47uf vs. 100uf-470uf power supply filtering.

[m_charles]

Hi!
Was just wondering. Why on some pedals is a 47uf used instead of a higher value? I've always used a 100 ohm resistor in series to a 100uf parallel with the V+ and Gnd.
Building my first t-bender and noticed the 47uf. I used my usual 100 ohm 100uf combo, worked fine, but the question is bugging me.
I just spent 45 min searching pasts posts (I do always try that route to avoid redundant posts) and got nowhere.
Also, if one used the 47uf, would it be kosher to also drop the series resistor to a 47 ohm?
thanks!
chuck

[Guitar_Ninja]

AFAIK it just affects which frequencies the RC network is dumping to ground. I always use a 100-ohm resistor in series with the battery with a 470uF cap to ground.

[Minion]

It might also have to do with the amount of current the circuit uses , the Bypass cap is there to filter out Ripple , the more current draw the bigger the bypass cap size needed to filter out ripple .....

[dano12]

The values you choose should be based on the "worst" wallwart AC-DC adpater you expect to to use.

- 100 ohms in series followed by a 100uf electro to ground would take care of a relatively well-behaved power supply.
- 100 ohms in series followed by a 100uf electro and a 47nf electro to ground has proven successful for me with really crappy power supplies.
- For higher-current circuits like amplifiers (i.e. small wattage amps based on JRC/LM368 parts, a pair of 220uf electros to ground can help not only with filtering, but acting as a small reservoir for peak loads
- For circuits which use 1/2 of the input voltage to provide a vref point, I like to add a smaller value electro just like the Rat does

If this doesn't make any sense (which I suspect, after the second chianti of the night, may be the case) I can draw some diagrams.

[slideman82]

Bigger the cap less the ripple, rectified voltage from AC to DC it's not pure DC... and a cap helps to make that AC after rectifier more like a DC (in fact the cap it works like a DC supply). Also, the cap and the resisitor makes a low pass filter, this means attenues (?) the AC signal from some frequency (obtained by the 1/(2*pi*R*C) ecuation), just to minimize hum (I think it has a freq of 100 Hz). So, bigger thet cap, better the filter (for this purpose).

BTW, main supply must have a 4700uF after voltage regulator.

Otta topic. what's AFAIK? and IIRC?

[m_charles]

Wait, then why not just always use, say, a 330uf? Am I understanding correctly if you're saying that the bigger the cap (within reason) the better the filter?
I'd bet I'm not understanding. I say this because as I fumble my way through the world of electronics I've become very weary of any blanket thoughts of "this always means this...etc". You wise men have taught me well...

So here's my next 2 questions:
1. Am I right about dropping the resistor value along with the cap? Somewhere along the line I picked up the habit of 100 ohm = 100uf, 220 ohm = 220uf. I know it's not a hard rule, but it seems to work. But I also never use really crappy power supplies, just the standard "boss" type. Or is the 100 ohm just fine as it is?
2. Why did "they" put it (the 47uf) in on the original units at all? Weren't the original Tonebenders batt. only? I thought with a pure isolated 9v source, such as a 9v batt, there was no need to filter. Why not (back then) just use a diode to protect the trannies and be done with it?
Ooops, make it 3 questions.
3. Can't find that article regarding power supplies on GEO. Saw a reference to it in my searches. Any help locating it?
Thanks so much for the help already fellas...
chuck

[Greg_G]

- For circuits which use 1/2 of the input voltage to provide a vref point, I like to add a smaller value electro just like the Rat does

Please tell us more about the reason for this...
Thanks

[Lurco]

http://www.diystompboxes.com/smfforum/index.php?topic=25548.0

IIRC = If I Recall/Remember Correctly
AFAIK = As Far As I Know

find all here: http://www.acronymfinder.com/

[dano12]

- For circuits which use 1/2 of the input voltage to provide a vref point, I like to add a smaller value electro just like the Rat does

Please tell us more about the reason for this...
Thanks

I don't know the exact technical reason for this, but it has proven useful. I have a box of crappy wallwarts. A few of them are regulated and don't interject noise into the circuit. On the other hand, a bunch of them do.

