Weird cracking sound at high gain.

[Bassman1]

G'day all. Been a long time lurker, finally bit the bullet as I might need some help here. I did a search but couldn't find a replica of my problem. I'm working on a custom bass overdrive as my first effect. I do mean custom, by the way, this is an original circuit and not a clone. Can't recommend this path as it's very very hard and bordering on masochistic,  but when you get into building because no one makes what you want, it's hard not to just skip the clone and kit stage. Anyway, I have my 2nd prototype done, and the effect works perfectly, except for this crackling sound once the gain is turned up, and it gets worse the higher the gain pot is set. The clipping circuit is just a soft clipping opamp with less in the feedback loop, and a 1Meg drive pot. Virtually no crackle at low gain, and it's only started once I had the effect on led wired in. Sorry for the long post. Help!

[antonis]

Hi and Welcome..

A schematic should be very helpful to locate the issue..
(e.g. abscence of HPF cap on gain stage of NFB loop or any other possible crackling cause due to DC flow through gain pot)

[Bassman1]

It's possible it could be DC across the pot but I do have decoupling caps everywhere. The effect works perfectly aside from the crackling, which appears to be gain dependant. I'm a little reluctant to post a full schematic as this is an original design I was looking selling but I could put up a partial schematic with  the problem area. It doesn't seem to be an issue  with the indicator led disconnected. I have a series resistor of 27k for the led and it's power and ground go straight to the dc jack.

[antonis]

It's possible it could be DC across the pot but I do have decoupling caps everywhere.

Decoupling caps are intended for AC shunt (by-pass) and are set in parallel with device for decoupling..
Coupling caps are intended for only AC pass (DC block) and are set in series with device for coupling..
(just to prevent terminology issues..)

So, post a partial schematic of Gain stage without items values if you wish to keep them secret..

[Bassman1]

 I'll send a schematic later on today. Just to clarify, the crackling isn't when turning the drive pot, but when playing, and is dependent on the opamp gain. I actually get very little crackling if any at low gain, but as I turn it up, the crackle appears, and the decay is weird. I can still hear the led clipping diodes, and aside from the crackle it works perfectly. I don't have to to play hard ttoo pass signal for instance, and it sounds exactly as it should at low gain.

[Bassman1]

 Just gave the pedal another listen and the drive pot is the likely culprit. The crackling is only when the pot is turned up and sometime the signal goes back to minimum gain (everything else works perfectly). No crackling at minimum gain that I can hear which should rule out bad solder joints. No crackle when turning pot which rules out dc in the signal. Can still hear nice overdrive mixed with the crackling when the drive pot decides to work. I'm going swap out the drive pot and will report back.

[antonis]

I'm going swap out the drive pot and will report back.

Well said..!! 

[composition4]

Are your sure you're not using a TL072 and hitting the rails with the internal gain?

[Bassman1]

Swapped the pot and all good. Indicator led now dead. Second one I've killed somehow in the last couple of days. Coincidentally the effect has always worked 100% when the led isn't working. Working led, crackly effect, dead led, perfect effect. I will now proceed to tear out my hair.

[antonis]

Do you, by any chance, use indicator LED as part of soft clipping configuration..?? 

[Bassman1]

Do you, by any chance, use indicator LED as part of soft clipping configuration..?? 

No, just the standard effect on or off LED.

[11-90-an]

I will now proceed to tear out my hair.

Fortunately, self-isolation gives us a lot of this stuff...

Maybe replace the LED and see what happens... Did you put a high-enough current limiting resistor?

[kraal]

Hello,
The problem here is that we have to guess what you've done, what is happening, how components are inter-related.
As already written, all this would be a lot easier with a simple diagram showing both your gain stage and how it is connected/related to your LED.
Take your smartphone, take a picture of your diagram, blur the parts you don't want people to look at and share it.
Meanwhile, here are some questions:

• Have you modeled your circuit in Kicad or any other tool, then ran an ERC (electrical rules check) (with your LED included in the circuit) ?
• Have you double checked your solder joints (measured and visually checked) ?
• Have you simulated your circuit ?
• How and where are you adding this LED indicator ? (is the LED separated from the circuit with a 3PDT Switch (i.e. no impact on your VCC) or is it part of the main circuit ? If it is part of your main circuit, is it located before, after the gain ?)
• Is the LED exposed to reverse voltage (that may destroy it) ?
• what is the value of your LED resistor?
• You're writing about a "gain pot", the gain of what component is it controlling? a transistor, opamp ?
• Is it really changing the gain (or for instance the bias of a transistor) ?

Best regards,

Michel

[duck_arse]

sorry, I can't resist .....

Swapped the pot and all good. Indicator led now dead.

so, actually, not all good.

..... the effect has always worked 100% when the led isn't working. Working led, crackly effect, dead led, perfect effect.

so, actually, not 100%, 0%.

circuit diagrams, layout diagrams, photos of what you've built - give us some sort of chance. also, late welcome, lest antonis smites me.

[antonis]

@Bassman1: You can see that debugging a hush-hush circuit build isn't the easiest thing in the world..