MXR MicroAmp clone

[markbee]

Hi there,
I'm building MXR MicroApm as my first project. In the list of the components that should be used is 500k rev. log. potentiometer. I have a big difficulty finding it, I live in central Europe and none of local shops offer them. I wanted to ask weather it could be replaced with linear pot of the same value?
Other thing is that I'm not sure which diode shall I use,  1N5819 or 1N914. I'll appreciate any advice or suggestion, thanks.

[lopsided]

hi,

i am from central Europe myself, I order stuff I can't get local at musikding.de ( http://www.musikding.de/product_info.php/info/p554_Alpha-Potentiometer-16mm-500k-rev-log.html )
You can with no problem use a linear pot, the only difference will be, that the useful settings will be more stuffed at one side of its rotation.
Any silicon diode should work fine, the 1N914/1N4148 is used by most of us.

J.

[pinio]

Yes, You can use 500k (or 470k) linear pot. I use 1N914 and effect its good.

[lopsided]

Any silicon diode should work fine, the 1N914/1N4148 is used by most of us.

ok, so this isn't entirely true. 1N914 is used mostly for clipping.
In the Microamp the diode is used for protection against reverse polarity and in that case the more popular choice would be something from the 1N400x family (usable with higher current then 1N914).
But i think in the case of microamp 1N914 will do the job as well.

[Mark Hammer]

The Micro-Amp uses a single chassis-mounted control: a gain pot.  The gain pot serves to set the volume as well as the amount of gain applied.  The circuit has two resistances on the output, a 470R in series, and a 10k to ground.  With a 56k feedback resistor, and a 500k pot in series with a 2k7 resistor on the ground leg of the op-amp, the gain will range from 1.11 at minimum to 21.74 at maximum.

Because the ground leg resistance is used to set gain, there is a requirement to  step through resistance very quickly, going from max to min resistance, in order to produce appreciable changes in gain.  For example, cutting the pot resistance by half, from 500k down to 250k, only change the gain from 1.11 to 1.22.  That's why the need for a reverse log pot: to get through the first 90% of resistance quickly.

If we were to vary gain via the feedback resistance, things proceed rather differently.

Let's say we make the 2k7 resistor 10k, and get rid of the pot.  Now let's make the 56k feedback resistor a 4k7 fixed resistor in series with a 100k pot, so that the resistance varies from 4k7 to 104k7.  At 4k7 (pot at min), the gain is (4k7+10k)/10k or 1.47.  Now increase the pot to 10k resistance.  The gain becomes (10k+4k7+10k)/10k or 2.47.  Nudge the pot up to 20k resistance, and the gain becomes 3.47.

The lesson?  When the gain of a non-inverting op-amp is set by the feedback resistance, instead of the ground-leg resistance, it doesn't take quite as much change in resistance to have an impact on gain.

So why NOT use feedback resistance?  I'm sure others will point out stuff I never considered, but one of the frequent reasons is that the feedback cap cuts top end as a function of the feedback resistance.  Holding cap value constant, the rolloff moves downward as feedback resistance increases.  It works the other way when using the ground-leg resistance, trimming off the bass end as the gain is increased by reducing the ground-leg resistance.  Ideally, one would like to keep bandwidth constant as gain is changed, but people make their design compromises, based on practical considerations, and go with that.  In the case of the Micro-Amp, bass rolloff is so low at min and max gain ( 0.07hz and 12hz, respectively) that there is little cause for concern.  Dropping the 4u7 cap to 1uf, and leaving everything else in the circuit stock, moves that to 0.3hz and 59hz, respectively, which still leaves all usable low-end for guitars intact.

The stock 56k/50pf pair puts the high-end rolloff just under 57khz.  Increasing the 50pf cap to 100pf would move that to 28.4khz, and 220pf would roll off starting around 12.0khz, which doesn't sacrifice all that much when you consider the  frequency range of most guitar speakers (whose response drops off sharply above 6khz).

What about that 470R/10k pair on the output?  The 10k fixed resistor could actually be turned into a 10k pot, or perhaps a 5k pot in series with 5k1 fixed resistor....after all, what's the point of making a booster silent?  I gather the 470R series resistor is there for a cogent reason, and increasing it beyond 470R by using a pot and sticking more resistance in series, would likely alter the output impedance.  But then, if you're boosting a guitar and using the Micro-Amp to push an amp into overdrive, having more top end is not necessarily in your best interests.

