Question - Cab Sims Theory?

[motterpaul]

Now I know what a cabinet simulator is used for - basically to eliminate the need for a speaker cabinet or even an amp in many cases.

But what I don't know is the electronic theory behind them, and I can't seem to find it anywhere. Is it merely an emulated amp circuit shaped by RC filters to simulate the frequency response of a guitar speaker?

Does the amp section then typically have an AIAB preamp followed by rectifier diode configs? Or do these pedals even have amp sections?

Just basically - what is the electronic theory behind the Tech 21 (for example) cabinet simulator?

[induction]

Most common DIY cab sims (Condor, etc.) are nothing more than active EQ's that approximate the frequency response of a given guitar speaker/cab. They do not rely on physical modeling of the speaker, they simple try to reproduce the response that results from the physical limitations of the speakers. Passive EQ's are not strong enough to produce these filters, so you can't generally dial them in with a TMB tone-stack. Any circuit billed as a speaker sim will probably not have a AIAB preamp included, but many AIAB pedals will have a switchable speaker sim included for use in direct recording or feeding a PA.

Another type of cabinet sim I've been studying lately is the (e.g. Marshall) speaker reactance emulator. These are also active EQ's, but instead of mimicking the response of the speaker cab, they mimic the interaction between the gain of a tube power amp and the speaker impedance. Here's a good article that explains the details. (Short, oversimplified version: guitar speakers have frequency-dependent impedance: a high-impedance resonance feature at ~100 Hz, and rising impedance with frequency. The gain of a solid state power amp is inversely proportional to speaker impedance, while the gain of a tube power amp is directly proportional to speaker impedance. Thus a tube power amp will give more bass and more treble than a solid state power amp when given the same input signal. This partially explains the tonal differences between solid state and tube amps, and explains why a tube amp will usually be louder than a solid state amp with the same power rating.) These types of sims can be used in conjunction with the above cab sims for direct recording, or can be used without a cab sim for reamping. (Beyond amp sim pedals, you can use a resistive dummy load in parallel with a voltage divider to get line level signal from the OT secondary of a tube amp, but the gain/impedance interaction will be lost and the signal will lack bass and treble. Using a speaker reactance sim on the line level signal adds that interaction back before you send the signal to a recording input or to an SS power amp. Using a reactive dummy load instead of a resistive one removes the need for the reactance sim.) Some 'tube voiced' SS amps also include this kind of internal filtering to add tube realism to the tone.

Neither of the above types of sim account for the compression that results from overdriving a guitar speaker. That can be simulated as well. All three of these types of sims could be put in one box as an all-in-one direct recording or reamping box.

For recording or direct-to-PA live work, software-based cabinet impulses have a better reputation than analog cab sims. The impulses are calculated from recordings of real cabinets/mics/rooms, so they are better approximations. They capture the differences between different models of cabinets and different mic placements, and sound more detailed and realistic-sounding than analog filters. Software abounds for creating and using your own impulses, and there are plenty of prepackaged impulses available for download.

[motterpaul]

Induction - great answer, thank you. I wondered how far an analog circuit could go towards reproducing the sound of a speaker beyond basic EQ filtering. It makes them kind of a one-trick pony - although it can be a really good trick.

Still, this makes me appreciate my tube amp more than ever, even though some of those cab sim pedals really do sound pretty good. I guess it is kind of like the fact that once I get the right amp/tubes/speakers and tone settings in my tube amp, I tend to stick with the settings I have and not vary anything much. Based on that, the right cab sim can do a pretty decent job.

[Silver Blues]

You just gave me a hell of an idea, man

[motterpaul]

Silver - I know, it really gets a person thinking, doesn't it?

These cab sims are "hot" right now - yet they are kind of "magic boxes" where no one really knows exactly  how they really work.

If it is just basic EQ - a person could put an amp in a box, add a bridge rectifier to push a pair of transistors (in place of power tubes) and sum it up with a speaker simulator.

I remember there is a circuit in the blog that had a 4-diode bridge rectifier that gets a certain sound. Miro calls it "crossover distortion"

http://tagboardeffects.blogspot.com/2014/01/the-tone-god-blade.html

[Silver Blues]

Yeah crossover distortion is a well-established concept. I've experimented with it a bit myself and it sounds just nasty (whether that's good or not is up to taste I guess but it's a really dirty and angular sound).

