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25 PF Capacitor sub

SparkyCal

Mar 11, 2020
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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

Feb 24, 2014
334
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Feb 24, 2014
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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.
 

bertus

Moderator
Nov 8, 2019
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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
 

Ylli

Jun 19, 2018
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Jun 19, 2018
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The application is not critical, either value would be OK. It I had to choose, I would go with the 27 pF.
 

kpatz

Feb 24, 2014
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Feb 24, 2014
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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.
 

SparkyCal

Mar 11, 2020
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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

Sep 24, 2016
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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.
 
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