Using rotary switch for MXR Micro Amp gain

[michaeleschuler]

I am considering the option of using a rotary switch to replace the potentiometer for an MXR Micro Amp build, to avoid the issues with unevenness of a reverse log pot. I have set up a spreadsheet to calculate gain levels for different resistor values, but I'm uncertain as to a good starting point for values, as I'm fairly new to electronics. I'm expecting that calculating dB gain and picking resistors that double the previous gain level would be optimal, but can I get an expert opinion on this?

[Mark Hammer]

The values would depend on what you want it to do for you.  Do you want it to provide buffering with just the slightest hint of gain, in order to keep the signal as clean as possible?  Do you want it to be able to bludgeon whatever follows it into submission?

The use of a reverse-log pot is because using the ground leg to alter gain implies initially big changes in ground-leg resistance to produce relatively modest changes in gain.  Once the ground-leg resistance starts to approach the feedback resistance (56k in this instance), much smaller changes in resistance yield noticeable changes in gain.  With 56k i the feedback loop, you'll only start to hear changes in gain when the Gain pot resistance drops down to maybe 60k or so.  Even there, 60k+2k7 will only give you a gain of just over 2x.  The use of a reverse-log pot is really to get quickly through the first 440k of resistance, and offer maximum dialability of that last 60k.  Quite frankly, you could use a 100k linear gain pot and do just fine, since it only takes a few degrees of rotation to get from 100k to around 50k.  And if you're fine with a wee bit of gain at the lowest setting, and do not intend to pummel the next pedal into obedience, replace the 2k7 resistor (which sets max gain) with 5k6 and use a 50k gain pot.  Min gain will be 2x, and max gain will be 11x, which is a reasonable range.  Any changes to bandwidth will be largely inaudible, and using a 50k pot will offer quite respectable dialability of gain.

[PRR]

Welcome.

Try this:

[Mark Hammer]

That'll also work just fine.  Typically, when the feedback resistance sets the gain, the feedback capacitor also sets the upper limit on treble.  If one of the roles of this circuit will be to push an amp or pedal into overdrive, I would recommend upping the feedback cap from 47pf to 820-1000pf.  That will provide automatic reduction of upper treble at higher gain settings.  Even halfway up, the treble rolloff begins around 6.5khz, and at max gain, using 1000pf, it would start around 3.2khz.  A rather shallow reduction in treble, but enough to keep the fizzies away.

Keep in mind the Micro Amp was designed and produced when we knew less about the behavior of amps.  Sending a warmer, less brittle, signal to the amp yields a more pleasing overdrive tone.  This is precisely why the Klon included a Treble control.  Bill told me himself that, even though it could provide boost, it was primarily for turning the treble down when feeding the amp with the hottest output.

[michaeleschuler]

Thanks for the replies. I am looking for a clean boost, so it would be great to have some more linearity around the point that distortion starts to happen, but as I alternate between humbuckers and single coils, that point isn't set. I will play with the suggestions you've both provided and see if I can tweak the circuit to something that fits my desired range.