Author Topic: MOSFET boost differences  (Read 258 times)


MOSFET boost differences
« on: January 20, 2021, 12:50:08 PM »
I'm putting together a EQD Life Pedal clone on a breadboard. I've got the whole thing wired up and it works great.

From what I understand, the stock boost circuit is based on EQD's Black Eye boost.

I have the stock boost wired up, but I decided I also wanted to hear what a SHO boost circuit would be like in that position.

I've tested (by ear) both boost circuits on their own and they seem to give pretty similar volume increase, the Black Eye circuit probably a bit more.

I used the schematic for the Life Pedal for the Black Eye boost part. But for the SHO circuit, I just built from the SHO schematic and connected it in. Maybe there's something that needs to be changed when it's placed like this in the Life Pedal?

The thing I'm noticing is that once I stick the circuit at the end of the Life Pedal, the stock boost circuit still gives a massive amount of volume increase as I turn the knob up.

The SHO does give a volume increase as the knob is turned, but it also seems to be adding more hair to the signal. Not what I would have expected, and I didn't notice it when the SHO was on its own.

Is there a reason this would be the case?

« Last Edit: January 20, 2021, 12:54:52 PM by befey »


Re: MOSFET boost differences
« Reply #1 on: January 20, 2021, 01:15:49 PM »
I'm no expert but:
the only significant difference between the two schematics is that the SHO uses a "gain" pot (variable R on the source) while the black eye uses a trimmer there and then a "volume" pot. The reason for this "improvement" is to avoid the crackle from DC on the gain pot.

The Black Eye has fixed gain, you fixed it by setting the trimmer. So any distortion ("hair") from the transistor clipping will be the same as you adjust the volume. The SHO clips more as you crank it.


Re: MOSFET boost differences
« Reply #2 on: January 20, 2021, 05:16:53 PM »
I guess it was my misunderstanding then.

I had assumed the SHO was a clean boost, but now that I read some of the description more carefully, it definitely says it adds distortion as you turn the knob.

The Life Pedal schematic I'm using doesn't have a trimmer for the boost part, just a 470o resistor there. I guess the way the MOSFET is set on there just doesn't clip the same way as the SHO does.

It does seem odd the Black Eye seems to get louder but doesn't clip, while the SHO doesn't get quite as loud but clips more. I'm using two different BS170s. I guess I'll have to check their measurements.

They really use completely different values of basically everything hanging off the MOSFET so I guess maybe it shouldn't be so surprising that they react so differently.

Rob Strand

Re: MOSFET boost differences
« Reply #3 on: January 21, 2021, 01:17:44 AM »
Measure the drain and source voltages on each circuit.    For circuits with an adjustable source resistor you will need to measure the voltages with the adjustment at each extreme.

I'm assuming you have built the  boost part of the Life Pedal schematic and the SHO.   The Black Eye I've seen isn't quite like the  Life Pedal  so when I read you original post it gets a bit confusing.  It would help if you posts links to the schematics you built.

The 5k vs 10k drain resistors and the 470k vs 10M gate resistors could affect the overall gain with a guitar input.
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