Micro Amp build/mod question.

[BDuguay]

....and it goes like this.
Say I wanted to make the lowest setting on the volume pot unity gain or so. What's involved in modding the circuit to do this.
My reason for this was inspired by brother-in-law expressing his confusion as to why you would ever use a micro amp, or similar type pedal to lower your level.
Y'all know what I mean?
Thanks,
B.

[Aharon]

The MicroAmp goes below unity?

[Mark Hammer]

The stock Micro-Amp has a gain control but no output level control.  As a noninverting op-amp stage, it *can't* go below unity if the resistance to ground is not infinity (or damn close).  In fact, the stock unit has a minimum gain of around 1.1 or so.

HOWEVER....

I never really gave much thought to it before, but there is a 470R/10k terminating network on the output.  It acts like a 10k47 "pot" turned down a bit such that the output is likely level matched for straight bypass when set to min gain.  Increase that 470R resistor to 1k, or drop the 10k to 8k2, and you'd be able to get outputs somewhat less than unity even though the op-amp always adds a bit.  Turn the gain control up a bit and you'll be able to nail bypass/microamped balance.

[BDuguay]

K.
Gothcha.
Perhaps I misused the term 'unity gain'. On my MXR Microamp, I need to put the level/gain knob past noon before the signal is louder than it is in bypass.
Know what I mean Vern?
Thanks fellers.

[Aharon]

Try a pot with a diferent taper?

[Mark Hammer]

There is another recent Micro-Amp thread where I worked out a lot of math regarding gain, pot taper, and suitable pot replacements.  Unfortunately, because it is a noninverting amp, the first 450k of that 500k gain pot is about as useful as the proverbial "tits on a bull" when it comes to adding more gain.  Personally, I don't know why MXR even bothered using it.  Maybe it was a case of having inventory to be used up or something. 

So, your experience of having to rotate that knob to get any discernible boost is spot on.  From one Ontarian to another, replace it with 50k, increase that 470r to 820r or 1k and you'll be much less frustrated.

Or just build a Crank.       Seriously, I'm impressed with how smooth the increase in gain is as the dual pot gets rotated.

[BDuguay]

I'm going to try that and see/hear how it goes. I figure I haven't got a 500k pot, let alone a 500k reverse log pot anyway so...
Tell/direct me to this Crank you speak of, I'm interested.
B.

[Mark Hammer]

http://img.photobucket.com/albums/v474/mhammer/TheCrank.gif

There is a vero layout and possibly a PCB posted somewhere around here.  The most exotic part is a dual-ganged 10k pot, but you can get those at Radio Shack.  Missing from the schem is a 1k resistor in series with the diodes in stage 2.  Enjoy.  Having seen your rig and style, I think you will.