Multiple clipping stage "spacing"

[WGTP]

I wasn't sure how to word this.  I've seen it mentioned, but not really discussed.

When multiple clipping stages are used, for example the various tube amp simulations, Joe Davidson's Obsidians and Vulcans, dual op amps, etc. what are your thoughts on the "spacing" or voltage differences at the input of the device that result in the various stages clipping.

I'm assuming the last stage usually clips first, followed by the next one, and the next one, etc.  We obviously don't want them all clipping at the same input level, but how spead out do we want them.

Do we want the last stage clipping with an input of 10mV, the next at 20mV, the next at

The ThunderChief might serve as a good point of discussion, since it is based on a Marshal, but I understand that the Jfets don't work the same as the real tubes, from this stand point.

I would think this would have a great impact on the dynamics, compression, response, feel, etc. off a particular device.

Thanks in advance for your response.   

[R.G.]

Welcome to distortion theory.

There are about a dozen ways to think of the effect of successive distortion stages. The input threshold is one of them. Another is how much overdrive you give the individual stages. Another is db of compression.

When one stage limits, the voltage to all succeeding stages is fixed. If you have a stage that clips at +/- 1.0V, then no succeeding stage will ever get an input signal more than that.  If you choose the clipping levels properly, you can ensure that there is an orderly transfer of clipping from the last stage, which clips first, to the preceeding stages one by one. Ideally, you'd make earlier and earlier stages clip more and more abruptly for a change in distortion character.

You have to watch the output level of each stage closely to get an orderly transfer from stage to stage. You may well have to diddle the levels on successive stages to get just the right amount of clipping for the following stages.

I think the spacing is just one of those design variables - where do you want changeover in the input signal.

[petemoore]

  I typed a couple times for over an hour...still thinking...
  This seems like the not so easy to phrase question, and the available answers [long, and 'hidden] are in schematics of circuits that match the description, the heart of the matter...every one is different, it's easier to think about than to relate in typed words, that's for sure.
  Disregarding amp nonlinearities, it would seem
  "I'm assuming the last stage usually clips first, followed by the next one, and the next one, etc."
  This has to be the case, if an early stage reached clip threshold before a sucessive one, effectively the signal would be compressed so that the latter stage would never reach threshold...so it's like a stairstep, last stage steps up to clipping at lowest input voltage to see clipping, then the one before gets enough when voltage gets 'there'.
  An interesting experiment would be to say 5k dual pot a couple clipping stages, one side of the dual pot connecting to ground some clipping diodes while the other side 'inserts' resistance between a later stages clipping diodes to ground...you could have a 'clipping early' tone at CW and a late clipping stage tone at CCW.
  I see that as what the circuits are really about, what the gain of the Q is, how it's voiced, how the output is attenuated, what is the CThreshold of the next stage etc. is what makes a Meteor sound like a Meteor and a TChief sound Plexi.

[WGTP]

I suppose another element of this, is the number of clipping stages used.  1 screaming or 5 cruising.  Of course the standard argument is that a tube amp has multiple stages, but does that mean a single stage can't generate a reasonable similarity

Again, thanks for your comments.   

[R.G.]

This is progressing right down the train of thought I had on successive distortion stages from either the previous incarnations of this forum or from Ampage.

A clipping stage has both an input level that it clips at, and an output level that it puts out. If the input level is less than the input clipping level, the output never reaches its clipped level. Now imagine two stages. If they both have gain over 1, then the second stage will clip before the first one will because it's being fed a bigger signal, having been amplified by the first stage.

Now we come to the critical question. If we feed a signal big enough to make the first stage clip, how big is the first stage's clipped output signal compared to the input clipping level of the second stage? If the first stage's clipped signal is smaller than the input clipping level of the second stage, then the second stage never clips at all, and the effective gain of the first stage is less than one, no matter what its real gain is. It cannot make the second stage clip because its output is limited below the level which would make that happen. If the first stage's clipped level is much, much higher than the level the second stage needs to clip, then the output of the second stage will always be a hard, sharp edged buzzy signal. This is because the massive input overdrive shoots through any interesting soft knee region too fast to have any information content. The signal is always hard edged. A clipped square wave is still a square wave.

When the clipped output of the first stage is close to the level which causes the second stage input to clip, then you start getting interesting transitions of distortion on changing signal levels.

You can mess with this with different clipping stages, but the simplest way to do it is with iterated identical clipping stages, with the signal level adjusted between them all.

[Steben]

If the "last first" theory is very usefull, you can make a row of for example an opamp and 2 FET buffers each followed by pair of diodes to ground.
Opamp sees a silicon pair, 1st buffer a pair of schottkies and last buffer a pair of germaniums.

[Caferacernoc]

No, I don't think a single clipping stage can sound like a multiple gain stage amp. Or it would, you know? A rat style pedal does not sound like a mesa boogie amp even though it has a similar amount of distortion. Besides the obvious tube vs transitor argument,  I think the difference is dynamics. Multiple stage high gain preamps have more compression than say hitting a old fender bassman with a clean boost pedal. It's also the big shards of glass vs the small crushed glass preference. A Peavey 5150 has a smooth, rich, creamy, compressed and controlled sound. My cranked '70's Marshall Jmp50 has a ton of distortion, but it's still more of a crunch. The bass is looser. The whole thing is more out of control, it sounds great but it's not smooth. The sustain comes from the volume. If you pick lightly it doesn't "play itself" like a Mesa Boogie.
But back to stomp boxes. Any circuit could have more or less distortion depending on how much you overdrive the clipping stage. And you can shape the distortion tonally to be more or less fuzzy. But I think that the less stages there are the more raw and dynamic it will sound. When you use multiple stages you can carefully control what and how the sound gets distorted. I think this is how the more complex pedals and amps can have that super complex mirange distortion with a tight bass. A rat pedal can only have tight bass by rolling it off it seems. Anyway, that's my opinion. And neither way is right or wrong, it's just what your preference is. Modern or old school. But I don't think one pedal can do it all IMHO.

[R.G.]

If the "last first" theory is very usefull, you can make a row of for example an opamp and 2 FET buffers each followed by pair of diodes to ground.
Opamp sees a silicon pair, 1st buffer a pair of schottkies and last buffer a pair of germaniums.

And what would that sound like? And why?

But I don't think one pedal can do it all IMHO.

Of course it can if it's big enough. Let's say I build a pedal which has inside it clones of the Peavey 5150, Marshall JMP50, and one of the Boogies.

The whole trick is to make a pedal that gets similar sounds with less than full replication of the equipment. But it's possible to do it all with one, albeit very large, pedal. In the immortal words of William Jefferson Clinton, "It all depends on what your definition of 'is' is.".   

[Caferacernoc]

Ha ha, I love these kinds of discussions. Let me rephrase, I don't think one pedal can do it all with only 3 knobs!

[aron]

My booster+ was 3 mini-boosters in series with a level knob between each stage. It sounded good but it was just a highpass filter away from sounding terrific.

Like R.G. and others have said for years, controlling the level and the harmonic/bass content between stages is the key to series overdrive/distortion devices.

It's in here as well:

http://www.diystompboxes.com/cnews/mods.html