So, putting this all together, it would not at all be unreasonable to replace the 500k reverse-log pot with a 3-position toggle, that made the ground-leg resistance either 41.7k (2k7 + 39k series resistor) for a gain of 2.3x, 9k5 (putting an 8k2 in parallel with the 39k) for a gain of   6.9x, and shunting the 39k resistor for a ground-leg resistance of 2k7 and gain of 21.7x.  Essentially, what that gives you is 3 ranges, each higher one roughly 3x the lower range.  Use the 5k pot+5k1 resistor arrangement noted above, and that will allow you to achieve all of the gain settings in the stock unit, from lowest to highest, but with more diabability and no need for an obscure taper pot to do it with.

[markbee]

Wow, you really gave it a thought, thank you for your comprehensive reply I'll have to think about it some more, but I'll probably try to get the original components so I dont have to mess with almost everything else. Thank you guys

[PRR]

> you really gave it a thought... I'll probably try to get the original components so I don't have to mess with almost everything else.

That may be the wisest idea posted here today.

Pedal designers usually had their reasons.

[PRR]

> I gather the 470R series resistor is there for a cogent reason

There's a recent thread, opamp pedal OK with a 10 foot cord but wacky with a 30 foot cord.

Cords are capacitors. Caps kill hi-freq response. Opamps already have declining hi-freq response. Put those two together with any third hi-drop, you have a phase-shift oscillator. With typical opamps and cable loads, the scream is above the audio band. We don't hear that, just the mangled audio as the opamp tries to handle 0.1V at 440Hz and 8V at 100KHz at the same time.

Inserting a few hundred ohms between opamp's NFB point and the cable puts the cable-caused hi-cut "outside" the NFB loop and damps this type of oscillation. There's side-effects but we can make them mild. For the Bi-FET chips in guitar-cord work, 300-up is a good plan.

I'm not sure you'd want to pad-down the output by making the 10K bleeder into a pot. I don't think this is supposed to clip/distort/fuzz. If it did, it would output way too much for guitar-amp input, and then you would need a knob to turn that down. However MXR surely sold distortion products, so this is the "clean boost", and apparently popular for that function.

Pot Taper.... it wasn't clear to me that 500K RA was "best". As Mark says, gain runs roughly from unity to 20. The way we hear, the "middle" or "5 on the knob" should probably be square-root of 20 or 4.5. We need 13K in the pot when at half-rotation.

Linear gives 50% of total resistance at half rotation. Conventional "audio taper" pots give 10%-20% of total resistance at half rotation. So if we want 13K at half, the total pot should be 130K. But that skews the minimum gain from near unity to about 1.4, significantly above unity.

When estimating the ear-effect of gain it is convenient to work in db. If you get the same db change per knob mark, you hear about the same change of gain per mark.

On this plot, the pale-blue line might be our "Ideal" gain/rotation curve.



The black line is a Linear pot. From zero to 9/10 rotation, gain changes VERY little; in the last tenth-turn the gain SOARS. This is not user-friendly.

The purple line is the suggested pot: 500K RA. At least we have a significant gain boost at half-turn. But the highest gains are still bunched-up in the last tenth turn.

Using lower-value RA pots brings up the middle. Indeed 100K RA lays very close to the "Ideal" line over most of the rotation. However these lower-value pots bring-up the minimum gain, from just over unity to a significant 1.2 or 1.5. Since this box may be useful just as buffering without gain, we may want to keep the near-unity-gain minimum setting.

What pot would we like to have?



We want the "Ideal taper". This is very different from the available RA or Lin pots. (No, it can NOT be loaded this way with the "Secret Lives" tricks.) A pot of this taper can not be manufactured with the usual techniques, too great a difference in resistances and thus pot-track mixtures. Any taper "can" be made but extra steps or pieces radically jack-up the cost. And pots are as cheap as they are only when the factory can make 100,000 of them without any special effort.

[electrosonic]

If you get the same db change per knob mark, you hear about the same change of gain per mark

That makes so much sense, that clears up a lot of confusion for me regarding pot tapers and volume.

Thanks,
Andrew.

[Thomeeque]

Hi there,
I'm building MXR MicroApm as my first project. In the list of the components that should be used is 500k rev. log. potentiometer. I have a big difficulty finding it, I live in central Europe and none of local shops offer them. I wanted to ask weather it could be replaced with linear pot of the same value?

Other option is to use non-reversed pot wired (and used) in reversed way - I do it all the time, it's really not that hard to get used to it (if you build your pedal for yourself).

Other thing is that I'm not sure which diode shall I use,  1N5819 or 1N914. I'll appreciate any advice or suggestion, thanks.

You don't have to use this diode at all, you can get polarity protection feature other way and it will give you little more dynamic range (especially in this application it seems like real waste to loose part of supply voltage this way).

T.