Pedal pops just on some amps

[ElectricDruid]

It is the LED. I disconnected it, and the pop is completely gone, even when both my amp and the pedal volume are at max.

Good work for finding that. This type of thing can be an absolute pain in the ar$e to track down.

[Lino22]

So what exactly is the source of the LED pop. I understand there is a sudden current spike when the LED is turned on added to the circuit current draw. How does it reflect to the signal though? Is it induced via crosstalk? Or is it a suddenly loaded DC that drops and causes the pop? I wish i understood it better so i could think of other ways of LED pop prevention.

[antonis]

In a high gain circuit, any current fluctuation results into signal amplitude alteration, right..??

P.S.
I presume LED and assosiated circuitry is placed right after +9V supply (before PCB power IN..)
(if not, rearrange it immediatly..!!)
I'd place a LPF rigt before PCB power IN..

[Rob Strand]

So what exactly is the source of the LED pop. I understand there is a sudden current spike when the LED is turned on added to the circuit current draw. How does it reflect to the signal though? Is it induced via crosstalk? Or is it a suddenly loaded DC that drops and causes the pop? I wish i understood it better so i could think of other ways of LED pop prevention.

Not a spike but a  *change* in current LED current ILED.  The LED current flows through the supply wires when the LED is off the voltage drop is 0A*Rwire = 0V.  When the LED is on the voltage drop is ILED*Rwire.  The most likely cause is you are passing the LED current through a sensitive ground wire.   Wire the LED to the battery (both the +ve and -ve wires).  If you wire switching the power through the input jack wire it directly to the input jack.  I if you have a big bypass cap on the PCB wire it to that.

You can add caps to the LED circuit to slow down the change in current so it is not heard.

[Lino22]

Thanks guys, i have just realized i feed the LED after the LPF filter on board. That is a silly mistake, i am glad you have mentioned it.

[Lino22]

Well, we have a new dimension here - microphony.
I rewired the LED and connected the pedal just guitar > pedal > amp, and it was clicking again when my amp was all the way up. I removed the DC jack to test it for microphony without any DC voltage, and guess what - it was clicking the same way.
So i installed a different 3PDT - with a smaller, red body that has a much smoother mechanical action. Normally i like to use Cliffs with blue body, they just feel more durable. Actually, they are very durable.

The little fellow was even worse, it has smoother action in a sense of pushing on it, but it is more microphonic than the big blue one.

The story will continue tomorrow.

[Rob Strand]

Well, we have a new dimension here - microphony.
I rewired the LED and connected the pedal just guitar > pedal > amp, and it was clicking again when my amp was all the way up. I removed the DC jack to test it for microphony without any DC voltage, and guess what - it was clicking the same way.
So i installed a different 3PDT - with a smaller, red body that has a much smoother mechanical action. Normally i like to use Cliffs with blue body, they just feel more durable. Actually, they are very durable.

The little fellow was even worse, it has smoother action in a sense of pushing on it, but it is more microphonic than the big blue one.

The story will continue tomorrow.

Is that with LED disconnected?

[Lino22]

No, the LED was in, but the DC was completely disconnected.

Another observation - i installed 8k2 - 100u - 360R AMZ filter on the LED. It got audibly better but as soon as i added a DE7 Ibanez delay after the tested pedal, the clicking got way stronger. Even when the delay was off. Even when i turned the volume on the tested pedal completely down (that removes the gain part). What a bunch of ghost stories.

[Lino22]

I have the LED connected like this





When i turned the trimpot all the way down, i burned the LED, 100u is probably big enough to burn it when the current is not restricted.

[Rob Strand]

Something isn't quite right with some of the finer points.  My suspicion it has to do with the ground wire used for the LED.

Can I suggest trying this:

When i turned the trimpot all the way down, i burned the LED, 100u is probably big enough to burn it when the current is not restricted.

Always a good idea to use a minimum series resistance on adjustable current to prevent 0 ohms case on the trimpot.

[Lino22]

I see, the devil may be in the detail
Let me try.

[Rob Strand]

I see, the devil may be in the detail
Let me try

I just realized what that detail is.   You need a resistor between +9V and the big cap, say 100R to 1k,  but you also need a resistor between the LED and the cap, that's the resistor that sets the current.

For your original diagram when you turn the LED on the cap is pre-charged to 9V and then a big thumping current flows through the LED and through the ground.   Not good for the LED or for popping.

The mod I drew may help but it's not solving the main issue.
In fact AMZ's original circuit has the idea correct except he puts the big small resistors in a different order.   I like big resistor second as it means the LED switching current is smallest when switching the ground lead (and returning to the ground of the LED cap) *but* the best form actually depends on which wire is switched as well!! - yes more details.

https://www.muzique.com/lab/led.htm

[Lino22]

Rob i am sorry there is a mistake on the original diagram, i drew the trimpot having the wiper and cold side shorted. The shotage is not there, thus there is a cap current restriction on the lower portion of the trimpot.
It was late and i overlooked it. It looks like this, with your suggestions in red now.




