Hello,
I'm looking for some ideas to emulate the sound of a tube power amp.
I found here some schematics but they are quite old.
Thank you
1. Phase split the signal
2. Asymmetrically clip both resulting signals
3. Sum the signals
Most clever embodiments integrate an envelope controlled clipping threshold to simulate clipping at plate (which's threshold is modulated by sag), grid conduction emulation with associated capacitive coupling circuits leading to DC bias shifts and crossover distortion under sustained overdrive plus an emulation of gain compression due to screen voltage modulation.
*hmm...use a pignose amp?*
thank you
but for the pignose amp, the aim is to use it in front of a cab simulator!
@teemuk : thank you. Do you know if there are some examples of these?
Search my posts from Last summer
J
Maybe some kind of FET differential amplifier, also you may take a look at Jon's "Snow Day OD". I've built it and IMO it sounds very close to a 20W Blackface at hi volumes.
Something I've been cooking up lately. This simulates voltage sag, sag modulated soft clipping, and effects of grid conduction clipping such as crossover distortion. Low damping factor is also simulated.
(http://oi60.tinypic.com/15z41o7.jpg)
Diodes after phase inverter stage simulate grid conduction, the other series diodes clipping at the plates. Reference voltage for this clipping threshold is derived from a differential amp that substracts output of an envelope follower from a steady DC reference. Last stage simulates response of an amplifier with low damping factor drioving a reactive speaker load. The circuit probably needs some tweaking, such as some pregain and some crude voltage limiting so that the input stages sans diode clipping won't get overdriven. Signal should never be allowed to swing above +/-13Vpeaks.
Something Peavey uses ("T-Dynamics"):
(http://oi61.tinypic.com/314rvk9.jpg)
This is actually a complete power amp that simulates effects of grid clipping and associated bias shifts (crossover distortion) and the moderately soft clipping at plates. Sag is not simulated. Low damping factor is designed in but it requires that the final power amp section and a loudspeaker load is included. You could likely convert that to run at lower voltages and currents to a simulated high-impedance reactive load. This circuit does not use phase inversion but relies on bipolar reference voltages and polarities of the diodes.
Quilter:
(http://oi57.tinypic.com/zjjus7.jpg)
Once again a complete power amp. This one simulates crossover distortion ("Zero crossing process") and sag modulated clipping at plates ("Soft clipping cell"). Low damping factor is again designed in to the final power amp stage with the aid of current feedback. Again no phase inversion per se but same goal achieved with bipolar reference voltages and diode polarities.
Wow it's more complicated than I thought it would be, I've owned a Peavey Envoy 110 and I think Peavy really did a good job at analog simulations.
I thing adding some kind of speaker cab simulator would be nice too...
thanks a lot
I have to take a closer look at it cause I admit it's more complicated than I thought it would be too!
Maybe one solution for me is to start with the runoffgroove Thunderbird soft clipping stage :
http://www.runoffgroove.com/thunderbird.html
and try to implement a even harmonic generator and a zero crossing processor?
Which embodiment would have the more impact in the sound?
Quote from: teemuk on March 05, 2015, 03:00:49 PM
1. Phase split the signal
2. Asymmetrically clip both resulting signals
3. Sum the signals
Most clever embodiments integrate an envelope controlled clipping threshold to simulate clipping at plate (which's threshold is modulated by sag), grid conduction emulation with associated capacitive coupling circuits leading to DC bias shifts and crossover distortion under sustained overdrive plus an emulation of gain compression due to screen voltage modulation.
What would be the easiest way to sum the signals?
Very interesting stuff there teemuk! - You are not by any chance The "Teemu Kyttälä" are you?
Once again I refer to KMG... Check out the micro power amp stuff. Click the "English" hyperlink if you can't read Russian.
http://milas.spb.ru/~kmg/index_en.html
More here
http://milas.spb.ru/~kmg/files/projects/fetpa/micro/xicon/sim/
Or you could do as i suggested. ..Build a miniaturised hybrid amp with a push/pull tube output stage, in a stompbox format. One opamp, one transistor, one glas bottle... You might like a different "preamp" but the tubestage works and makes it sound and feel like a tubeamp instead of a fuzzbox... You could even start from the phase inverter(transistor) and use any stlmpbox as preamp...or place the tube stage alone in the loop of a solidstate amp to give it some tube character.
here is the thread
http://www.diystompboxes.com/smfforum/index.php?topic=107868.0
j
Modern amps have most of the distortion done with preamp.
My aim is to build the more compact as possible pedalboard to use instead of my amp head + cab for rehearsals or little gigs where I don't want to carry my amp+cab.
I'm building a tube stompbox preamp, I plan to buy a cab sim (ada gcs2?). I'm thinking of adding a compact "power amp sim" stompbox, not really for distortion, but only to sweeten/soften the preamp distortion (I don't know exactly which word to use, you know sometimes preamp distortion can be a little raw and harsh).
- A simple option can be the Runofgroove Thunderbird (only the last stage) with maybe adding teemuk embodiments (but as I'm a newbie I have to understand how all of these work!)
