Pros and Cons of OD circuits

[uneumann]

There’s no right of wrong on this topic, since it’s largely a matter of personal taste, but certain circuits have become classics for generating preamp distortion and therefore we can conclude that a good number of people like them. I’ve personally played around with a lot of OD circuits – mostly preamp circuits - and it may help to group them into categories. I’m including SS and tube methods since both appear in pedals and amps.

I know there are also output stage OD preferences, but I don’t quite know how to categorize them other than fixed-bias or cathode bias. Comments and suggestions on this are welcome.

I’ll admit that there are no hard rules about this, but a thread where people add comments and suggestions about what works for them (or things to avoid) would be helpful. I’m starting this with my personal views and current favorites (marked with *), but I really hope others contribute since I for one am really curious about other views.

DCCF: *
Pros: probably the most common and popular OD circuit. Sounds good for low and moderate drive.
Cons: At high-drive it loses high end due to clipping and it can block badly if the input cap is too large.

Cold bias unbypassed stage:
Pros: provides modest and slowly increasing distortion as drive increases. Rarely blocks.
Cons: Audible amount distortion is low due to the unbypassed cathode.

Cascaded common cathode stages:
Pros: produces high-gain, clipped output. This can produce high-distortion metal tones and amp feels.
Cons: challenge to dial in EQ and coupling between stages. Noise is difficult to control.

Diode clipper: (back to back (BB) diodes shunting signal to ground)
Pros: passive and inexpensive. Capable of moderate distortion.
Cons: sensitive to drive level and at high drive it loses high end due to clipping.

BB Diodes around feedback of inverting opamp:
Pros: Capable of moderate clipping distortion.
Cons: sensitive to drive level and at high drive it loses high end due to clipping.

BB Diodes around feedback on follower opamp: *
Pros: crossover distortion peaks at moderate signal levels. Distortion reduces at lower and higher signal levels.
Cons: limited headroom

BB Diodes around resistor to ground on follower opamp:
(never tried this but seems feasible)

Others?

[Helmholtz]

I wonder why you feel that clipping causes loss of high end.

[uneumann]

I wonder why you feel that clipping causes loss of high end.

Good question. Some of this is my subjective observation, so I'm open to arguments. I've come to understand that there are two things (at least) that happen with clipping.

1 - High harmonics are generated - so you could argue that there are more high freq's at the output. This is true, but the high harmonics all relate to freq that is clipping. It's basically a fundamental and it's harmonics.

2 - When you pluck a string, there are little wiggles at the peaks of a waveform going into the clipping stage. These are harmonic and non-harmonic to a fundamental. Zollner helped me understand this. These string harmonics (or partials) are significantly reduced due to the clip flattening them. There is less of them getting to the output. The same happens when you hit two notes - or a chord. The lower bass notes typically dominate the amplitude of the waveform going into the clipper so the high notes (wiggles with lower amplitude) tend to be lost due to clipping. I'm not saying they are all lost, it's just that they tend to be suppressed. This leads to boring OD (IMO) where the tone is monotonous. This also relates to the reason why bass is cut before an OD stage. You try not too overpower those string partials.

[Helmholtz]

Interesting, thanks.

I tend to prefer symmetrical clipping. It gives me a punchy attack and most importantly cleans up nicely as the note decays.
Apart from that I like the compression/sustain involved, but actually don't like to hear much fuzz.

[uneumann]

Interesting, thanks.

I tend to prefer symmetrical clipping. It gives me a punchy attack and most importantly cleans up nicely as the note decays.
Apart from that I like the compression/sustain involved, but actually don't like to hear much fuzz.

Do you have a favorite OD circuit that gives you what you like?

[Helmholtz]

Do you have a favorite OD circuit that gives you what you like?

I love a JTM45 and the distorted (though not the clean) sound of a Marshall 2203.
So that would involve the DCCF.

And I like a Tubescreamer or a Zendrive with a Fender BF amp.

Of course there's always some power amp distortion involved as well.