I've got a little mixer running into my computer and my firs thought was combine an amp-in-a-box preamp like the Ginger with one of those reactance sims and maybe also the compression sim as a sick DI box for bass into computer. (Don't do anything fancy or professional so don't need fancy equipment either.) Beats trying to bring over my amp head every time I want to get decent tone and I could even put a balanced output in it to have a line-level signal via XLR.

[strassercaster]

[strassercaster]

This is an awesome thread questions and answers very deep and though provoking. I will have to read/reread this several times but I have a question. I have played with several speaker simulators not to mention line 6 pod and the like. For me nothing seems to replicate the air moving. Is there any cab sim that really nails this aspect. For example long ago when i record a heavy breather singing i sometimes edit out the breaths. for a couple of projects i felt this was essential. looking back and listening now it really steralized and dehumanized the tracks. there is something about the feel of a speaker  moving the air into the microphone. I have some great mics vintage jmp 50w and 100w heads that sound amazing in the room but I havent ever been able to get them captured to satisfaction. i have just used my line 6 pod out of laziness but i really dont like it because mostly its seems sterile and not moving air is a big part of it IMHO. havent stayed up to date on whats out there now anything thats got it all?

[motterpaul]

Thank you for complimenting my question

But it was Induction's great answer that made it.  

Anyway - I know what you mean about moving air. I am pretty sure Induction is looking at this with his new project because he said he is analyzing the Marshall amp/cab interface.

I would think, though, it isn't that hard. Like he said, Tube Amps will reproduce a wider freq. range than SS amps, which basically means that low end punch comes from loud tube amps. I'll bet you a well placed LPF (low pass filter) would do the trick - something with a center frequency of about 80 to 100 Hz.

[motterpaul]

Silver Blues wrote

Yeah crossover distortion is a well-established concept. I've experimented with it a bit myself and it sounds just nasty (whether that's good or not is up to taste I guess but it's a really dirty and angular sound).

I think I made that circuit too, but a long time ago. The funny thing is that a bridge rectifier is nothing like a crossover. Maybe that is what they call it is pedal theory, but a rectifier is not supposed to affect the sound, all it does is change AC waveforms to DC so the output tubes can amplify the signal. Then the Output Transformer changes the signal back to AC so the speakers can push it out.

There is no crossover in a guitar amp. And any clipping or overdrive you get beyond the rectifier happens in the output tubes (usually only when the amp is on full) - not in the rectifier. The "original" Marshall sound was all about cooking the output tubes, plus the fact that they changed the first preamp tube to an ECC83 instead of an ECC81 (i think, anyway, a 12ax7 instead of an 12au7) so a rangemaster could push it into distortion.

Think about it - Marshall went to a solid state rectifier in about 1965 - it has never been an integral part of their sound.

[Silver Blues]

Actually a rectifier can significantly change the sound of a guitar signal. This is a common method of creating an octave-up effect.

I also think you're confusing this concept with a "crossover" as in a device that splits one signal into distinct frequency bands which isn't what we're talking about here. This type of crossover distortion actually can happen in a tube power amp, for example if your output tubes aren't matched (this is the mechanism described in the page I linked actually).

The tonal changes resulting from the rectifier in a tube amp have to do with power delivery (I'm sure you're familiar with the concept of "sag"), and the relative lack of delivery speed and magnitude of output of a tube rectifier compared to solid state. It's not distortion but it does contribute to the sound.

Additionally, see circuit #3 in this article.

[rocket88]

Just to add to silver's comment. A rectifier can and does make a HUGE difference in an amps tone and how it reacts to the instrument being used. Tube rectifiers typically react slow, in comparison to solid state rectifier, and have a "sag" or looseness to them, and in many cases changing to a solid state rectifier will also increase the headroom and output of the amp. Replacing a tube rectifier with a solid state rectifier will tighten up the low end and clarity of the amp. Read Saying that the rectifier material does not make a difference in tone or character of an amp is like saying the tube type doesn't make a difference, which it does.