- KMG micropower seems another good option (and maybe if the embedded cab sim is good I can use it instead of buying an ada gcs)
- Johan project could be another good option but initially I didn't plan to use tube to make it as compact as possible. And I already have the "tube sound" from 2 12ax7 in the preamp. The only thing I'm looking to achieve is to add a little bit of the power amp stage feeling in the sound to sweeten the preamp distortion and make the preamp + amp sim + cab sim act more like a real amp.
While you're doing all that building , why not just build a Tube amp ...??? They're really easy and not at all as expensive to construct as one might expect... :icon_eek:
Quote from: StarGeezers on March 07, 2015, 07:09:56 AM
While you're doing all that building , why not just build a Tube amp ...??? They're really easy and not at all as expensive to construct as one might expect... :icon_eek:
Try reading:
Quote from: Sheldon on March 07, 2015, 05:04:07 AM[...]
My aim is to build the more compact as possible pedalboard to use instead of my amp head + cab for rehearsals or little gigs where I don't want to carry my amp+cab.
[...]
I don't need to build an amp since I already have an amp!
I just want a compact pedalboard with preamp + power amp sim + cab sim to run direct in the PA when I don't want to carry my amp + cab!
@J0K3RX : are there some explanation somwhere on the KMG micro amp schematics?
I've always thought that depending how hard you drive it, the power amp section added a little bit of "warm distortion", less raw and more musical than the preamp one.
But I can't understand what is doing this.
As far as I can understand it, a push pull power tube section is attenuating most of the even harmonics, but I thought that it was those even harmonics that give the warm and musical tone?
It seems nobody from experts gonna explain it so a word from the idiot guy: :icon_mrgreen:
Actually both even order and odd order harmonics can be musical, kind of...
The warm feeling of the tone is about how much harmonics is added to the raw guitar signal. let me explain it this way, 2nd order harmonic is always the same as original note but one octave higher, so definitely it's musical. 3rd order harmonic is not the same note as original note but that doesn't mean it's not musical, because it will be always another musical note and will form a chord with original note, so on 4th and 5th orders...
But higher orders mostly aren't related to any musical note and tend to add some junky,fizzy,... taste to the tone.
So why tubes are more musical? That's because they tend to add a huge load of low harmonics (musical ones) to the signal and a little high order junk! And they do it in both clean amplification and clipping, but at higher signal levels and especially clipping condition the amount of generated harmonics will rise and the sound will get warmer and warmer.
Transistors work in another way, they do not add much harmonics to the signal at clean level, and at clipping they produce a huge load of ugly harmonics (high order ones).
Even order harmonics are produced by any type of assymetrical amplifier stage, like a triode gain stage, phase inverter, SE output stage. So if we want to generate even orders with transistors we need a "Clean Assymetrical Amplifier" stage.
PP output stages form a composite curve, and they tend to amplify the signal nearly symmetrical so they will cancel out the most of even orders, but odd orders still are there and they are part of PP tone!
I think it's best to focus on a particular amp's output stage and try to simulate all of its characteristics...
I had a similar idea, about a year ago. I read somewhere that the JFET transfer curve is more like a pentode than like a triode, and combined with the knowledge that tube amps are almost-always transformer-coupled, threw a JFET phase-inverter at a RadioShack transformer into a corner of my breadboard. It barely worked, and only sounded slightly better than crap, but was functional enough that I drew-up a generic class-AB push-pull power-amp topology in a sim. According to SIMetrix, it should work... I'm hopeful, seeing as I also simulated the proof-of-concept circuit, and the sim told me that *that* would never work (to be fair, I did have to crank my amp to even get bedroom-level output).
That sim file is on my other (currently broken) computer, though, and would almost certainly sound bad without tweaking anyway... meanwhile my breadboard has been perpetually occupied with pre-build tweaking of known-good circuits, so the project has stagnated. I expect to be finished messing-around with CMOS overdrives and/or buy another breadboard (or several) by the end of the month, though, and will at that point resume work on the concept.
CAVEAT EMPTOR:
This is *way* above my pay-grade, but on the other hand, it's also the exact reason why I hope my boss (me) will one-day give me a promotion. I assume there's a reason why my idea hasn't been done to death, but I'm the stubborn sort, and I've found that prop-wash is quite effective at keeping dust bunnies out of a car's engine bay, so I'm quite content to persist anyway... even if it's a fool's errand, I'm sure the topological similarity to a real tube amp will at-least give it some mojo ;).
I have to admit I'm totally lost and I don't know where to start !
If I understand correctly, if I want to simulate the effect of the power amp stage in a modern push pull amp, I have to :
* split the signal
* invert phase of one side with a inverter op-amp
* use different diodes to do assymetrical clipping =>simulate Tube grid bias shift and Tube distortion (only first orders even and odd harmonics)
* combine the signals (substract???) so remains the signal with mainly third and some fifth order, and a little bit of second and fourth order, to simulate the fact that a push pull stage is never perfectly symetrical
And then trying to simulate sag compression and current feedback?
I'm trying to understand KGM Micro Amp Schematics that J0K3RX suggested but I admit I have difficulties to understand each part.
http://milas.spb.ru/~kmg/files/projects/fetpa/micro/MicroSchSS.pdf
@ Johan : as I'm exploring each possibility cause I'm trying to learn how things work, what is the role of the b (lower case) polarity and how is it created?
Its the bias/4.5volt for the opamp
thank you. In fact it's the top right part of your schematics that I don't understand. The b polarity runs through the 100k pot and depending of its resistance it gives a PP or SE character to the tone?
