How to Determine a Capacitor’s Outside Foil

Non-polarized metal film capacitors still have a “desired” orientation. Installing the capacitor one way will have a shielding property that may help the circuit control noise.

Background

In some boutique amplifiers, the seemingly non-polarized capacitors have a marking on one side. This marking indicates the capacitor lead that is connected to the outside foil inside the capacitor. This is important because the outside foil being connected to a low impedance node (like ground) of the circuit creates a shielding effect on the capacitor. When the outside foil is connected to a high impedance node, the circuit will attract noise, from something like the heater/filament circuit, which is highly undesirable.

Imagine having ten of these capacitors inside your amp, every single capacitors were facing the wrong way attracting noise. That’s a lot of noise. However, having the capacitors facing the right way, shielding the noise somewhat, will have a lot lower noise floor.

Tools and Materials

You don’t need much to find the outside foil:

  • Oscilloscope – $40 unit I use.
  • BNC to alligator clip cable
  • Bunch of capacitors to test

Procedure

  1. Fire up the oscilloscope
  2. Plug in the BNC cable to the oscilloscope
  3. Clip the alligator clips to each of the leads of the capacitor to check
  4. Adjust the oscilloscope to show the full signal
  5. Lightly hold the capacitor with your fingers, let go of the other hand and read the oscilloscope
Red lead on top, somewhat noisy.
  1. Switch the red and black leads, hold the capacitor with your fingers again and let go of other hand, re-read the oscilloscope
Black lead on top, noise seem much quieter. Black lead is connected to the outside foil!
  1. When the noise is the lowest, the black lead is clipped onto the outside foil
  2. In this example, the outside foil is the lead on top with the black mark.

Conclusion

Connecting the outside foil lead of a non-polarized capacitor can help lower the noise floor in your circuits, acting like shielding around the capacitor itself. Inserting the capacitor the opposite way will make the capacitor act as an antenna for noise. Inside vacuum tube amplifiers, AC power and AC filament heater supplies are the noisiest.

You can find the outside foil for your capacitors fairly easily, using an oscilloscope and your fingers. At the end, if you were to take home one sentence:

When the noise is the lowest, the black lead is clipped onto the outside foil.

You might have to say this to yourself a few times to get the outside foil right. Enjoy!