[Chuck H]

There are plenty of newer distortion circuits that sound (to me) like food processors, for lack of a better analogy. The notes are rendered into an amalgam of harmonics that can only equal the root intention. Almost a synthesizer tone. Though it does allow for a lot of right hand technique, which is fun. But the loss of high end noted is a result of the blender effect. Some amps do it better than others, but the leveled playing field for frequency dynamics makes it hard to cut through with pick attack alone. I'm speaking strictly about the uber gain monsters of the last fifteen years. Though the extremely "cold clipped" stage is largely responsible for this sound and that was being done by Soldano over twenty years ago.

I think the list and considerations are not comprehensive. Tone is such an amalgamation of the arrangement of circuits and how they react to each other that any of the above aspects can be included in a "good" clipped tone. Though not necessarily all at once as some designs have done. And again, this is all subjective, right?

I like a saturated tone. I also like a tone to be dynamic enough that as a player I can express myself without being stomped down by compression. And I really don't think any one circuit is responsible for the better or worse of either. It's all in the combination and even more how a given player responds to what musical properties are available.

For myself I'll agree with Helmholtz that I prefer a little (or a lot of) power tube clipping. Though I've played enough well designed amps that are designed around preamp clipping to say that it's not a necessity. I've also designed and built a few amps that do something good both ways. As good as the best? Maybe? Subjective.

What I CAN do is say what's worked for me personally and that may not be anybody else's opinion. But here goes...

An early 80's RAT into a 1959 is a hard act to beat.

I have a design that's very similar to what's known about the TrainWreck circuit (which includes a cold clipped stage) that I dearly love.

For years I played through, and was happy with 50W Marshall JMP master volume amps with a boost pedal. I liked that tone well enough though it wasn't my favorite. But I have to say now that I got more positive feedback on my tone with that rig than any other I've ever played through.

I think the Peavey Rockmaster/Ultra 60/120 series amps had exceptional overdrive. So did my early Fender Prosonic. These are both strictly preamp clipping designs and I liked them.

What I notice looking at the above list is that everything you mention above is included in one way or another in each scenario. So what does that mean? There's more than one way to get there. And you only know it when you arrive.

[Enzo]

When I was on the road in the 1960s all we had was a basic Oster blender. But now days when I want tone, I can chose from Ninja, Foodie, and of course MAgic Bullet. Modern food processors can do many things the old Oster could not.

[loudthud]

BB Diodes around feedback on follower opamp: *
Pros: crossover distortion peaks at moderate signal levels. Distortion reduces at lower and higher signal levels.
Cons: limited headroom

I assume you mean non-inverting gain stage like the Tubescreamer. This circuit has an interesting defect, it always adds a replica of the input signal on top of whatever distortion is created by the diodes. It sounds ok in a narrow range of signal levels.

I think for good overdrive you need to cut or limit bass before the circuit, then boost treble after the circuit but not so much that you get the fizz sound.

[pdf64]

I assume you mean non-inverting gain stage like the Tubescreamer. This circuit has an interesting defect, it always adds a replica of the input signal on top of whatever distortion is created by the diodes.
…

I think one reason the straight sound out of a TS etc is somewhat naff, especially with higher output pickups, is that that buffered input signal that appears at the output, is then passed through a low pass filter & tone control. So it’s a muffled version of the input, with the clipped signal sitting on top of it.

It may be worth noting that the 2nd ‘cold clipper’ stage of a typical 2203 / 2204 doesn’t tend to actually clip much, unless the guitar signal is boosted, eg with a TS.

For a smooth transition into mild overdrive, I think it’s worth noting that cascaded common cathode stages whose clipping stages have unbypassed cathodes (eg 2203 / 2204) may be worth putting in a separate category. Of course the massive treble boosts in those circuits tends to work counter to smooth, mild overdrive.

[Helmholtz]

I think one reason the straight sound out of a TS etc is somewhat naff, especially with higher output pickups, is that that buffered input signal that appears at the output, is then passed through a low pass filter & tone control. So it’s a muffled version of the input, with the clipped signal sitting on top of it.

