Importance of IV knee graph of a Diode in soft clipping applications

[Vivek]

Everybody agrees that the shape of the IV knee of a diode is very important.

In all my SPICE simulations so far, I found the maximum current through the clipping diodes in the feedback loop is normally around 100 to 250uA

I drew red lines in the graph to show the region of the IV curve that we are operating in.

Seems hard to prove there is a big difference or impact of IV curve in such a small range.





What did I overlook ?

[Steben]

Change the scale at the right side to get at +/- 300 µA max. Then you'll see.

[ThermionicScott]

I take it R666 is a metal resistor?   

[iainpunk]

Everybody agrees that the shape of the IV knee of a diode is very important.

i don't
in the bedroom, playing alone, sure, one might sound different than another, and there is plenty of change noticeable in the feel,
but when you are out there on a stage, in a rehearsal room, at your local bar's jam session or somewhere else where you play at decently high volume, together with more musicians... you aren't going to notice the difference. the sound difference of a different speaker is about 10x as large. placing the amp in a corner or along a straight wall has more impact on the sound than the knee of the diode. so... yes and no, there is a difference in sound and feel, but its nowhere near ''very important"

Seems hard to prove there is a big difference or impact of IV curve in such a small range.

in that bedroom situation, the difference will be biggest in the small beginning of the diode curve. as the currents are higher, the difference will be less and less noticeable, since we're going towards a square wave anyways, it doesn't matter much how we get there.

cheers, Iain

[Fancy Lime]

I take it R666 is a metal resistor?   

Thanks Scott, you just made my otherwise rotten day

Change the scale at the right side to get at +/- 300 µA max. Then you'll see.

+1

Andy

[PRR]

Everybody agrees that ...

I disagree.

Unidirectional diodes are mostly an exponential (which looks the same after scaling) with shunt and series resistors (which are intended to be insignificant).

They are not THE SAME, but you can adjust drive and circuit resistances to make it Real Real Close.

Note that the wide range of music and the exponential function makes a Linear scale pretty misleading.

[Vivek]

I take it R666 is a metal resistor?   

Noice !!!!!

You are my type of guy !!!!

I love puns, play on words, mixed metaphors, cryptic crossword puzzles, etymology ..............

[Vivek]

Everybody agrees that ...

I disagree.

Everybody agrees that the shape of the IV knee of a diode is very important.

i don't

cheers, Iain

My original contention was the the role of shape of diode IV knee is overstated in clipper applications.

So would you agree that exact shape of diode knee is quite uncritical in clipper applications since we operate in very small region of the IV curve

and can change knee with compliance resistors in any case.

[antonis]

So would you agree that exact shape of diode knee is quite uncritical in clipper applications..

If so, then C3 should be sheer useless...

[Steben]

Most diodes follow basically a log function with a fixed internal resistor.
Add a resistor to a classic silicon diode, double the gain and you will get close to any germanium sound.

The change happens to an extend once you use zeners and active devices as clipping diodes.

[iainpunk]

..... since we operate in very small region of the IV curve
and can change knee with compliance resistors in any case.

no, it doesn't matter what part of the vi curve were in, the knee is not important however we try to change i. distortion sounds like distortion regardless of curve, especially when you are in a live mix, it all gets mushed out. what really matters is the frequency shaping before and after the distortion, this, i didn't understand the first few years of building pedals, before i joined the forum.
the only place where knee and curve and such things matter is when you play at low gain and low volume in a bedroom or so. if the volume is high enough, you'll lose that difference in feel and sound (yes, even when wearing ear protection).

cheers, Iain

[Fancy Lime]

..... since we operate in very small region of the IV curve
and can change knee with compliance resistors in any case.

no, it doesn't matter what part of the vi curve were in, the knee is not important however we try to change i. distortion sounds like distortion regardless of curve, especially when you are in a live mix, it all gets mushed out. what really matters is the frequency shaping before and after the distortion, this, i didn't understand the first few years of building pedals, before i joined the forum.
the only place where knee and curve and such things matter is when you play at low gain and low volume in a bedroom or so. if the volume is high enough, you'll lose that difference in feel and sound (yes, even when wearing ear protection).

cheers, Iain

I would add that what matters in the bedroom also matters on tape. Wait, that came out wrong... I mean, the differences between diode characteristics are subtle but audible under good conditions. In a live situation, all that matters is volume. Well, almost. A really loud guitar will sound awesome, no matter the diodes.

