Germanium diodes start clipping at lower voltages 300mV, compared to 600mV of silicon diodes and 1.2V of LEDs, that means that for germanium diodes, clipping starts at lower levels and hence distortion happens in a wider dynamic range.
Germanium diode clipping has a more rounded I-V response than silicon and hence clipping is softer.
I recently did some research trying to find an alternative solution to germanium diodes.
Germanium diodes are expensive compare to silicon diodes and difficult to find in big distributors and out of specialized shops.
I was thinking if there is an alternative to replace germanium diodes while still having the same clipping response:
- clipping at low signal input levels
- softer clipping than silicon diodes
I am not sure if this is a subject that somebody has already raised but I think that I could have found an answer to replace germanim diodes that I would like to share with you. Let me know what you think.
A germanium diode could be replaced by a Schottky diode in series with a resistor to obtain very similar results.
I've been doing some LTSpice simulations comparing I-V response of both circuits and though exact overlap is not possible, the two goals previously mentioned can be met: clipping at low levels and softer clipping.
The figure below shows the I-V response of a 1N34A germanium diode (red trace) and the I-V response of a BAT54 Schottky diode with series resistors betwen 5 to 500 ohms (gree traces)

BAT54 diode is an inexpensive widely used Schottky diode that can be found in a dual configuration in the same package for positive and negative clipping. SMD packages are also available.
The following figure shows the soft clipping response of both circuits:

The most adequate series resistor value for soft clipping seems to be around 250 ohms (for a 1kohm opamp input resistor)
and the hard clipping response:

The most adequate series resistor value for hard clipping seems to be around 14 ohms.
The schematics used for these simulations are shown below:

Additional information can be found in my blog rezzonics.blogspot.com
These simulations must be confirmed by real experiements, though, but it looks promising.
Let me know your thoughts.