Please look on the development below, i added some details. I am very sorry if i confused you.

[Lino22]

Here is the circuit at the very beginning, before i started fiddling with it.




This one pops a lot.

[Lino22]

Here is the circuit after i introduced the AMZ led pop filter




This one pops less, but it still pops.

[Lino22]

Finally i have a concrete observation that is worth mentioning 

Here is the LED circuit with the filter, the way it looks now




I do have the 3PDT wired like Antonis said, with both input and output grounded, and i did measure the DC leakage, which my multimeter shows to pop up 0.6 - 1 mV and quickly dying as it slips to the GND. Microphony, (tested with disconected DC jack) is there but it is bearable. The pop is absolutely caused by the LED.

Now what i noticed is, it pops super loudly when i take pauses between on/off. As i measured, the cap C1 charges up to the full DC voltage (17.5V) and when i switch the LED on, it discharges, causing the pop.
To prove it, i kept quickly turn it on/off while measuring the voltage on the cap - the C1 will charge just a bit and the pop is not that loud.

The weird thing is the cap is there to suppress the current change, and the pop it causes, but then, at the same time, it discharges and causes the pop.

Is it just out of balance? Or would a large resistor across the cap help it to discharge? Because the remedy seems to become the illness.

[Lino22]

Right now, i tried this variant and no help ... the discharging cap still causes pop.

[Rob Strand]

I am very sorry if i confused you.

No problem, it's easy to do.

Here is the circuit after i introduced the AMZ led pop filter

This one pops less, but it still pops.

The take home message is things which we expect to improvement the situation are working (as expected) but they are not enough.

I have a very strong feeling the true problem is elsewhere.   Something like a high resistance connection on one of your ground wires or solder points (many chances for that on a vero build).  You might even have an outright break on the board and it's getting the ground via two paths, say from the output jack.  Another possibility is if you are switching power with the input jack that switch point might have a bad contact - you can temporarily bypass that switch by soldering a wire across it at the input jack.

Plenty of possibles all a little tricky to debug.

Now what i noticed is, it pops super loudly when i take pauses between on/off. As i measured, the cap C1 charges up to the full DC voltage (17.5V) and when i switch the LED on, it discharges, causing the pop.
To prove it, i kept quickly turn it on/off while measuring the voltage on the cap - the C1 will charge just a bit and the pop is not that loud.

The weird thing is the cap is there to suppress the current change, and the pop it causes, but then, at the same time, it discharges and causes the pop.

Is it just out of balance? Or would a large resistor across the cap help it to discharge? Because the remedy seems to become the illness

The bottom line is the circuit should not be so sensitive to start with.   That's the thing way at the top of my list.   By reducing the LED current (or LED current change) we are just hiding the true problem.   The LED circuit has become a tool to expose the issue.

The pop with time thing is related to how much current is flowing.  The more current change the more pop.

So where you are at now is really reducing the change in the LED to nothing so the problem (whatever it is) doesn't get exposed.   We can go further.  Keep your last circuit and then short out the LED in bypass mode using the other set of contacts on the LED switch.   My feeling though is you should start poking around elsewhere because we are on a path of burying the problem and not solving the root cause.   One way might be to solder one end of a thick wire to ground near the 220uF at the output of the converter.   Use one of the loud popping LED circuits.  Then try connecting the other end of the added ground wire to other ground points on you build.   You may actually hear pops when touching the end of that ground wire to the ground points alone without the LED being involved.  That will be a big flag that the ground point isn't solid.   If that doesn't happen then see if you can find  places where connecting the fat ground wire reduces or stops the LED pop.   Then work back and try to find a bad solder joint or link on the grounds.

I'm focusing the ground but there's also a the chance you have a problem on the +9V rail.
Sorry I had to edit this a few times.   Not switched on today.

[Lino22]

I was trying the different GND spots and no result. They were all the same. So i built a Fuzz Face, just to make sure. The same thing. When the amp is all the way up (even the preamp) the pop is there.

The only difference between "with the AMZ filter" and "without" is, that the "turn off" pop is suppressed. The "turn/on" pop is very loud and the filter has no positive impact on it.

When i remove the LED, there is no pop.
When i turn the preamp half way down, i can barely notice it.

Sorry I had to edit this a few times.   Not switched on today.

For some reason, i haven't managed to write a post without editing it yet. Something i am still waiting for.

[Lino22]

Out of sheer despair, i restricted the current flow, and it did help quite a bit.







Then i connected in a T-Rex little yellow fuzz, and noticed, it pops like hell. So it maybe the preamp all the way up that creates extreme conditions.