Electronic – Issues with HUM on DIY audio signal switcher using a relay

The simplest way would be to simply feed it through any logic IC which features Schmitt Triggering.

A Schmitt trigger has what is called input hysteresis.

This includes many of the 7400 series of logic ICs. There is a list Here, just Ctrl+F for "schmitt".

Alternatively, if you really want to do it yourself, it's pretty simple to achieve with an Op-Amp.

The way this circuit works is fairly simple.

• Take the above schematic, and imagine the "input" pin is at -V.
• Since the voltage at the "+" input is less then the voltage at the "-" input, the value of "Output" will be Low (in this case, approximately -V).
• Now, "Input" begins to rise. However, notice that there are two resistors. To cause the output to change to High, the voltage at the "+" input has to increase until i is greater then the voltage at the "-" input. This means that the value of input has to increase above the value of the negative input by an amount determined by the ratio of the resistor divider.
• Now, as soon as the "+" input is higher then the value of the "-" input, the value of output will go High (in this case, it will be approximately the value of +V).
• Since the output is also fed into the input through the resistor divider, once the output has gone positive, the threshold to cause the output to go negative changes as a function of the resistor divider.
• When the Voltage at input decreases, the same happens in reverse: Because the end of the voltage divider is now at +V, the value of input has to go below the value of the - input to cause the output to become low.

All together, this is called positive feedback (as you can see, the output is connected to the positive input. As a result, the transition threshold is affected by both the output state and the input state

The term for this, e.g. the output as both a function of the input and the previous output is Hysteresis.
When hysteresis is implemented in a circuit like this, it is called input hysteresis.

It is not correct to have 0dB represent "off". dB is a relative scale and 0 dB, effectively, means "the same as some reference value", and your reference value can't be silence.

A more common implementation of such a control would be to have 0dB represent maximum volume and negative values represent lower output. This is what you see on all professional audio equipment analog and digital. You would then use -Infinity for no output. Many applications approximate this by making the slider go from 0 at the top to -96 [1] dB or so at the bottom. When the user slides the slider down all the way, the application will turn off the output.

[1] 96 is chosen because that's the approximate dynamic range of 16-bit audio. In the presence of dither, this value has no real special meaning, though, so some applications use an even larger value.