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MXR MicroAmp Analys isMXR MicroAmp Analys isThe M-133 MicroAmp is a clean boost/volume pedal, part of the first Reference Series by MXR released between 1973 and
1984. The original stompbox did not have power-on LED or A/C connector. Jim Dunlop bought the MXR licensing rights and
currently manufactures reissues of the classic MXR effect.
The MXR Micro Amp is designed to be a transparent clean volume booster; it does not color or modify the guitar tone. The
circuit is in fact an un-distorted redesign of the previous M-104 MXR Distortion+ pedal.
The main application of this pedal is to do louder solos. It can also supply a permanent boost in a long effects chain or cables
where signal drop is a problem. Placing it before the amp the signal boost will drive the preamp harder and into more saturation.
Table of Contents.
1. MXR MicroAmp Schematic.
1.1 Power Supply Stage.
1.2 Op-Amp Amplifier Stage.
2. MXR MicroAmp Frequency Response.
3. Resources.
1. MXR MicroAmp Schematic.
The MXR Micro Amp circuit can be divided into two blocks: Power Supply Stage and the Op-Amp Amplifier.
This simple and in-expensive design will boost the guitar signal within the audio range up to 26dB , giving a clean flat frequency
response.
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The circuit is built in a double layer PCB with the potentiometer, led, jacks and footswitch mounted directly on the PCB, this is
useful in order to simplify wire connections and save money during mass production. There are several PCB releases in green
and red color with minor component modifications due to manufacturing component resourcing.
MXR MicroAmp Bill of Materials / Parts List:
C1 0.1u
C2 47p
C3 4.7u
C4 1u
C5 15uD1 1N4001
R1 22M
R2 10M
R3 1K
R4 56K
R5 500K pot
R6 2.7K
R7, R8 100K
R9 470
R10 10K
U1 TL061
Jack IN / Jack OUT
1.1 Power Supply Stage.
The Power Supply Stage provides the electrical power and bias voltage for the 4.5V floating ground reference.
The 9V supply will feed the op-amp, with a simple 100K resistor divider (R7, R8) 4.5 Volts are generated to be used as a
bias voltage.The resistors junction (+4.5V) is decoupled to ground with an electrolytic capacitor C4 (1uF) which removes all ripple from
supply voltage.
The diode D1 protects the pedal against reverse polarity connections.
The stereo in jack is used as on-off switch, switching the battery (-) terminal to ground when the guitar jack is plugged.
1.2 Op-Amp Amplifier Stage.
The signal booster is a non-inverting op-amp stage which provides high input impedance, voltage gain and signal filtering:
The operational amplifier is in a c lassic non inverting topology, the resistances R4, R5 and R6 set the voltage gain as it will be
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seen in the Voltage Gain Section. Several capacitors C1, C2, C3 and C5 will filter the guitar signal, always keeping a flat
response as it is explained in the Frequency Response Section.
The 22MΩ input resistor R1 next to the input jack to ground is a pull-down resistor which avoids popping sounds when the
pedal is switched on. The input pull-down resistor becomes the maximum input impedance of the pedal.
The (+) input is biased to 4.5V through the R2 resistor (10MΩ), keeping the virtual ground at 4.5V and being able to amplify
bipolar guitar input signals.
MXR MicroAmp Input Impedance.
The input impedance is defined by the formula:
6.8MΩ is very good input impedance, not loading the guitar pickups and preventing tone sucking. As a rule of thumb the inputimpedance of a pedal should be 1MΩ minimum.
MXR MicroAmp Output Impedance.
The output resistor network composed by R9 and R10 will limit the output current; even if the output jack is connected to ground
the op-amp will see a load of at least 470Ω, limiting the output current and protecting the operational amplifier. The TL061 has
an internal output resistance of around 100Ω, but anyway an additional extra protection is good.
The output impedance is defined by the formula:
539Ω is good output impedance, keeping signal fidelity. It is a good practice to keep output resistance of a pedal below 10KΩ.
MXR MicroAmp Voltage Gain.
The voltage gain is defined by the non-inverting operational amplifier and the output voltage divider formed by R9 and R10:
For single coil pickups which output is around 100mVpp, the maximum gain will boost the signal to 2.06V, without any
problems. Humbuckers which output is around 1Vpp, could be clipped due to the op-amp supply limits (9V) using the maximum
gain.
In the image above, the output signal voltage level is shown sweeping the volume potentiometer. The gain goes from 0 to 26dB
as calculated before.
2. MXR MicroAmp Frequency Response.
The tone response in a boost/volume pedal is featured to be transparent, not modifying the contents of treble or bass. Anyway
some signal filtering is always needed to keep the low frequency hum and the high harsh harmonics out of the audio scope.
To do so, 4 capacitors are used to place 4 poles and tailor the tone response:
C1 together with R3 and the input resistance of the op-amp forms a high-pass filter, with a cut-off frequency of:
The small 47pF C2 capacitor across the feedback loop works as a low-pass filter, avoiding instability and softening the
corners of the harsh harmonics mellowing the response.
The capacitor C3 and the series resistors R5 and R6 from the (-) input to ground act as high-pass filter, attenuating
frequencies below the f c cut-off frequency:
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The out capacitor C5 acts like a high pass filter together with R9 and R10. Being R10>>R9 the cut-off frequency is:
In the above graph, the gain potentiometer is set in the mid position (250K Ω). It is clear that Micro-Amp does nothing to
deliberately constrain either top or low end, it is essentially a flat booster.
The black signal is the guitar input signal.
The red signal is the signal after the first high pass filter C1 R3 with f c=0.15Hz. Hum from guitar is removed here. It can be
seen how the red line is rolling-off in the bass frequencies below 3Hz.
The yellow signal is the signal after the op-amp stage, filtered with the high-pass formed by C2 and the low pass by C3.
The blue signal is the output signal at the jack, the gain is sligtly lower due tu the R 9 R10 voltage divider. Additionally the
high pass filter C5 with f c=1.06Hz removes useless bass harmonics. This resulting output signal is completely flat in the
audio spectrum but removes the excess of bass and treble frequencies.
MXR MicroAmp Op-amp
The TL061 op-amp used in MicroAmp is very close to TL051, TL071 and TL081. TL081 is the base model, TL071s are less
noisy, TL061s have low current draw and TL051s have slightly better datasheet. If the battery life is not a restriction, the best bet
is the TL071.
3. Resources.
MXR Micro Amp True Bypass Modification by Mike Bland.
MXR MicroAmp clone Thread in DIYstompboxes.
MXR Clone Project in GGG.
My sincere appreciation to Shawn Hale for your support.
Thanks for reading, all feedback is appreciated
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