Capacitor Question ???

Quote:

---

Originally Posted by Simon Barden

With the volume control, the full-range of the signal is being attenuated as you would expect.

If you look at the wiring, the connection to the tone pot is made from the connection on the volume pot that also connects to the pickup output. So if you use the tone pot, some of that signal is going down to earth via the tone capacitor. Because it's the resistance of the pickup that forms the actual R part of the RC tone circuit, and there's obviously something wrong with the pickup, the low-pass circuit cut-off frequency is affected and probably all (not just the high frequency part) of the signal is passing to ground through the tone circuit. Because the tone pot is normally selected as a linear pot, then unlike the log/audio taper volume pot, it doesn't take much turning to significantly lower the pot resistance.

---

Thank you for your info Simon. I have a lot to learn about pots and caps. I am still trying to understand the difference between linear and taper in pots and values and volts in capacitors.

The 'volts' in capacitor ratings is easy to understand. The rating is the maximum voltage the insulation between the different layers that make up a capacitor can withstand before breaking down and allowing current to pass across them (basically destroying the capacitor). This is normally a DC voltage rating, though if it has AC in front of the value, then it will be a capacitor normally designed to go in AC power circuits. Sometimes a capacitor will have both AC and DC voltage ratings.

As AC voltages are normally given as RMS values (root mean square), this gives the average voltage level of the AC voltage, but the peak voltage (the one that affects the insulation levels) is 1.41 times higher than the RMS value (assuming a mains style sine-wave). So a 240v AC RMS mains voltage actually has a peak voltage level of 240 x 1.41 = 338.4v. So anything designed for mains voltages would need to have an insulation rating of at least 350v DC, preferably higher, to allow for normal variations in mains voltages.

But in capacitors, this is not the only consideration. As the voltage present nears the insulation rating, the actual capacitance value starts to reduce. At 95% of the rated insulation value, the value of the capacitor decreases significantly to around 20% of its nominal value e.g. with an applied voltage of 19v DC across it, a 10uF 20v DC capacitor would have an actual capacitance of around 2uF.

This isn't a linear effect, and only happens when the voltage reaches a significant percentage of the insulation rating. But it is standard practice to use a capacitor that's got an insulation rating of at least 200% of the highest voltage expected in that part of the circuit.

The higher the insulation rating, the more costly the capacitor, so it's natural for mass produced goods to have capacitors selected on a circuit board exactly for the maximum design voltages present.

On a guitar/bass, it's not really a problem as the highest voltages likely to be generated by even the highest output passive pickups is around 1v AC, with 0.1v AC being a more standard pickup peak value, so any capacitor rated above 3v DC should be fine. If it's a higher insulation rated capacitor (a lot of the Sprague Orange Drop capacitors sold are between 100v to 600v rated), it doesn't really matter because you are only buying one of two of them for your own guitar. If you were a factory making 20,000 guitars a year, you'd certainly fit something cheaper to your lower-end models and maybe fit Orange Drops to the more expensive models as a selling point.