
Originally Posted by
soundguruman
OK you asked for it...
Ceramic disk caps induct 60 cycle fields, and inject that 60 cycle right into the audio path. That's why you should replace them, particularly, if they are used in the audio path.
These are most noticeable in the application of the "bright" cap used across the volume control, or on the "bright" switch. Also the hi pass filter, used for the treble control. You will also spot these in other places, within the circuit. The closer the cap is located to the input stage, (or IN the input stage) the more 60 cycle will be introduced into the audio path.
The 60 cycle surrounds the heater wires, where these are used for the preamp, is inviting 60 cycle to be amplified.
That's one big reason we use DC heaters, especially for the first two stages of the preamp. The 60 cycle does not have the opportunity to enter the highest gain stages of the preamp, and therefore, is not amplified.
You are puzzled as to why a capacitor can INDUCT? I thought capacitance was the opposite of inductance.
They do, in fact modern capacitors are deliberately designed to be "non-inductive." The capacitor acts as an inductor at frequencies outside of it's intended operating range. It's just like an antenna. The antenna picks up 60 cycle, from the heater wires mainly, or the power transformer. There is no shielding to prevent it on a ceramic disk.
Your guitar pickups, pick up 60 cycle, from electrical equipment, and that 60 cycle is amplified also along with the music. Same exact problem.
Trivia: In silver face and black-face fender amps, you will always notice a low, 60 cycle hum, especially when the amp is cranked up.
Guess where at least half of the hum be coming from? If you do not know by now, start at the top and read again.
Notice: In an amplifier the parts talk to each other. They do not care where the wires go. Try for a moment to realize there are many other signal paths besides the ones connected by wires.
Any time you put two wires side by side, you have made a capacitor. Now the AC traveling through those wires is mixed together, it does not care if the AC is 60 cycle, music, anything, it still mixes together. Not the current, but the voltage, yes. Now that the 60 cycle has entered the audio path, it is amplified, you hear hum.
There is AC voltage on the chassis of your amp. It is not a lot, but it's there. Now you are laying a wire on the chassis. That tiny little AC voltage is inducted into the wire, and amplified.
that is why, if we want low noise, we need to start with non inductive capacitors and non inductive resistors. Then the parts talk to each other less.
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