IMO
It has a lot to do with the output tubes, output transformer and speaker interaction. This mess is hard to model
There are amps that are IC until the output transistor/tube transformer out
BJTs might be better to use instead of jfets for preamp gain stages
Quote from: Sheldon on March 08, 2015, 10:34:48 AM
I have to admit I'm totally lost and I don't know where to start !
If I understand correctly, if I want to simulate the effect of the power amp stage in a modern push pull amp, I have to :
* split the signal
* invert phase of one side with a inverter op-amp
* use different diodes to do assymetrical clipping =>simulate Tube grid bias shift and Tube distortion (only first orders even and odd harmonics)
* combine the signals (substract???) so remains the signal with mainly third and some fifth order, and a little bit of second and fourth order, to simulate the fact that a push pull stage is never perfectly symetrical
And then trying to simulate sag compression and current feedback?
I'm trying to understand KGM Micro Amp Schematics that J0K3RX suggested but I admit I have difficulties to understand each part.
http://milas.spb.ru/~kmg/files/projects/fetpa/micro/MicroSchSS.pdf
I don't think assymetrical diode clipping will do any favor in producing more even orders, because it's just clipping of "already symnetrically amplified signal". We need "assymmetrical amplification" to do this particular job. Also don't forget that tubes do have internal capacitances, so they will add low pass filters in the path.
See if you can find a PDF or hard copy version of The Stompbox Cookbook.
I believe that we've mostly figured out "tube sound" since people have tiny hand-sized amplifier emulator/simulators these days, but there is a section on the considerations of all the variables regarding what makes tubes sound "tubey". You might find it helpful if only as reinforcement. It may only be an analogue for a stomp box builder without software or hardware coding and microprocessors, but it can sound reasonably close.
I think rule #1 might be to prevent hard clipping as much as possible. In transistor terms, its the saturation or cutoff where no further changes take place. In preamp tubes and output tubes, as far as I know, no matter how hard they are driven, there is still at least a minimal change in the signal occurring at any given Time, because of the complex interactions of transformers, microphonics, operating point changes, etc. What is called soft clipping, the thing which tube amps sound is famous for, is the distortion of the signal so that it fails to follow the input accurately. Hard clipping, which happens because BJT transistors are so accurate at following input signal changes, is the point at which the signal completely stops following the input signal at all. Tubes still do follow, if at least highly inaccurately. I haven't really seen any information yet that shows true hard clipping in tube based amplifiers.
The section on tube emulation in TSC is a good read.
Quote from: sajy_ho on March 07, 2015, 06:09:27 PM
It seems nobody from experts gonna explain it so a word from the idiot guy: :icon_mrgreen:
Actually both even order and odd order harmonics can be musical, kind of...
The warm feeling of the tone is about how much harmonics is added to the raw guitar signal. let me explain it this way, 2nd order harmonic is always the same as original note but one octave higher, so definitely it's musical. 3rd order harmonic is not the same note as original note but that doesn't mean it's not musical, because it will be always another musical note and will form a chord with original note, so on 4th and 5th orders...
But higher orders mostly aren't related to any musical note and tend to add some junky,fizzy,... taste to the tone.
Definitely. It is my personal opinion that this is why metal is mostly power chords, possibly even where the term comes form, because with all that "power", you need to play fewer notes to have a sound that isn't garbled :P. After all, you're getting "extra notes" with the distortion, so you can play less on the guitar, and then use different combinations of the same notes of a chord for different musical sounds.
Quote from: Sheldon on March 07, 2015, 03:57:06 PM
I've always thought that depending how hard you drive it, the power amp section added a little bit of "warm distortion", less raw and more musical than the preamp one.
But I can't understand what is doing this.
As far as I can understand it, a push pull power tube section is attenuating most of the even harmonics, but I thought that it was those even harmonics that give the warm and musical tone?
Single output vs push-pull is also considered in TSC. Pre-amps are also given some attention. From what I remember off the top of my head, the claim was that more tubes in the pre-amp sound warmer because there's more cascading going on, softer clipping, than higher gain stages with fewer tubes. I believe the push-pull reasoning was along the lines of the shift in operating points, IMD, and such.
thank you
One thing I find tricky is to simulate push pull amplification without actually amplifying the signal cause my aim is to have a line in and a line out
QuoteI think rule #1 might be to prevent hard clipping as much as possible.
A myth. People are brainwashed to this concept after they have been saturated with -drawn- images of what tube amps output in some weird imaginationland. Reality, that revelead by oscilloscope captures and such, is often totally different than drawn pictures.
If you actually scope output of a tube power amp you typically see plenty of hard clipping and other "nasties" like crossover distortion. Fortunately all this sounds much better than what it looks.
Given characteristic curves, pentode tubes are damn near FETs and beam tetrodes damn near BJTs so hard clipping is to be expected. Couple this to global negative feedback and hard clipping is guaranteed. So yes, tube power amps hard clip, like this Fender amp here:
(http://theinside.net/misc/OT.JPG)
...or like this Trainwreck clone here:
(http://home.polstra.com/amps/wreck1/scope/IMG_0688.jpg)
(http://)
Actually when voltage can no longer swing upwards things just clip, whether they are BJTs or tubes. It's the limit. The tube isn't suddenly going to overcome it by some magic. If you look at it, tubes are equally limited by saturation and cutoff states. We can notice a slight "softness" in overall curvature of the characteristic curves but as I said, pentodes are damn near FETs and beam tetrodes damn near BJTs so don't expect miracles. The most prominent thing explaining why tube power amps behave like they do lies within the overall circuit architecture not in the active devices.
