LPB1 circuit analysis

The LPB1 (Line Power Boost 1) is a boost pedal that was commercialized in 1968 by Electro Harmonix. It is the first boost pedal using a silicon transistor. Indeed, previous boosts like the Dallas Rangemaster Treble Booster were using germanium transistors. Another novelty with this pedal was that it was boosting all frequencies and not only trebles, making it the first clean boost available!


The circuit is really the most simple one you can imagine for a boost:

LPB1 schematic
First, there is a 0.1uF coupling capacitor that prevents parasitic DC current from the guitar to go in the circuit. With the R2 resistance, it forms a high pass filter : by changing its value, you can modulate the amount of bass going through the circuit. If you increase C1 value, more bass will go through, and vice versa.

Then, there are two resistors forming a voltage divider (R2 and R1), to provide a certain voltage to the base of the transistor. Here it is : R2/(R2+R3)x9V = 43/(43+430)x9=0,81V at the transistor's base.

The silicon transistor is a 2n5088 (originally a 2n5133 - same transistors that were used in the Big Muff later), wired as a common emitter. R4 and R3 will define the amount of amplification. If you increase R4, amplification will be larger. If you increase R4, there will be less gain.
A second 0.1uF coupling capacitor that prevents DC current from the battery to go out of the circuit. Finally, a 100k potentiometer wired as a variable resistor defines the final volume.

If you look carefuly, you can see that the last stage of the Big Muff circuit is exactly the same! A LPB1 circuit is used to increase the final volume.

Here is a global schematic for what does what: (click to enlarge)
1 Comment
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Thank you for the schematic and analysis. As an ammateur electronic gadgets DIYer I tried to follow but hit a snag in the following two points:

i) from where the biasing of the transistor comes

and

ii) the function(s) of R3 and R4. Can you help me please?

In i) it appears that biasing comes from R1 and R2 but then later you say that R3 and R4 also bias Q1.

In ii) it appears that R3 and R4 work ..."against" each other!? To me it sounds like instead of complementing they "balance" each other. So what if I want to introduce a variable resistor in place of one of these resistors to provide a control for increasing/reducing gain? At the same time if R3 and R4 affect the biasing in conjunction with R1 and R2 then how introducing a pot will affect the entire design, thus final output? Thanks for your time and explanation.

Answer