Mosfet Booster trimpot wiring question

[trumpus]

Hey,

So i built an AMZ Mosfet booster (using a board I purchased directly from Jack) a few months ago.  I did a cross-country move right after I finished it, and only had timwe to plug it in (direct to amp) to make sure it worked (which it did).  When I recently got around to integrating it into my setup, I realized that there was a problem.

When I used it in the effects loop of my Mesa DC-3 OR in my chain of effects in front of the amp, there was a loud popping and a volume drop which crecendo'd to the set volume, right after i turned it on.  I think I isolated the problem to how it interacts with a buffer (the effects loop is buffered and in my direct chain, it is placed AFTER a signal buffer).  When i run it guitar > mosfet >amp, it behaves normally.

My questions are these:

1. Will substituting the R2 with the 100K trimpot enable me to tweak the bias so that it can be used with a buffer, or does this pedal (like some of my others) not play well with buffers?

2. For trimpot wiring:  The 100K trimpot I have has 3 leads coming out of one side of it.  From the top, these leads are labeled 3-2-1.  There are 3 corresponding holes on the PCB board where the trimpot should go.  How do I wire the trimpot?  Should I insert the 3 leads from the component side of the board (so that when I am looking at the side of the board with all the resistors and caps, I see the knob of the trimpot) or from the solder side.  Does it even matter?

Furthermore, what exactly will tweaking the trimpot allow me to do?  What does it change?

Brian

[davebungo]

Ho,

I assume that the popping occurs when you switch on your MOSFET booster (not your amp).  Popping is almost always caused by a sudden transient flow of current (or charge) somewhere in your circuit or even between circuits as a capacitor (dis)charges.  As it does not occur when you have guitar->booster->amp, then this suggests that having a DC path to ground at the input (in the case of your guitar, your pickups will provide this) corrects the problem.  I would have thought that placing a large value R say 1Meg right at the front of the MOSFET boost circuit may help, but without the circuit in front of me I can't confirm this. 

[trumpus]

I e-mailed Jack about it, and he told me that the Mosfet board has a pulldown resistor built into the output and under no circumstances should I add one to the input!

I really just need to know about the trimpot wiring, and whether or not adding a trimpot will help this pedal play nicer with a buffered signal?

Thanks!

Brian

[davebungo]

"Under no circumstances"?

I don't see why not if it fixes your problem.  Didn't Jack sort out your trimpot question?

[amz-fx]

Don't put a pull-down on the front of the pedal.  Period.   Okay, put a 10M if you absolutely are dying to put one there but nothing else.

It is better to change the input cap to a non-leaky one that to put in the input resistor.

There is a drawing on the sheet of instructions that was shipped with the board on how to add the trimpot.  It is the pcb layout in the bottom left corner....  R2, 100k trim.  Just fit the legs of the trimpot in the 3 holes provided.  It doesn't matter which way the trimpot is turned, it's not polarized.

The trimpot has nothing to do with buffering.

regards, Jack

[trumpus]

Thanks again Jack,

So then what exactly is the benefit of adding the trimpot (ie., what does adjusting the bias actually do?).

Brian

[trumpus]

Anyone?

Can anyone tell me what the benefit of the trimpot is (ie, what exactly does adjusting the bias do?)

What situation would i need this for or how would i use it to adjust the pedal?

Thanks,
Brian

[geertjacobs]

I recently found this very nice article from R.G. Keen that gives some more insight:
"Designing and building your own Mosfet Boosters"

[amz-fx]

Can anyone tell me what the benefit of the trimpot is (ie, what exactly does adjusting the bias do?)

It allows you to adjust the idle conditions of the circuit, usually measured by checking the dc drain-to-ground voltage. That allows you to adjust the circuit for the greatest voltage swing before clipping.

You don't really have to have the trimpot as the resistors in the parts list will do the job most of the time.

regards, Jack