DIY guitar preamp in stompbox makes oscillate as hell

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abbey road d enfer

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+ I fitted DC-DC converter and plugged DC jack of 9V 300mA adapter.


Now the board can deliver guitar sound to next stage.

It make some noise like boiling kettle. (like file below)
But it doesn't work right.
the adapter supply 8.93DC
but DC-DC converter make
+5.946 VDC 225mVAC
-5.963 VDC 237mVAC

demo file (file from my blog)
Well, so it shows that the converter has a hard time providing teh rail voltages.
You need to find out if it's one of the positions that creates the problem or if the problem happens when the number of IC's exceeds a certain value.
You need to start with no IC's in place then populate them one after each other until the rails fall down.
 

freebird92

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Well, so it shows that the converter has a hard time providing teh rail voltages.
You need to find out if it's one of the positions that creates the problem or if the problem happens when the number of IC's exceeds a certain value.
You need to start with no IC's in place then populate them one after each other until the rails fall down.
Thank you!

+ I take 12V 2A adapter.
DC converter makes +- 9V DC

When I turn Volume pot(C50K) output voltage and frequency varies. (measured at output jack)
When I turn to 0(full ccw) 0VAC
When I turn to CW it goes to 0.13VAC 1kHz. And supply voltages drop slightly.

Isn't OP2B causing problems because it changes like this depending on the volume knob?
 

Bo Deadly

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I removed all bypass capacitors(include 0.1uF MLCCs).
All of them? Why? You need bypass capacitors. I would leave the two 47uF closest to the supply source and I would also keep the 0.1uF MLCCs (for some op amps they are required to keep the amps stable).

The problem you seem to be having is that the SMPS and circuit are interacting in a bad way which causes it to oscillate which causes stress on the SMPS which causes it to oscillate and so on. So you need to isolate the SMPS from the circuit with series resistance. The best way to do that with minimal voltage drop is a capacitance multiplier.

But for the moment you could just use 47R in place of the inductors to get an RC with the 47uF for a filter cutoff of 72Hz which is well below the oscillation frequency.

volume full
1 685mVAC
2 26mVAC
3 29mVAC

5 100mVAC
6 48mVAC
7 6.8VAC

Volume 0
1 -6mVAC
2 -6mVAC
3 -6mVAC
5 0V
6 20mVAC
7 0V
I don't know what these numbers mean.

Okay! I'll try it according to your advice.
Does "1K resistors across output of SMPS" mean
1.
+15V - 1K - GND
-15V -1K - GND
This.

Anyway, The CMs cause some voltage drop. But would it not matter if it was designed like this?
No, it doesn't matter because the small SMPS don't regulate the rails that well anyway. So you're going to loose some voltage no matter what you do. The voltage drop of a single transistor CM will be 0.6V which is hardly a problem. Combined with the normal droop of these really small SMPS, you'll end up with maybe 12-13VDC depending on load. That's quite fine for your circuit. We are talking about guitar after all.
 

freebird92

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Finally I made success!
Now it can work with dc-dc converter. and make about +-15V

I replaced inductors into 47R resistors.
Re-installed decoupling caps. and add 1K resistors across DC-DC conveter's output.
But it doesn't work.

When I measure DC-DC converter's input voltage, it was only 3V. Before L1 it was about 8.7VDC.

So I did
1. I removed electrolytic cap(C30 in schematic). - failed
2. I replaced inductor into jumper(L1 in schematic) - failed
3. I removed MLCC(CM9) - failed
4. I removed zener diode - failed
5. I replaced diode(D3: it was 1N5819 actually) into jumper - Success!?

When I remove the 1K resistors across converter's rail, It works.
I didn't restore bypass MLCCs yet. But it works without noise.


I wonder the problem was happened because the diode in my box was defective.
Or usage of schottky diode in front of dc-dc converter was bad idea.
Or the combination of what I removed made some bad comprehensive effect.

When I find out the reason, I will report here.

Anyway I am grateful to everyone who was willing to take the time to help me!
 
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chilidawg

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I didn't restore bypass MLCCs yet. But it works without noise.
Regardless of that, you should put them back in.
So I did
1. I removed electrolytic cap(C30 in schematic). - failed
2. I replaced inductor into jumper(L1 in schematic) - failed
3. I removed MLCC(CM9) - failed
4. I removed zener diode - failed
Ah... I thought it is common sense to make sure the components are not dead every time you test a circuit? 🤦‍♂️
Or usage of schottky diode in front of dc-dc converter was bad idea.
What was the reason you put one there anyway?
 

abbey road d enfer

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When I measure DC-DC converter's input voltage, it was only 3V. Before L1 it was about 8.7VDC.
I would think this is the major cause for malfunction. The inductor you used has too much DC resistance for the current that the circuit draws. You did not mention what type it was.
I would bet you used a wrong value with too much DCR. No 10uH inductor would create so much voltage drop.
 

freebird92

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Ah... I thought it is common sense to make sure the components are not dead every time you test a circuit? 🤦‍♂️
Yes It is common sense. But I couldn't think that was dead. Since another board has symptom.
So I thought it was problem of board it self rather than problem of component.

What was the reason you put one there anyway?
I wanted to protect it from receiving voltage from the opposite polarity adapter. I wired DC jack as center negative.

Regardless of that, you should put them back in.
Yes I will! Thank you!!


I would think this is the major cause for malfunction. The inductor you used has too much DC resistance for the current that the circuit draws. You did not mention what type it was.
I would bet you used a wrong value with too much DCR. No 10uH inductor would create so much voltage drop.

I was really surprised.
The inductor was axial leaded one.
After listening to you, I measured the DCR of the inductor I have. It was almost about 100R.
And most of all, it was 10 mH.
I measure other inductors I have. 10uH one has only 0.6DCR.
I gave up reading the color band because I have color weak, But I should have at least tried to measure the value.

Thank you!!
 

Bo Deadly

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After listening to you, I measured the DCR of the inductor I have. It was almost about 100R.
And most of all, it was 10 mH.
When that would be the problem then.

When selecting inductors / chokes start by finding one that supports 2x the current you will actually be drawing through it. Then find DCR low enough so that the voltage drop isn't too much for whatever the downstream circuit needs. Then hopefully you can find a part with a high enough inductance to filter out whatever you're trying to filter. Fortunately, in your case the switching frequency is 500kHz so there should be many applicable parts. It's going to be a tiny little SMD chip. It's not a critical part either. It's just supposed to stop noise from going backward and then down into other circuits. But if you're powering just the one DCDC converter with another ACDC supply, it's not particularly critical. If you're powering 8 of these boards with the same ACDC supply, it might be important.
 

freebird92

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When that would be the problem then.

When selecting inductors / chokes start by finding one that supports 2x the current you will actually be drawing through it. Then find DCR low enough so that the voltage drop isn't too much for whatever the downstream circuit needs. Then hopefully you can find a part with a high enough inductance to filter out whatever you're trying to filter. Fortunately, in your case the switching frequency is 500kHz so there should be many applicable parts. It's going to be a tiny little SMD chip. It's not a critical part either. It's just supposed to stop noise from going backward and then down into other circuits. But if you're powering just the one DCDC converter with another ACDC supply, it's not particularly critical. If you're powering 8 of these boards with the same ACDC supply, it might be important.

It was still a newbie, so I didn't know which way was the best or the problem in which situation.

Since I learned about the inductor this time,
I will try to use the inductor according to proper calculation from next time.

And now my project is using the Capacitance Multiplier that you told me recent time.
Thank you.
 
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