Bypass popping noise issue

[knutolai]

This thread originates from another thread I started. However the bypass noise issue was not relevant to the topic of the thread so I though I'd start a new one. Original thread: http://www.diystompboxes.com/smfforum/index.php?topic=102316.20

So! Im building a op-amp circuit for a digital pedal with gain and blend parameters. The circuit itself is working as intended, the bypass popping noise being the only exception. The project is cased in a plastic enclosure and all offboard components are safely fastened to the enclosure (no potential unwanted connections).
First of all, here's the schematic:


Now I've done some reading on the possible reasons for the popping noise (articles listed at the bottom of my post). I'll go through them one by one.

LED on/off switching noise:
The indicator LED might cause noise when switched on and off. I tried removing the indicator LED completely from the circuit. No difference.

Open circuit input/output capacitors:
As suggested on forums all around the web I've added pull-down resistors at both the input and output of the pedal. No difference. Also it's mentioned that if the circuit has a high gain the popping sound will be amplified, however when 'P1' is set all the way towards 'Vr' and 'P2' is set all the way towards the output of 'U1A' the popping sound is just as persistent.

Noisy heavy-duty bypass switch:
People seem to blame these for creating noise when used to bypass a circuit. My bypass configuration is a standard "true bypass" setup:

To evaluate this possible cause of the unwanted noise I changed the big heavy-duty switch with a smaller switch designed with lower current signals in mind:

I've checked that both switches work as they are supposed to with my multimeter.

The noise might be caused by another module in the setup:
My setup consists of my Danelectro guitar, the pedal with the issue and a MicroCube amp. I tried changing the pedal with other homemade pedals (some with heavy-duty true bypass switches) and the issue disappear along with the problematic pedal. I feel certain this pedal is the cause of the noise.

The circuit might not be connected the way it should:
I've revised my PCB three times now, checked it with my schematic and the PCB map designed on my computer. I've checked for short-circuits and missing connections and found no issues. Everything seems to fit. I'm starting to suspect my design, though correctly implemented, is seriously flawed. In my head it looks correct though   

So these are the issues I've seen mentioned around the internet. Does anyone have any other suggestions on things I could try out? I'm open and thankful for any suggestions on alternative bypass-methods. I'm getting rather desperate on finding any solution as the deadline for my project is nearing. 

The articles:
http://www.geofex.com/Article_Folders/box_pop.htm (open circuit capacitors)
http://www.muzique.com/lab/led.htm (Noisy LEDs)
http://www.muzique.com/news/noisy-3pdt-switches/ (noisy mechanical switches)

[R.G.]

Popping is always an indication that a voltage level is changing suddenly when a switch opens or closes.

Take your voltmeter. Measure the voltage on pin 14 of the quad opamp. Does this voltage change when you flip the bypass switch?

Do the same for pins 1, 7, and 8.  If the DC voltage on one of these pins changes when you press the bypass switch, that is what you're hearing.

An oscilloscope will let you see the AC transient, but I don't know if you have an o'scope handy.

Does the reference voltage Vr change when you press the bypass switch?

Do you turn the DAC on or off with the bypass switch?

[knutolai]

Take your voltmeter. Measure the voltage on pin 14 of the quad opamp. Does this voltage change when you flip the bypass switch?

Do the same for pins 1, 7, and 8.  If the DC voltage on one of these pins changes when you press the bypass switch, that is what you're hearing.
Does the reference voltage Vr change when you press the bypass switch?

The DC voltage at pins 1, 7, 8, 14 as well as Vr does not change when I flip the bypass switch.
The DC voltage had a static reading of 3,6 volt at pin 1 and 14, and 3,7 volt at pin 7, 8 and Vr.
Is that the value Im looking for? I was expecting to read something closer to 4,5 volt. Might this be the root of the problem?

Do you turn the DAC on or off with the bypass switch?

The DAC is always turned on.

[R O Tiree]

Measure the voltage at pin 3 as you flip the switch. If there's no change there, try all the others. Find the one that's changing and that will inform how to proceed.

[knutolai]

Measure the voltage at pin 3 as you flip the switch. If there's no change there, try all the others. Find the one that's changing and that will inform how to proceed.

Ive measured all the pins now. None change in value when I flip the bypass switch. The values are as follows:
Pin 1 = 3,65 (output 1)
Pin 2 = 3,65
Pin 3 = 1,91
Pin 4 = 7,36 (this is the supply voltage, had expected something higher as I use a 9 volt power supply)
Pin 5 = 3,5
Pin 6 = 3,7
Pin 7 = 3,7 (output 2)
Pin 8 = 3,7 (output 3)
Pin 9 = 3,7
Pin 10= 3,7
Pin 11= 0 (Ground)
Pin 12= 3,49
Pin 13= 3,69
Pin 14= 3,7 (output 4)

[R O Tiree]

How about the junction of Pot 1 and D1/D2, etc?

