Re: Cap orientation help needed

Posted by induction on Apr 27, 2015; 8:52am
URL: http://guitar-fx-layouts.238.s1.nabble.com/Cap-orientation-help-needed-tp19761p19913.html

Any phase issues (reversals or shifts) with caps are not relevant to this particular question. Phase shifts are an AC effect, and polarized cap orientation is only a question of DC bias.

For our purposes, you can just pretend that the cap is jumper for AC, and an open circuit for DC. Just figure out where the DC voltage is higher and put the positive leg there. There are a few common scenarios that you see over and over (I'm assuming a positive supply voltage for all of these):

1. Filter caps: These often go from V+ to ground or from Vref to ground.
Solution: Just put the negative leg on ground.

2. Coupling caps: These caps are in series with the signal and connect different stages of the circuit and allow us to introduce or remove a DC shift at the circuit input, output, or between subcircuits.
Solution: Pedal inputs and outputs should have a DC voltage of zero. Internal subcircuits are often centered around a reference voltage. The reference voltage will be positive for circuits with positive supply voltages, and negative for circuits with negative supply voltages. Figure out which side of the cap touches the part of the circuit with a DC shift and put the positive leg there. Both of the 100n caps in question on the Diefet are an example of this. The one connecting the Deep control to the Volume control is an output cap. The Deep control is connected to the final op-amp stage (DC biased to Vref) and the Volume control is connected to thecircuit output (ground). The other one connects to the input of the final op-amp stage (again, DC biased to Vref) and the other side connects to the tone stack. Notice that there is a DC path to ground on this side (through the TMB pots) but no DC path to the supply voltage, because the signal path from the previous jfet stage passes through other caps before it reaches the 100n. Thus the DC bias on this side should be zero.

3. Hi-pass filters: These are also in series with the signal, but there may not be a DC shift across the cap. Solution: Don't use polarized caps here.

4. Tone stacks/tone controls: These are connected to resistors and potentiometers in networks that can be simple or very complex.
Solution: Look for DC connections (through resistors or active components, but not blocked by other caps) to the supply voltage and/or to ground. This one requires some expertise or experience.

Needless to say, you must look at a schematic when you are working this out. Trying to figure out correct cap orientation from a layout is difficult and error-prone. This means you must find the schematic for the circuit, and you must be able to read schematics to figure it out yourself. What if you can't read schematics?
Solution: Learn to read schematics. Or ask someone who can read schematics to tell you the answer.

Also note that the answers will be reversed in circuits with negative supply voltages. With bipolar supplies, the correct orientation will depend on exactly where the cap is placed, but the analysis will be the same.

In very tough cases, you can just sim the circuit and calculate the DC voltage on both sides of the cap.