MXR - Microamp  [schematic]

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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Browser
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Post by Browser »

If the real schematic breaks any rules please delete thanks.

The pot is 470 something, I forget.
I did not pull the diode just saw a 48 but I don't think it matters much.

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Post by Browser »

oops I forgot the 100 ohm

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Post by RnFR »

thanks for adding that! i'm surprised we didn't have that circuit covered already.
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Post by lolbou »

RnFR wrote:i'm surprised we didn't have that circuit covered already
Might be because it's readily available everywhere else. :wink: But now it's here indeed!! :D
I like this booster (built two from tonepad's layout)...
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Post by IvIark »

Here's a vero layout I did for some axial components I wanted to use:

Image

There seems to be some differences between the schematic I used and the one Browser posted above. I've got 1K and 10M after the input cap, Browser has 10K and 1M, the voltage divider for Vref I have 2 x 100K, Browser has 75K and 100K, mine has a 15uF from the opamp output compared to 22uF.

Is yours based on the current model Microamp Browser?
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Post by okgb »

It is a little different but you may want to check out the
CAE / MXR schemo in the booster thread in this section

viewtopic.php?f=1&t=10060
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Post by Browser »

IvIark wrote:Is yours based on the current model Microamp Browser?
Bought it new last year from Musicians Fiend. I pulled the resistors to verify check values. Busted a lead because they are paper-thin.

This is all I saved if you want the actual layout:

Purple is top, blue is solderside

@ IvIark I left out the zero but the input cap is 0.1uF just in case.

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Post by IvIark »

Cool thanks for that. They make small changes like this from time to time, so I wondered if yours was the modern version schematic and the one I had was older, and that seems to be the case.
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Post by soupboneguitar »

I changed the op-amp in my microamp to a JRC4558.Sounds a lot better know.

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Post by Browser »

Not verified

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Post by Jack Deville »

Here'a a layout I knocked up for dual opamps.
Total PCB size: 2" x 1.2" Layout is unverified.
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Parts layout
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Transfer
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Post by Browser »

Since there are several changes it would be helpful if you added the revision number to your schematic. The changes are the sole reason for the initial posting, thanks.

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Post by Browser »

Verified

The pot was backwards. I tested some chips and surprisingly found the UA741CP cleaner then the original. IMO: CN for clean - CP for distortion - TL071 too boomy *** I only tested one of each chip ***

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Post by Browser »

I accidentally deleted the schematic.

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Post by marshmellow »

Jack Deville wrote:Here'a a layout I knocked up for dual opamps.
Total PCB size: 2" x 1.2" Layout is unverified.
Two things:

- The unused opamp will be happier, if you tie it to VCC/2 instead of ground. Here and here is why.

- You could use it to buffer VCC/2 for the main circuit by configuring it as a voltage follower and then connect the output to R2. Still the same parts count, but improved.

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Post by Jack Deville »

Doh.

You're right. So much for that pretty tracking, eh?
Thank you for catching that, marshmellow.

Here is a quick and dirty revision.
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Rev. B Transfer
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Rev. B Scheme
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Post by jay42 »

I only post this because Paul Rivera's set of pet peeves about the MicroAmp are evident from the Buf series he produced in the late 70's. Before the gig at Fender he was doing all sorts of things to make a living. Modifying MXR boxes was one of them.
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Post by roseblood11 »

Hi,

i´d recommend to use a better IC (OPA134) and a 18V power supply, it really improves the sound and the headroom.
Or use a voltage doubler:
Image

The interesting point of this booster design is that the volume is at unity gain when the pot is turned completely ccw. That makes it easy to add a switch there - so you can switch between booster and buffer modes...

The Micro Amp is one of my ugliest designs:
http://forum.musikding.de/cpg/albums/us ... design.jpg

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Post by minnow »

Tonepad's got a couple of MicroAmp layouts.
I'm interested in the dual microamp (i'd like to have stereo out) and I have a question.
Image

How should I wire the input signals? - can I just solder two wires to the input jack, each end to IN1 and IN2 respectively?
Or should I feed the signal into a splitter first (like the AMZ JFET Splitter) before soldering onto the FX board?

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Post by dickierex »

jay42 wrote:I only post this because Paul Rivera's set of pet peeves about the MicroAmp are evident from the Buf series he produced in the late 70's. Before the gig at Fender he was doing all sorts of things to make a living. Modifying MXR boxes was one of them.
What are Pauls pet peeves? I've had a Microamp for about 10 years now, and it's been the only pedal to stay on my board in that time - I love it. I just made one from a GGG board, and in my initial testing it sounded great, too. I noticed it spec'd a TL061 IC, whereas the original had a UA741CP. I've only briefly compared them, but they seemed to be very very similar. Considering they both seem to be general purpose opamps, I guess this should be expected. Does anyone have any info regarding the differences in these? My limited research (and knowledge on the subject) seems to indicate a better slew rate on the 061 - was this maybe the reason for this chip rather than the 741? Does anyone have a more recent model of Microamp, and does it have a different IC now?

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