Besides, now that we talk about "change" in signal after sustained ovedrive we aren't neccessarily discussing about softness of clipping characteristics. Not at all. If you overdrive a typical push-pull tube power amp the signal will typically soft clip distort just a little then very quickly hard clip. THEN interesting stuff begins to happen: Voltages may sag due to increased current draw and this will furthermore reduce clipping threshold, making clipping even harder. Additionally voltage drop at screen circuit may introduce little bit of gain compression. Now the gradual voltage sags and increased distortion adds more and more harmonic distortion as overdrive sustains. It's not just static clipping to a static clipping threshold.
The signal is still hard clipping but due to such changes the clipping thresholds have changed, in fact they change in interaction with overtall signal envelope, and this affects harmonic patterns of the distortion. Continue overdriving and power tube grids begin to conduct and draw current, this both clips the signal at their grids and causes DC offsets (such as bias) change. This typically introduces crossover distortion to the signal so again more and more harmonics (now from crossover distortion) are getting layered to the distorted signal. Furthermore the harder we drive the power amp, we may have rising asymmetry in the PI output and other little details that have an effect on pattern of the harmonic distortion. Essentially we have several distortion/clipping mechanism that interact with overall envelope of the signal, not just with static thresholds of voltage. All this while our signal is still getting hard clipped, in fact all the time progressively more and more. Hard clipping does not exclude anything from this process. But we have a system where sustained overdrive not only increasas amplitude of harmonics but continuously changes the entire pattern of the harmonic distortion.
Preamps have similar mechanisms where clipping process is spread across multiple stages and asymmetry of clipping and capacitive signal coupling causes DC offset modulation. Such modulation naturally has further effect on overall asymmetry/symmetry of clipping and therefore also to harmonic patterns of the clipping distortion. The behaviour is totally different than static clipping to, say, just a single diode clipper where increased overdrive doesn't really change the harmonic pattern of the distortion per se, just increases amplitude of the harmonics.
Again, soft clipping is not a requirement. The phenomenon takes place as long as you have some asymmetric clipping and capacitive coupling between stages. In fact, if you break down a typical tube preamp it is quite common to find out that clipping in it happens in just one to three stages: Few of them clipping hardly at all and one practically clipping so much that it has nearly a square wave-ish output.
This is why I need to invest in oscilloscope. Thanks, Teemuk :icon_biggrin: I believed in those squashed sine wave diagrams for "tube sound" mainly because FETs and tubes are not linear devices, and their gain tends to fall as their voltage swings get bigger.
How does the trainwreck clone compare to an equally overdriven BJT section? Does BJT maintain constant wiggle like on the plate of the clone?
Today I began simulations with LT Spice, especially Johan's project, but with no success!
teemuk
those are great scope shots. can you get a shot of a screen grid above/below a plate trace? been googling for one lately with no luck :) thanks!
Quote from: Sheldon on March 09, 2015, 05:09:19 PM
Today I began simulations with LT Spice, especially Johan's project, but with no success!
Is there an accurate 12au7 model in LT Spice? When i tried it a few years ago there were no tube models at all in there.
j
I simulate a 12au7 triode stage quite correctly, but when I try to simulate your schematics there are a lot of gain attenuation and the output signal is far thinner as the input one when I try with a guitar wave input
From memory, the valve models I have tried with LTSpice don't give sensible results at low voltages.
Sheldon. What do you hope to learn from a simulation regarding Johan's design? Can you replace the triodes with e.g. BJTs to get a feel for what's going on?
A stupid thought, perhaps, but i have to point it out as i made that mistake myself..
..i drew the opamp +input at the top, and i think LT spice have them on the bottom of the opamp....?
J
@Johan : I verified and the opamp I drew the opamp in lt spice as in your schematics
@samhay : In fact I'm quite a newbie in electronics (I had a few lessons when I was in engineering school but it's not my job) si I decided to simulate Teemuk, Johan... schematics with LT spice so I can try to understand part by part how they work!
fair enough - it's a good general approach to understanding how these things work.
I found some old LTSpice simulations of circuits based on Johan's design. One is below - save the text as a .asc file and it should work once you add TL072 and 12au7 model files (.sub). The 12au7 model I used gives some gain with a 9V supply. I don't trust the amount of gain, but at least it shows it working.
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Quote from: Sheldon on March 07, 2015, 03:57:06 PM
I've always thought that depending how hard you drive it, the power amp section added a little bit of "warm distortion", less raw and more musical than the preamp one.
But I can't understand what is doing this.
...
After several years of asking the same to myself (about the little bit of "warm distortion" in the power amp section) I'm pretty sure I have sorted it out. I'm talking about the "near-clean" warm sound which appears when the amp volume is set at certain level where the warmth appears, and if you take back the volume a bit then this "magic" dissappears. At this volume it is not about phase inverter saturation, not about power supply sag, not about output transformer saturation, not about speaker breakup (however some speakers may show breakup effects at low volumes), and certainly not about speaker thermal compression.
