Asymmetric clipping turning into Symmetric for bigger signals

[Vivek]

I had read somewhere that, for small signals, the tube amp clipping is Asymmetric

But for larger signals (or with sag) the signal is clipped in a Symmetric way


If above is true, I did not detect too many distortion pedals that behave in this way

Even those circuits that have diodes in feedback loop followed by "hard clip diodes" to ground dont adjust their circuits such that the hard clip diodes come into play only for louder signals

What have I missed ?

[iainpunk]

let me explain it with this picture:



the difference in clipping thresholds becomes insignificant in relationship with the 'projected amplitude' if the gain is high enough. it will just ''square out'' no matter the threshold.

cheers, Iain

[Vivek]

Interesting way on how this can be achieved !!

But I never saw this circuit in any pedal

or a method to achieve this in any pedal

( Assuming its true that tubes clip Asymmetricaly for small signals and symmetrically for larger ones)

[Vivek]

the difference in clipping thresholds becomes insignificant in relationship with the 'projected amplitude' if the gain is high enough. it will just ''square out'' no matter the threshold.

cheers, Iain

Do you mean ".......... if the input signal is high enough."

[teemuk]

I had read somewhere that, for small signals, the tube amp clipping is Asymmetric

It can be either way. Importantly harmonic distortion has different structure relating to amplitude, which creates time variant dynamic effects.

An average fuzz face has similar effects, BTW, so do many discrete designs with more than one asymmetrically overdriving stage for instance.

If above is true, I did not detect too many distortion pedals that behave in this way

Well, how many pedals are just variants of e.g. Big Muff, Tube Screamer, DS-1, etc. Originality is rare in this industry but in small amounts it does exists.

What have I missed ?

Maybe... In the end all distorting effects sound like distortion. Emphasis, pre and post, likely makes more difference than minute differences in harmonic spectrums. Dynamic effects and touch sensitivity are fun but may introduce plenty of complexity to the design and in the end majority of markets may just be more appealed by yet another TS clone in a flashy package.

Here's another question in similar vein to ponder at: Why is PPIMV such a revered and popular mod to tube amps when in the end it does nothing a plain shunt diode clipper couldn't do and is equally devoid of all dynamic effects associated with tube power amp clipping?

[idy]

+1 on teemuk for pointing out the Fuzz Face does this.
I like this: Opamp with feedback loop clipper single MOSFET with diode in series against the polarity of the body diode. Clips in one direction only...until *gain* or *input level* gets high enough to drive (rail to rail or MOSFET) opamp to clip.

Or just put an asymmetric OD pedal in front of a symmetric one and adjust levels and gains.... Or use an asymmetric device to drive a tube amp....like that high tech gadget called the "Range Master."

[iainpunk]

'projected amplitude' is input signal times gain ignoring headroom
the circuit i used is an extreme example, but it also goes for smaller differences and yes, for tubes.
the rational behind it is that if you make a signal big enough, the clipping will make it a square wave, no matter what the difference in clipping threshold is.
this only holds for a single gain stage with a full clipping pair, if you clip asymmetrical with low gain, and then block the DC and then go through super high gain, it will still be asymmetrical. (remember that blocking DC makes sure that the area underneath the wave shape is the same on both sides, not the amplitude)

but don't worry the theory above only fully applies to sine waves, a clean guitar signal is asymmetrical by nature so symmetrical clipping is a myth anyways.

if you want a circuit that clips only one side of the signal, look no further than a Range Master!!!!

cheers, Iain

[Vivek]

the Fuzz Face is a fortuitous combination of circuits that combine initial soft clipping with asymetrical clipping that changes toward symetrical clipping under drive.
--RG KEEN 1998
http://www.geofex.com/Article_Folders/fuzzface/fffram.htm




The gist of the Fuzz Face remains in the simple circuit that uses eleven components (2 transistors, 4 resistors, 3 caps and 2 pots) and the astonishing tones created with them; delivering a soft asymmetrical clipping that changes to hard clipping in both semi-cycles under the fuzz pot action.






Only small move towards Symmetry !


https://www.electrosmash.com/fuzz-face

[Vivek]

If I understand correctly

When we have a stage with diodes in the feedback look, the output of that stage is close to as many diode drops as diodes you had in the feedback loop

Then if we feed this stage to shunt diodes to ground, the hard clipper is taking a signal of roughly one diode drop and clipping it at one diode drop ie not really adding much.


So I added a voltage divider to get gain from the same Opamp, to get higher signal than the diode drops

and now I pass it through hard clip LED

So idea is

Small signals are Asymmetrically clipped by the diodes in the feedback loop of the Opamp. They are lower that the Vf of the LED so they escape to the next stage, unharmed.

Larger signals are symmetrically clipped by the LED.

Would this work to get some level of dynamism and time variant harmonics  ? 




