better clipping protection than V-I limiter?

[phatt]

I want to try to design an SS amp that has some reasonable limiting on the preamp stages, but it is very hard to "softly" clip due to the crest factor problem.  The amplitude of the guitar signal's peaks are very large compared to the average signal.  So I haven't figured out any good approach that seems worth building.  I have tried the easy circuits (like diode limiters) that you see in the old schematics but they always sound very bad compared to a good tube amp or the digital processed modelers.  Anyway, I keep looking.  Thanks.

Forgive my somewhat ignorant observations here,,,,,
but why not use an Output Transformer? 0:)
Phil.

[Neosho]

Hi phatt:

You asked, "why not use an output transformer?"  Good question...

I am not sure if it would help or hurt.  It seems like it would be very good at preventing DC offsets from getting to the speaker and damaging it.  Instead, the amp's transformer would become damaged.  At least you wouldn't be frying speaker coils due to a transistor getting shorted to the power rail, so that's some progress.

I know that old amps like Acoustic used transformers, but maybe it was because they were limited on the availability of transistors (only NPN? was available for awhile).

The hi-fi amp books I have read argue against output transformers.  I have Doug Self's book and Slone's book.  But their interest is sound reproduction.  The cons are: transformers add cost, aren't really needed, cause phase shift, make the amp more narrow band, add weight and mechanical complexity.  Certainly you never see output transformers in newer SS amps built by Fender and Tech21, etc.

I have seen the theory that transformers have some ability to saturate non-linearly, and that is part of vacuum tube tone.  I think there is even some patent where the author introduces DC offset on purpose to try to imitate tube tone.  I don't know if these things are provable or just wishful thinking.  I know the some of the tube people think that it's helpful to use a high class transformer to get the best sound, and there are differences in the construction that are important.  Mercury Magnetics sells an upgrade transformer kit for the Valve Junior for asking price of $299... seems like a lot of money for an amp that can be seen on craigslist for $100.

Do you have a theory about output transformers?

[Brymus]

Hey Phatt check this out
http://www.bonavolta.ch/hobby/en/audio/2sk135se.htm

And this It uses a transformer with a set of MOSFETs
http://www.turneraudio.com.au/solidstateamps4-50w-mono-mosfet.htm

I found them in this thread http://www.diystompboxes.com/smfforum/index.php?topic=79034.0

That Turner guy has alot of neat designs and ideas look at his rebuild of that old JBL into an instrument amp.
And check out his pre-amp designs really cool.
I hope this helps.

[ponchojuan]

Teemu's comment is quite accurate.  Proper V-I limiter specs should not effect the amp signal significantly  if done correctly.  Trying to overdive transistors like they are tubes gets quite ugly.  They are not the same and don't respond  the same as they are driven out of design range.   I can't tell you how many bad amp designs in the early 1970s tried this a failed badly.  This being said, you can design circuits that mimick the distortion characteristic of tube amplifiers (clipping, compression, power sag, etc...) designs from the 1980s started this and it still continues, but cheap DSP has made this sort of old school.

Why not go the other way>  use solid state to better manage the characteristics of tube circuits?  Probably not very cost effective for commercail application but sure a fun possibility for a cool one off amp with lots of effects control.

[phatt]

Hi phatt:

You asked, "why not use an output transformer?"  Good question...

I am not sure if it would help or hurt.  It seems like it would be very good at preventing DC offsets from getting to the speaker and damaging it.  Instead, the amp's transformer would become damaged.  At least you wouldn't be frying speaker coils due to a transistor getting shorted to the power rail, so that's some progress.

I know that old amps like Acoustic used transformers, but maybe it was because they were limited on the availability of transistors (only NPN? was available for awhile).

The hi-fi amp books I have read argue against output transformers.  I have Doug Self's book and Slone's book.  But their interest is sound reproduction.  The cons are: transformers add cost, aren't really needed, cause phase shift, make the amp more narrow band, add weight and mechanical complexity.  Certainly you never see output transformers in newer SS amps built by Fender and Tech21, etc.

I have seen the theory that transformers have some ability to saturate non-linearly, and that is part of vacuum tube tone.  I think there is even some patent where the author introduces DC offset on purpose to try to imitate tube tone.  I don't know if these things are provable or just wishful thinking.  I know the some of the tube people think that it's helpful to use a high class transformer to get the best sound, and there are differences in the construction that are important.  Mercury Magnetics sells an upgrade transformer kit for the Valve Junior for asking price of $299... seems like a lot of money for an amp that can be seen on craigslist for $100.