In build various hi-gain circuits, the large value electro from + to ground works, but it was only with the addition of the second smaller electro from vRef to ground that AC line noises disappeared completely.

[cpm]

Wait, then why not just always use, say, a 330uf? Am I understanding correctly if you're saying that the bigger the cap (within reason) the better the filter?
I'd bet I'm not understanding. I say this because as I fumble my way through the world of electronics I've become very weary of any blanket thoughts of "this always means this...etc". You wise men have taught me well...
So here's my next 2 questions:
1. Am I right about dropping the resistor value along with the cap? Somewhere along the line I picked up the habit of 100 ohm = 100uf, 220 ohm = 220uf. I know it's not a hard rule, but it seems to work. But I also never use really crappy power supplies, just the standard "boss" type. Or is the 100 ohm just fine as it is?

No, you ar making a low pass fiter, with a frequency rolloff at: 1/(2*pi*R*C)
So if you keep the RC product the same the filter wont change: eg. from 100R*100uF to 47R*220uF.

2. Why did "they" put it (the 47uf) in on the original units at all? Weren't the original Tonebenders batt. only? I thought with a pure isolated 9v source, such as a 9v batt, there was no need to filter. Why not (back then) just use a diode to protect the trannies and be done with it?

Even with a battery you may want to avoid current peaks to leak throug the power wires into another parts of the circuit (like a prior stage on a high gain chain that would make it oscillate), or have a small bucket for available power when current usage peaks.

- For circuits which use 1/2 of the input voltage to provide a vref point, I like to add a smaller value electro just like the Rat does

Please tell us more about the reason for this...
Thanks

You mean on the Vref? This is made to clean audio from the Vref rail. If there's not a cap there, the audio signal leaks into it through the bias resistors


Sometimes you dont need a big filtering, like opamps (with a nice CMMR noise rejection), so you just use a smaller, cheaper, less bulky, cap. But in a big muff for example i would use the double 100r/220uF, to avoid it humming as hell.

Its all down to undertand how the circuit and its components work.

[Greg_G]

You mean on the Vref? This is made to clean audio from the Vref rail. If there's not a cap there, the audio signal leaks into it through the bias resistors

[dukie]

If this doesn't make any sense (which I suspect, after the second chianti of the night, may be the case) I can draw some diagrams.

Please draw some diagrams Dano, cause sometime words play some trick on me.. 
Cheers!

[dano12]

If this doesn't make any sense (which I suspect, after the second chianti of the night, may be the case) I can draw some diagrams.

Please draw some diagrams Dano, cause sometime words play some trick on me.. 
Cheers!

Here are the variations I've seen and used, everything from simple to more robust.



It seems there are many ways to do this. I would encourage feedback on alternative schemes, or deeper explanations of the various merits of the arrangements shown above.

[cpm]

ah yes, the smaller cap improves the filtering because electro usually have higher resistance on higher frequencies, so a small ceramic will complement well

i would like to know about using inductors. I've seen them on some charge pumps.
I have a buch of 100uH that may have some use there.

[cpm]

that one with both big electros across the Vref to rails, i think is common for bipolar supplies or whenever the Vref is sourcing or sinking current.

[slideman82]

Wait, then why not just always use, say, a 330uf? Am I understanding correctly if you're saying that the bigger the cap (within reason) the better the filter?

There's something called production costs, my friend, and it's obvious manufacturers have to lower those costs. A 47uF is not a bad filtering, but that leaves you the power supply problem. If it's a 9V battery, no problem, you don't have hum beacuse of the supply, but that forces you using a well regulated PS. My tip: use 100uF in your stompboxes, and 4700uF in your PS.

You can use two LPF in series for filtering, using 2 x 47 or 100 ohm and 100uF caps for noisiers circuits, for example, a Big Muff.

[Mark Hammer]

Wouldn't life be wonderful if you could undo a screw or two, pop the lid off a wallwart, and see what's in there?

