Power cap question.

[digi2t]

Hi all,

I was wondering, is there an importance as to the size of the power filter cap on the 9 volt positive side of a pedal? I'm looking at a diagram which spec's a 47uF cap, but could I use a 100uF instead? Or, vice versa? I'm thinking about combining 3 different pedals into one enclosure, and each one (except one, 47uF) has a 100uF filter cap directly off the positive to ground. Can I eliminate the 2 others, use one 100uF on the positive rail, and then feed the circuits?

Is there a certain formula that dictates the size of the cap, or is 100uF some sort of "standard" insofar as 9vdc power filtering caps go? It's usually the size I see on most schematics.

Cheers,
Dino

[Sanguinicus]

Testing my memory here. There is a simplified formula I saw a while back. It's basically the ripple voltage multiplied by something to give you the right value. It really depends on how noisy the supply is. If you're only running off a battery, you don't need a filter section. It helps to have one though. I think If you used 100uF, you should be fine.

Connect it up and find out! if you've got one, connect a CRO to the supply and see how noisy it is. Insert caps to suit.

[Hides-His-Eyes]

You can't make that cap too big. I use 220us because that's the most capacitance you can get at 25V without going to the next size of capacitor up.

[amptramp]

A simple approximation is CV = Q = IT where:

C is capacitance in farads
V is voltage drop in volts between alternations on the power supply diodes
Q is charge in coulombs
I is current in amperes
T is time in seconds between charge additions which is half period for a fullwave rectifier

An example:

A pedal takes 15 mA and uses a 100 µF cap and is operated from 60 Hz from a fullwave-rectified wall-wart.  The period of the supply frequency is 16.666 msec (milliseconds = 0.001 seconds) and the half period is 8.333 msec.  The voltage drop is the required value so

V = I*T/C

V = 0.015 * 0.0083333 / 0.0001 with values in amps, farads and seconds

V = 1.249 volts drop between alternations.  Note that you are going to have a fairly high capacitance in the wall-wart, so you have to add this capacitance to the amount you put in the unit.  There is nothing unusual about finding a large value like 1000 µF in the wall-wart itself, so that is why you don't tend to get ripple anywhere near this high.  If you are powering an op amp, there is a decent amount of ripple rejection, usually 80 db, but for most discrete circuits, ripple rejection is much lower.  However, even with an op amp, ripple rejection specs usually apply when operated from dual positive and negative supplies.  If you are not happy with the ripple you have, get a wall-wart with a higher current rating - it will have more capacitance.  Remember that the current will split in proportion to the capacitance so if you have most of your capacitance in the pedal and not enough in the wall-wart, you will get large pulse currents going down the wiring between them.

If you want to design your power supply and get a visual representation of what is going on, check out: http://www.duncanamps.com/psud2/index.html

[defaced]

I was wondering, is there an importance as to the size of the power filter cap on the 9 volt positive side of a pedal? I'm looking at a diagram which spec's a 47uF cap, but could I use a 100uF instead? Or, vice versa? I'm thinking about combining 3 different pedals into one enclosure, and each one (except one, 47uF) has a 100uF filter cap directly off the positive to ground. Can I eliminate the 2 others, use one 100uF on the positive rail, and then feed the circuits?

Probably not a good idea.  That cap also decouples the positive rail, so in your case, it will help noise and junk from one pedal from affecting the other. 

[Mark Hammer]

Think of the power cap as a kind of "spare battery", a reserve supply that sees you through momentary demands.  Owners of high-powered automotive amplifiers know that if you want to "stiffen" the power the car battery and electrical system can provide to the amp (often in the 300W+ range), you stick on another couple of thousand microfrarads, or even get yourself one of those "supercaps" in the farad range.

If the effect/circuit is such that you need it to be resilient in the face of sudden current demand, then a larger value cap (e.g., 470uf+) is probably warranted.  If the effect is such that its performance under "sagging" power supply is essential to the desired tone, then you'll want a power cap (if you want one at all) that holds less reserve supply, like 22uf-47uf.

In general, though, 100uf is probably about just right for the majority of things people here build.  That presumes an acceptably-regulated power supply or battery, however.  If one is using a "plain vanilla" wallwart, the amount of regulation, and supply ripple, may be poor enough to justify adding some smoothing at the pedal itself.  In which case aiming for 470uf is not a bad idea.

One option is to use what I like to call a "power distribution box", where a single wallwart feeds a small plastic box, housing a biggish power cap (1000uf), a status LED, and a small handful of jacks from which power cables can be run to individual pedals or small clusters of them.  The common power cap provides the needed smoothing, such that using larger caps for individual pedals is not required.