Case in point, if you take a sunn 2000s, which has not 1, but 2 tube rectifiers, and rated at 120rms/280 peak watts, will gain approx 50watts believe it or not. This will also help give it an even more hifi sound, more articulate lows, when driven a crisper distortion, and make them even more of a monster as far as output goes. Now the increase in headroom can let you run kt88's in place of the 6550's more efficiently.

Additionally, if you think about it, in the example given about Marshall's change to a solid state rectifier absolutely have them the articulate crunchy distortion they're known for. Without that change they may not have been the amp of choice of the hard rock and metal scene, and may not be what they are today.

[motterpaul]

Sorry, I thought I was clear that I was talking about bridge rectifiers, not tubes. Of course a tube rectifier can affect the sound. And I see what you mean about "crossover" distortion in terms of a sine wave. That is a new term to me in this context.

So, "crossover distortion" is more or less a natural occurrence of having output tubes that are correctly biased,  and it really shouldn't be used as any kind of reference point for biasing an amp (a correctly biased amp has it, and trying to eliminate it makes for a very cold-biased amp).

On the other hand, doing it on purpose doesn't seem to yield great results either (in my opinion). The ZVex Machine uses it.

Here is a demo:

<nabble_embed>https://www.youtube.com/watch?v=pk5POcGAUjY</nabble_embed>

[rocket88]

I was simply responding to the comment you make about marshall amps changing to solid state rectifiers in 65, which was a switch from a tube rectifier. I'm simply pointing out that you're wrong in stating that the switch to a solid state rectifier had no effect on the tone of a marshall and that it's essential to the Marshall's tone and part if the reason they are what they are today. You don't need to backtrack on the statement, it won't show weakness or make it seem like your dumb, it's called learning.  Just say you learned something you didn't know and thank you.

motterpaul wrote

Think about it - Marshall went to a solid state rectifier in about 1965 - it has never been an integral part of their sound.

To expand and make the connection to what I said before and a bridge rectifier, a bridge rectifier is just an arrangement of diodes either Si or Schotkey that can replace a tube rectifier. One of the big reasons for the appearance and widespread use of a bridge rectifier configuration is it's cheaper then using tubes, or a center tap.

As far as crossover distortion it is something specific to push/pull amps, not effects pedals. This sums crossover distortion the best.

From Aiken Amps:
"Crossover distortion is the term given to a type of distortion that occurs in push-pull class AB or class B amplifiers. It happens during the time that one side of the output stage shuts off, and the other turns on."

[Silver Blues]

Yeah, basically this. Although I was also pointing out AMZ's simulator circuit and in that sense I have to agree with Paul in that it really doesn't sound very good at all.

[rocket88]

Ah, for some reason I completely missed that section in the AMZ article, and I've read it more then a few times. Interesting how it can be applied to a pedal, and never knew that, so thanks for pointing that out silver. Always glad to learn something new . I've only known it in the realm of amps, especially since I've got a few plans in the works to build a couple, and it cross over is something you want to prevent. I wonder the thought behind putting that in as a form of distortion would be considered. I guess to each their own, right?  

[motterpaul]

Here is what I was saying...

In my experience I have never seen anyone change to any rectifier tube and get more "good" gain. Yes, you can get different sounds by going to a lower gain tube (such as a 12au7 instead of a 12ax7 - which people sometimes do), but that was not what Marshall wanted. They wanted a more transparent rectifier - so they went solid state. It merely translated the sound of their pre-amp design to the output tubes without coloring it in a bad way. When I said "it was never an integral part of their sound" I was referring to tube rectifiers (or rectifiers in general) which I discussed in the previous sentence. I am sorry that statement was unclear.

What is a bad way? You see it to this day when people discuss the Mesa dual rec. People generally refer to them as bassier and fizzier on the high end - not the good kind of distortion. Fender uses tube rectifiers because they generally specialize in a cleaner, more full fidelity sound. Marshall had a more mid-ranged focus in mind, and the SS rec worked better - that was all I meant to say.

