Weird Fatpants issue

[jkokura]

Hey guys, Brian in particular.

You know that Fatoss I'm building Brian? I have included the mod where I've subbed a 5kB pot in for the 3k3 resistor to include a gain control, on both of these I'm building. On one of them, I put a 470 resistor in before heading to the pot, one I didn't do anything.

In both cases, I'm getting a little bit of gain at CCW, then as I turn CW the gains goes up as per normal! Only, when I hit about '3 o'clock' the gain drops like crazy down as if the pot is full CCW. I can deal with it, but I'm just wondering if you have any suggestions. Never had this happen to me in any other build.

Jacob

[madbean]

In the FatPants, when you turn the gain control up you are decreasing the overall resistance between the source pin of the JFET and ground (the 470R sets the minimum resistance, i.e.maximum gain, attainable in ver.2). It is this decrease that varies the bias of the transistor which pushes it into higher amounts of amplification.

Here's how I understand it: In the absence of any resistance in there, you only have the 22n bypass cap between the source pin and ground which decouples it and eliminates the necessary bias needed for the device to work properly (as a source biased amplifier).

So, you do need some kind of minimum resistance in there if you also have a bypass cap. It sounds like in one case you do (the 470R) and one case you don't. In the case that you don't have that stopper resistor the explanation is obvious, considering the information above. In the case that you do have a stopper resistor, I am puzzled, because that should be the same set-up as the FP ver.2. It sounds like you have wired the gain control correctly given that you get amplification as you turn it up (to a certain point that is). So, it may be that the 470R needs to be changed to a different value, or possibly the drain voltage needs to be re-adjusted. Are you getting around 4.5 on the drain?

[jkokura]

Fixed it!

I used a 2k resistor instead of a 470r. It works great now.

For other peoples reference:

If doing a gain pot on a v1 fatpants, use a 2k resistor to lug 2 of a 5kb pot, and send lug 3 to ground.

Jacob