You're right that the cleanish sound you get with a TS when rolling back the guitar volume is rather muffled (part of this is due to the cable capacitance anyway).
But that's more or less common with most OD or distortion pedals employing LP filtering after the clipping stage.
Probably this effect can only be completely avoided with a distorting single channel amp like a JTM45.

I can't see why the small added unity gain clean signal in a TS should make a difference here. The way I see it, the clean content just lifts the threshold for clipping a bit.
After all any clipped signal has more or less clean signal content.

BTW, an 80s RAT can sound great, but only when used with a buffer before it.
Otherwise the input capacitance of the RAT of 1nF (!) adds to the cable capacitance and makes the sound dark and unusable for me.

Of course the pre and post filtering always has a strong influence on the distorted sound.

[uneumann]

I assume you mean non-inverting gain stage like the Tubescreamer. This circuit has an interesting defect, it always adds a replica of the input signal on top of whatever distortion is created by the diodes. It sounds ok in a narrow range of signal levels.

I think for good overdrive you need to cut or limit bass before the circuit, then boost treble after the circuit but not so much that you get the fizz sound.

Interesting comments from everyone. The variety of perspectives is appreciated. I violently agree on many points - especially the need to EQ before the OD stage (cut bass) and the systems nature of all of this.

As for this circuit, I think of this "defect" as an asset. (Just my personal view.) What I find interesting and useful about this circuit is that while many OD circuits introduce harmonics by clipping or compression, this one introduces a form of cross-over distortion. At low input levels, it's clean since the diodes aren't conducting yet. At moderate input levels the diodes start conducting and the crossover distortion kicks in. As the input gets larger, the crossover is still there, but the output continues to follow the input level so the % distortion actually decreases and you don't hit that completely saturated and compressed behavior.

This is very different than clipper behavior. The two are complementary and that's potentially worth exploiting (IMO).
Just for reference and making sure we're talking about the same thing, the circuit below is an example.

[Helmholtz]

As for this circuit, ....

Can you show pics of distorted signals at different levels and frequencies?

Not sure if I like the sound of crossover distortion. But I haven't experimented with that yet.
Maybe I should bias my amps rather cold?

[uneumann]

Can you show pics of distorted signals at different levels and frequencies?

Not sure if I like the sound of crossover distortion. But I haven't experimented with that yet.
Maybe I should bias my amps rather cold?

You won't see much in the wave shapes. If you like output stage distortion, you probably like crossover distortion. Cathode bias output stages in particular are prone to cross-over distortion (CD).
Most people think of CD in the hi fi context where it's an annoying low level grainy effect. In guitar amps it's different, it only becomes prevalent at high signal levels where bias shifts create it. If you look at most full power waveforms from guitar amps, you'll see a small discontinuity at the zero crossing. That's it. It often start before clipping, and only gets more pronounced with clipping. It contributes harmonics to the output. If you bias cold or warm you alter the point where CD starts. It depends on what you like.

In the circuit I posted, the crossover distortion is different in that it's a vertical stretching of the zero-crossing area of the waveform with compression at the tops. Basically the diode network produces a nonlinear response that stretches the low level (zero crossing) area of the waveform. The pic below shows input/output of the stage. At low levels, it's linear with high gain. The first kink reduces gain when the first diodes conduct. The second kink is where the second diodes conduct and gain is pretty linear from there. So the portion of a waveform (around zero) that falls into the nonlinear area is what's distorted.




A clip stage, on the other hand, is linear up to clipping and then the response flattens out (becomes horizontal) from there. Those are two very different behaviors.

[Helmholtz]

You won't see much in the wave shapes.

I think I might, please post them. Different signal levels, different frequencies.
The only thing I couldn't see in sine response would a dynamic or f-dependent effect.
But feel free to use a periodic triangular signal.

If you like output stage distortion, you probably like crossover distortion. Cathode bias output stages in particular are prone to cross-over distortion (CD).

I like to bias my amps for almost straight slopes with a triangular signal at full output, so no essentially CD.

In the circuit I posted, the crossover distortion is different in that it's a vertical stretching of the zero-crossing area of the waveform with compression at the tops.

I'm not sure what you mean here (maybe language barrier).

And please explain the picture shown.