What is "amp-like distortion" anyway?

[Fancy Lime]

Hi there,

so a friend asked me recently, if I could build him an "amp-like distortion pedal" and I said "sure, if you can tell me what that means?". But we could not really decide what it means beyond similarly "meaningful" terms like "tube-like", "Marshally" and so on.

But it got me thinking: what, if anything apart from marketing hogwash, would you guys consider "amp-like distortion"? And: as opposed to what? Fuzz, I guess...

Thanks,
Andy

[garcho]

check this stuff out:

http://runoffgroove.com/articles.html

then check this stuff out:

https://fuzzcentral.ssguitar.com/schematics.php

See some patterns and differences?

If you friend doesn't know, then for him, this doesn't necessarily apply, but I would imagine a lot of people think "tube like" and "amp like" are synonymous. Look up some "tube like" "tube fuzz" "tube distortion" etc., schematics. Can you breadboard him something and have him listen to it?

[EBK]

check this stuff out:

http://runoffgroove.com/articles.html

I can't remember if ROG provided a link to the paper that inspired the Fetzer Valve, but I do know they missed the sequel, which is also worth reading: 
https://www.researchgate.net/publication/290534843_Triode_emulator_-_Part_2 (scroll down on the page and you will see the paper).

[dschwartz]

To me, a typical "pedal" distortion is voiced to work with the amp's preamp and it's very simple and tipically used as a boost for soloing. dist +, DS1, Rat, BMP, etc. Usually consists on a single gain stage and clipping diodes.
And "amp like" distortion would have more gain stages, a more refined tone, a tone stack or a tone control resembling a tonestack response, or generally speaking, voiced to imitate or approximate an existing amplifier..like BSIAB, distortus maximus, dr. Boogie, etc.

[cnspedalbuilder]

I am about 95% sure that your friend--if s/he is like most guitarists I know--means that they want touch sensitivity. That is, they want that sweet spot where you can control the amount of clipping with pick attack. Of course, your pedal can't do that on its own...one has to take into account the whole chain, and the amp in particular will be the final arbiter! It also depends on the typical volume levels used with that amp. With a dimed Marshall plexi, for instance, you will be in a completely different boat than a silverface Fender twin with volume at 3. The solution could be as simple as a boost pedal, like a Boss FET preamp or a Klon Centaur. 

It is also possible that your friend wants a pedal that emulates an overdriven amp, as opposed to a pedal that will push his/her amp into overdrive. That's more like what dschwartz is talking about.

Either way, you should check out the guitar rig if possible to get a sense of what you're dealing with.

[EBK]

One significant possibility:
Your friend really wants a new amp but doesn't really know it or thinks they can't afford a good one.

[GibsonGM]

Ask them what their wet dream tone is; what is THEIR definition of "amp-like distortion".  For me, it could be the intro to AC/DC "Walk All Over You".    That's not a pedal - that's the combination of whatever ODs, amp gain settings, power amp distortion that was used (as several others have said).   

They may be trying to 'get a tone', and want 1 box to do it - won't work.  Has to take into consideration their whole chain.  Now - 1 box MAY transform their chain into their dream, however!  That's where YOU might come in 

[Fancy Lime]

Hi folks,

thanks for the brain food!

@garcho
Well, I know the ROG stuff but to be perfectly honest I never found their various "amp in a box" designs awfully convincing sound wise. The usually (not always) adapt the topology of a given tube amp to use with FETs. The results sound good, no question,but not really like the amps they emulate. I mean, of course they don't, a stompbox is not an amp. Nevertheless, there is a character difference in the distortion that I am not quite able to put into words. So if the ROG designs are "really" amp-like? I just don't know. Hence my question, I'm trying to find out what the term means sonically, not so much in terms of topology.

@EBK
Thanks Eric! I didn't know the original paper. Always love to have a proper sciency explanation.

