i had thought that a variable resistance between a diode and ground acted to raise or lower the diodes threshold, but it acts as a sort of 'softness' or blend, and softens the hard clipping... interesting
What it actually does is this:
A pair of clipping diodes to ground clip because they are (conceptually) open circuits to ground until the voltage across them gets to their conduction threshold, then they clamp the voltage to their conduction voltage. This is an oversimplification, of course, as the conduction of diodes is not abrupt, but has a knee, but go with it for a moment.
This means that diodes to ground are always used with some current limiting device in series from the signal source to the diodes. This is often a 10K resistor. The resistor limits the current that flows when the diodes conduct and their incremental resistance changes from almost an open circuit down to a few ohms. The signal is invariably taken between the junction of the two diodes and the limiting resistor.
If you add another resistor in series with the diodes, then any current that flow through the diodes also flows through the resistor, and that causes a voltage by ohm's law. This voltage is added to whatever the diodes are causing. This resistor voltage is a replica of the signal voltage, and is added back into the output signal by the resistor being in series with the diodes. The net effect is similar to some of the original signal being added back into the clipped signal. So the output becomes a sum of the clipped, diode waveform plus an addition of some amount of the part of the input signal that's bigger than the diode clipping threshold. This has the effect of making the diodes not clip as hard.
Another way to look at it is that the reason diodes can clip at all is that they are a voltage-variable resistor. Their resistance to tiny, tiny signals changes from very high, almost open circuit, if the DC voltage across the diode is near zero, to intermediate values as the diode starts conducting, to finally a quite small value, maybe 10 ohms, when the diode's forward voltage is pushed fully into conduction. Diodes can and have been used as voltage variable resistors. They're used this way in RF a lot, and some Vox solid state amps use them for a tremolo modulator. Anyway, inserting a resistor in series with them limits how low the diode+resistor resistance can go, no matter what the diode by itself is doing.
Usable values vary from zero ohms up to about the value of the current limiting resistor. At some point the diode clipping becomes hardly noticeable. The added resistor becomes a limit on how much the diodes can clip. The down side is that the output signal voltage grows as you increase the "softness" resistor, so it starts smallest and most distorted with the added resistor at zero, and the signal gets bigger and softer at the same time. This is very odd - your ears expect the opposite to happen.
it occurs to me that with a bunch of resistor/diode combinaitons you could have alot of control . like very minimal soft clipping for small signals
and added soft clipping for medium signals, and even more soft clipping for really big signals, and still retain the original signal dynamics...
the signal would never need to hit a hard ceiling and never really get squared... just rounded...or compressed?...
are there any circuit designs that do something like that ? i just don't recall seeing diodes set up in a row like that... set up to turn on one at a time depending on signal threshold...
It's been used. This is a simple form of a diode-clamp wave shaper, which was common in desiging waveform generators at one time. I used a series of diodes with resistors into nodes in the middle as a form of soft clipper. It works fine. You just need a bigger signal to work with, because the smallest step from diode to diode is the forward voltage of the diodes, 0.6V or so for silicon. This means for a soft clipper, you need on the order of 4-6 diode drops as the final clipper, and that means your signal needs to be 2-4V peak to have any noticeable soft clipping. But it does work, and you *did* just invent it, whether it's ever been used before or not.