Waveforms for 40mv, 60mv, 150mv, 300mv input signals.




Maybe this is somewhat similar to having LED and (Si diodes with compliance resistors) in parallel, in the feedback loop of the Opamp. The Si kick in first and the LEDs kick in later. I was hoping that having Shunt LED clip harder than LED in loop.

[PRR]

...for small signals, the tube amp clipping is Asymmetric ... But for larger signals (or with sag) the signal is clipped in a Symmetric way....

Drive a 8 foot wide car into a 6 foot wide garage.

You can go in centered, and bash both sides the same.

Or off-center, bash one side more than the other.

In practice, you go in off-center, the crush on the tight side tends to force the car more-centered. So lop-sided at first and both-sided for major damage.

[Vivek]

Thanks PRR

I understood the analogy.

Could you please give me ideas on how to achieve this via opamps and diodes.

I assume this needs DC biased diodes, so that they clip asymmetrical initially.

I did see a keyboard limiter that was studied by RG Keen, it had DC bias applied to the limiter diodes. I didn't see DC biased diodes in clipper circuits.

[teemuk]

Opamp will force its output to retain the DC voltage bias (in your example's case 0V) regardless of asymmetric clipping. You get more time variant shifting if you DC couple the opamp output from the LED clipper.

You can, to some extent, control hardness of clipping with the series resistor preceding the diodes (clipper is, after all, a resistive voltage divider). Higher values produce harder clipping.

[Ben N]

...for small signals, the tube amp clipping is Asymmetric ... But for larger signals (or with sag) the signal is clipped in a Symmetric way....

Drive a 8 foot wide car into a 6 foot wide garage.

You can go in centered, and bash both sides the same.

Or off-center, bash one side more than the other.

In practice, you go in off-center, the crush on the tight side tends to force the car more-centered. So lop-sided at first and both-sided for major damage.

A video would be helpful.

[Steben]

...for small signals, the tube amp clipping is Asymmetric ... But for larger signals (or with sag) the signal is clipped in a Symmetric way....

Drive a 8 foot wide car into a 6 foot wide garage.

You can go in centered, and bash both sides the same.

Or off-center, bash one side more than the other.

In practice, you go in off-center, the crush on the tight side tends to force the car more-centered. So lop-sided at first and both-sided for major damage.

A video would be helpful.

Start crowd funding ild say 

[Steben]

On topic. Basically most classic push pull tube amps tend to have a base of class A stages in the preamp. These stages tend to add non linear response albeit small. This adds even order harmonics without actually audible clipping. It is what people call tube "character". The perfect clean sound. As the signal becomes bigger, the addition gets bigger. Again most tend to like this. Very close to very very soft clipping at one side. Yet at some point this signal will saturate the following push pull power stage which reaches its limits. This dramatically introduces symmetrical clipping adding odd order harmonics at a fast pace. this overcomes the subtle non-linearity of the preamp.
That is the classic (master volume) amp.

Modern amps with lots of pre amp gain exhibit a different character since at high preamp distortion the duty cycle shift comes into play. This can even deliver a dynamic character with the power amp saturated, because the duty cycle shift still comes through the odd order power amp distortion. In a way this can give again a addition of even order character beyond the odd order power amp.

The fuzz face - god bless its soul - is besides its low impedance effects a very dramatic creature. It goes from clean non linear to almost unidirectional soft clipping into hard clipping with duty cylce shift at once. So a fuzz face is the mix of added even order into dynamically more of that and lots of odd harmonics.

Now if you look at it, the rangemaster style boosters in th eearly days tended to give another mix because these have the low impedance effects of a fuzz face (but with the treble boost) and non linearity of the class A stage boosting the preamp not only the power amp.

This means
all the classic legendary tones almost
all have
all these elements.

[teemuk]

It seems one can actually patent an asymmetric clipper followed by a symmetric one.   
https://patents.google.com/patent/US8471136B2/en
(Fulltone "Plimsoul")

Yet, I must wonder how defendable rehashing Roland Corporation's circuit idea from 1983 (e.g. Boss HM-2) is in practice...

[Vivek]

I had saved that Patent in my list of interesting patents, but with a different number

https://patents.google.com/patent/US20120192703A1/

[Steben]

Now I think of it an opamp clipper is on its own in 1 stage already able to do that as long as you design your diode clipping well enough. You can make one half hard clipping and the other soft with the same hard clipping.

For example with this:

With a signal large enough the result will be dominantly the two diodes conducting in each direction

[Elektrojänis]

Could a non master volume push-pull amp actually work another way round?

Like when you start pushing it the power amp saturates, but because its nature it can only do it symmetrically. If you push it more the preamp starts to clip too and that can get asymmetrical. (And if the duty cycle goes off enough the power amp cannot make it symmetrical any more.)

[Vivek]