Do you have a theory about output transformers?

Hello Neosho,
                  I think *Brymus* found some good ones to study,,
Thanks Brymus Very good stuff there. :tu:
*Neosho* asks,,  Do I have a Theory?  Oh Yuk I hate those things.
For me it's called Trial and Error,, (mostly ending in Error though) 
I'm just a hobby geek,, I'm useless at maths and many other things in life but music and the workings of Audio systems have always facinated me.
So ever so slowly I just absorb stuff.

To cut to the chase, most muso's covert the sonic result of output stage compression/distortion found in *some* tube amps. (not all I should add)
It would seem that no matter how many preamp stages and or distortion gizmos are used it just can't quite deliver the same sonic result.

I can only assume this is what you are chasing.  Join the  :duh club
If I could do this trick with SS I would but no one has ever been able to reproduce *THAT Sound* via a SS system.

First lets establish what we are shooting for.
I'll roughly quote the words of *Kevin O'Conner*,
"If you wish to Maximise Tubyness,, a Small PP Common Cathode bias,, no FB,, and moderate HT."
The one big advantage of valve power stages is there very large range of dynamic compression.  The Above just maxes that effect.
So a super clean 100 Twin is a mugs game if you are looking for all the inbetween dynamics. I built a tube Amp on that advice and indeed I got exactly what I wanted.
So if you want to achieve it with SS then that might be a good refference point.

If I was to have any theroy it would be THUS:
Transformer coupling (ie, the classic PP output stage) is unique in that it *Floats* the AC signal on a HIGH DC Volatage potential.
In my limited ability to explain such things all I can think of is a concertina effect or spring where the AC signal can rise and fall and (via some quirks in transformers) can at times rise momenterily *Well above* the DC inside a transformer primary winding.

Surely this Just can't happen in the modern DC coupled SS Amp where the signal (at the output) is basically at Ground.
There is also a Very Rigid PSU in SS and hence the signal litteraly *Slams into a brick wall* of rigid DC,,, Limiting the ability of ANY large signal swing. The result is a hard clip along with a whole lot of other things.
I'm probably talking to a lot of experts so no need to rehash the obvious.

There are some points that don't get much attention one being the *Response Curve Difference* between SS an Tube output stages.
A SS power stage is almost always *Dead Flat* often from very low (ie, 10 Hz or lower) whereas Tube power stages of calssic valve Amps can have quite dramatic curves. You can tweak DC powerAmps a bit but all the major tone tweaking has to be done in the pre stages. Interesting to note that Valve amps of a past era often had masses of bass in the preamp section and often very steep rolloffs below 100Hz in the power stage.
I'm far from the expert but my guess is it's gonna sound a lot different.
I'll leave the better qualified folks to debate whether that be good or bad.

The other point that gets missed is pentodes have a *Screen Grid* !!!
Try and find a SS device with one of them??
*Brymus* might be interested that although the *Turner* stuff looks very slick (the big PP 50 watter, which is likely a good SS HiFi amp) It probably whould not deliver the goods for guitar amps.
Just take a squizzy at some *Early fender deluxe* or similar tube Amps and *Take note* of the BIG Screen drop resistors in some of those schematics.

By far the fastest cheapest way to make a Tube Power stage *Compress* (early breakup) is to Drop the screen voltage. This has a huge impact on how the power stage responds to the incoming signal.
Modern valve Amps tend to run insanely high HT ,,have very little screen drop (ie, the screen is only a couple of volts below the plate). It becomes darn near  impossible for the output to compress unless it's fully dimmed so they *Have To* add many fancy tweaked up preamp gain stages and master volume knobs just to get some kind of distortion.
Hence it all becomes needlessly complex and a never ending battle to extract a good dynamic range to work in.
(some refer to this as,*the sweet spot*)


Back to OT's;
Whether you wish to design SS or Valves there is one *Transfomer Fact* I've discovered in my neverending journey.

Obsolete Electronics web pages (now gone I believe) had a very good little article about the Primaries of OT's and how they relate to *Guitar amp Tone*.
It seems that the *Lower primary windings* tend to **Accentuate the Second Harmonic content**
And it is widely accepted that these are the harmonics we like to hear.
The artical had refference to the original artical which was written in the 1940's so it was well known way back then.

