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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":

Schematics of 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?

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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 :-)

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  • \$\begingroup\$ "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." Can you kindly explain in more details? \$\endgroup\$ – user2513881 May 9 '15 at 4:14
  • \$\begingroup\$ @user2513881 you'll find some information here: buildyourguitar.com/resources/lemme \$\endgroup\$ – Nils Pipenbrinck May 9 '15 at 8:53
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The purpose of a guitar pedal isn't to generally feed any significant amount of power to the output, but rather to feed a signal to a guitar amplifier or other device that expects to be fed by a guitar. One could probably within reason scale resistor values down and capacitor values up in the indicated portion of the circuit without affecting things too much, if one were using very good-quality potentiometers.

There is, however, an advantage to using larger resistances and smaller capacitors when driving high-impedance inputs: most pots have a certain amount of wiper series resistance which may vary in irregular fashion when the pot is turned. Better pots will have lower and more predictable resistance than cheaper pots, but the amount of wiper resistance, in ohms, will be relatively unaffected by the value of the pot. If e.g. the pot used for the "treble" control above has a wiper resistance that randomly varies between 10 ohms and 1K, this would have little effect if the signal is fed into a 500K volume pot (it would cause volume to vary by about 0.2%), but could have a rather large effect if it were fed into a 10K volume pot (it could cause volume to vary by about 10%). Because of this, when using potentiometers to scale signals that are fed to high-impedance inputs, it's often desirable to use the highest-value potentiometers that are practical so as to minimize the relative effect of wiper resistance.

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  • \$\begingroup\$ So is it better to use 250K in MID? \$\endgroup\$ – user2513881 May 8 '15 at 17:19
  • \$\begingroup\$ The potentiometers interact with each other; if one were going to change any of the potentiometers while keeping the circuit behavior the same, one would have to scale them all by the same factor; further, if one scaled all the pots down by a factor of ten, one would have to scale all the capacitors up by a factor of ten to compensate. \$\endgroup\$ – supercat May 8 '15 at 17:52
  • \$\begingroup\$ Guitars often have a 500K volume pot on their output, so amplifiers should work well when fed from such a pot. Scaling the "MID" pot up to 250K would require scaling the volume pot to 5M, and amplifiers might not work as well when driven from something that high. \$\endgroup\$ – supercat May 8 '15 at 17:55
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High value potentiometers are common in these circuits because guitar pickups are essencially just a long coil of thin wire ie a high-Z source. Don´t forget it´s impedence that counts here, not just the resistance you´ll measure with a multimeter. Using too low a value will lose frequency response.

Then you have your valve amp, common for electric guitarists, where the input impedence can be as high as 1M, so for best matching, 500kOhm has become a bit of a standard.

Here´s a short note on the subject: SOS link

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