Simple FET Help

[Fangda]

Hi, ive recently taken this - http://www.till.com/articles/GuitarPreamp/ - preamp and added both hard and soft diode clipping... the only problem is that I am unsure of what values to use for the two pots and the resistor in the soft clipping circuit...

Any help or suggestions you may have would be much appreciated... Im intending this to go onboard my guitar, so it has to remain fairly compact...
Thanks in advance!

[Dragonfly]

I havent done the calculations, but the "stock" Tillman only has about 3dBs of boost....so if you add shunted diodes you may not even reach unity gain.  Something to think about....

[Zben3129]

Do you have this on breadboard / mind soldering and desoldering if it is wrong?

I would try 500k for the soft clipping and 100k for the hard clipping. Also, I would reccomend putting the resistor in series with the pot instead of in parallel, and to use something like 47k.

You could add a small (47pf) cap across the soft clippers aswell to let through more treble.

PS. I apologize in advance if any of the above is wrong / stupid. Im no EE, I just try to get a grasp on what is going in in circuits i build / look at. And a lot of the time, I'm wrong  

Zach

[Fangda]

Ah... thats slightly inconveniant... any suggestions as to how i could boost the signal more?

And, yeah ill try out your suggestions for the pots and the cap in the soft clipping... and is there any point of putting that resistor in series with the pot? Why wouldnt I just increase the pot's value..?
Then again, i dont know a whole lot about this, so any help is much appreciated =D

[Zben3129]

As for putting the resistor in parallel, it would greatly reduce the amount of gain, as the maximum resistance would always be less than the resistor (or pot, if it is lower in value).

As for the resistor in series with the pot, this makes it so that even with the pot all the way at 0, there is still some resistance in the clipping section, which is desireable in a soft clipping circuit.

(Someone of greater knowledge please correct me if I am wrong  )


Zach

[Fangda]

Ahh, ok, thanks.
Hmm, the only problem now is that the boost is going to be too low... As dragonfly said, I may not even achieve unity gain...

[Zben3129]

I think what Andy was saying is that the hard clipping section will be trouble with the 3db gain. I think the soft clipping portion will be fine.

Andy knows what he is talking about, however, and I am, well, "attempting" to understand all this stuff, so he will give you a better answer than this I'm sure


Zach

[Fangda]

Haha, dont worry, you know much more than I do...
Im still in the 'trying to learn all the terms' stage... =(

Hm, so if I cut out the hard clipping, it would be fine?
I guess ill wire it up on a breadboard this weekend and see if it can handle the hard clipping...

[earthtonesaudio]

You'll probably want a capacitor either at the input or between the "soft clipping" section and the 6.8k resistor, or there will be DC going into your guitar.

And just as a subjective comment, when I think of "soft clipping," it's usually in the negative feedback of an op-amp with signal going into the non-inverting input.  The diodes in the feedback loop of an inverting circuit like you have here, would be more like a Big Muff clipping stage.

As for getting more gain so you can actually hear the clipping, make the 6.8k larger or the 2.2k smaller, or put a cap from source to ground, etc, etc.

But a 100mV guitar signal only needs to be amplified by 7 to get silicon diode clipping.  Adjustable gain up to maybe 20 could be more useful.

[Fangda]

Hm, add a pot in series with the 6.8? (now made higher?), and a cap in series too?

[earthtonesaudio]

Add a pot in series-sure.
Add a cap in series-no.
A cap in series with the + power connection is about the same as disconnecting the power.
I was talking about a cap to keep DC out of your guitar.
The stock Tillman doesn't need a DC blocking cap because the input is referenced to ground, but you've added resistors and diodes from the input to the positive supply.  That will affect the bias.  It could still work with the right values, but even then you have the problem of DC on the input.  Therefore you should add a cap to block DC at the input, or a cap to block positive DC from reaching the gate.

[Fangda]

Oh, so add a cap at the input...
what value for the cap, just a small one?
Sorry if I sound stupid or im going in circles... I simply have very little knowledge and experience with this subject... I just started tinkering and learning about it a week ago