It's the power amp nonlinear transfer function which adds mostly third order harmonics. The "ideal" transfer function would be a straight line. The practical transfer function in a push-pull valve amp looks rather like an "S" shape to each side of the origin of the graph. Negative feedback sort of straightens this wobbly transfer function, but not entirely. Amps with no feedback at all (like the Vox AC30) have very warm "clean" sounds because of this. Search around for testimonials of people that have removed negative feedback from their amps, giving their amps a more "vintage" and warm tone instantly.
You may see a practical implementation of this in a future ROG project.
I think I have issues with my op amps
none of them works as they supposed to work!
Quote from: stm on March 10, 2015, 08:39:08 AM
After several years of asking the same to myself (about the little bit of "warm distortion" in the power amp section) I'm pretty sure I have sorted it out. I'm talking about the "near-clean" warm sound which appears when the amp volume is set at certain level where the warmth appears, and if you take back the volume a bit then this "magic" dissappears. At this volume it is not about phase inverter saturation, not about power supply sag, not about output transformer saturation, not about speaker breakup (however some speakers may show breakup effects at low volumes), and certainly not about speaker thermal compression.
It's the power amp nonlinear transfer function which adds mostly third order harmonics. The "ideal" transfer function would be a straight line. The practical transfer function in a push-pull valve amp looks rather like an "S" shape to each side of the origin of the graph. Negative feedback sort of straightens this wobbly transfer function, but not entirely. Amps with no feedback at all (like the Vox AC30) have very warm "clean" sounds because of this. Search around for testimonials of people that have removed negative feedback from their amps, giving their amps a more "vintage" and warm tone instantly.
You may see a practical implementation of this in a future ROG project.
Exactly! I'm talking about the same "Warm Sound". A sound that is not very distorted (from aspect of clipping distortion), but contains a lot of nice sounding harmonics.
For now I'm trying to simulate Seljer schematics in Johan's thread :
http://www.diystompboxes.com/smfforum/index.php?topic=107868.msg981504#msg981504
It's basically the same idea with JFET, so I don't have to rely on lt spice tube simulation.
For now I'm not able to have the same scope signal as him (mine are less assymetrical, more clipped on the top), but it's quite interesting as it brings even and odd harmonics, with less even harmonics (I assume it's the effect of the phase splitter) so I think it's like a push pull power amp.
I'd like to know how is it possible with the mosfet to adjust the quantity of harmonics. I assume that the ratio odd harmonics/even harmonics is due to the signal assymetricality? So I have to play with the jfet bias voltage?
And how to adjust the global quantity of harmonics? For instance, how to attenuate harmonics past the 5th order?
Quote from: Sheldon on March 10, 2015, 04:25:54 PM
For now I'm trying to simulate ...
Simulating a circuit is fine but you should never, I repeat, never, avoid testing a circuit on the breadboard based on the sims alone.
Try this on the breadboard:
(http://www.diale.org/img/lion_sym.png)
and feed it with a hot clean signal... You'll have a surprise (http://www.diystompboxes.com/smfforum/index.php?topic=93930.0)!
You should also try Tim Escobedo's bootstrap:
(http://www.diale.org/img/bootstrap_for_gain_sym.png)
and also Arsenio's Novo - Tube Sound Overdrive
(http://www.diale.org/img/arsenio-novo-TSO.png)
Have fun!
P.S.
Arsenio's - Tube Sound Overdrive is my favorite.
P.P.S.
Have a look at Teemu's book: http://www.thatraymond.com/downloads/solidstate_guitar_amplifiers_teemu_kyttala_v1.0.pdf
Specially from pg. 18 to 26.
Quote from: Sheldon on March 10, 2015, 04:25:54 PM
And how to adjust the global quantity of harmonics? For instance, how to attenuate harmonics past the 5th order?
Maybe by adding several low pass filters at the end of chain, like this one:
http://www.runoffgroove.com/thunderchief.html
Quote from: Sheldon on March 10, 2015, 04:25:54 PM
And how to adjust the global quantity of harmonics? For instance, how to attenuate harmonics past the 5th order?
You don't. At least with an analog circuit, but you could do it digitally.
I think the question is not how to
remove attenuate it, but how do *all* harmonics scale to one another. What are the ratios of the 2nd, 3rd, etc...to the fundamental?
The 5th isn't all that bad it all depends on the amplitude ratio to the other harmonics.
I have a question
If you have a transistor phase inverter feeding 2 jfet for soft assymetrical clipping, what is the method to bias the transistor and the jfet?
Quote from: teemuk on March 06, 2015, 09:43:51 AM
Something I've been cooking up lately.....
Thanks - this gave me a push to dust off some old ideas about envelope-controlled clipping and actually get them working.
Have started a new thread: http://www.diystompboxes.com/smfforum/index.php?topic=110346.0
QuoteP.S.
Arsenio's - Tube Sound Overdrive is my favorite.
Arsenio Novo = Bazz Fuss ;D
http://www.diystompboxes.com/smfforum/index.php?topic=98141.msg857451#msg857451
mac
I have to ask, what are you going to do with just a tube power amplifier simulation circuit?