[digi2t]

I was wondering, is there an importance as to the size of the power filter cap on the 9 volt positive side of a pedal? I'm looking at a diagram which spec's a 47uF cap, but could I use a 100uF instead? Or, vice versa? I'm thinking about combining 3 different pedals into one enclosure, and each one (except one, 47uF) has a 100uF filter cap directly off the positive to ground. Can I eliminate the 2 others, use one 100uF on the positive rail, and then feed the circuits?

Probably not a good idea.  That cap also decouples the positive rail, so in your case, it will help noise and junk from one pedal from affecting the other. 

Actually, I should have been more clear here. I should not have said "3 pedals into one enclosure". What I meant was, 3 different effects, but all on veroboard, and all fed off the same +9 rail. My apologies. That's why I'm thinking that only one 100uF cap upstream of all would be OK. I'm looking at the Skyripper as an example, which is 3 circuits being fed by one +9 rail. You've got the buffer section, the rangemaster section, and then the fuzz section. It uses one 47uF filter cap. I would like to build something similar, but the circuits that I want to amalgamate (on their own) each show a 100uF filter cap. Hence, my thinking is that if I stick a 100uF cap right after the power, I can then chain my circuits Skyripper style after it down the rail. At least, that's how my brain is processing the scheme.

Am I on the right track here, or completely out to lunch?

[merlinb]

What I meant was, 3 different effects, but all on veroboard, and all fed off the same +9 rail. That's why I'm thinking that only one 100uF cap upstream of all would be OK.
Am I on the right track here, or completely out to lunch?

If you're using a battery or a regulated power supply then you don't need enourmous PSU caps in the effect itself. It is, however, good practice to add say 10uF close to the effect circuit to decouple the amplifier stages, if there happens to be a long cable between the effect and the wall wart. With a discrete transistor circuit it may be wise to use something a little larger (47uF say), as they have much poorer PSRR than opamps.

If you are using an unregulated PSU then the sky's the limit as far as local smoothing caps. Again, a discrete transistor circuit will need more than an opamp circuit, perhaps even thousands of microfarads! Use whatever is necessary to eliminate audible hum. (In other words, use a regulated or battery supply where possible! Also be aware that a big cap, like >100uF will drain a decent amount of charge from a battery every time you turn the effect on!).

In your case it would be good practice to put a 10uF-47uF cap on each of the three veroboards, provided you're using a battery or regulated supply.

[defaced]

I was wondering, is there an importance as to the size of the power filter cap on the 9 volt positive side of a pedal? I'm looking at a diagram which spec's a 47uF cap, but could I use a 100uF instead? Or, vice versa? I'm thinking about combining 3 different pedals into one enclosure, and each one (except one, 47uF) has a 100uF filter cap directly off the positive to ground. Can I eliminate the 2 others, use one 100uF on the positive rail, and then feed the circuits?

Probably not a good idea.  That cap also decouples the positive rail, so in your case, it will help noise and junk from one pedal from affecting the other.  

Actually, I should have been more clear here. I should not have said "3 pedals into one enclosure". What I meant was, 3 different effects, but all on veroboard, and all fed off the same +9 rail. My apologies. That's why I'm thinking that only one 100uF cap upstream of all would be OK. I'm looking at the Skyripper as an example, which is 3 circuits being fed by one +9 rail. You've got the buffer section, the rangemaster section, and then the fuzz section. It uses one 47uF filter cap. I would like to build something similar, but the circuits that I want to amalgamate (on their own) each show a 100uF filter cap. Hence, my thinking is that if I stick a 100uF cap right after the power, I can then chain my circuits Skyripper style after it down the rail. At least, that's how my brain is processing the scheme.

Am I on the right track here, or completely out to lunch?

You were perfectly clear originally, though that cap does more than just filter.  Decoupling is not the same as power smoothing.  While most people only see the power smoothing function of that cap, the other roles it plays need to be considered with the project you're planning because you're supplying all three effects from a single +9v rail.  The reason you need a cap, as Merlin has suggested 10u to 47u even in regulated/battery setups (i.e., pure DC no ripple to be smoothed out conditions), is because of decoupling.  Decoupling didn't make sense to me when I started, it was one of those mystical "WTF does this mean" words. If you do some research on it (google "decoupling capacitor"), hopefully it will make more sense.  Basically you're electrically isolating the circuits from each other, or rather, trying to minimize their interaction outside of what you want them to do.  

[digi2t]

OK, many many thanks to all for clearing this up for me. As suggested by defaced, I did do some more homework on the decoupling aspect, and the concept is much more clear to me now. Special thanks to amptramp for the math lesson. This noob went to bed a bit less stupid the other night 

I`ve decided to respect the original schematics of the effects I`ll be intregrating, and I`ll add a cap on the rail, just before the drop to the effect. This way, everybody in the box should be happy. I don`t use batteries, but rather, a daisy chain power supply. I`ve got more than enough power on tap, but the ``interaction`` aspect really got my attention. I should avoid any problems this way.

Thanks all.
Cheers,
Dino