Stacking/paralleling caps - same value?

[phrarod]

I'm playing around with stacking caps ie non polars with inner to outer foil.

Is it important to have two caps of equal value? ie 2-220uF etc? Or is it ok to stack different values like 180uf+700uF?

[Arnulf]

Obey the voltage ratings of the lowest rated device and you'll be fine.

It is common to parallel different sized capacitors when trying to offset deficiencies of one capacitor type (say paralleling larger electrolytic cap with smaller foil one for better high frequency response).

[phrarod]

Thanks much appreciated. Am I correct thinking that by paralleling them or stacking them I'm lowering the overall impedance and getting better performance?

[phrarod]

A friend corrected me. I am paralleling them not stacking them.

[refference]

I have had excellents experiences , paralleling caps for power supplies filters purposes .
The power supply total impedance falls and you will have a better frequency response
and bandwidth .
Example : Instead to use a 220 uf x 450 v cap , I use 5 x 47 uf x 450 v paralleled .
You can mix values too , BUT is very advisable that you use all capacitors rated to the same voltage value , and if possible ( not an obligation ) all the same brand .

Carlos

[phrarod]

What's your feeling on using this method for cathode bias?

[Fenris]

Originally Posted by phrarod

What's your feeling on using this method for cathode bias?

If you mean the bypass cap on the cathode bias, it's highly recommended, within reason. For example, with an EL84 I prefer a high quality 1000uF electrolytic paralleled with a good film capacitor in the 1 to 4.7 uF range. Even better would be five 220uF electros with a 10uF film, a 1 uF film and a very high quality film/foil 0.1uF cap, but this gets expensive and bulky and the improvements may not be noticeable.

[refference]

Originally Posted by phrarod

What's your feeling on using this method for cathode bias?

Yes . No problem , you can use this method too , and more you must ( or
you can ) by-pass all the stuff , with a good quality film capacitor perhaps
0.47 uf or 1 uf x 63 v , as said above .

My only disagreement point from Fenris reply , is to use very large capacitors
on the cathode bias , due to the " blocking " occurrence .
It occurs because a very large capacitor , is not able to charge and discharge
in the same velocity of music transitions , thus generating distortion .
To minimize the blocking effect , I use a cap. calculated for a - 3dB cut off
at + / - 10 Hz

[phrarod]

Interesting. Could you provide the method of calculating the caps?

[refference]

Of course ,

Ck = 1 / ( 2 . pi . f . Rk ) , where :

Ck = cathode by-pass capacitor value ( in Farads )
pi = always the same 3,1416
f = the low - 3 dB cut off frequency , in this case 10 Hz
Rk = Value of cathode resistor ( in ohms )

( 1 Farad = 1,000,000 uF )

It is not an EXACT method to calculate Ck , but the total error won't be more than 3% .

Regards ,

Carlos