25 PF Capacitor sub

[SparkyCal]

A circuit I am building calls for a 25 pF capacitor. There is no polarity shown, so I assume it is a film capacitor. It is a guitar pedal distortion circuit.

I don't have 25 pF capacitiors . You'd think with all these kits I bought I'd be able to cobble one up, but I am not sure I can. Does anyone see anything in there I can use?

I have the following kits:

https://www.amazon.ca/gp/product/B01GFE11ES/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

https://www.amazon.ca/gp/product/B07D1J5LSD/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

https://www.amazon.ca/gp/product/B07F7ZN695/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

https://www.amazon.ca/gp/product/B00OJPU24Q/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

https://www.amazon.ca/gp/product/B07K7NLSH2/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

 

[kpatz]

What's the smallest capacitor you have? The smallest in the kits you linked (your 1st link) are 100pF.

If you have a couple 12pFs you could parallel them.

In a pinch you could connect 4 100pF caps in series to get 25pF but that's a kludge.

Maybe find some locally (if you have any electronics stores in the area) or try Digikey/Mouser.

 

[Ylli]

It is a guitar pedal distortion circuit.

Are you sure it is 25 pF and not 25 uF? Can you post a snipped of the schematic showing where it is used?

 

[Hunter64]

"There is no polarity shown..."

 

[bertus]

Hello,

Show the schematic.
It can be the value is not that critical.
If you really want 25 PF, take the 10 and 15 PF capacitors from kit 3 in parallel.

Bertus

 

[SparkyCal]

It's this same one I have been rebuilding on stripbaord. https://www.electronicspoint.com/forums/threads/a-stretch.293212/page-9#post-1801204. C3

I was able to salvage two capacitors that total 25Pf from another iteration.

 

[SparkyCal]

I hope I am not driving you folks nuts with my questions.

I was at the elctronics store today, so i decided to pick up some 25pF capacitors. They only had 22pF and 27pF. I bought some of each. Which would you think is best to use for this circuit?
https://www.electronicspoint.com/forums/threads/a-stretch.293212/page-9#post-1801204

 

[Ylli]

The application is not critical, either value would be OK. It I had to choose, I would go with the 27 pF.

 

[kpatz]

Ditto, the difference will be negligible if any. I think it's just to prevent the 741 from oscillating at high frequencies, by damping them. Basically a low-pass filter, with a high cutoff judging by the small capacitor being used.

 

[bertus]

Hello,

As @kpatz said, the capacitor is used to avoid oscillation of the opamp.
Read this article for more info:
https://e2e.ti.com/blogs_/archives/b/thesignal/archive/2012/05/30/taming-the-oscillating-op-amp

Bertus

 

[SparkyCal]

Thanks guys. i used the 27pF. Bertus, thanks for the article. I am making it a point to go back to this circuit once I compete it, and begin to understand it theoretically. I think that is an important part of my learning. Otherwise, i am simply doing a paint by numbers thing.

 

[Audioguru]

The 27pF capacitor parallel with the 1M opamp's negative-feedback resistor simply cuts audio frequencies above 1 divided by (2 x pi x 27uu x 1M)= 5926Hz at -3dB, making sounds a little muffled. R6 and C5 also cut frequencies above 1 divided by (2 x pi x 10k x 1nF)= 16kHz at -3dB.

I never calculate with 2 x pi since 1 divided by 2 x pi is simply 0.16 times. So 0.16 divided by (27uu x 1M)= 5926Hz and 0.16 divided by (10k x 1n)= 16kHz.