Fuzz Face NPN Clone with Tone Control question

[bushidov]

Hi Guys,

Sorry to continue asking noob questions, but I think this one will be a short and sweet one.

I have an attached schematic that I have confirmed works pretty well, but I am not exactly sure how one particular part is working.


On the Tone Control, I see VR3, which is the tone pot, giving the player the ability to switch between the C4 and C5 caps. For an example, assume the pot is all the way to the C4 at 33nF or all the way to C5 at 220nF, how is that changing the tone? I know from listening and running an analyzer, it appears to cut off high end. The larger the cap, like C5, the more high end it cuts. The small the cap, like C4, the less high end it cuts.

This makes me think it is just an RC low pass filter, and even the math seems to add up that it is an RC low pass with C being either of those two caps, and R4 (1K). I mean, if I do the math with the 33nF, I'd get:
fc = 1 / (2 x pi x R x C) = 1 / (2 x pi x 1000 x 0.000000033) = 1 / 0.000207345 = 4.8kHz

So, it starts the cutting at around the 4.8K mark, which is about what the analyzer is hearing.

And as I go to the larger cap, it does cut more as it adds the parallel capacitance of C5.
fc = 1 / (2 x pi x R x C) = 1 / (2 x pi x 1000 x 0.000000253) = 1 / 0.00159 = 629Hz

What doesn't add up is that I thought RC filters had to take signal to ground, not high (9V in this case). Am I incorrect on this or are my assumptions above incorrect? Or can you actually make an RC filter go to high instead of ground?

Sorry for the electronic questions. Just trying to understand how this circuit works.

[GibsonGM]

Good question bushidov, and welcome.  I can't claim to have expertise with all the math and so on, tho I do have the basics down.   

The very simple, non-technical answer is:  AC signals don't care if they are returned to ground or "+"...if you think about it, the power supply is actually a short circuit for AC, + and - are only separated by a low resistance capacitor in most cases!  AC signals have both + and - components, too...they are not of a certain polarity.   We are only concerned with the concrete idea of 'ground' and 'power' for the DC part of our work, usually.  Most designers DO use ground in the conventional way, but you don't have to...

Think about reactance here, rather than 'resistance'...

Hope that helps!

[mac]

If this is for your personal use, forget about maths, use a breadboard and try some caps here and there until you find the ones you like.

What doesn't add up is that I thought RC filters had to take signal to ground, not high (9V in this case). Am I incorrect on this or are my assumptions above incorrect? Or can you actually make an RC filter go to high instead of ground?

The treble cut I always use for silicons versions goes to Vcc.



mac

[bushidov]

Yeah, I used it on a PCB I made myself, and it works well. Just trying to understand why.

[mac]

Yeah, I used it on a PCB I made myself, and it works well. Just trying to understand why.

The signal goes to the volume pot between the 500 pot and 8k2 resistors.

If the 0.047uf cap bypass the 500 pot then the highs go to GND via Vcc -> big filter cap -> Gnd.
The corner freq is given by 500 x 0,047

If in the middle, as in the schem, only half of the pot is bypassed, Besides, there are 250 of resistance in the way of the highs.

mac

[PRR]

> I thought RC filters had to take signal to ground, not high

Ideally, DC power rail IS an AC ground. Since the network blocks DC (caps on all paths) it "should not matter".

DC rail is never a perfect AC ground. In this case, if there is high frequency crap on the DC rail, it couples into your signal, which could be annoying.

Again because the network blocks DC, it could just as well go to DC common and output jack shell. If it was actually wired as drawn, it could be to save an inch of wire, or it could just be a try-and-see hack which was not fully thought-out.

[bushidov]

I think PRR nailed it with that last bit. I was converting a PNP circuit to NPN and didn't notice that bit didn't get flipped to ground, so it accidentally got left as high.

However, as I said earlier, it still worked. After I made this post, I hacked my circuit and added a switch that flipped the ends of those two caps to ground or to the 9V rail so I could see if I could hear a difference. I really couldn't hear any difference across the sweep of the tone pot. But PRR is also correct in potential risk.

PRR, is there a way to make that worse case scenario happen on purpose so I could test the circuit on it, or because I have C1 and C3 (input and output caps blocking DC in between), that circuit is "technically" safe?

[PRR]

Try it on a crappy power supply.

However the typical crap-supply is riddled with 100/120Hz and lower harmonics. This tone control bleeds above 600Hz. So it may have little effect.

Until you get to bad switcher supplies. Large >20KHz whine will come right in. That won't affect *this* unit, but may upset the next in the chain.

[italianguy63]

Shouldn't that .047uF cap be significantly larger to be effective?

MC