The Wattage Conundrum - My Take

It's all an interesting discussion in any case. This business of how waveforms get distorted in a particular amp is top of mind for people who model such things. The designer of an amp that attempts to duplicate a tube guitar amp has at least a couple of choices on how he / she might want to do it.

One type of modeling amp tries to take a look at all aspects of the original amps design, warts and all...then tries to duplicate that with solid state circuitry (not digital circuitry...analog transistor circuitry). This is how it was originally done, and it is exceedingly hard to get right. In principle, you can take a gain stage for example, figure out exactly what is going on, and there is no reason on earth that you can't duplicate how that gain stage is working by adding the sorts of non linearity that the original design had. The problem is that there is so much feedback amongst components, in even the simplest amp designs that it's near impossible to get right. Just too many variables to account for that aren't represented in component data sheets. So, you try to measure away and hope that you are accounting for as much as you can. The results are usually less than pleasing.

For the other type of modeling amp, the designer takes the attitude that he doesn't care if the amp is composed of transformers, tubes, leaky capacitors, inductors and resistors, or if the amp has little green Martians inside making the amp do what it does. It treats the amp (or the amp-speaker combination) as a black box. The modeler measures the input / output relationship of the black box under a variety of input conditions to the point that an accurate representation of the original box can be made. This requires all sorts of digital signal processing, so it is by its nature a digital (sampled data) thing. So for those of you who can hear the digital nature of a CD for example, you'll hear the same thing with these. But these things do work surprisingly well, and they have the advantage that a single amp can model any amp that you want. They do (in my opinion) a very decent job of including all of the power supply / transformer types of things that are hard to accomplish with an analog model, to the point that I find it hard to tell in a recording which is the original and which is the modeled amp. Live, I do think that I can feel a difference. I attribute it to the fact that the modeled amp isn't developed from my particular crappy playing, and no model has infinite fidelity, so I can just feel the difference. The amp doesn't respond to me like the original. Or maybe it does, and I just think it doesn't. It's pretty close in any case.
 
Totally agree, Aristotle...there are some that do an amazing job. You have to leave your "purist" hat at home, but you'll be happy you did if you want to evolve your tastes. I might use the word amazing.

Things have changed considerably from the day when TI and Fujitsu were the only players in the DSP game. Even working on slick military applications, those little (OK, they really were huge, relatively) complex ICs made some incredibly cool hokus-pokus possible. Those are the projects we have to thank for today's music applications. From combat avionics to Marshall Plexi emulation...amazing.
 
Thanks... I should have mentioned why I went on this tangent in the first place in Les' thread. My original point (that I forgot to make) is that the amp modelers go to great pains to emulate the pre-amp / power amp relationship in a traditional tube amp. For the "bedroom" player, this isn't ideal. It strikes me that there is room in the world of modeling amps to satisfy the home player by actually deviating from the accurate model in a way that keeps the general feel of a high wattage power amp while skewing it somewhat at lower output SPLs. There may be people doing this, but I'm not aware of them...
 
How is the Kemper not doing this? You can turn the output down -- which is pretty much straight -- and you get everything except the speaker break-up at lower volumes.
 
Les, I agree completely and was just pointing out the different ways amps can be designed to sound the way the designers wants them to not only sound, but feel and respond. Part of my point that I may have missed, was that the things I was discussing don't just apply to small amps, there are big glass amps that do the same thing. All these factors go into what makes an amp the way it is.

There's a really good thread about this at The Gear Page right now, with more than a few amp builders weighing in. Pretty good stuff if you are into it. To me, the cool thing about it is, whether you are into it or not, reading in one of these discussions that goes into more details, gives you an idea as a player of what amps that you haven't tried, that might be good for you.
 
Les, I agree completely and was just pointing out the different ways amps can be designed to sound the way the designers wants them to not only sound, but feel and respond. Part of my point that I may have missed, was that the things I was discussing don't just apply to small amps, there are big glass amps that do the same thing. All these factors go into what makes an amp the way it is.

There's a really good thread about this at The Gear Page right now, with more than a few amp builders weighing in. Pretty good stuff if you are into it. To me, the cool thing about it is, whether you are into it or not, reading in one of these discussions that goes into more details, gives you an idea as a player of what amps that you haven't tried, that might be good for you.

You're so right!

Appreciate everyone who is weighing in on this.
 
For the other type of modeling amp, the designer takes the attitude that he doesn't care if the amp is composed of transformers, tubes, leaky capacitors, inductors and resistors, or if the amp has little green Martians inside making the amp do what it does. It treats the amp (or the amp-speaker combination) as a black box. The modeler measures the input / output relationship of the black box under a variety of input conditions to the point that an accurate representation of the original box can be made.

There is another way that's becoming popular in the studio plugin world, that was (as far as I know) first done by Fabrice Gabriel for Slate Digital. Instead of taking the input-vs-output-only approach, which is what companies like Waves originally did, they model the individual components in the device that's being modeled, how the individual components affect the signal, and so on.

This approach has subsequently been taken by Brainworx, a German company that makes modeling plugins, and also by Universal Audio, who've got a team headed by one of the profs at UC Berkeley, doing the same thing.