Now add another *Known Fact*,,Some Early Marshall Amps had primary windings as low as 1700 Ohms pp (plate to plate)  (That's for a quad 100watter). A world away from HiFi Specs,, LOL.
Todays Marshall's it's more like 3000 Ohms pp. (I think?) and they sound nothing like the old ones.
Some early Marshall's are now revered for there magic tonal qualities.
To me it's obvious you don't want an OT that is capable of 300kHz bandwidth for great guitar sound.
So paying big money for a top end OT complete with all the fancy interleaving and exotic mu metals is likely a conterproductive waste of money. Tube or SS!!

I have built a 6V6 Amp firstly using a 6k6 OT and not happy I searched for a long time to understand my problem.
Long story short;
Without altering anything in the 6V6 circuit,,, I swapped for a 2K6 OT,,, BINGO!!! Insant tone heaven was the result.
So from my humble experience there is no *Theory* anymore it's more like concrete evidence that the Transformer plays a BIG BIG role.

Even without a *Screen grid* I tend to think a SS Amp would benifit greatly from a transformer coupled output,, much like the idea of the Turner Amp.
The b*t** for me is I will never possess the knowledge/skill needed to design a Tranz Coupled SS poweramp from scratch.
I've released my fair share of magic smoke from perfecty good Semiconductors, LOL! Even tried some back to back zeners across the feedback in dc poweramps and although it does compress it's nowhere near good enough.
*Joe's* come up with his neat little diode trick but looking at the wave forms produced I doubt it will be a serious contenter as it lacks the ability to saturate and square wave that is seen inside the OT of
a Valve Power Stage.

FWIW, I have opted for a Hybrid, that being a Modular Reamping setup where I soak a small Valve Amp,,, re eq and reamplifiy it via a big SS powerAmp.

For any short comings this system may have,,For $Cost outlay$ and importantly absolute control and flexability this is darn hard to beat.

Being able to Re EQ *After* the power tubes makes for a setup that delivers closer to *any toneshape, dist, any wattage level I wish to play at*. I can play in the sweet zone all night long.

Oh well that's bout it,, I wish you luck in your goals and hope you get it to happen.
Don't forget to have fun and play music also, Phil.

[teemuk]

Just to make few quick points:

- It is certainly possible to design a solid-state (power) amplifier that saturates gradually instead of slamming into "brick wall" clipping, as you say. It's not even extremely difficult and many examples of commercial applications can be found that do exactly this.
The downside is that this design principle sacrifices some of the amplifier's power reserve to extend and achieve this non-linear region of operation. Say, you have a power supply capable of supplying a 60W continuous average amp. On the other hand, you use a soft clipping scheme that makes the amp roll off into clipping already at 30 watts average. When the output slams above 120W peak you get the "brick walling" effect. You have a lot of "dirty" headroom now, just like in tube amps that have not been linearized to provide the maximum power the supply allows. On the other hand, instead of a 60W amp your amp is now only a 30 watter. What do you think will sell better, the bigger rating or the smaller, albeit the smaller might sound as big and more pleasing? The markets have proven that as long as we talk about solid-state amps, the first one.
- Solid-state amps do not have a linear response. Most of them have almost similar response as tube amps. Current feedback schemes to increase the output impedance have been used since roughly late 1960's and these days they are almost a standard. With mixed mode feedback a power amp will have almost similar frequency response to loads as a tube amp. Preamps are not linear either. Most modern high gain amps have serious bandwidth limiting where the low frequencies may start to roll off already at about 1 kHz, the highs at 5 kHz. The tone controls (or additional circuitry) deliberately introduce midrange scoops, bass and high boost, etc. Even most of the jazz amps are far from being linear from their frequency response.
- Transistors do not have screen grids but those are elements that can be simulated. Pritchard has many interesting applications of such that base on varying bias current of a transconductance amplifier.
- I don't see the big deal in a transformer coupled amplifier swinging past supply voltage limits. Yes the signal at the collector/plate can swing above B+ but there is no magic in it because the signals zero crossing is centered at DC voltage equivalent to B+. That's just how the thing works. Equivalently a bootstrap capacitors may shift collector voltage of VAS above Vcc limit or grounded emitter amplifiers modulate the power supply rail voltage with a voltage equivalent to output signal. Nevertheless, in all those cases the supply voltage still imposes a certain limit to maximum voltage swing and exceeding this limit will cause clipping. Anything that deviates from this operation is abnormal and may damage the amp. Most tube amps have reverse diode shunts at output tube plates to protect transformers and power tube sockets from this and they will short to ground if induction creates serious voltage peaks. Likewise solid-state amps have diodes hooked from output to supply rails for protection against such "kickbacks".