For rehearsals or small gigs, instead of carrying an amp, I'd like to use a pedalboard with tube preamp -> power amp sim -> cab sim
Like a preamp into a torpedo cab that have a power amp sim section that brings beef up and warm up a bit the preamp distortion.
For the moment I have to admit I'm a bit discouraged.
Teemuk schematics seems nice but too complex for me.
With lt spice I finally undertood how bjp-op amp phase inverters work, how to have assymetrical clipping with op-amps or jfet (even if I have trouble with biasing), how to sum the two push-pull signals. But at the end of the days, it gives nothing more that some "cold" harmonics, nothing like the Torpedo Cab power amp simulation.
I think I should try to implement a feedback inverter with presence and depth filters, but for the moment, I don't know how!
this might be a job for a diode ladder type soft clipper... i messed with a few different variations and you can get a pretty smooth 's' shape or 'saturation' type shape with it... it's not a common trick, other than your typical ' diodes with a resistor in series' type softness control, or warp control. this approach takes the transfer curve a step further and allows you to stack several thresholds, each with additional soft clipping added. so the signal hits the first lowest threshold and bends a little, then hits the next and bends a little more, then the next and bends a little more etc... like an s curve... there are a few ways to do it, and it offers some stepwise control of the curve, so you can adjust it to bend/clip the signal 10% say, then at the next threshold an additional 33% then at the next 75 % etc... you need a decent voltage swing to use the diode ladder type setup, as you need to go through 3 or 4 diode thresholds without hitting the rails, but theres another way to do it where you set up the soft clippers one at a time and just voltage divide the signal down after each stage to make it the right size for each separate soft clipper...
to simplify :icon_eek: :)
first stage: b2b leds(1.2 threshold-ish) with a small value resistor in series to ground... clips just the peaks alot say 85% but not quite flat.
second stage ; voltage divide to where the already clipped off part is juuust above this stage's clipping threshold : si with a medium value resistor in series... clips the signal say 50% (anything above the si diodes is half clipped)
third stage ; voltage divide to where the already clipped off part is juuust above this stage's clipping threshold : ge' with a large value resistor in series... clips the signal only 20% say -bending the transfer juuust a little bit...
so a signal big enough to get through all the thresholds will...
1st. hit the ge's and gently bend a bit... a little compression, and a little low order hamonics added ( 20%)
2nd. hit the si's and bend a bit more ... more compression and more harmonics added (50)%
3rd. hit the leds and bend alot ... compressing peaks and adding higher order harmonics (85% clipped)
a big signal will get progressively 'rounded' a small signal will just get a bit bent and a medium signal will get softly squashed but still have dynamics... never hit a hard ceiling... the smoothness of the curve keeps the added harmonic content from getting buzzy, by having less high order harmonics added till you slam it, but you dont have to slam it....
i should have just drawn it up... :icon_rolleyes:
i do have a falstads simulation of the idea i could post if you're interested. lemme know. that way you could play with the values etc... to get the curve you want...
of course I'm interested!
I'm so lost that I will be glad to test and learn any idea!
is it possible to add a negative feedback simply by adding an op amp before the phase inverter fed with input and a signal taken from the output?
I tried it with lt spice with different resistors values but it seems I've done something wrong!
heres a falstad sim of the idea...
go here http://www.falstad.com/circuit/ (http://www.falstad.com/circuit/) go to file/import from the app window and copy/paste the code into the import window.
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thank you
Hi guys,
There's "pure BJT" simulation of tube push-pull amp in the link below.
You can find stompbox version as well as 1,5 Watts RMS output stage there (also on the picture below plus output waveform)
http://forum.gtlab.net/cgi-bin/yabb2/YaBB.pl?num=1427446870/20
(http://i.imgur.com/UJeqTNN.png)
(http://i.imgur.com/x8WHjYa.png)
> As far as I can understand it, a push pull power tube section is attenuating most of the even harmonics
No. It does not *add* even harmonics. Any evens which come in are passed like any other tone.
A push-pull stage will always ultimately add odd harmonics when it overloads.
> but I thought that it was those even harmonics that give the warm and musical tone?
Very substantial amounts of even harmonic are scarcely audible. While even I can tell the difference between a Fender input triode and another input with so much NFB that it is "clean", the difference is very subtle.
ODD harmonic distortion is the sound of rock-n-roll, the Power Chord.
Single-ended stages may add 2nd harmonic to medium-strong signals, but *always* add 3rd harmonic as they get into overload.
Come home drunk in a loose-steering car and park in the garage. If the steering slop is one-sided you will tear-up one side of the car. If both-ways you tear up both sides. Even if one-sided slop, if you are drunk enough you will tear-up both sides of the car as you bang from wall to wall. So don't drink and park.
High-order distortion is typically weak and masked by low-order until you get to Gross Overdrive. Yeah, we do that. Up to the 9th(?) harmonic is on the musical scale, the 11th and up is nasty; also for guitar fundamentals a spurt of partials up around 3KHz is ear-piercing and some may say "unmusical". Bandwidth limitations (guitar speaker top-cut) and rounder clipping reduces this, but also reduces the "good" 2nd and 3rd harmonic.