From several years of first-hand experience, I can attest to the fact that these plugin models sound significantly more authentic than the black box approach you've described. Some of them sound downright amazing. In comparison, the black box models sound very two-dimensional. If, for example, you compare the Waves Pultec model with the Universal Audio Pultec model, you will hear a significant difference in the dimensionality of the signal. It just sounds better. More real. Almost as though you're listening at a higher digital resolution.

I have a Slate plugin that models analog tape. It does exactly - and I mean so close that I wasn't able to tell them apart - what my $15,000 former analog mastering machine did to the signal. Their model of the input section of a Neve console was so dead-on that I was able to sell my Neve summing mixer (and buy a pretty nice guitar! ;)). For a dyed-in-the-wool analog studio gear person, that was pretty amazing.

However -- and this really amazes me, because I simply do not understand why this should be the case -- Brainworx and Universal Audio have also recently modeled several guitar amps. Because I have certain bundled products, I have these recent releases. And while they've improved on earlier releases of guitar amp models, they still sound papery and two-dimensional. I would ordinarily add the disclaimer "to me, YMMV," but the fact is that I know I'm not hearing things. Compared to the rack gear processor models, they simply suck.

I think in part that there is something to the notion that signal processing from a piece of rack gear is conceptually different from gear that generates the original musical signal. We aren't really supposed to hear much from a signal processor, like a compressor or an EQ. The idea of the original hardware was to subtly affect a signal, not generate a musical signal in the first place.

But a guitar amp generates a musical signal. It's essentially operating as part of the instrument, the sound production chain as opposed to the reproduction chain.

In reproduction, I think I'm able to tolerate alterations to the signal that I am simply unable to tolerate in a piece of gear that creates the musical signal in the first place. And I think it's more difficult to hear problems in, say, a modeled EQ than in a modeled guitar amp.

People have asked me, "If you can use plugins on your DAW, why can't you use a modeled guitar amp?" Well, of course, I can use a modeled guitar amp, and I've done so. Every time a new one comes out, I try it at the very least, and as I said, some of my bundles simply include these models as they are introduced.

But I don't use them in my final tracks because I think they inhibit what I'm trying to accomplish, they don't help it. On the other hand, I feel much more secure in my conclusion that what I'm hearing with recent signal processing plugins is acceptable, if not downright good.
 
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We hosted a Super Bowl party for 30 friends (including 3 guitarists and 1 drummer), had a fun time, all cleaned up, so getting back to the web now. Will try to provide a better explanation for Les.

The THD versus input level plots that Les showed last Friday are not tube transfer functions. THD plots are an AC measurement of an amplifier circuit, usually with a 1KHz sine wave audio signal. Tube transfer functions are a DC measurement of a tube only (no audio signal), where the input is the grid voltage and the output is the plate current. They are found in all tube data sheets, a 6V6 example is shown below.

6V6%20Transfer%20Function_zpsqizklqan.jpg


Each line represents a different grid voltage ("Ec1"). A change in Ec1 changes the plate current ("Ib"). So Ib = F(Ec1) where F is the tube transfer function. For F to be linear (no distortion), the lines should be 1) straight, 2) parallel, and 3) equal-distant from each other. In the 6V6 example, this is not the case so we can expect some distortion at normal signal levels. These imperfections in the tube transfer function are the first distortion type. This distortion warps the waveform, but it does not chop the peaks off because there is no clipping yet.

The second distortion type is when the tube plate hits the power supply rails. This is the clipping seen at high signal levels - either the plate voltage goes down to zero or up to the B+ voltage. It can't go any farther, so the peaks are chopped off the output signal. At these extremes, it doesn't matter whether F was linear or not, because the tube is operating "off the chart" at the X or Y axis of its transfer function. The THD distortion will "go vertical".

I believe there are two types of distortion because they have different harmonic effects on the waveform caused by different physical mechanisms. Using negative feedback circuits can reduce the first type, but not the second type. At low signal levels the first type dominates, and at high signal levels the second type dominates. With guitar playing dynamics, both types may be present at the same time.

I think the first (tube) THD plot Les shows is only the first distortion type at normal signal levels, because the THD % is too low. The shape of the distortion plot also matches the tube data sheets at normal signal levels. If the tube had hit the power supply rails, the THD % would be much higher because of clipping! For example a high gain amp which saturates a sine wave into a square wave could have a theoretical THD = 48.3%. By increasing the input signal level, the first (tube) THD plot will go vertical just like the second (transistor) THD plot.

I did not say the Champ behaves differently when it clips. I just said "the combination of a soft output tube, soft output transformer, and soft power supply allowed sweet saturation without harsh clipping". I do believe its power transfer efficiency is pretty low, which means soft clipping. One trick is their use of higher primary impedance output transformer than the tube data sheet recommends. A transformer company recently reverse-engineered a 1958 Champ 5F1 output transformer and measured a 17,000 ohm primary impedance. This puts it off the chart below, enhancing the 2nd harmonic over the 3rd harmonic. Very soft indeed!

