# Capacitor Question - Polarity - sort of



## Lincoln (Jun 2, 2008)

I'm playing around with some mods to the normal channel on a fender AB763 DR circuit. One of the things recommended is replacing the 25uf/25V VI cathode cap with a .68uf cap. I couldn't find an electrolytic capacitor that small, so I ended up with the cap shown in the picture. 








The banding on the one end (to me) signifies the negative or "outside foil" of the cap. The million dollar question - is it OK to use this non-electrolytic capacitor in an electrolytic application? banded end goes to ground, right?


----------



## dtsaudio (Apr 15, 2009)

That is non-polarized cap. It doesn't matter which end goes to ground. There is some truth that the outside foil should go to the lowest impedance (ground) but that has to do with noise immunity (which is minimal at audio frequencies) and a bit of internet myth.
Prepare to lose some bass response with that mod.


----------



## dcole (Oct 8, 2008)

You will be fine using it in that application. The difference in using a non-polarized vs electrolytic in this position comes down to the production of capacitors at this point. A 25uF film capacitor is physically large and expensive compared to a 25uF electrolytic.


----------



## Lincoln (Jun 2, 2008)

dtsaudio said:


> That is non-polarized cap. It doesn't matter which end goes to ground. There is some truth that the outside foil should go to the lowest impedance (ground) but that has to do with noise immunity (which is minimal at audio frequencies) and a bit of internet myth.
> Prepare to lose some bass response with that mod.


Thanks for the response. I realize I might not like this mod, but I figure the only way I'm going to figure out what all this stuff actual does is by playing with it. I don't use the normal channel anyway if I mess it up real bad.


----------



## keto (May 23, 2006)

Is there *ever* a case where you *should* use an electrolytic over a non-polarized, or vice versa?

My first instinct is that a cap with either do the filtering job or it won't, based on rated specs, so why should polarization or not matter? I don't remember reading about this specific topic back when I did a bunch of cramming style reading.


----------



## dcole (Oct 8, 2008)

> Wikipedia: Standard electrolytic capacitors are polarized components due to their asymmetrical construction, and may only be operated with a higher voltage (ie, more positive) on the anode than on the cathode at all times. Voltages with reverse polarity, or voltage or ripple current higher than specified (as little as 1 or 1.5 volts may suffice), can destroy the dielectric and thus the capacitor. The destruction of electrolytic capacitors can have catastrophic consequences (explosion, fire).


What you get with an electrolytic is a nice balance between uF/physical size/cost. These capacitors as mentioned above are polarized due to the nature of their construction. To make a non-polarized electrolytic, you need to put two in series with one turned 180 degrees. This causes the overall capacitance to be cut in half, as series capacitors divide rather than add like resistors. It will also be physically larger than the average polarized cap.

Different types of caps are used in different types of situations depending on the needs. For filtering power supplies, electrolytic caps are used as they have large uF for small physical size and handle the currents found in power supply sections. Film capacitors are used in the signal path as they again fit the range of uF used for size requirements and tend to have less distortion then other caps. Ceramic capacitors are used for bypassing power supplies for integrated circuits again because they are made for the uF range required and size available. An engineer/designer will use these and many other variables to determine what capacitor construction/material type is required for the job being done.


----------



## ezcomes (Jul 28, 2008)

This vid has been shared before...but it can always be watched again...


----------



## dtsaudio (Apr 15, 2009)

Ah this guy again. 
While his demonstration has some merit, where he screws up is in his terminology and his failure to take into account the circuit that film caps are often used in.
First his terminology. The cap does not have polarity. Polarity denotes that the cap needs to see a DC voltage where the + and - voltages are aligned properly on the cap. This is your polarized electrolytic.
All he is demonstrating is that the cap, made up of rolled layers of conductive material and a dielectric (insulator), will have an inner and outer layer. When used at a high impedance (source and load) the outer layer will be prone to pick up outside interference. How much this will add noise to a circuit will depend on the gain and impedance of the circuit. He has basically an open circuit (high impedance) and the cap is picking up some noise. Valid demo there. But in real life it may not make any difference as the noise is very small and the circuit may not have enough gain to amplify the noise.
In the case of using it like Lincoln wants to do, ie as a bypass cap, one end is ground, the other end is attached to a 1.5k resistor. If you look at any capacitor spec, there is a specification called ESR (electrostatic resistance). In a film cap it is so small that at at audio frequencies it is virtually a dead short. So it shunts all signal (including noise) to ground while its DC resistance is so high it doesn't change the operating point of the tube.
In short it will make no difference at audible frequencies which way this cap is put in the circuit.


----------



## Lincoln (Jun 2, 2008)

ezcomes said:


> This vid has been shared before...but it can always be watched again...


Yeah I stumbled onto that video after I had built a few amps. Scared the hell outta me. The pro's here had a good chuckle about it anyway.


----------



## ezcomes (Jul 28, 2008)

i didn't say i bought it...just posted it...


----------



## jb welder (Sep 14, 2010)

I figure if Fender & Marshall didn't worry about the orientation of the non-electrolytics, I won't either. 
If they could gain an advantage by doing so, I think they would have.
For any such cap in amps we consider 'classic', there is a 50/50 chance of it being 'backwards'.
For all we know, maybe amps that have a certain mojo are _because _some particular non-polar cap is 'backwards'.


----------