I also read that Aiken article and more, and it says crossover "distortion" is a natural occurrence that you can see when looking at the combined waveform coming off the rectifier, and you can change it by changing the bias on the output tubes. Some techs believe that biasing with an eye to eliminating that distortion is a good method, but Aiken was saying that method results in a vastly cold bias setting. So basically crossover distortion is just a design anomaly that in most cases is best ignored, it doesn't make the amp sound better to eliminate it or to increase it. The best approach is to just bias your tubes for optimal performance.

So, yes, I learned something, or to be more accurate I recalled something I had forgotten because I read it a long time ago, that that form of waveform "distortion" is called "crossover" distortion because it happens at the point where the waveform crosses over from positive to negative. And it is "amp theory", not "pedal theory."

Frank - I am just curious if you are willing to talk about your idea whether you are looking at somehow utilizing something else (besides crossover distortion) into a cab simulator. That was just one example I mentioned of something a "cab simulator" could do - but as I said, I would not recommend it.

If you want to look at a circuit that is said to emulate the sound of hot output tubes and a good transformer look at the Catalinbread Wiio
http://tagboardeffects.blogspot.com/search?q=wiio

I built this one and it really does have a nice midrange "singing" quality -

[Silver Blues]

I quite disagree that the change in tonality caused by a tube rectifier is "bad coloration". The bloom and compression can sound extremely good in many cases.

I may be wrong on this postulation, but consider that the early days of electronics were tubes and germanium semiconductors because that was the only technology was available. Once silicon-based electronics became mainstream there was a mass movement toward this because the manufacturing cost was cheaper, the reliability was better, and they could be made smaller. There was a point where you could go to the grocery store and find a rack of replacement tubes for common electronics. To be honest the move away from tube rectification may have simply been a drive to lower unit cost because a half-wave or even a full-wave rectifier made from diodes is a lot cheaper and has fewer ways to go wrong than a tube-based system. The change in sound just goes along with this. The continuing prevalence of solid-state rectification even in modern amps may also have something to do with the fact that the effect is subtle and more noticeable the harder you push an amp. I'd also argue that a solid-state rectifier is "cleaner" than a tube rectifier because of it's more consistent power delivery.

The crossover distortion produced in the power amp doesn't have anything to do with the rectification but rather the "push-pull" nature of the class B or AB amplifier; if you don't compensate for it, there will be a point where both halves of the pair are off and that's where you get the distortion (the push switches off before the pull switches on or vice-versa). More consistent rectification won't really help that issue because it's about bias and not supply (you've mentioned this though).

[rocket88]

Silver Blues wrote

I quite disagree that the change in tonality caused by a tube rectifier is "bad coloration". The bloom and compression can sound extremely good in many cases.

I may be wrong on this postulation, but consider that the early days of electronics were tubes and germanium semiconductors because that was the only technology was available. Once silicon-based electronics became mainstream there was a mass movement toward this because the manufacturing cost was cheaper, the reliability was better, and they could be made smaller. There was a point where you could go to the grocery store and find a rack of replacement tubes for common electronics. To be honest the move away from tube rectification may have simply been a drive to lower unit cost because a half-wave or even a full-wave rectifier made from diodes is a lot cheaper and has fewer ways to go wrong than a tube-based system. The change in sound just goes along with this. The continuing prevalence of solid-state rectification even in modern amps may also have something to do with the fact that the effect is subtle and more noticeable the harder you push an amp. I'd also argue that a solid-state rectifier is "cleaner" than a tube rectifier because of it's more consistent power delivery.

The crossover distortion produced in the power amp doesn't have anything to do with the rectification but rather the "push-pull" nature of the class B or AB amplifier; if you don't compensate for it, there will be a point where both halves of the pair are off and that's where you get the distortion (the push switches off before the pull switches on or vice-versa). More consistent rectification won't really help that issue because it's about bias and not supply (you've mentioned this though).

pretty much spot on and without going into all the detail previously, this is what i was trying to alluding to.

paul, as for the comments:

"You see it to this day when people discuss the Mesa dual rec. People generally refer to them as bassier and fizzier on the high end - not the good kind of distortion. Fender uses tube rectifiers because they generally specialize in a cleaner, more full fidelity sound. Marshall had a more mid-ranged focus in mind, and the SS rec worked better - that was all I meant to say. "

again, you're pretty far off bass. fender amps do not give a cleaner, full fidelity sound due to the use of tube rectifiers, it's due to the type of tubes they use. fender amps are known for their clean tones and the classic "california" cleans from back in the day, which came due to the use of 6L6 tubes. fender's are not known for good crunch tones, mostly due to the 6L6 and how it distorts. IMHO fender amps don't do good OD tones, but have beautiful crisp cleans. when you look at the classic british amps, like marshalls, they did not have 6L6's widely available and cost effective, hence the use of EL34's, and the change in component values that allowed their use. the el34's are what really contributes to the british crunch.

in the article i linked to by Soldano, they clearly layout what a tube rectifier will sound like and why, and compares the same info about solid state rectifiers.

"A tube rectifier has internal resistance. The more current that travels through a tube rectifier, the more the voltage drops. When the voltage drops, the power of the amplifier also drops. The tube rectifier has the drawback of not being able to provide a consistent voltage when it’s under load. The other drawback is that the tubes themselves run hot, and can be relatively short-lived. Unfortunately, modern day sources for rectifier tubes are not very reliable and, even in their prime, these tubes were usually the weak link in most amplifiers.

An amp with a tube rectifier tends to sound much spongier in the bottom end. Low frequency notes take more current through the power tubes to reproduce. This increased current causes a voltage drop in the rectifier tube and the amp loses power. So, when more power is actually needed, the amp gives less. Because of this, a tube rectifier amp will sound spongy and more distorted at high volumes. This, probably more than anything, is what gives a vintage amp its sound and color."

since tube rectifiers create a spongey bottom end, they will not have a higher fidelity sound.

while solid state rectifiers do the following:

"A solid state rectifier has no internal resistance whatsoever. It has a very consistent fixed voltage drop that occurs both at zero or full current (approximately .7 volts). When an amplifier needs power at low frequencies, there will be no limit to the current that travels through the rectifier. This results in an amp with more headroom that is punchier, more articulate and able to deliver the goods in the bottom end."

a solid state rectifier will give a higher fidelity sound, with clearer, crisper bass. the mid range nature of the marshall is due to the values that were changed when trying to clone the fender bassman and the use of el34's rather then 6L6's. the solid state rectifier tightens up the bass and removes the sponginess of a tube rectifier.

the mesa dual rectifier is a completely different animal. it has a a switchable rectifier so you can choose between type of rectifier in the amp, ie tube or solid state. as for whether or not the distortion is good is ones opinion, but the dual rectifier is highly regarded for it's crunch and distortion, and known for having very thick, heavy sustain/distortion while retaining clarity, not having a bassy and fizzy top end.

in addition i think you're missing what a rectifier does. PGS did a short article about them, and what they do, due to many having a misconception of what they actually do.

"This is a question I’ve heard a lot.  What does a rectifier do?  If you don’t know, don’t feel bad, you’re about to find out.  A rectifier changes alternating current (AC), like a wall outlet, to direct current (DC) that is required to run all electronics inside your amplifier.  That’s it, it’s really that simple.  The AC voltage comes from the wall into the transformer of your amp where it is then run through the rectifier to get DC voltage.  This is before all audio circuitry so an important thing to realize is that NO AUDIO SIGNAL PASSES THROUGH THE RECTIFIER.  It is purely for voltage conversion.  So how does it affect the tone if no audio passes through it?  We’ll be discussing that very soon, keep reading."

the mesa dual and triple rectifiers use multiple rectifiers because a single rectifier can not handle the current they are pushing for the wattage of those amps. mesa is not the only company that has used multiple rectifiers because of this issue, case in point the old Sun 2000s.

[motterpaul]

(note: before you type a long reply to this - note that I changed something I said about Fender &  their rectifier in a new post -- sorry for the confusion).


Thank you, I appreciate the time you both put into this. You are right that signal does not pass through a rectifier (I did just learn this, sometimes I do carry misconceptions in my head - although I am sure I have read rectifier theory before and learned this before as well). But rectifiers do affect tone.

