Question: How do the diodes protect the input of amplifier from excess voltage in the circuit below?

[![enter image description here](https://i.stack.imgur.com/fO89q.jpg)](https://i.stack.imgur.com/fO89q.jpg)

Answer: 
1N4148 diodes are made of silicon and conduct very little until the forward voltage reaches 0.6 V or so. 

* On positive half-cycles D1 will remain "open-circuit" unless the audio signal exceeds 0.5 to 0.6 V. If the signal exceeds the forward voltage of the diode then the signal will be clamped at the forward voltage. You will hear distortion.
* On positive half-cycles D2 is reverse biased so it won't conduct.
* On negative half-cycles the situation is reversed.
The 1N4158 will have this behavior (assuming N=1 for doping profile, and 1mA at 0.6 volts)

10mA @ 0.658 volt; incremental R = 2.6 ohms

1mA @0.600 volt; incremental R = 26 ohms

0.1mA @0.542 volt; incremental R = 260 ohms

0.01mA @0.484 volt incremental R = 2,600 ohms

0.001mA @0.426 volt; incremental R = 26,000 ohms

0.0001mA @0.368 volt; incremental R = 260,000 ohms

What does this table tell us?
Consider the 0.0001mA line; for the 1N4148 to conduct that current, the potentiometer
wiper must output 0.368 volts, at which point the diode has an incremental resistance (we can this the small\_signal model) of 260,000.

You can compute the worst case effect, at 0.0001mA, by improving the model with a 260Kohm resistor from potentiometer wiper to Ground.

Given your pot is 10Kohm,the worst case effect (assuming the Rsource into the 1uF is zero ohms, is with wiper set to the middle, whereupon you can model the pot at 5K || 5K or 2.5K.

The diode's incremental R of 260,000 ohms forms a voltage divider with the pot at 2.5Kohm.

This voltage divider will add about 1% distortion: none at the zero crossings of a sin, and maximum (1%) at the + and - peak of the sin.

By the way, notice the diode voltage is the scaled logarithm of the diode current. This behavior continues down to nanoAmp and picoAmp current levels.

**THERE IS NO ABRUPT THRESHOLD FOR DIODE CURRENT**