@dschwartz
Yes, amp-like tone controls rather than just a high roll-off and responsiveness to picking dynamics instead of flattening all the dynamics are certainly a part of what I would intuitively consider "amp-like" distortion. But about multiple gain stages, by which I suppose you mean multiple overdriven/distorting stages: That is something I often use in pedals to get a more dynamic sound, but a lot of tube amps seem to have just a single overdriven stage, or at least one that does the overwhelming majority of the distorting. If you look at the Sunn Model T, which must be my favourite overdriven amp tone, there is just 1) one gain stage, 2) one overdriven stage, 3) the tone control that cuts the volume so much that the following stage will hardly overdrive, 4) a recovery stage, 5) the phase splitter, and 6) the poweramp. Schematic:
https://4.bp.blogspot.com/-VMofNzunqMQ/WhwIGL0br3I/AAAAAAAAE7w/j0QHv1mpyqomaC5g0C1sUmJJQDWaWB0DgCLcBGAs/s1600/Sunn%2BModel%2BT%2B2nd%2BGen_ed.GIF
Although I am far from a tube expert, that looks to me like the second stage does almost all the overdriving. The recovery stage and phase splitter may contribute some as well but probably only when the gain control is set very high and the bass and/or treble controls are way past noon. The poweramp and powersupply are so oversized that they will only clip at insane volumes. So my question is: Are multiple stages actually involved in the way that real amps make their distortion?
If you take a look at Benoits Model T simulator, here:
https://www.coda-effects.com/2017/12/black-hole-sunn-model-t-preamp.html
There is also just one stage that does the overdriving. And I think the pedal sounds fairly convincing. Not exactly like a Model T but not too far off at least at lower gains, which is where amp distortion shines, in my oppinion.

@cnspedalbuilder, EBK, GibsonGM
So I got back to my friend. What does he want the thing for: Straight to console recording (so it'll need a cab sim) and as an additional channel on his Marshall. He likes the clean and crunch channels on the Marshall but finds the lead channel to smooth and "80's sounding" and would rather have an additional channel that is similar to the crunch channel, so he still has three sounds. Ok, I can work with that. What does he consider "amp-like" and opposed to what? "Not the typical distortion pedal sound, more organic and natural." Hm, what is his reference for a "pedal sound"? His Metal Zone. Alrighty, I think I can manage "more organic and natural than a Metal Zone".

Thanks guy, I think I know a few things I'll have him try out.
Andy

[StephenGiles]

In 1968 I was using a Selmer Selectortone and an Ampeg 2 x 12" combo, which I could not get to distort with my diy guitar. A friend suggested using a mic preamp to boost the guitar signal which I did and it overloaded the 2 amps nicely. So why not try a mic preamp?

[Mark Hammer]

"Amp-like distortion" could variously mean:

- pre-amp distortion
- power-tube distortion
- output transformer distortion
- speaker breakup
- some combination of two or more of these

I think until the specific elements are identified as the targets, it is difficult to make any recommendations.  I will note that the many excellent ROG emulations are specifically pre-amp designs.

Supro has recently begun to include transformers in some of their overdrive/coloration pedals, but that tends to be the commercial extent of attempts to mimic amplifier behaviour more explicitly, apart from digital modelling.

[GibsonGM]

What I like to use for direct recording, for 'amp tone', is my GTFO tube preamp (search on the forum, you'll find it).    That goes to whatever 'tone control box' I am in the mood for at the moment.   Basically, I will feed the output to a marshall "contour" control for hard stuff, or a FMV-style for more classic sounds.

I'll combine this with OD at the front end if needed, or to fatten up a strat when using less preamp distortion.  The GTFO has an 'active high control' a la Merlin's preamp book, so I can boost or cut highs in the last stage, too.

This goes into my computer via a multi-effects pedal w/USB out (set 100% clean).    I record this way, and then use a cab sim in the software.  That does most of the heavy lifting, given that there are like 5,000 impulses to choose from... 

If I need to monitor, I will split the clean signal from guitar and run that to an amp that I set at a decent level to hear over what's coming at me in the cans....

At times I'll also run the line out from my 18W Marshall >>usb > Computer and just record that way.

Maybe something in there is what your friend is looking for?

