Suggesions on MicroAmp ?

[guillaume_75]

Hi !

I'm about to build a MXR MicroAmp. I have a on/off/on switch that I'd like to use. Any suggestions on how to use it ? Maybe to connect some resistors/capacitors that would change tone, or diodes that would clip a little ?

Thanx for your help !

[martinbertrand]

Hi Guillaume,

I would try 2 different diode pairs : maybe 2 leds or an asymetric pair of 1 1n914 in one direction and 2 1n914 in the other. or any match/mismatch that may ever imagine!

you could also have a pair of diodes with a small "smoothness" port wired in series with the diodes to control the amount of Grit. going to the diodes pair.

Let us know how it goes.  :wink:

[guillaume_75]

Thanx for the tip ! But where do I connect the diodes pairs ? As in the MXR Distortion+ ?

[Phorhas]

Now that would just make it a Dist+ and not a microamp

but it could become a Two-In-One thingy

[Mark Hammer]

It's intended to be clean, so I'd suggest letting it do what it does best, and not co-opting it to do something different poorly.  That being said, there are all sorts of things that might be useful mods to make it more flexible.  

For instance, the stock unit provides a high-end rolloff around 60khz (1/[2*pi*.056*.000047).  Great for flat frequency response, but do you always want or need it?  Sometimes you want less bandwidth at the high end, perhaps for coloration, perhaps for noise/hiss reduction.  So, your toggle might be used to add other caps in parallel with the 47pf feedback cap.  In the middle position, nothing else is added in so it's stock.  In the side positions, you could tack on a parallel 220pf cap (220pf + 47pf gives a rolloff around 10.6khz) or 390pf cap (390pf + 47pf gives a rolloff around 6.5khz).

Alternatively, sometimes you want to use a box like a Microamp to boost a voice mic, or maybe an acoustic guitar mic, where you may not need or want bottom end coming through the mic.  The stock Microamp has a lowend rolloff at max gain of around 12-13hz (even lower at lowest gains).  If you replace the 4.7uf cap currently in series with the pot and 2k7 resistor with a 0.22uf (220nf) cap, you'll get a rolloff around 268hz at max gain (lower down with less gain).  Use the toggle to either stick with 0.22uf  or add in parallel caps to yield rolloffs at lower corner frequencies (e.g.,  a parallel 0.47uf cap yields a rolloff around 85hz at max gain, and a parallel 1uf cap yields a rolloff around 18hz at max gain).  Note that toggling picofarad range parallel caps in the feedback loop will not likely produce any loud popping, but microfarad-range parallel caps in the leg going to ground might.

Finally, the Microamp is intended to provide as little loading on the front end as possible (in service of retaining all the bandwidth that the device aims for) by means of the 22Meg input resistor.  A toggle can be productively used to load down the guitar by adding other resistors in parallel with the 22Meg one.  Note that players who use single coil pickups will often opt for lower value volume pots because they find larger value ones too "brittle-sounding".  Being able to provide the loading in the box itself would let one use 500k or even 1Meg pots, and change the loading externally.  Here, a 220k and 100k resistor in parallel with the 22Meg one (toggled in of course) might deliver slightly different resonances from a guitar by virtue of looading it down differently.

[Kleber AG]

Very interesting, thanks Mark, many great tips there!
Cool

Regards
Kleber AG

[petemoore]

The circuit displays the differences in OA sounds...
 I have one with a 4558 and one with a 5534 [5534 is PDClean]...they sound a bit different.

[guillaume_75]

Great tips, Mark. I think I'll go with adding caps !