You said "again, you're pretty far off bass. fender amps do not give a cleaner, full fidelity sound due to the use of tube rectifiers, it's due to the type of tubes they use. fender amps are known for their clean tones and the classic "california" cleans from back in the day, which came due to the use of 6L6 tubes. fender's are not known for good crunch tones, mostly due to the 6L6 and how it distorts. IMHO fender amps don't do good OD tones, but have beautiful crisp cleans. when you look at the classic british amps, like marshalls, they did not have 6L6's widely available and cost effective, hence the use of EL34's, and the change in component values that allowed their use. the el34's are what really contributes to the british crunch."

I think you mean "off-base" unless that was a pun intended. But you are quoting me out of context (which is something that I personally really dislike when anyone does it, so I am not singling out you). What I actually said was "Fender uses tube rectifiers because they generally specialize in a cleaner, more full fidelity sound."

But I did not say, and I guarantee you I never meant to imply, that the rectifier alone gives them that, I only implied it is a part of attaining their goal of fuller freq response aka - more bass and treble. I said "full-fidelity" meaning MORE bass and treble, not "high fidelity" which means "more accurate" frequency response - a bridge rectifier gives you that. But although tube rectifiers may make the bass sound muddier, I would never say the bass of Fender amps sounds muddy, would you? But tube rectifiers do give MORE bass, so my point is that the goal was MORE bass, not a "tighter bass." But where Fender really wanted and succeeded in attaining clarity and "high fidelity" was in the mid-range and high frequencies. Largely due to the different preamps and speakers, and yes, also the 6L6s.

By the way, have you ever put a JBL D120 (a classic Fender speaker choice) into a Marshall cabinet? I have and it sounds like TOTAL ASS and nothing will make it sound any better. But they sound great in twin reverbs.

But you are a little off-bass ( about 6L6s as being only for clean amps, I use 6L6s in my higher gain Marshalls (the dsl100) - the 5150 EVH uses 6L6s exclusively, and so do Mesa Boogie and Soldano use them (edit: although not exclusively) . I prefer 6L6s in my new Marshalls for the headroom, but I still get all the great gain of my new model Marshall preamps.

Also for the record, the original Marshalls (back when full-output tone mattered more) used 5881s and KT66s, which sound closer to a 6L6 than an EL34. EL34s are a little more hairy, I agree, but not in a good way to me. Marshall only switched to EL34s when they introduced the higher output Dagnall transformer to replace the Drakes in about 1968, because the Dagnalls had 50-watts of output (vs 46 iirc)  and the EL34s matched better. When they were shipped to America often the el34 was replaced with a 5188 by the distributor.

This is what I believe - in the old days when your on-stage amp was your main sound source you needed the most power you could get. In those days an amp's tone was judged by how it sounded when fully cranked and the output section was where you got a lot of the "magic" tone.
 
But today such stage volumes (on amps) are not needed, and in fact are not acceptable - so the tone focus has moved to preamps. At the same time, modern amp makers shoot for a cleaner output section (those NOT chasing the vintage tone) and the fact that bride rectifiers are cheaper, have better (not fuller) fidelity and are more dependable does not hurt.

I have a single channel Marshall 2204 as my main amp - a model they started making in the early 70s (although my 80s model has changed slightly, it is still basically the same circuit). It is not a super high gain amp (think AC/DC) although it is good enough for rock & roll. But later amps (The DSL Marshall series, and the Peavey & Fender 5150 EVH models) have much more gain in the pre-amps, and actual master volume controls, so you never need to turn them all the way up to get "that tone." That makes the effect of saturation in the output tubes less critical in modern amps.

Even the 200-watt Marshall Major with its 4 KT88s was not really a "high-gain" amp, but it sounded fantastic if you could ever hear one cranked (in a studio for example). Stevie Ray Vaughn had one for recording only. His sound is not high gain but it is powerful. I actually owned one for awhile in about 1973, it was a beast. I blew it out trying to muffle to sound of the speakers with mattress (hey, I was a kid) - but it was the only way I could even use it without getting a visit from the police.

Also - by the way - original Marshalls (Superleads with EL34s) were even made as PA systems with no change in the electronic design except adding more pre-amps (as many as four with HiZ inputs)

So - to sum up. Thanks for all the discussion about rectifiers, although it was a little off-topic. I learned some things and thank you for that , but I also just want to say that a lot of this is opinion - and you have yours and I have mine - so since you elucidated on yours I am just returning the favor.