[2719] The Underdrive

Date: October 30th, 2020 | Comments : [5] | Categories: DIY.

I never knew this was a thing until I saw a forum post about it this week. Apparently it is the anti-boost, which means that the output can only be attenuated. Sounds easy enough… a potentiometer and switch in a box will do the job but can lead to loading problems and tone loss, so I quickly designed this circuit:

The output can be adjusted from zero to unity gain with the R4 potentiometer. A 10k log taper will give best results. The high input impedance and fairly low output impedance will preserve the guitar’s tone in almost every situation.

I believe this can be constructed on my Multi-Purpose Transistor pcb, so I will check into it and post the findings in the comments below.


5 Responses to “The Underdrive”

[787290] Marcello Says: 3:50 pm, November 9th, 2020

let’s call it a buffer

[787291] admin Says: 9:01 am, November 13th, 2020

It is buffering the loading that would result from just using a volume pot, which is what the people on the forum wanted…

[787295] George Neyrey Says: 9:00 am, November 20th, 2020

Nice! A simple solution to a simple problem. Think I’ll retire my gargantuan, antique BOSS FV-100 volume pedal that’s on one of my boards and give this a shot!

[787345] Artie Says: 5:12 am, June 14th, 2021

So, is the input impedance 2.2M, or 1.1M? I’m not sure how to calculate input Z in this instance.

[787349] admin Says: 9:33 am, June 29th, 2021

Input Z is about 733k if you use the pull-down resistor, but 1.1M if you do not.

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