Modifying an MXR Microamp

[anacrusis]

My MXR Microamp pedal boosts the signal just fine, however, when the volume is at zero the normal guitar volume still comes through completely, you can't turn it down at all..

Is there any way to modify this so that when the volume is at zero, no sound at all comes through? or if there is a way to at least be able to lower the volume at all?

[Güero]

Jumper in place of R6

[anacrusis]

wow thanks so much!

[Thomeeque]

Jumper in place of R6

Nope, it would just lead to "infinite" boost at max.

MicroAmp is non-inverting amp, where AC signal gain (considering C3 & C5 to be big enough and C2 small enough to be ignored) equals:

G_opamp = 1 + R4/(R5 + R6)

G_total = G_opamp * R10/(R9 + R10) = (1 + R4/(R5 + R6)) * R10/(R9 + R10)

Minimal gain (R5=500k) = approx. 1.

There is no way how to modify it to go to zero (turn it down at all) by simple part replacement*. What you can do is to set lower minimal gain (but never to zero), e.g. by altering output R9/R10 divider or by introducing input divider. But it has some drawbacks too. What do you need this mod for exactly?

T.


* EDIT: Actually you could if you would not mind to replace R9/R10 divider by volume pot (but it's simple only on paper ).

[anchovie]

or if there is a way to at least be able to lower the volume at all?

Your guitar's volume control?

[FlyingZ]

Make r5 a trimmer then replace r9 and r10 with the old pot. BTW that schematic is very outdated if ever truly correct.

[Thomeeque]

Your guitar's volume control?

Depends on where is his MicroAmp placed in his signal chain.

Make r5 a trimmer then replace r9 and r10 with the old pot.

This could lead to loss of dynamics in some situations (pedal could by internally overdriven thanks to preset gain >> 1 without chance to adjust it from outside).

IMO anacrusis should clarify what exactly he needs this mod for first.

BTW that schematic is very outdated if ever truly correct.

Is there a new one? Or do you know what is different now?

T.

[petemoore]

  You could set the level and leave that where.
  Then add an end volume control, a 10kA pot across where R10 was, wiper-output.

[Fender3D]

lol guero 2 - netiquette 0! 

Tom is right, but you might use this mod to get rid of volume when at "0"



hmm... 500k's a too much large value maybe...
then out impedance will vary...
what about adding a volume pot instead of R9/R10?
You'll get a nice 2 knobs pedal 

[Güero]

 

damn, you right!

[Thomeeque]

Tom is right, but you might use this mod to get rid of volume when at "0"

Hat's off, Federico, brilliant!!

hmm... 500k's a too much large value maybe... then out impedance will vary...

He could maybe still left something on R10 place (even maybe those 10K - I'm always bit puzzled with log/rev-log thinking, but IMO most of it's rotation would be "right side" of R5 pot ignorably big and "hit in" only close to "0").. then out impedance would not vary that much..

what about adding a volume pot instead of R9/R10?
You'll get a nice 2 knobs pedal  

Yep, that's probably the cleanest way, but there's definitely potential in your idea above..

T.

[Fender3D]

Thanks Tomas, but it's not my stuff...
Peavey used it plenty in their mixers and amps 

@Guero
cya on top of Mercy Hospital and watch out those boomers!!! 

[Güero]

@Guero
cya on top of Mercy Hospital and watch out those boomers!!! 

Mercy Hostipal top A.K.A.  HELL on Earth

heheheheheheheheheheheheheheheh

I hate the Smokers more than the Boomers 

[Mark Hammer]

The simplest, most trouble-free, arrangement would be to simply replace R10 with a 10k log pot.  The stock circuit never provides less than a gain of 1.11x, which is fine if you're aiming to use it only for boost.  But if a person want to also use it as a sort of volume pedal preset, and achieve less than unity, the non-inverting configuration won't easily do it for you.

If implementing a second control knob is a non starter, then just replace R5 with a trimpot set for whatever you imagine the most boost you might usually ever want, and let the new output volume pot take its place on the chassis.  That way, you'll be able to go from max boost to less than unity when you want.

[Thomeeque]

The stock circuit never provides less than a gain of 1.11x

1.06x

[Mark Hammer]

Okay, I'm here to learn.  By my calculations, [56k+500k+2k7] / [500k+2k7] = 1.11

What am I not factoring in?

[Thomeeque]

Okay, I'm here to learn.  By my calculations, [56k+500k+2k7] / [500k+2k7] = 1.11

What am I not factoring in?

The output divider R10/(R9 + R10)

[Mark Hammer]

OK, so we're differentiating here between gain of the op-amp stage (my calculation) and total gain available at the output (your calculation).  Fair enough.