As RG says, popping is always an indication of a voltage changing suddenly as you engage/disengage the switch. Anywhere from a few mV upwards will do it...

It will be worth probing every point in the circuit, now, to see what's changing. Be as accurate and critical as you can be with all the readings.

[knutolai]

Ok so I've measured every junction of the circuit and found only two places where anything happens:
The input and output jack.

On the input jack the voltage springs from +-0,001 volt to 0,006 volt. Then it falls down to +-0,001 volt after a short time.

The output difference is quite a lot bigger! when the circuit is bypassed it measures 0,001 volt. when the switch is toggled the voltage increases to 0,713 volt (R12, output junction measures the same voltage). ...THEN I noticed I had attached C7 the wrong way around(!!!)  I feel so silly!

So after replacing the capacitor the voltage at the input 'jumps' max. 0,003 volt, and the output max. 0,006 volt when flipping the switch. The popping sound is still audible, but very reduced. I'd say its now at least tolerable!
Thank you very much guys! This is the first time I've had to troubleshoot a circuit. I've learned a great lesson!

[Jdansti]

If you haven't already, you might want to check the polarity of the other electrolytics.

Question for R.G.: If we assume that the remaining popping sound is not caused by a construction error, is it possible that the pulldown resistor (R19/1M) could be changed to see if it eliminates the popping without throwing the input impedance off too much? If so, what value would you recommend?

Edit: I should have also asked the same question about R13/100k.

[R.G.]

Ok so I've measured every junction of the circuit and found only two places where anything happens:
The input and output jack.

On the input jack the voltage springs from +-0,001 volt to 0,006 volt. Then it falls down to +-0,001 volt after a short time.

The output difference is quite a lot bigger! when the circuit is bypassed it measures 0,001 volt. when the switch is toggled the voltage increases to 0,713 volt (R12, output junction measures the same voltage). ...THEN I noticed I had attached C7 the wrong way around(!!!)  I feel so silly! 

We've all put in parts backwards. It's part of the process. After a while I came to *expect* that I had one or more parts in backwards. 

So after replacing the capacitor the voltage at the input 'jumps' max. 0,003 volt, and the output max. 0,006 volt when flipping the switch. The popping sound is still audible, but very reduced. I'd say its now at least tolerable!

There's still something going on, but you now can enjoy the effect while running down the last few issues. Good Work!!

Thank you very much guys! This is the first time I've had to troubleshoot a circuit. I've learned a great lesson!

We're all here to learn - maybe especially me.

[knutolai]

If we assume that the remaining popping sound is not caused by a construction error, is it possible that the pulldown resistor (R19/1M) could be changed to see if it eliminates the popping without throwing the input impedance off too much? If so, what value would you recommend?

I tried lowering the input impedance. I couldn't hear any difference. Tried with 100k and 2,2M

If you haven't already, you might want to check the polarity of the other electrolytics.

They are the right way around. first thing I checked when I realized what was wrong 

[R O Tiree]

If you don't mind me asking, what are the functions of C11, C12, D1 and D2? If it were mine, I'd perhaps move C12 to the other side of C1 (in parallel with R19) to provide RF rejection, and then get rid of those other components.

[knutolai]

If you don't mind me asking, what are the functions of C11, C12, D1 and D2? If it were mine, I'd perhaps move C12 to the other side of C1 (in parallel with R19) to provide RF rejection, and then get rid of those other components.

C11 and C12 was suggested by PRR in the other thread (http://www.diystompboxes.com/smfforum/index.php?topic=102316.0) See his replies on the first page. I'm not a proficient circuit designer, but according PRR it would be a measure to reject RF. The circuit is housed in a plastic enclosure and not very well shielded.

The diodes D1 and D2 is meant to protect the circuit from high-voltage accidental input signals. Seeing as the unit that does the digital processing is relatively expensive I though this would be a good measure.

[R O Tiree]

Could you try lifting one leg of each of those components to see if the pop goes away? C11, C12, D1, D2.

If that works, put C11 back in then, if that works OK, C12. See what happens?

[knutolai]

Could you try lifting one leg of each of those components to see if the pop goes away?

I'll try this later and get back to you guys
It got a little busy with the other aspects of my assignment

[allesz]

Hallo, I pop  in to add that, on the popping pedal issue, Jack Deville wrote (on one of his sites, I don't remember wich one) about adding a small resistor (I think he uses 1k) in series pre input cap and after output cap.
You have r14 that's 220 ohm, so you may just try increasing its value, then you can add a resistor before c1. Of course keep the pulldown resistors and the ground connection of the input on bypass.
Mr. Black says (if I remember correctly) that the problem is that, when you switch the effect on and off, the connections inside the switch floats free for a little moment, and this can lead to popping problems.