Anyway, this has been completely off topic so my apologies.

[Neosho]

Hi phatt,

As far as what I am chasing, probably it is what happens with most people who mess with guitar amps.  You read reviews, get a cheap example, it is OK but not perfect, read more, get another amp, it is OK but not great, read more, and so on.  Several months ago I was trying to build circuits and see how they sounded but can't get motivated lately.  The amp mfgrs already have so many excellent products, and right now prices are very good, so I am sure I won't be able to rival them.  But it is fun to learn about the concepts... maybe I'll get motivated again.

Thanks for sharing your views.

Hi Teemu,

One thing I wonder about is whether the marketing of "30W versus 60W" might someday be less important.  There seem to be a lot of sales right now of the little 5W tube amps like the Valve Jr and Blackheart.   Even with the higher cost amps, the Mesa Express 5:25 / 5:50 and Egnator Rebel are selling because people realize it just doesn't make sense for most people to own a 40W or 50W tube amp.  Maybe something like this will happen for SS.

[phatt]

Neosho,
       Before you part with your money on new gear just get a simple sequence of events like this;
A Parametric EQ>> dist pedal or box>>> A Graphic EQ >> into a SS Amp.  It might help you grasp just how important *ToneShaping* is.

Even with such a basic setup EQ before and after Dist will sound far more convincing than just relying on overated promises of advertising blab. insert fancy brandname Amp.

FWIW I've just recently been inside both a Tech21 Trademark 60 and a Fender Performer 1000.
Both these amps seem to be using CFB among other tricks and Seriously niether Amp comes remotely close to the Control over dynamics I have available via my Setup.
The P1000 was dated 92 and trademark is about the same era.

Neither of these amps have any ability to develop a sweet spot between clean and compressed.
Yet I have the original marketing phamphlets for the Trademark60 and it's just so OVERHYPED and overrated.
I doubt if the other makers are any more or less *Honest* in their appraisial of their own equipment.
Phil.

[phatt]

Hi Teemu, Thanks for your input, Obviously you have far more knowledge than I will ever attain and I seriously respect your ability and taking time for us less knowledgeable folk.
Judging by Neosho's last comment you are not far off the topic

My main point was The **PowerAmp Frequency Response curves** surley most of what you refer to is *Preamp tone shaping*.

Unless I'm seriously misguided the Response Plot of a modern DC coupled *PowerAmp* is almost always flat, I do understand that some tweaking of this is possible but probably not to the extreme that is achivable with Old Valve circuits.
Yes, I'm fully aware that in the *PREAMP* Stages of modern SS gear their is a lot of tone tweaking going on.

No one seems to give any thought to the SS powerstages cept for some defined impeadence (or current FB) which in my experience does almost nothing to the tonality of the output.

With Guitar Valve PowerAmps, Transformers can be (Are) wound to different specs to alter these curves. then there are interstage coupling tweaks and other places in the power stage that alter the Freq curve/line.
In some classic Valve circuits it seems there maybe a 40Db or more drop from 100Hz down to 10Hz. I've not found a SS DC PowerAmp where this kind of cut is possible.

To say the least; "Big tone shaping curves" is not an inherint trait of DC power stages and although I obviously have limited knowledge on Exact details I have fiddled around long enough to pick up on some of the basics.

I assume you get to play with more toys than I ever will so point me to a SS circuit that can reproduce an Early Marshall powerstage Comp and tonal Honk.
I won't live long enough to build every theroy that comes along and most times you have to take a half educated guess at what is worth wasting time and money on.

I've not stumbled across anything cept for the Valvetronic concept (looks slick, sounds average) that comes remotly close to the sound I'm wanting.
Thanks Phil.

[R.G.]

There is one thing you always need to keep in sight when you're thinking about how to get tube sounds out of solid state gear, or nicely compressed and/or distorted and/or whatever that you think a good tube amp does.

What we mostly listen to guitarists on is a solid state amp. It's the one in our stereos, or the one in the group's PA system.

