power filtering: evrybody omits the resistor from the RC filter -- is that OK?

[Rob Strand]

On the whole the OP's questions and misunderstandings have been answered.

As with most things if you think about it there's more to it than meets the eye.

If you have heap of ripple, say in the extreme where the input is a rectified but unfiltered waveform (ie. no pre-diode filter cap) the diode + cap will peak detect.   The ripple will now be set by post diode filter cap.    Because the diode is peak detecting the output voltage will be high.    The ripple perhaps a bit more.  The resistor cannot peak detect, since the output voltage of the filter is the average DC level the the output will be 0.64 times the peak.   So even with very small (no resistor) the output voltage will be less than the diode case.  Also we would expect the output ripple to be quite high for resistor.

Now go the other way suppose we have a tiny amount of input ripple, say < 100mV.   The diode won't turn off, the diode essentially passes the output current all the time and the output cap doesn't act much as a reservoir.  In that case the output waveform with the diode would be expected to track the input.  So it doesn't actually filter any/much ripple.    The resistor will still act as a filter and reduce the ripple, almost by the same reduction factor regardless of the size of the input ripple.

So there's only a small range of ripple voltage where we can make comparisons.   For large ripple the diode wins and for small ripple the resistor wins and with very small ripple the diode does very little.
EDIT:
So it seems this is the case.

For the example circuit I gave earlier.  Very roughly:   if the input ripple is less than about 1.45Vpp the resistor produces less ripple and when the input ripple is more than 1.45Vpp the diode produces less ripple.

(The conclusion (rule of thumb) I gave earlier that the diode is half as good is only true when the input ripple is about 1Vpp.   I probably came to this conclusion because the I considered input ripples of 0.5Vpp to 1.0Vpp.)

[R.G.]

Replacing the polarity protection diode with a resistor in hopes of getting more filtering is nearly as useless

I thought I understood this, but I don't understand this sentence. The diode doesn't provide any effective resistance, so doesn't make a good filter with the capacitor - we agree about that. After all, that's not it's job or why it's there. But *replacing* it with a resistor will absolutely get you more filtering, won't it? Bigger R or bigger C or both is better filtering.
Ok, you'll lose the *polarity protection* that the diode gave you, but that's a separate thing, and the reason why you really need *both* the diode *and* the resistor.

RG? Is that not right?

The key word there is "nearly". As noted before, a resistor drops the voltage at the same time it filters. There is an inherent limit on how much you can drop voltage into a pedal circuit before it starts to affect the circuit, for better or worse. The amount of filtering available is limited by the need to make the resistor "low" to avoid voltage drop and "high" to get better filtering. It's a double bind. Sure, every milli-ohm of resistance lowers the corner frequency of an R-C high pass filter; but should we string many feet of wire in series to the filter capacitor just because the milliohms add up?

A test of reason should apply: will a small resistor added to the bulk filter cap in the pedal get you to a frequency cutoff you need? Will adding a 10 ohm resistor help? A 100 ohm? Is that a tolerable voltage loss?

I personally don't like inserting a diode drop for just this reason. I developed a couple of polarity protection circuits that protect against reversal and have voltage drops under about 10-50mV for just this reason. I only accept a Schottky protection diode under duress. 

This whole thread is based on some faulty premises. These are that (1) protection diodes have anything to do with filtering, and (2) that replacing the >protection diode< with a resistor can get you any usable filtering. It would be far better to take a systems view and ask what and why are you filtering?

Is your DC power into the pedal so humm-y and noisy that you have to design your pedals to fix that? Wouldn't it be better to use a quieter power supply? Those exist - I know one of them intimately - and are not expensive.

If you're stuck with a noisy DC supply, some modern LDO regulators could give you, say, 8Vdc while suppressing hum by 46-50db. Maybe better than messing with swapping resistors and diodes.

I sympathize with the OP, it's fascinating to just swap a couple of parts in isolation and think that you've done an improvement. But if that was really possible, the original circuit designer, if they were good, would already have done it.

[j-pee]

... the OP thinks that the diode and cap are there to filter.

With respect, I don't

The OP keeps seeing vero board layouts with "9V -> diode -> 100uF cap to ground" blocks...
and he thinks that the layout creators did that because they thought a diode would be suffice (in the place of the resistor)

The OP, of course, might be wrong about that (too)


Take this example:
V 1.1 versus V 2
https://tagboardeffects.blogspot.com/2023/01/t-rex-mudhoney-v11.html



But PRR's argument about PSUs being silent these days is absolutely convincing! I'll adapt that approach!
(I'm really not a big pedal builder, nevertheless)

~

[R.G.]

With respect, I don't

No problem. I should have read more closely.

The OP keeps seeing vero board layouts with "9V -> diode -> 100uF cap to ground" blocks...
and he thinks that the layout creators did that because they thought a diode would be suffice (in the place of the resistor)

With respect, I think that may be mistaken. We usually can't know what was in the layout creators' heads when they put in a diode. I believe that they put in that diode for reverse polarity protection, or just because they saw it a lot on other layouts/pedals and didn't think about it much.

On the technical side, a diode is near worthless for filtering, and a resistor will have a tough time being both large enough to place the low frequency cutoff above line frequency to filter hum and simultaneously small enough not to steal too much voltage from the DC supply voltage. They are opposed requirements.

It is only my personal opinion, which may be wrong, that there should be neither a diode nor a resistor in that position. Instead, using a quieter DC power supply would be superior to both.

But PRR's argument about PSUs being silent these days is absolutely convincing! I'll adapt that approach!

And he's right about that. I agree completely with you adopting quiet DC power supplies. If I were in your position, I would also remove the and insert a wire where it was. If polarity protection is needed/wanted, I would use either a bipolar or MOSFET polarity protector, circuits for which are at geofex.com.