Adding a bit of distortion to MXR microamp

[Pavezer]

Hi!

I've been using a MXR microamp in my effects loop for boosting up my solos. It works great, but I would like to have some tiny bit of extra distortion in addition to boost. I'm not a pedal enthusiastic and I'd like to keep the floor as clear as possible, so I'm not greatly interested in getting another distortion pedal on the side of Microamp. My microamp is DIY and I'm willing to mod it to achieve a bit of extra distortion. It doesn't have to be controllable, so possibly a a bit of clipping might be enough.

Here's the schematic:
http://www.generalguitargadgets.com/diagrams/microamp_sc.gif

So, maybe I'd like to try some clipping circuits, asymmetric or symmetric in opamp negative feedback or in output. Would this be a bad idea?
Does someone have an rough idea which would be better scene to start with or any suggestions or possible problems which migh accure?
All the suggestion and comments are very welcome!

Thanks a lot in advance!

[drewl]

I built a MicroAmp and D+ in the same circuit by adding a second switch to switch in the clipping diodes and to change the opamp feedback.
So, start experimenting with the feedback resistor and you should be able to get alot of gain out of the amp.
Maybe put a couple resistors on a switch to kick in more boost.

[Mark Hammer]

A stock Microamp has a maximum gain of 21.7.  While that is enough to produce clipping in an amp that is already pushed hard, it is a far cry from what is needed to produce clipping on its own.  For example, the Tube Screamer has a max gain of 118, and the Distortion+ has a max gain of 213.

So, the first thing you'd need to do would be to increase gain, in order to make productive use of diodes anywhere in the circuit.

Probably the simplest thing to do would be to install a switch that put some diodes in the feedback path and increased the feedback resistance at the same time.  Happily, that only takes a SPDT toggle or stomp.

Let's go for something that achieves a bit of asymmetry but still delivers a hefty enough output that you can add to the clipping with the amp (IMHO many of the most satisfying overdrive tones come from a modest but synergistic contribution of pedal, amp, and speaker).

A combination of a single germanium and silicon diode will do that,  So, wire up the switch of your choice with the common/center lug tied to the output of the op-amp.  Put your Ge/Si back to back pair between one of the outside lugs of that switch and pin 2.  At this point, the switch will now place clipping diodes across the feedback path.

What about the extra gain needed to make use of them?  For that we'll need to replace the existing 56k resistor with a higher value unit.  Since you only want a bit of clipping, I'll suggest 470k.  That'll give you a gain of 175, or somethng between a TS and a Dist+.  Enough to get the Ge diode conducting at many settings, and the Si conducting at medium settings and above.

But how do you get back to your modest gain in the Microamp mode?  Easy.  You have a spare lug on that toggle.  Solder a 62k or 68k resistor between the spare lug on the switch and pin 2, and the combined parallel resistance of the 470k and additional resistor will put you back to your more modest max gain of 54.8 (with 62k) or 59.4 (with 68k), which is clearly in the ballpark.  The stock 47pf cap will provide a rolloff at 7.2khz whioch will likely keep too much fizz.  Change it to 100pf for a rolloff just under 3.4lhz and a somewhat smoother tone.  This will have no audible impact on the clean setting.

The truly cool thing, however, is that in one toggle position you get Microamp and no clipping diodes, while in the other you get clipping and bigger gain.  If you want, it can be a 2nd stompswitch with status LED to let you know which mode you're in.

Thanks for the question.  I enjoyed this.

[jacobyjd]

lol...that sounds like a sweet project. Very useful.

[Mark Hammer]

I maded a boo-boo.  See quote below for corrected version.

But how do you get back to your modest gain in the Microamp mode?  Easy.  You have a spare lug on that toggle.  Solder a 62k or 68k resistor between the spare lug on the switch and pin 2, and the combined parallel resistance of the 470k and additional resistor will put you back to your more modest max gain of 21.3 (combined parallel resistance of 54.8k with 62k) or 23 (combined parallel resistance of 59.4k with 68k), which is clearly in the ballpark.  The stock 47pf cap will provide a rolloff at 7.2khz which will likely keep too much fizz.  Change it to 100pf for a rolloff just under 3.4lhz and a somewhat smoother tone.  This will have no audible impact on the clean setting.

Addendum:  If you wanted to have a preset variable boost, you could make that 470k feedback resistor a 270k fixed resistor and 250k linear pot.  That way, you could have a gain hike that ranges from a min of 1.5 (with the 500k gain pot turned to max resistance, and the added 250k pot turned to min resistance) to a max of 186.2 (500k pot set to zero ohms, 250k pot set to max resistance).  In principle, you could set the two pots such that the additional boost is well within what the Microamp could normally do, and so does not result in any appreciable clipping even though there are diodes in the feedback path.  Think of it as a 2-level clean boost.

If the basic u-amp is set for a gain of, say, around 10 with the stock clean feedback path (500k pot is set to around 3.5k), then maxing the secondary gain control (feedback resistance will be 520k when you hit the switch) will get you a gain of around 88, which should be enough to extract noticeable grit out of the Ge diode on one half of the waveform.

