MXR MicroAmp Analysis

The 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.

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.

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    15u
D1    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 classic non inverting topology, the resistances  R4,  R5 and R6 set the voltage gain as it will be seen in the Several capacitors C1, C2, C3 and C5 will filter the guitar signal, always keeping a flat response as it is explained in the

• 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:

$Z_{in}= (R_{1}//R_{2})//(R_{3}+Z_{in\: opamp})$

$Z_{in}= (22M//10M)//(1K+10^{6}M)=6.8M$

6.8MΩ is very good input impedance, not loading the guitar pickups and preventing tone sucking. As a rule of thumb the input impedance 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:

$Z_{out}= R_{10}//(R_{9}+Z_{out\: opamp})$

$Z_{out}= 10K//(470+100)=539$

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:

$G_{v}= (1 + (\frac{R_{4}}{R_{5}+R_{6}}))\cdot (\frac{R_{10}}{R_{9}+R_{10}})$

$G_{v\, min}|_{R_{5}=500K}= 1\:(0dB)$

$G_{v\, max}|_{R_{5}=0}= 20.6\:(26.2 dB)$

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:

$f_{c1}=\frac{1}{2\pi C_{1}(R_{2}//(R_{3}+Z_{in\,opamp}))}$

$f_{c1}=\frac{1}{2\pi 0.1uF(10M//(1K+10^{6}M))}= 0.15Hz$

• 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.

$f_{c2}=\frac{1}{2\pi C_{2}R_{4}}$

$f_{c2}=\frac{1}{2\pi\cdot 47pF\cdot 56K}= 60.4\: KHz$

• The capacitor C3 and the series resistors R5 and R6 from the (-) input to ground act as high-pass filter, attenuating frequencies below the fc cut-off frequency:

$f_{c3}=\frac{1}{2\pi C_{3}(R_{5}+R_{6})}$

$f_{c3\: max}=\frac{1}{2\pi\cdot 4.7uF\cdot (2.7K)}=12.5\: Hz$

$f_{c3\: max}=\frac{1}{2\pi\cdot 4.7uF\cdot (2.7K+500K)}= 0.06\: Hz$

• The out capacitor C5 acts like a high pass filter together with R9 and R10. Being R10>>R9 the cut-off frequency is:

$f_{c2}=\frac{1}{2\pi C_{5}R_{10}}$

$f_{c2}=\frac{1}{2\pi\cdot 15uF\cdot 10K}= 1.06\: Hz$

In the above graph, the gain potentiometer is set in the mid position (250KΩ). 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 fc=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 R9 R10 voltage divider. Additionally the high pass filter C5 with fc=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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