etet wrote:
Why is a linear pot being used for the volume control vs a log?
First, I always specify a log A potentiometer for a volume control. Is there somewhere in the thread where it said to use a linear taper pot for the control?
In any event, now for a little more detailed explanation. The human ear is a logarithmic detector. This means that as absolute sound pressure level increases exponentially, the human ear will register it as a linear progression. This is why the standard unit of measure for sound is an exponential scale is decibels (dB) relative to a reference. So the issue for audio circuit design becomes how do I get an exponential transfer characteristic in the voltage realm from a linear progressing on the control shaft position? The answer is to use a log taper potentiometer.
Consider these plots for a linear taper potentiometer wired as shown in the 4S circuit.
Attachment:
Linear Pot.jpg
The first plot on the left is a fractional voltage out as a function of shaft position. As expected, the voltage transfer characteristic is linear. However, the second graph shows how this transfer characteristic would be heard by the human ear. In this case the major volume change is bunched up at one end of the shaft positions. In effect almost all the "volume control" function occurs on only half of the shaft rotation; the later half resulting in very little change.
Now consider the same plots for a logarithmic taper potentiometer.
Attachment:
Log Pot.jpg
Here you can see that the plot on the left for the voltage transfer characteristic has the vast majority of the voltage change taking place on the latter part of the shaft rotation; the earlier part contributing almost none of the adjustment range. However, now look at the dB scale. The human ear will hear this voltage transfer characteristic as a perfectly linear progression from very soft to full volume. Here the "volume control" function is evenly distributed across the entire range shaft positions.
This is the slightly longer explanation as to why one should always use a logarithmic transfer characteristic for volume control. I hope this help's to clear things up a bit.