ohms), both of which have a Gain Bandwidth Product of 4.5 MHz.
(From Wikipedia)
In a non-inverting unity gain buffer configuration, the input impedance of the buffer is the input impedance of the op-amp itself, and resistance noise is minimized / none.
One other factor, though, is the input capacitance of these op-amps, 4 pF in the case of both these example op-amps. This capacitance itself would load the incoming signal, if the signal frequency is very high.
As the question does not state the desired bandwidth, the suitability and capacitance impact of the suggested op-amps can not be verified. Using a spice model for one of these in the simulation may help in this.
Based on subsequent comments, for a 100 MHz unity gain bandwidth desired, the Texas Instruments OPA355 or OPA356 would work, with their GBW of 200 MHz, and input impedance of 10 TeraOhms coupled with capacitance of just 1.5 pF.
I am designing one pre-amplifier
if you are designing a pre-amp: then there are a couple of things you need to take in consideration
Usually (but not always)
(I just find it handy to have some DI boxes ready, knowing your outputs/inputs, and wire accordingly when the situation requires... much easier than catering for every possible combination)
can some one tell me how the guitar inputs differ from line inputs ?
Generally speaking Guitar Outputs are "Line Level" "Unbalanced" "Hi-Z" If the Guitar pickup/electronics are "active" (e.g. there is a battery or so)
If it is a Guitar without active components, then it is likely to be "Mic Level" "Unbalanced" and can be either "Hi-Z" or "Lo-Z" depending on pickup/built-in-mic.
Some "effects" used with guitar may increase even over Line Level, which means you need to attenuate, which can be done either by DI or on the Mixing Desk by "pads" or "attenuators" (terminology differs from brand to brand)
... my 2 cents ...
[EDIT] As this question is migrated from sound.stackexchange.com to electronics.stackexchange.com, this answer is given from a "sound production perspective" without going into further electronics design. While I do have some electronic skills, I would not edit this answer further as I feel I would not be skilled enough to advise on electronic subjects like designing an audio-pre-amp to use with Pro-Audio applications. Please take that into account while you consider voting or commenting on this answer.
Best Answer
In general when building stomp-boxes you want high input impedance and low output impedance.
A passive guitar pickup usually has 6 to 15 kOhm impedance.
Typical values for inputs are 1 MOhm for the input. That's what has been traditionally used in the input stage of tube amplifiers. The value is so much higher than the output of the pickup that you will see almost no change in tone by plugging your guitar into this input.
You can go lower, but the lower you go, the more the input affects the tone. I wouldn't go below 10 times the pickup impedance (so 150kOhm would be my limit).
The LM368 has a input impedance of 50kOhm. If you directly connect your guitar to it, you'll load your pickups quite a bit. This results in a shift of the resonant frequencies of the pickup and some loss in treble. If you on the other hand connect this to the output of another stomp-box you may just lose some gain.
For comparison the good old Tubescreamer effect has a input impedance of 510kOhm.
Regarding output impedance of a stomp-box: There is no real standard here. Some effects are driving the output with a impedance of 10kOhm. That's fine and in the ballpark of what the pickup itself has. On the other hand some effects have the output as high as 100kOhm.
The schematic you've posted has a output of roughly 1Meg. That is a lot. The reason for this is, that there is a passive tone control in front of it. If the volume pot would be of a smaller resistance the pot would not only control the voltage but also influence the tone stack.
Overall I don't like the design. I would expect a simple buffer amplifier between the tone-stack and the volume pot. That would allow you to drop the resistance of the volume pot to something reasonable.
On the other hand I have never heard the effect, and the impedances might be just right for the tone. A good distorted guitar tone is the sum of all imperfections down the signal path after all :-)