It is also true that you can bend the behavior of diodes by adjusting the circuit around them. Or you can choose the diode according to what purpose it shall serve in a given circuit. It you really want to get a specific clipping characteristic, you will have to select the diodes, as well as the voltages and currents they are operated at quite carefully. And even then, it only really matters under ideal listening conditions.

I for one always design with ideal listening conditions in mind. I think it is fun to tweak those tiny little details that almost no one will ever hear. But that is a personal choice. Or OCD. I also vacuum my apartment although I have a dog and everything looks like under Satan's sofa again five minutes after I'm done. It's a bit pointless but it feels good to know I've given it my best.

Andy

[Steben]

I agree on the difference in application. A low to mid gainy circuit will have lots of signal crossing the knee. 10x1N4148 in series will not sound as 2x3V Zenders and that will not sound as a 6V Zener diode. And there is capacitance as well...  Of course this only applies to the possibility of high supply voltage. The lower the headroom, the less design choices.

DO not forget transient curves of typical amps are different as well! A typical vox with non feedback cathode bias push pull ("creamy", "shimmer") has a much more curved transition into clipping than feedback fixed bias brothers of most marshalls and fenders ("crisp" "tight").

[Vivek]

It you really want to get a specific clipping characteristic, you will have to select the diodes, as well as the voltages and currents they are operated at quite carefully. And even then, it only really matters under ideal listening conditions.

Andy

Almost no distortion pedal DC biases the clipping diodes to particular operating points

At low signals, the diode is working at 0 current

At high clipping, the diode in feedback loop passes somewhere between 100 to 250uA.

I never saw any circuit where the designer decided that the  clipping diode is biased around 5ma with no signal, and varies around it when signal is passed.

( Solid state diodes in feedback loop)


So I do not agree that any pedal builder choses the voltages and currents passing through the clipping diodes.

[Vivek]

Thanks

Would it be possible to mark the values of current on the y axis

And also to show only 0 to 350 uA on the y axis since that is the range of diodes in the feedback loop of an opamp

[Steben]

Thanks

Would it be possible to mark the values of current on the y axis

And also to show only 0 to 350 uA on the y axis since that is the range of diodes in the feedback loop of an opamp

[Steben]

And of course already posted:

[Steben]

..... since we operate in very small region of the IV curve
and can change knee with compliance resistors in any case.

what really matters is the frequency shaping before and after the distortion, this, i didn't understand the first few years of building pedals, before i joined the forum.

On top of that, those EQ details are very important in combination with the guitar. A strat usually has a flat to almost scooped character (in between pos) and in that way loves mid hump stuff (like most overdrives, brown sounds, rats...) more than tele's or humbucker glued necks. I always found strats suck with overdrive circuits that are cheered for their "neutral sound".

[iainpunk]

what matters in the bedroom also matters on tape

hahahaha, thats a good one.
but on that note, if i'd be in a studio recording "good sounding guitar", i don't think i'd use an overdrive pedal. ill rent or loan a good tube amp, and maybe put an overdrive pedal in front of the already overdriven amp, but in that situation, the diodes won't matter either.

cheers, Iain

[iainpunk]

I always found strats suck with overdrive circuits that are cheered for their "neutral sound".

i always found that those circuits suck for every guitar you put through it. the only way i like a Klon is everything on max with a rangemaster in front of it.

cheers, Iain