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Forum Home > Layout Gallery - GuitarPCB Build Documents, Mods and Tips > Big Muff Pi - Opamp

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Brand new fabricated version with extras.

Not only does it sound great but it is small enough to fit into a 1290B size enclosure.

Board Dimensions (W x H) 2” x 1.65” ca. 50.2 mm x 42mm

BMP Opamp Build Instructions

Big Muff Pi  - Audio Demo


In regards to the optional values in the Build Documents above for those who are interested in more detail please see this "Copy and Paste" to a Forum Question by Tonmann below.

Hopefully this answers the question.

Some of the value changes were done to keep the number of different component values to a minimum.  This is useful for people who buy in quantity (bags of resistors) rather than just single components.  These value changes in terms of circuit / sub-circuit performance are minimal and hardly perceptible.

The Tone control (pi filter) changes were made to create a scoop at mid frequencies when the Tone pot is at 50%.  I'll return to the tone control section after I've briefly covered the other component value changes.

R2, R3 C1

These components set the gain of the first stage (IC1A) with C1 setting the roll-off point (the point at which frequencies start to attenuate) for bass frequencies.

With the original values the gain of IC1A is 5.9 (+15.4dB) with the roll-off point at about 19Hz.

With the new values the gain of IC1A is 7 (+16.9dB) with the roll-off point at about 34 Hz.

With a difference of about 1for the gain and both roll-off points well below anything that even a bass guitar produces, I would say that there is very little difference between the two sets of values.

R6 R7 R8 P1

R6 and R7 set the gain of the second stage (IC1B), P1 sets the output level and R8 prevents the output level from being set to zero.

The original values of R6 and R7 produce a gain of about 10 (+20dB) while the new values produce a gain of about 11 (+21dB).  With both sets of values there is a slight "knee" or hump at about 1kHz before the high frequencies start to roll off - the new set of values produce a slightly larger knee.  Reducing the value of C3 will flatten the knee - my prototype notes say 3n9 but they go on to say that there wasn't much noticeable difference at the output of the circuit.

The value of P1 is not critical; without getting too "techie" - if the output impedance of the op amp is 100Ω (a 4558 op amp is about 75Ω ;) a 10kΩ pot will "grab" 99.01% of the output signal at 100% rotation, a 100kΩ pot will "grab" 99.9% of the signal - not much difference between the two.

Looking over my prototype notes it would seem that a log pot is preferable over the original lin pot.

R8 prevents the pot pot from being completely turned off - any small value resistor (anything up to about 10% of the pot value) will do.

R9 R10 C5

Like the first stage, R9 and R10 set the gain for the third stage (IC2B) and C5 sets the roll-off point for bass frequencies.

With the original values the gain of IC2B is 57.3 (+35.2dB) with no roll-off point i.e flat frequency response down to 0Hz

With the new values the gain of IC2B is 58.9 (+35.4dB) with the roll-off point at about 58 Hz - below anything a guitar can produce.

With C6 in the circuit the high frequency roll-off point for IC2B is about 2.3kHz for the original values and 3.35KHz for the new values.

P3  R13

M prototype notes say that there was no audible difference between a 50k and 100k Volume pot and that whether R13 was in the circuit or not made no difference (at least to my ears).  I left R13 in the circuit as it was part of the original design and in case my ears were deceiving me.


Any low value resistor up to 100Ω

The Tone Section

Rather than go through the whole tone section in detail, you can see the effect changing the values has by looking at the last page of the BMP Op Amp V1 Build Instructions or:

Here is another option resembling the Version 5 BMP Opamp:

You will need a 220nF (or larger) capacitor and a DPDT switch in addition to the standard components.

I would also suggest using a piece of vero board as you have to connect one lead of C7 and one of C9 together and run the connection to lug 3 of the DPDT switch - this will make soldering much easier.

Follow the wiring in the above diagram.

Courtesy of Wilkie:

Bypassing the tone control of the BMPOA for a Tone TwEQ board

Just connect the Input (T pad on In) of the Tone TwEQ (1 wire) to the Pad A on the BMP board and ignore all components downstream from that point.

Again, from the A pad of the BMPOA wire directly into the Input of the Tone TwEQ. (T Pad on In)

Finish your build by  using the Output of the Tone TwEQ as your main Output. (1 circuit 2 boards)

This would add the active tone circuit of the Tone Tweq to the BMP OP AMP V2 and also replace the final VOL pot of the BMPOA. So now use the Volume on the Tone TwEQ.

That is it.

Or simply wire both completed builds in series inside the enclosure, use a trimmer set to (0 or 1) for the Tone TwEQ Volume and voila you have a simple combo build.

October 30, 2012 at 1:56 PM Flag Quote & Reply

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