_______________________
> Transistors work in another way, they do not add much harmonics to the signal at clean level
Naked BJT transistors distort *worse* than tubes or FETs. Just below hard-clipping they will do 26% of 2nd, versus 5% for tubes. However they have SO much nonlinearity and gain that we almost "never" run them without significant negative feedback. When we do, it is because we *want* hard clipping as soon as possible, and never work much lower.
_______________________
The waveforms you see in steady-state 'scope tests are not what you will get with guitar. Usually (not always) there are caps, and an overdriven stage will charge-up these caps as it overloads. For tube interstage coupling caps, a typical response is a burst of distortion on the initial transient and then the stage un-biases itself to a different, sometimes lesser, level of distortion, and lower still when/if the note decays. In the extreme a tube grid with a HUGE signal can de-bias itself so far it quits amplifying, "blocking". Dancing on the edge of grid-block has been a key tone for some classic guitarists. In any multi-stage amplifier chain you can get a variety of tones this way, but you can also "stun" the amp so it is tricky business.
_______________________
All "simple" distortion mechanisms are VERY level-critical. 3rd harmonic typically rises as the *square* of the signal level. The idea of a distortion effect which will take "any" signal of any level and deliver a constant racket is probably folly, or at needs a very complicated mechanism.
"... Dancing on the edge of grid-block has been a key tone for some classic guitarists. In any multi-stage amplifier chain you can get a variety of tones this way, but you can also "stun" the amp so it is tricky business. ... " - that's why the famous Tube Screamer is so popular.
It is not clear whether you want to simulate an amplifier with self-bias through a common cathode resistor in the output stage or fixed bias through a negative supply to the grids. The overload characteristics are quite different. If your output tube requires -14 volts bias and you feed in a signal that that goes positive 15 volts, the grid will act like a diode and conduct. When the level goes back down, the extra electrons pulled out of the grid will increase the bias and the amplifier may cut out briefly (sometimes called blocking or gulp distortion) with a time constant set by the RC time constant of the grid resistor and coupling cap as well as other source resistances. If the amplifier is self-biased, this may increase the current through the cathode resistor and whatever capacitor is across it so you have another RC time constant that may overload. It sounds like the same thing is going to happen in both cases, but consider a waveform with a positive spike and a longer, lower negative side. With fixed bias, the tube conducting during the positive side will pull grid current but the other tube will not, so only one side blocks and you get a lot of even-order harmonics. With the common cathode resistor, both sides block because bias on both tubes is affected equally and you get mainly odd-order harmonics.
What do we intend to simulate, fixed bias or cathode bias?
Similarly, the long-tailed inverter (basically a differential stage with a cathode follower feeding a grounded-grid amp) and a triode phase splitter will have different harmonic generation. Any imbalance adds to even-order harmonics.
What do we intend to simulate, long-tailed or phase splitter or maybe one of the more esoteric ones like the Van Scoyoc cross-coupled inverter or the humble paraphrase inverter with its built-in delay and frequency limitation on one side only?
Also, tube capacitances are quite low and do not vary all that much with the bias point. Semiconductor capacitances are so high that even MOSFET output stages require high drive current just to charge the capacitances. Furthermore, the capacitances get higher at lower voltages meaning that a waveform that looks OK at or near DC deteriorates at high frequencies.
Finally, tube gain varies with 3/2 power of bias whereas FET's are square law and bipolar transistors have an exponential change of gain with bias. You might get something that sounds vaguely like a tube amplifier with solid-state devices, but only empty-state devices give you the sound of tubes.
> ... to emulate the sound of a tube power amp.
As someone said, long time ago, tube amps are a continuum of things. You can emulate some of those things, or others, but not ALL those things, but wanting all, you really need ALL the tubes (and btw the output transformer).
Note also that not all tube amps sound the same.
Emulation of "tube power amp" is a ill defined quest.
This question pops up, every now and then, to everyone that builds stompboxes... You need to go beyond that, be creative and use low voltage devices to create something different (it may sound like any other existent device ;) ).
A simple way of getting power amp distortion is to build an amplifier, the speaker plays a important role on the final outcome.
Or, simply, go digital.
> Semiconductor capacitances are so high that even MOSFET output stages require high drive current just to charge the capacitances. Furthermore, the capacitances get higher at lower voltages meaning that a waveform that looks OK at or near DC deteriorates at high frequencies.
I'm a big fan of high power MOSFETs, they are in a certain way simple to work with and with simple math (only need to solve quadratic equations no need to use the Lambert function). You can work out a SRPP in a A4 piece of paper (anything really, A4 is big).
A simple way of getting power amp distortion is to build an amplifier, the speaker plays a important role on the final outcome.
Another experiment: get a power resistor 22Ohm, an IRF something MOSFET (510, 610, IRFP240, ...) and a laptop power source (18V, 6A). Two caps: input cap 100n-1u, ouput cap 470u-1m. One 10k to 100K resistor and try:
(http://www.diale.org/img/Untitled.png)
Use any booster/OD pedal as a pre-amp.
Does it sound like a tube amp?
Cheers.
P.S.
The booster/OD pedal will complaint.
It is similar to a statement made about high fidelity: show me a transistor stage that will swing 50 volts at less than 1% distortion with no feedback. The 6SN7 can do that.
Also, trying to get a clean push-pull stage with little crossover distortion. Not difficult with tubes, not easy with transistors.