[dschwartz]

If your friend wants to git straight to the board from the pedal, then of course it has to sound like an amp.
The cabsim part is the most important for a convincing amp tone. You can use a RAT circuit, plus an amp tonestack, and a good cabsim and you'll have something pretty "ampy"

[merlinb]

IMO 'amp-like distortion' varies its clipping threshold depending on the envelope of the signal. This helps to preserve the dynamics of the signal and can create a 'swirl' effect as the duty cycle changes and shifts.
As opposed to 'pedal-like distortion' which normally has a rigidly-fixed clipping threshold set by a couple of diodes, creating a constant-amplitude signal and no swirl effect.

That said, one of the best 'amp-like distortions' I have ever heard is the Sans Amp GT-2, which is purely TL072 opamp clipping!

[Fancy Lime]

@dschwartz
Yes, I agree, cabsim is going to be very important. Does anyone happen to have a large collection of analyzed frequency response graphs of different cabs with different mikings for reference? Hm, would be nice wouldn't it. So how do I go about designing a cabsim if I don't want to just copy one (just copying things without properly understanding them always makes me feel a bit yucky)?

@merlinb
I always wondered how they managed that with TL072s or if that was just an error in sche schematics, because wehenever I played around with opamp clipping, TL072s were by far the worst sounding. I mean, absolutely unusable rubbish, nails in a blender, dentists drill on a chalkboard. NE5532s sound nice, as do most BJT opamps, actually. TLC2262 rail to rail mosfet opamps sound great when kicked into clipping. But I never got the TL072 to sound anything other than awful. Maybe I need to take another swing at that some time, keeping in mind the GT-2. It's all in the tone filtering in the end, I guess.

Andy

[Ben N]

"Amp-like distortion" iw whatever the pedal builder is trying to sell.

[Rob Strand]

Does anyone happen to have a large collection of analyzed frequency response graphs of different cabs with different mikings for reference?

You can find impulse responses on the web.   If you do an FFT on the impulse you can get the frequency response then plot the graph.    The problem with these is you don't know what was measured or how they were measured.  They could be corrupted by overloading or noise.  (I've seen many frequency responses on the internet where clipping has clearly emphasized the high frequencies.)

There's also the issue of what exactly you are trying to capture in the impulse (and that impacts how to measure it in the first place).  If you just want the frequency response of the mic + speaker then you would test with the speaker driven by a solid state amp.  However, the interaction between the amp and speaker is part of the overall frequency response.   So maybe you should test with a tube amp + mic + speaker.   The question is do you want to capture the low and high roll-off of the amp?  And if you just want to plug into a box and make it sound like an amp should it include some mid cut representing the tone controls?

See reply #122, and #123 on page 7,
https://www.diystompboxes.com/smfforum/index.php?topic=106185.0

A lot of cab sims have the low-end peak much lower than a real speaker in a box.   That comes about because of the mic + proximity effect.

[ElectricDruid]

He likes the clean and crunch channels on the Marshall but finds the lead channel to smooth and "80's sounding" and would rather have an additional channel that is similar to the crunch channel, so he still has three sounds.

For that part, I'd be looking into modding the Marshall. Partly because I'd love to mod a Marshall amp, I admit. But surely a couple of well placed tweaks could prevent the poor beastie from sounding too 80s?

[bartimaeus]

I may be wrong, but it seems like most people looking for an "amp-like distortion" have a clean amp (probably a Fender) and an overdrive (probably a TS9) and wonder why turning on the overdrive doesn't feel like switching to the dirty channel on an amp. Usually a Timmy is what they're looking for, something with decent sensitivity that doesn't color or compress the signal too much. Either that or they want a MIAB.


Of course, we're definitely at a point where companies will call anything amp-like, so it's getting increasingly muddled...

[merlinb]

Usually a Timmy is what they're looking for,  Either that or they want a MIAB.

What's Timmy and MIAB

[bool]

Timmy=simplified Tim(2x pn-drop in a nfb loop); MIAB=marshall-in-a-box.