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eblc1388:
Your should add the 100R ~ 470R resistor as shown in the image.


dark_hawk:

--- Quote from: eblc1388 on October 17, 2021, 04:12:03 pm ---Your should add the 100R ~ 470R resistor as shown in the image.

--- End quote ---

That's exactly how I put them. And they have no effect on the switching noise.

eblc1388:

--- Quote from: dark_hawk on October 17, 2021, 04:14:00 pm ---That's exactly how I put them. And they have no effect on the switching noise.

--- End quote ---

Try adding one resistor in the +9V line and one 100uF capacitor from +9V to Gound wire instead to see if there are any improvement?

dark_hawk:
That seems to have solved it.
Adding a 100uF cap between +9V and ground seems to have solved it, even without the resistor. I could've swore that I tried adding a capacitor at the input at some point and it didn't fix it then.
I increased the 100uF to a 270uF. Hope that's OK.
Thank you very much for you help.

If it's not too much trouble, could you please explain why I should not add a resistor to the ground? and What is the purpose of R18 in your modification?

eblc1388:

--- Quote from: dark_hawk on October 17, 2021, 05:00:20 pm ---If it's not too much trouble, could you please explain why I should not add a resistor to the ground? and What is the purpose of R18 in your modification?

--- End quote ---

I'm glad you've finally got the circuit working in the manner you wanted.

Ground is a common reference point for the rest of the circuit and we wanted its potential to be as steady as possible. If you add resistor in series with the ground line on part of your circuit, then any current flow in this ground resistor will create a potential difference between the two parts of the ground lines inter-connected by this resistor. As you have found out, this will have serious side effects like making your TR1 turns ON randomly without audio present. So normally we seldom place resistors between the ground lines of different part of the circuit. This is difficult to explain clearly, hopefully someone can provide you with more informative comment on this subject.

The purpose of R18 is to make TR2 more stable in regard to noise triggering. Without R18 TR2 will turn on if the output of the opamp ever goes higher than 0.7V. By adding this resistor, TR2 would require at least 1.4V of steady opamp output for it to turn on. So its purpose I would say is to increase the noise immunity of TR2 against noise.

 

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