Author Topic: what happens when you short a capacitor?  (Read 4520 times)

ode2no1

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what happens when you short a capacitor?
« on: December 12, 2012, 12:15:20 AM »
about 7-8 years ago i built a sonic distortion from ggg and in doing some random poking around i noticed that if i shorted C9 the sound got so much thicker...so i ended up putting a toggle switch in there to get the stock or fatter sound. i never really know how or why this worked for me, but now i would like to know what happens when you short a cap. does it basically take the cap out of the circuit and replace it with a jumper? does it change the value of the cap in the circuit?

here is the schematic

http://www.generalguitargadgets.com/pdf/ggg_isd9_sc.pdf?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

R.G.

Re: what happens when you short a capacitor?
« Reply #1 on: December 12, 2012, 12:35:50 AM »
what happens when you short a cap. does it basically take the cap out of the circuit and replace it with a jumper?
Yep, that's what it does.

Quote
does it change the value of the cap in the circuit?
No. A wire is fundamentally different from a cap, as it conducts DC as well as AC.

Quote
here is the schematic
C9 is in the tone control circuit, on the treble side, so it lets through more signal in general, not just treble.

Exactly what happens when you short a cap depends more on the rest of the circuit - which may be ANYthing - than on the cap.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

ode2no1

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Re: what happens when you short a capacitor?
« Reply #2 on: December 12, 2012, 01:07:04 AM »
thanks so much R.G. you're awesome. another quick question...what would happen if both caps in the tone circuit were replaced with jumpers? would it work?

PRR

Re: what happens when you short a capacitor?
« Reply #3 on: December 12, 2012, 01:39:57 AM »
> what would happen if both caps in the tone circuit were replaced with jumpers? would it work?

It would not be a "tone control".

It would be a "volume control" (but you already have R15 for that).

C9 R12 cut-off the lows.

R13 C8 cut-off the highs.

Tone knob R14 allows you to select the low-cut, the high-cut, or a range of compromises.

With both caps short, R14 wiper "up" now gets a full-range sound. R14 wiper "down" gets a short-to-ground where C8 is, no signal at all.

You can try it(*).

(*) Do NOT go randomly shortiing caps. Just for example, in this circuit, shorting C2 will put DC onto your guitar. The guitar absorbs current and gets hot. The pot will "scratch" from minor roughness interrupting the large DC voltage. Most likely Q1 will be badly mis-biased and won't amplify. So you may not know you are potentially cooking your pickup. (Actual damage is unlikely, but still not something you should risk.)

For a more dramatic example: short C11 and plug into a fresh battery or a good strong power supply. The battery will get HOT, about the edge of skin-burn. But a strong power supply will cook wires until they melt and burn.

Caps cost several dimes. Jumpers are a penny. If there's a cap in a well-designed circuit, it is probbaly there for a reason. You may disagree with the reason. You find you like this C9 shorted. In THIS case, that's OK. In other cases, stuff can get unhappy or even damaged.
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PRR

Re: what happens when you short a capacitor?
« Reply #4 on: December 12, 2012, 01:48:35 AM »
What is usually safe: make the cap 2 octaves bigger. 2 octaves is 4X frequency. Make C9 a cap 4 times bigger than 0.027uFd. Looks like around 0.1uFd. That should be a big change of audio, yet preserves any DC-block function you may not have noticed.

In this case, C9 R12 work at about 900Hz (stuff below 900Hz is reduced). 900Hz is well up the guitar range. Fuzz/distortion effects often cut bass so the fuzz stands-out clearer.

Changing C9 to 0.1uFd shifts the bass-cut to 250Hz. That still shaves the lowest notes. But the times you would use fuzz/distort are times you are playing higher up the frets/strings, or emphasizing overtones more than fundamental.

If you find that you want ALL the bass, 0.3uFd will pass 82Hz with hardly-any loss. Go 0.47uFd to be sure. Since half-uFd caps are large, you might try 0.22uFd and see if that's good enough.

OTOH, if a 4X bigger cap is too much (perhaps boomy muddy thunky-funky), split the difference. If 0.027 is too thin and 0.1uFd is too fatt, try 0.047uFd.
« Last Edit: December 12, 2012, 01:50:07 AM by PRR »
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ode2no1

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Re: what happens when you short a capacitor?
« Reply #5 on: December 12, 2012, 03:11:50 AM »
thanks PRR. honestly...these days i wouldn't go poking around like a dummy shorting things. but this was probably my 3rd build ever and i didn't know much at all...not that i'm a pro now. i keep thinking back to that pedal though. i wouldn't mind rebuilding it and wondered if i should just replace the cap with a jumper or if shorting it still included some sort of capacitance which could be replaced by a different value cap.