[Pavezer]

Thanks guys!

I don't have any parts lying around at the mo, so this is exactly what I was wishing for an answer. I don't have to order too many parts, and on the other hand, I'll be sure that i will order them enough to start with 

Drewl, thanks for the idea of combining the two. I didn't check out the dist+ schem before, but as I noticed now, they are so similiar that this might be a very useable solution.

Mark, you got some great idea there. Your reply is somewhat perfect, thanks for that! I wasn't even thinking about having (needing) a variable boost option, but now I think I might like it. This would make the pedal more versatile and not so attached (or tuned, as we might say) to only one amp. As you said earlier, good distortion tone is about the combination of things, i do agree with that, so this would make the pedal useable with several applications. I'm also a bit concerned about the gritness after the mods (it's a bit too trebly already, i think) and i will definitely do some tweaking with the low pass filtering. I might also be interested to put in a switch for smooth, which would increase the value of feedback capacitor, obviously.

I will try some of the solutions and will reply about my experiences.

Thanks again!

[drewl]

It really is one of my most useful pedals, a nice clean boost that can be footswitched into a ripping distortion!

[Mark Hammer]

Just occurred to me that even with a max gain of under 200, this should be potentially dirtier than a Dist+, simply because the Micro-amp has no bass trimming as the gain is increased.  That will provide more signal to hit the diodes.

[Pavezer]

It really is one of my most useful pedals, a nice clean boost that can be footswitched into a ripping distortion!

Drewl, did you use the dist+ schematic values in your pedal? And is the diode clipping placed in the end of chain just as in dist+?
And supposingly your distortion amount is just on/off with a switch, no pots involved?

[drewl]

If you look at the schematics for both on the Tonepad site, they are both quite similiar.
I simply added a 3p3t footswtich to switch the diodes at the output on/off while the other set of contacts switches the feedback resistor.
The 3rd set of contacts switches an LED for which mode you're in....

[Pavezer]

Well, I just realized that varying the negative feedback ohms with a pot will affect the filtering. I might not be happy with that. So... it might not be a good idea to have a potentiometer. On the other hand, I'm not wishing to have a very high amount of distortion, even if the distortion would be set to full, and I'm not really sure how much it would affect the filtering when the pot is turned.. any idea?

OR maybe I should add a tone pot too? Possibly I should have the negative feedback filtering set a bit high (brittle) and control it with added tone circuit as the distortion and the filtering varies. Does this make any sense?

I was wishing to proceed with a simple solution (as it is already possible), but a distortion control seems so intriguing that the whole project (or small tweaking) is getting out of control. Again. 

[Mark Hammer]

The change in filtering with a variable feedback resistance is actually a good thing.  As gain goes up and clipping begins, one starts to produce higher-order harmonics that don't sound very good.  Rolling more high-end off as clipping is increased is a very common strategy, used in the venerable TS-9 and all its derivatives.  Moreover, as the pedal starts to become used more for overdriving the amp, as opposed to producing a sound of its own, you want the top end to be more restrained so that the amp behaves itself and overdrives in a pleasing manner.   Some might argue that the 3.4khz rolloff provided by the circuit I suggest isn't enough top-end rolloff.

[drewl]

I mainly used all the D+ values, and there's not too much variance in frequency response between the two modes.
Mine has a simple tone control only because the box I used had 3 holes!



http://www.tonepad.com/getFile.asp?id=6

[Pavezer]

Ok, thanks mark!
There's definitely a learning curve here

Drewl, good to hear that a step in feedback resistance doesn't affect the tone that much in practice!

Thanks for the info guys, I appreciate it.
I surely have a direction where to head. I'll order some parts.

[MikeH]

Since this has basicall turned into "how to make a dist+ from a micro amp 101" I was wondering:  Comparing the 2 schematics, the 2 designs are biased a little differently.  Microamp uses 2 100k resistors to create a reference voltage and a 10M reisitor tied to the opamp input, while a dist+ uses 2 1M resistors for vref and a 1M resistor tied to the opamp input.  What type of sonic differences can be expected with the different biases?  I know that the resistors used to divide for vref don't really matter, but what of the others?

[Mark Hammer]

Well, for starters, do keep in mind that the two circuits use different op-amps.  The Dist+ traditionally used the unadorned lowly LM741, while the Micro-amp used the, then-new, TL061.  The biasing requirements may well be different for the two chips.  On the other hand, biasing requirements can be different from how biasing is used.  I've always been of the opinion that somehow the Dist+'s tendency to NEVER clean up, even when diodes are totally omitted, may well stem from the sort of "current-starved" bias voltage it uses.  I may well be wayyyyy off on that one, but you have to admit the tiny Vref bias current provided to the Dist+ is pretty unique in the annals of pedaldom.

[drewl]

FWIW I used a 4558 because I have a ton of them and it sounds almost identical to my original MXR, the main difference probably being the clipping diodes.