Capacitor Question for the techies

[Evil Hoodoo]

So, I'm building a Fuzz that requires a 25uF electrolytic. Schematic here: http://eu11.stripper.jp/pulcino/blog/images/fuzzrite.gif

What is the purpose of such a high value? Could I just use a smaller value, or is it required because of the 9V power?

In addition, what is a good substitute for ceramic caps? Are Polypropylenes suitable?

gracias

[nexekho]

Unless I'm missing something, it says down the bottom that C6 should be a magnese dioxide capacitor, and that all the others should be ceramic.  No electrolytics on there.

[Evil Hoodoo]

If it's polarized, isn't that the same thing as an electrolytic? They're not interchangeable? In the circuit, my sprague electrolytic seems to be working fine, although I wonder
if this fabled manganese dioxide will influence tone. Thanks for your help.

[CynicalMan]

That capacitor has two functions. Firstly, it filters AC out of the power. A poorly-regulated supply can have AC and noise on the power line, and if it has a high resistance, the capacitor can help to filter them out. More importantly, it grounds any AC signal from the circuit on the power line. If the power supply has a high resistance, then AC signal can pass through the power line, which can cause oscillations. With a good, regulated power supply, this capacitor isn't necessary except for in the most unstable of effects.

An electrolytic would work fine. You won't hear any difference. Same goes for polyprops instead of ceramic caps.

[PRR]

I have NO clue what a "manganese dioxide capacitor" is. That stuff is used in batteries, not capacitors. I suspect a translation error.

It is clearly an ordinary electrolytic such as Aron's 22uFd or 100uFd electrolytics. Price isn't big.

> What is the purpose of such a high value?

25uFd isn't "large". Here's 9,900uFd 25V, and there's much larger caps on that site.

> Could I just use a smaller value

Yes. If your battery is very fresh, no cap is needed; however as the battery fades you may get wonky sound. With power-pack, the cap could reduce buzz from a cheap wallwart; however you often need a lot more than 25uFd to damp wallwart buzz.

> What is the purpose of such a high value?

It holds the power supply steady. Same function as the foundation under your house. Some places you can lay boards on gravel on bedrock and the house stands solid. Try that on the beach, tide works the sand out, maybe the whole house. Try that here on my land, between soft rock and wet/frozen clay, the house will rack and slant in a couple years.

A good house often stands on 40,000 pounds of concrete. A good circuit often stands on a big electrolytic. Often 50uFd for high-volt tubework, 500uFd for small 12V stuff, 5,000uFd for speaker amps.

[CynicalMan]

> What is the purpose of such a high value?

25uFd isn't "large". Here's 9,900uFd 25V, and there's much larger caps on that site.

Yawn. http://www.ioxus.com/documents/5000FPrismaticDataSheetRev07122011.pdf

[PRR]

> Yawn

I wanted to avoid getting into Farads... mental translation to microFarads is not a common skill here.

However, 5,000F is interesting. Back in the middle of the last century my father told of hazing the new-guy by sending him to borrow a "one Farad capacitor". No such thing (then), it would be as big as a garbage can. All the old guys knew the joke. When asked, they'd search their bins and say "all out, try Fred up on the 13th floor". The new-kid would get the run-around until someone spilled the beans or took pity.

[amptramp]

The OP should be cautioned that the value of the capacitor is OK if you are using a battery or a regulated power supply.  But if you are one of the majority that uses unregulated wall wart supplies, note that the pulse current during the short period twice each cycle that the diodes conduct will be split between the wall wart capacitor and the capacitor in the pedal in proportion to the capacitance.  If you go a lot larger in the pedal electrolytic capacitor across the power supply leads, more pulse current will flow through the power supply leads.  In the power supply, there is a large capacitor that stores charge and is replenished only during the brief time that the voltage at the diode input exceeds the voltage at the output.  For a low-power circuit such as this, it may be only a few degrees for each alternation of the line.  Pulse current flows through the diode which is time averaged to be equal to the average current in the load.  You can have pulse currents of an amp for ten degrees around the maximum positive and negative amplitude of the AC input depending on the amount of voltage sag you allow between alternations.  The amount of sag is set by: CV = Q = IT where C is capacitance, V is voltage drop between alternations, Q is charge, I is average load current and T is time between alternations e.g. 1/120 second for 60 Hz supplies.  When the voltage has sagged, the pulse current flows until the capacitor is charged up to the diode voltage.  You can play with timing and values using:

http://www.duncanamps.com/psud2/index.html

Most people twist the leads so that once you get far enough away from the power lead, the positive magnetic field from one loop will be cancelled by the negative field from the adjacent loop.  Another idea would be to use coax for power wiring to completely contain the magnetic field.  In all cases the rectifier pulse produces harmonics in the audio band.  So if your capacitor in the pedal is equal to the capacitor in the wall wart, half the pulse current goes down the line.  If it is 1/20 of the wall wart capacitance, only 1/21 of the current goes down the line.

[Mark Hammer]

And you DO realize that a 22uf or 33uf (both extremely standard vaues) make a perfectly acceptable substitute for the 25uf shown?

[Evil Hoodoo]

Really great responses. Thanks for all the useful info everyone!