Whatever mystical system of tubes, power supplies, transformers, exotic wiring, turret boards, point to point wiring, or voodoo-based deals with the devil that has gone into making some magic guitar sound, it is by far more common to listen to that through a solid state PA system at a concert or through a stereo amp from the recorded CD or other medium.

So it is clearly **possible** that a solid state amp can sound like a tube amp if it's fed the right signal and drives that signal through nice, linear speakers. The whole trick is what to feed that SS power amp at its input so that its response times the response of the speakers has the right tonal quality and you get to hold and play the guitar at the front end that ends up in the signal to the power amp. The entire tube vs solid state argument can be reduced to this.

[armstrom]

Way off topic here, but just to respond to R.G.'s last post:

This is why I plan to build a nice, low wattage tube amp that has exactly the sound I want (Maybe a Fender Champ design, AX84 P1, or even a 1W firefly) then get the best dummy load you possibly can and feed it into something like a LM3886 power amp.
The trick is finding a good dummy load and voicing the power amp/speaker combination correctly. My end goal is to have an amp that I can switch between having the tube amp drive the speaker directly or have it drive a dummy load and let the SS power amp drive the speaker. The only noticeable difference should be the maximum volume available (at least that's the goal!). We'll see if I ever really get there

It all boils down to how do you want to achieve the "tube tone" in a louder, lighter and more reliable amplifier package. I see two possibilities, you can either get the tone in the same way people have been doing it for decades, then just make it louder with a high quality SS power amp. Or do you invest lots of time and effort trying to "emulate" the sound of a tube amp without using tubes?  Depends on what you're after really. It's almost a guarantee that the cost of building a hybrid amp will be significantly higher than an emulating/modeling amp just because of the "iron" required by the tube section.

It seems to me that the "tube sound" has almost been relegated to only a few genres of music (dirty blues, for example) and a set of purists who must have it for what ever reason. If you look a areas that used to be dominated by tube amps (hard rock, heavy metal, etc...) you have to admit the trend seems to be moving toward using extensive effects systems (pedals, rack-mount modelers, whatever) feeding into a super clean solid state amp like a JC-120 or something similar. The wall-o-marshalls just doesn't seem to popular any more

It's just my $0.02 though. I've likely completely missed the mark on this.

[phatt]

Way off topic here, but just to respond to R.G.'s last post:

This is why I plan to build a nice, low wattage tube amp that has exactly the sound I want (Maybe a Fender Champ design, AX84 P1, or even a 1W firefly) then get the best dummy load you possibly can and feed it into something like a LM3886 power amp.
The trick is finding a good dummy load and voicing the power amp/speaker combination correctly. My end goal is to have an amp that I can switch between having the tube amp drive the speaker directly or have it drive a dummy load and let the SS power amp drive the speaker. The only noticeable difference should be the maximum volume available (at least that's the goal!). We'll see if I ever really get there

It all boils down to how do you want to achieve the "tube tone" in a louder, lighter and more reliable amplifier package. I see two possibilities, you can either get the tone in the same way people have been doing it for decades, then just make it louder with a high quality SS power amp. Or do you invest lots of time and effort trying to "emulate" the sound of a tube amp without using tubes?  Depends on what you're after really. It's almost a guarantee that the cost of building a hybrid amp will be significantly higher than an emulating/modeling amp just because of the "iron" required by the tube section.

It seems to me that the "tube sound" has almost been relegated to only a few genres of music (dirty blues, for example) and a set of purists who must have it for what ever reason. If you look a areas that used to be dominated by tube amps (hard rock, heavy metal, etc...) you have to admit the trend seems to be moving toward using extensive effects systems (pedals, rack-mount modelers, whatever) feeding into a super clean solid state amp like a JC-120 or something similar. The wall-o-marshalls just doesn't seem to popular any more

It's just my $0.02 though. I've likely completely missed the mark on this.

Hi armstrom ,
No I don't think it's too far off,,, in fact you are closer to tone heaven than you think. 0:)

You are not missing anything, It's by far the most useful Amp configuration I have used, EVER!!
If you wish I will post some drawings of my *PhAbb ReAmp* setup to show how easy it is to impliment such a system.

My tube Amp section is just a 2 knob simple affiar built from old radio parts.
All the other tweaking is done via simple SS gear.
Compared to the cost of big name gear (SS or Valve) my setup is dirt cheap.

Cheers Phil.