The fundamental problem with these beasts is that they are sealed black boxes, and the adjustments we have to make in our pedals all have to be optimized on the basis of listening and detective work, as opposed to being able to look at the damn circuit and think:  "Hmm, full-wave rectified, plus a 3-pin regulator, but only 220uf smoothing.  I think it'll benefit if I throw another 100uf-220uf on there."

And you thought goop on effects pedals was a pain?  Sealed wallwarts are every bit as big a pain.

[cpm]

Sealed wallwarts are every bit as big a pain.

not as painful as mains shock, for one thing i might see as a not so bad idea

[Minion]

Wouldn't life be wonderful if you could undo a screw or two, pop the lid off a wallwart, and see what's in there?

The fundamental problem with these beasts is that they are sealed black boxes, and the adjustments we have to make in our pedals all have to be optimized on the basis of listening and detective work, as opposed to being able to look at the damn circuit and think:  "Hmm, full-wave rectified, plus a 3-pin regulator, but only 220uf smoothing.  I think it'll benefit if I throw another 100uf-220uf on there."

And you thought goop on effects pedals was a pain?  Sealed wallwarts are every bit as big a pain.

I recently opened up a cheapo 12v dc wallmart with my dremel to use the transformer in a project that needed 12v AC , inside there was just a small transformer and a very small PCB with 4 1n4001 diodes , no resistors or filter caps ... I was quite surprised , but then again it wasn"t for use in audio equipment , I believe the walmart was from an old calculator .....

[R.G.]

 Three equations. Sorry, the universe does seem to work on the basis of equations. Maxwell's Equations explain all of electro-magnetics in four equations.

In this case, they're simpler.
1. V = I*R (that is, the voltage dropped across a resistor is the current times the resistor)
2. I = C*dV/dT (the current into/out of a capacitor is equal to the capacitor times the rate of change of voltage in the cap in volts/second)
3. F = 1/(2*pi*R*C) (the rolloff frequency of an R-C network is the inverse of two pi times the product of R and C)

If your pedal pulls a current I of 20 ma, and you put a resistor in front of it between it and the power supply then the voltage the pedal sees is going to be less. It will be V  = 20ma times R less. A 100 ohm resistor will make for two volts less! A 10 ohm will make for 0.2V less. And if your pedal pulls 1ma with the bypass LED off and 20 ma with the bypass LED on, then a 100 ohm resistor will change the power supply from 8.9V to 7V when you switch the bypass. A 10 ohm will make it change from 8.99 to 8.8 when you switch. Hey - you think that could make for switch popping if that's fed in to the collector resistor of a bipolar or JFET? 

But if we use a capacitor there, it will slow down the rate of change of that supply voltage. Instead of an instant (and LOUD) change, the cap will throttle the voltage change by holding and releasing charge. The rate of change of the power supply voltage will change from instant to dV/dT= I/C = 19ma/C where C is in farads. A 100uF cap will hold the change rate down to at worst 0.019/100*10E-6 = 190V/Second, which looks bad, but if you think of it as 0.19V/millisecond, it's more palatable. Would you rather stub your toe on a vertical-edged solid rock, or one with a slow slope? 

Even better, the resistor is helping keep high frequency stuff out of the power supply. For a 100 ohm resistor and a 100uF cap, the frequency where the resistor and cap is reducing incoming power supply grack by half is F = 1/3*pi*R*C = 1/(6.28*100*100*10E-6) = 15.9Hz. That's GOOD! Any power supply noise is likely to be at lowest 120Hz, so that's reduced by a ratio of 0.117; it's only one ninth as big as it was.  To do the same thing with a 10 ohms series resistor the cap has to be ten times as big, so you you get the same incoming interference reduction with 10 ohms and 1000uF as you did for 100 ohms and 100uF... but you don't lose a couple of volts of DC to the resistor.

I think this sketches out most of the tradeoffs. Big R = more immunity from noise with smaller filter cap, but more DC loss. Big C is more immunity from noise, and slowing down transients. Filtering of noise in the incoming DC is better with big R and big C. Get all you can, or all you can afford/fit in the box, consistent with how much current *and current change! * your pedal has.