Mosfet Boost Pot question

Started by trumpus, June 10, 2006, 08:18:44 PM

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Hey all,

I'm in the midst of finishing up my first build, the Mosfet Booster.  I got it wired up today, and fired it worked on the first try - but not perfectly!

Everything seems to receive signal alright, however, the pot seems to work backwards!  From the top, it gets more boost when it is turned counter  clockwise, and less when it is turned clockwise!

Any thoughts?  TO fit it in, I had to sit it with the lugs to the right (when viewed from underneath).  Does this have anything to do with it?  Is it wired up wrong? When viewed from inside the box, the wire to the board is on the right most lug, and the two on the left are grounded.

I've also noticed that the boost is less pronounced at the lower end of the spectrum (from 12 noon to 5 o'clock - the lower end for me because the pot is reversed).  Is this normal?  I'm using a 5k linear taper pot as the instructions said...

Any other thoughts?


If you are building this from the GGG site,
I beleive it has the pot wired backwards.
Just hook it up completely opposite to the way you have it
wired currently and you should be good to go.

The wire from the board to lug 3, 1 and 2 to ground.


I am wiring it based on the instructions that came with the board i purchased from Jack Orman at AMZ.  I wired it the same way you have pictured in your drawing!

Should I wire it opposite?



Mine is wired as I showed you and the volume increases when the knob is turned clockwise.
Give it a shot.

Mark Hammer

Whichever way you wire it up, gain increases as the resistance provided by the pot gets smaller.

One of the things I like about the AMZ-MFB is that it really only takes a SPST switch to go between some preset boost and no boost.  Here's how.

As noted, the degree of boost depends on the resistance provided between the 100uf cap and ground.  When there is either no cap to ground, or the cap has no connection to ground, the gain is drastically reduced.  Given that it is a 100uf cap that can store a lot of charge, it is NOT the sort of cap you want to connect and unconnect because it will pop...big time.

But what if there was a fairly large resistance between the cap and ground?  Would the circuit behave as if there was effectively no cap there?  The answer is pretty much yes.  If the lower resistance, as a parallel path, was connected and unconnected, would there be a pop?  The answer is pretty much no, because the cap would always have a path to to ground to drain off.

So, imagine you had a 100k resistor between the negative lead of the cap and ground, and a 10k pot, wired up as a variable resistor.  One end of the pot is tied to the cap and the other goes to a switch, which can either lift the ground connection or close it.  You could preset the boost on the pot.  When the switch is open, it reverts back to the lower gain state, and when the switch is closed, the gain is given by whatever the combined parallel resistance of the 100k resistor and pot might be.   And no pop.

So why and how is this useful?  First, the swapping between boost and no boost doesn't have to involve bypassing the entire circuit.  This means you get to keep the high input impedance provided by the MFB...all the time.  In light of the various discussions here about whether true bypass is a good idea or not, the advantage is that you can use the MFB as your impedance buffer that stays on all the time, and use true bypass with all its advantages, without any of its disadvantages.  The other nice thing about this arrangement is that since it only needs two contacts to switch between boost and unboosted state, you can use the "lowly" DPDT stompswitch to simultaneously engage the boost function, and turn on a status LED, something you'd normally need a 3PDT to do.

Alternatively, if you want to get fancy, there is no reason why you couldn't use a DPDT (or even SPDT if you don't need status indicators LEDs) to select between two alternate preset pots for two different boost levels, and use another DPDT to lift the ground connection from those two pots, yielding a buffered nonboost setting and two different boost levels.



Edit:  Changed the pot wiring, and initially it didn't work.  I loosened the jacks a but (as I had just tightened everything up for the final test) and everything worked fine!

The pot contols the boost in the correct direction now. All is well - great first build!

I need to get better at laying out the enclosure (in terms of where everything fits).  I'll post pics in a bit - but basically, everything fit in, but i'd prefer the pot to be a bit lower/more centered.

The only other annoying part of this build, was the fact that i "splurged" for the heavy-duty 9V connectors this time - and after all my wiring, it didn't fit in the box with the batery (a little too thick and a little too tall).  I swapped it out for the cheaper connector and everything is hunky dory!

Great little boost - I ended up using a 1N757A Zener diode and a BS170 transistor.  When I purchased parts, however, i got a 1N5239 diode and a 2N7000 transistor as spares.  Would swapping these out and messing around with combinations change the tone at all?

One small problem - when I engage the effect, there is an audible pop - I have heard of wiring a "pulldown" resistor to the input jack, ,but i'm not sure exactly what this is or where on the jack it should connect.  Any input ? (no pun intended).

Finally, I also purchased the 100K trimpot, but use it initially, instead using the resistor.  What exactly does the trimpot do!?  On a technical, newb-related note, the 3 holes on the PCB for the trimpot are in a straight line, evenly spaced.  The trim pot leads are in a triangle orientation - is it ok to bend the leads to fit the holes, or should I make the connections with wire?  Additionally, how is the trimpot oriented with respect to the PCB. Which lead goes in which hole?



from the FAQ on the topic of the pulldown resistor

Bypass - I get a pop when switching my effect in and out. How do I stop this?

The typical method is to put a "pulldown resistor" from signal to ground at the front and end of your circuit. This is implemented as a 1 meg resistor from signal to ground. Just put one at the beginning of your circuit board (before the input cap or start of circuit if no input cap) and another at the end (after the output cap or end of circuit). Look at the many GEO layouts if you need to see a layout using these resistors. If you are not using high quality switches, it could also be the switch.

heres a schematic where you can see it at the start (the 1m by the input)

a trimpot is just like a regular pot, but you don't have a knob for it and instead adjust it with a screwdriver. Its used for things you don't have to adjust very often.


Can I just wire to the input and output jacks or does it physically have to be wired to the board?

Re: trimpot - i guess I was asking what THIS trimpot specifically controls, not what a trimpot does in general, sorry!


Alex C

Quote from: trumpus on June 11, 2006, 12:47:42 PM
Can I just wire to the input and output jacks or does it physically have to be wired to the board?

R.G. says:
QuoteThe signal lugs on the box are not tied to the effect circuit when a pedal is bypassed. The whole point of pulldown resistors is to pull the input/output capacitors to ground when the effect is bypassed, and they can't do that if they're permanently mounted on the in/out jacks. They have to be permanently attached from the outside cap of the effect circuit input and output to ground to be effective.


Hmmm...not 100% confident I know enough to do this...

Anyone have any pictures of a pulldown resistor wired correctly?  My board doesn't have any extra pads or holes - so i guess i just don't exactly understand where to physically wired the resistor to, then...

Sorry - i know it's newbie stuff...



  I just put in some pulldowns.
  I twisted two 1meg resistors together and soldered where they meet to a ground wire, one 1megs' end goes to circuit input lug [right on the switch], the other 1meg to circuit output [on switch.
  This I like in case I want to change circuits around, and it makes a nice quiet/switch in box, those resistors stay with that switch, and minimizes heat buildup in the switch when wiring inputs and outputs. Also I find it's alot easier to trim a solid core resistor wire [I leave about an inch to wire to], into a new switchlughole, and never makes the mess stranded wires can turn into.
Convention creates following, following creates convention.


Very Cool.

Thanks for the info...i'm ure I can handle that!