Yes, it's not as easy if you insist on following the pointless design rule of no feedback. Are we trying to attain high fidelity or some characteristic power tube distortion? If the former, feedback is your friend, and a $5 chipamp will gladly swing 50 V @ 0.1% THD. But I think this thread is about the latter.
^ The input impedance is too low for what a "regular" OD would expect, but the opamp of the Shaka HV would do the trick...
Try and breadboard it... I could draw the complete schematic if you would like to have a go.
P.S.
Or: http://www.diystompboxes.com/smfforum/index.php?topic=107374.msg975651#msg975651
Quote from: teemuk on March 06, 2015, 09:43:51 AM
Quilter:
(http://oi57.tinypic.com/zjjus7.jpg)
Once again a complete power amp. This one simulates crossover distortion ("Zero crossing process") and sag modulated clipping at plates ("Soft clipping cell"). Low damping factor is again designed in to the final power amp stage with the aid of current feedback. Again no phase inversion per se but same goal achieved with bipolar reference voltages and diode polarities.
Hey Teemuk, where'd this schematic come from? Is that part of the Tone Block 200?
Quote from: Sheldon on March 07, 2015, 05:04:07 AM
Modern amps have most of the distortion done with preamp.
My aim is to build the more compact as possible pedalboard to use instead of my amp head + cab for rehearsals or little gigs where I don't want to carry my amp+cab.
I'm building a tube stompbox preamp, I plan to buy a cab sim (ada gcs2?). I'm thinking of adding a compact "power amp sim" stompbox, not really for distortion, but only to sweeten/soften the preamp distortion (I don't know exactly which word to use, you know sometimes preamp distortion can be a little raw and harsh).
- A simple option can be the Runofgroove Thunderbird (only the last stage) with maybe adding teemuk embodiments (but as I'm a newbie I have to understand how all of these work!)
- KMG micropower seems another good option (and maybe if the embedded cab sim is good I can use it instead of buying an ada gcs)
- Johan project could be another good option but initially I didn't plan to use tube to make it as compact as possible. And I already have the "tube sound" from 2 12ax7 in the preamp. The only thing I'm looking to achieve is to add a little bit of the power amp stage feeling in the sound to sweeten the preamp distortion and make the preamp + amp sim + cab sim act more like a real amp.
Run your tube preamp into a laptop and use impulse response with a good software cab sim. Or use something like JamUp Pro and just use the power amp and cab simulation, you can run that on an iPhone! Or, buy a Torpedo setup. I doubt you will find better in a stomp with all of the options that these would have!
QuoteHey Teemuk, where'd this schematic come from? Is that part of the Tone Block 200?
http://www.google.com/patents/US20130136278
Why not run a 0,5 - 1 W tube amp (for example with ECL / PCL range) into a dummy load with high impedance output tap into SS?
I think it is the way Eddie Van Halen did it, but smaller.
Hello everyone!
I'm looking for ideas and information about tube power amp emulation. There are some brilliant thoughts here regarding the subject by teemuk and others. It's a pity that teemuk's schematic example from the post #6 (first image) is no longer available. I'd really love to have a look at it.
Quote from: teemuk on March 06, 2015, 09:43:51 AM
Something I've been cooking up lately. This simulates voltage sag, sag modulated soft clipping, and effects of grid conduction clipping such as crossover distortion. Low damping factor is also simulated.
(http://oi60.tinypic.com/15z41o7.jpg)
Wayback Machine doesn't help (recovers only the third image).
If anyone has that schematic, please repost it here or PM me. Any help would be appreciated.
Quote from: teemuk on March 06, 2015, 09:43:51 AMSomething I've been cooking up lately. This simulates voltage sag, sag modulated soft clipping, and effects of grid conduction clipping such as crossover distortion. Low damping factor is also simulated.
(http://oi60.tinypic.com/15z41o7.jpg)
Diodes after phase inverter stage simulate grid conduction, the other series diodes clipping at the plates. Reference voltage for this clipping threshold is derived from a differential amp that substracts output of an envelope follower from a steady DC reference. Last stage simulates response of an amplifier with low damping factor drioving a reactive speaker load. The circuit probably needs some tweaking, such as some pregain and some crude voltage limiting so that the input stages sans diode clipping won't get overdriven. Signal should never be allowed to swing above +/-13Vpeaks.
Something Peavey uses ("T-Dynamics"):
(http://oi61.tinypic.com/314rvk9.jpg)
This is actually a complete power amp that simulates effects of grid clipping and associated bias shifts (crossover distortion) and the moderately soft clipping at plates. Sag is not simulated. Low damping factor is designed in but it requires that the final power amp section and a loudspeaker load is included. You could likely convert that to run at lower voltages and currents to a simulated high-impedance reactive load. This circuit does not use phase inversion but relies on bipolar reference voltages and polarities of the diodes.
Quilter:
(http://oi57.tinypic.com/zjjus7.jpg)
Once again a complete power amp. This one simulates crossover distortion ("Zero crossing process") and sag modulated clipping at plates ("Soft clipping cell"). Low damping factor is again designed in to the final power amp stage with the aid of current feedback. Again no phase inversion per se but same goal achieved with bipolar reference voltages and diode polarities.
I came across a post you made from 2015. I am curious what you found out and if anything ever became of this?