Thor schematic missing a cap at the beginning?

[rikkards]

Hi,
I am presently building the Thor and noticed that the schematic at runoffgroove is missing a cap at the Input. I assume that it would go between the 1M and 33K or before the 1M or is it not needed? Also I assume a 0.1uF?
Thanks!

[soulsonic]

It's not necessary. I built mine without and it was fine.

[rikkards]

Interesting. I would have assumed you needed one to isolate the DC voltage or is this not the case?.

It's not necessary. I built mine without and it was fine.

[slacker]

The way Q1 is set up in the Thor there is no DC voltage on the gate of the FET so you don't need a decoupling cap.

[rikkards]

Thanks! That explains it!

The way Q1 is set up in the Thor there is no DC voltage on the gate of the FET so you don't need a decoupling cap.

[PerroGrande]

I included one on my Thor build, but that is because I'm a creature of habit.  It is technically not necessary as Slacker pointed out.  The gate of Q1 self-biases and only as a reference resistor to ground.

Nevertheless, I never assume what type of signal will be presented to a given circuit, so I include stuff like this.

A possible example of how this could happen isn't too hard to dream up. If the previous pedal, for example, had a defective output cap it could present DC to the input of Thor.  Likewise, if the pedal used CMOS switching (which requires the signal to be biased) and did not include a coupling cap (under the assumption that the next stage would have it), it could present DC to the input. 

If you do want to include one, 0.1 uF is perfectly good for that.  The impedance presented by Thor will largely be dominated by the ground reference resistor, which is at least 1 meg.  The high-pass filter created by these would have its 3dB point at 1.59 Hz.  You could even go a bit smaller with the cap and still be well within the range of audible frequencies on the low side.