6V6%20Load%20Resistance_zpsa2uhbul9.jpg
 
Thanks, Kred! I really appreciate the explanation and clarification.

It's great to have folks on the Forum who can explain this material in detail.
 
"It was a Fender Champ with a whammy bar!" Wow there was a lot of detail in this thread and it was very interesting to read.

I was in the low wattage camp for years when I had Strats and p90's. which sounded really great with my low watt amp. Changing to PRS with some fairly hot pickups killed that idea pretty quickly. I think my go to rig will be a an archon 50 with the 25 watt step down and a chorus pedal to split the signal to the low watt amp for effects.
 
"It was a Fender Champ with a whammy bar!" Wow there was a lot of detail in this thread and it was very interesting to read.

I was in the low wattage camp for years when I had Strats and p90's. which sounded really great with my low watt amp. Changing to PRS with some fairly hot pickups killed that idea pretty quickly. I think my go to rig will be a an archon 50 with the 25 watt step down and a chorus pedal to split the signal to the low watt amp for effects.

That sounds like a cool idea, actually.
 
I have been on both ends of the spectrum, I have gigged with a 100 watt Orange amp and with a 5 watt Bad Cat minicat II and both approaches have there pro & con but I have settled on a 40 watt Fender Super Reverb as my current main amp because it has enough headroom, love the tone of 4x10's, and I like to tilt it back on it's legs on stage to project the sound up. As others have stated, 30-40 watts seems to be where people like it.
 
..a chorus pedal to split the signal to the low watt amp for effects.
I used to do the same thing but with big amps...the Boogie into an old Digitech RDS1900 with the out of phase output into my Twin head. Thru the old Boogie 2x12 cabs, it was a Neil Schon clone machine. At the time, it was epic but now it's way too big.

Incidentally, I've gigged a Marshall Class 5 with the MkIII with mixed results. There's just ZERO headroom in that little 5 watt amp.
 
Doesn't the cabinet also have an impact? I drag a dual rectifier cabinet around because I believe the "thump" makes a difference. However at many gigs I go to the guitarist uses a small amp miked to the pa. The pa is providing all the head room and the amp is essentially only a bit more than a monitor. Hmm a lot to learn about amps.
 
Doesn't the cabinet also have an impact? I ...small amp miked to the pa. The pa is providing all the head room and the amp is essentially only a bit more than a monitor.
In one scenario, yes. But if you're playing in, say, a quintet that only runs certain drums and vox thru the PA, everyone else has to fend for themselves...self amplified. Drums and vocals set the performance level and the others have to self govern to get the mix correct. Dynamics become a tremendously important aspect of the band. So, having an amp that can be cleanish and climb over 2 other guitars and a bass is important and not a job for a small amp.
 
So why would you only run vox and drums through pa when you can run your low wattage , light weight amp through the pa and use just one pa channel?

I think this is a question more and more guitar players ask themselves. It's not my current choice because I like the sonic variety I get from the three channels in the mkV and also the thump from the big cabinet, but I think I am increasingly in a minority.
 
Can't speak for Boogie, but for club gigs where you're playing through the typical small club PA (or the typical PA that you bring yourself) when you mic guitars through the PA, with vocals, and a touch of drums and bass, it's just too mushy. In my experience, so long as you can work the mix right, it sounds way better to have the main guitar sound in club gigs coming from the actual amps.
 
Thanks. That's helpful and comforting that at least when I lug all the weight around there may be some benefit.
 
So why would you only run vox and drums through pa when you can run your low wattage , light weight amp through the pa and use just one pa channel?

I think this is a question more and more guitar players ask themselves. It's not my current choice because I like the sonic variety I get from the three channels in the mkV and also the thump from the big cabinet, but I think I am increasingly in a minority.

Can't speak for Boogie, but for club gigs where you're playing through the typical small club PA (or the typical PA that you bring yourself) when you mic guitars through the PA, with vocals, and a touch of drums and bass, it's just too mushy. In my experience, so long as you can work the mix right, it sounds way better to have the main guitar sound in club gigs coming from the actual amps.

Technically, a Big A hit the nail on the head. The other factor for me is that I play in under- budgeted bands with complete sh*t for PA. I'm so sick and tired of old Peavey boards, amps, and speakers I could throw up! So, I know that MY gear will make me sound way better than if I were thru FOH. Now, you might say that I could solve this problem by buying some decent gear and quit complaining. Valid point. But then I'd own the PA and would have more stake than just the guitarist that also sings some backup. I'm not managing or running another band, thank you. /rant :D

Back to topic...on one hand, I control my tone, and on the other, I get to keep my amp in its sweet spot by playing loud enough to project. Plus, we all have to pay more attention throughout the gig, adjusting our personal volume for the sake of the mix. Keeps us on our toes.
 
In part from the info in this thread which caused me to experiment, I've downsized my own wee home rig, losing an amp.

Out went the Vox AC4, and I put the Celstion Blue it had into the Egnater Tweaker in place of the G12H speaker it came with.
Now I have that amp working better at lower room volumes, and the blue makes the AC type tone stack do a good flavour of a Vox clean sound.
 
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