better clipping protection than V-I limiter?

[Neosho]

Does anyone have links to a better design than the usual V-I limiter?  I think the V-I limiter would degrade the sound of a guitar very much.  First, even in an ideal simulation you can see it cuts off the tops of the signal, making distortion that is obvious.  In the real world the V-I limiter would also cause DC imbalance because the components that limit the positive peaks are going to be different than the negative simply because of part-to-part tolerance variation.  Also in the real world the music signals are not symmetrical sine waves that would clip evenly.  Instead the positive peaks of the music signal might be very narrow while the bottoms are wider.  So you might only limit the positive peaks, thus causing another source of DC imbalance.  The DC imbalance is not good for the speaker, so the supply will be affected, and this will lead to further degradation of the limiting. 

I also don't like the idea of the V-I limiter because you have two control loops fighting for control of the signal (the global negative feedback versus the local control of the limiter).  It seems like timing problems will arise during transitions (into & out of limiting).

So is there a better method?

[R.G.]

A lot depends on what you consider better.

V-I limiters are not there to limit the signal from overloading the amp. They are there to prevent damage to the amp. It doesn't much matter what a damage-prevention method sounds like, if not having it means smoke and flames  - or just dead, permanent silence - pouring out of the amplifier box.

Your observations on the sound and limitations of V-I limiters are correct. So you need to ask yourself: would you rather listen to the V-I limiters sometimes inserting a bit of grak or to have to send the amplifier into the shop to have the output transistors replaced whenever the V-I limiters operated?

[teemuk]

I think there's also the issue that the very simplest VI limiter (the one with just resistor divider circuit controlling the base) cannot be adjusted properly to detect the safe operating area of a device. The safest SOA approximation will cause clipping during normal operating conditions, the unsafe and inaudible one is, well, unsafe.

This phenomenon has been known since the 1970's and the circuit can be improved tremendously as is documented for example by Eric Mendenhall in his patent and AES whitepaper. Built properly, the VI limiter can be made perfectly inaudible under normal operating conditions.

[Joe]

You can use zener diode limiting (look at some Fender SS schematics). This arrangement may not handle a short for as long, though.

[Neosho]

...It doesn't much matter what a damage-prevention method sounds like, if not having it means smoke and flames  - or just dead, permanent silence - pouring out of the amplifier box...

...would you rather listen to the V-I limiters sometimes inserting a bit of grak or to have to send the amplifier into the shop to have the output transistors replaced whenever the V-I limiters operated?...

Hi R.G.,

I am not sure the picture is as back/white as you paint it.  I was hoping someone would reply with a link to some superior limiter design, something that was better than the typical V-I limiter.  I think we all recognize that vacuum tubes don't use V-I limiters, and when overdriven into clipping they do not usually cause smoke & flames.  So it seems to me there ought to be a SS limiter that avoids "grak" yet also protects against damage to the outputs.

[teemuk]

http://www.schematicsforfree.com/archive/file/Audio/Circuits/POWER%20AMPLIFIERS-LINEAR/GIBSON%20POWER%20AMPLIFIER%20PROTECTION%20CIRCUIT%20PATENT.PDF

http://www.schematicsforfree.com/archive/file/Audio/Circuits/POWER%20AMPLIFIERS-LINEAR/AUDIO%20POWER%20AMPLIFIER%20OUTPUT%20STAGE%20PROTECTION.PDF

[R.G.]

I am not sure the picture is as back/white as you paint it.  I was hoping someone would reply with a link to some superior limiter design, something that was better than the typical V-I limiter.  I think we all recognize that vacuum tubes don't use V-I limiters, and when overdriven into clipping they do not usually cause smoke & flames.  So it seems to me there ought to be a SS limiter that avoids "grak" yet also protects against damage to the outputs.

Ah, so the question is really, "why can't we somehow overdrive transistor amps like tube amps and still have them live?"

OK, that's fine. It is entirely possible to do that. You just have to define what you want to happen.

First, V-I limiters are not so much intended to protect against overdrive as to protect against abusive loads. You may freely overdrive a SS amp to any degree you like with no V-I protection on it at all, as long as the reactive nature of the load does not force the output transistors into failure. Overdrive does not kill amps, at least not well-designed ones. The voltage/current/ phase on the output of the amp may do that, though. The Thomas Vox amps and others designed before about 1970 *were* sensitive to overdrive because they were marginal to start with, and were designed before people really understood second breakdown and how to both prevent it and ameliorate it (i.e. with limiters).

Second, V-I limiters are actually an economy move. The real solution to being able to drive any load, including nominal short circuits, is to keep adding output transistors to increase current output and power dissipation ability. When you add enough voltage/current capability in the output stage to drive the overload non-distructively, V-I limiting is not needed. There are, after all, solid state *welders*. However, it is much cheaper to used a few $0.05 signal transistors, resistors and diodes than to keep adding pairs of output devices at $5.00 a pair and also heatsinking to support some overload point.

Third, you need to separate issues. Making a solid state amp, any solid state amp immune to input overdrives is simple: you put a limiter ahead of the amp and feed the amp a signal that is pre-limited not to drive the amp into its own overdrive. Thomas Vox did this with their "big head" models, and many pro audio and some hifi makers have done this. The key word is "soft clip". The amp never knows it's being driven into overdrive, and indeed is not. The limiter ahead of the amp feeds the amp a pre-limited signal that's within the amp's linear range, so the amp's native clipping never surfaces.

This is different from what happens on the output when you either short the output or connect a funny load to it. Shorting the output can easily enough be handled by limiters without affecting audio, as there isn't much audio output from a short circuit to hear anyway. Funny loads are the one that needs either V-I limiting or enough output devices to drive them anyway. And actually, tubes *can* be killed by funny loads too. They're just different funny loads. As in - never run a tube amp unloaded, as this can cause an oscillation that can make the output tubes eat themselves. In fact, never run it too lightly loaded. And some loads can cause voltage spikes at the plates that arc across the tubes or tube sockets. The "protection circuits" for this tend to be either R-C snubbers across the OT primary or diode clamps to ground from the ends of the primary.

So no, the issue isn't black and white, but neither is it gray enough to simply put in a better V-I limiter. It's more purple and green. 

[phatt]

Hi RG,
        Arrh!! Beautifully explained in plain english.
Easy to see why you have so many chip points

Interesting to note that some of my first attempts at SS pwramp builds sounded quite good when driven hard. They do start to rattle and roll when cranked up but still not as good as a valve output section.

Just 7 BJT's and no saftey circuit cept for a PTC (Poly sw)

I guess it's a case of ;; if you want a good guitar amp build a cheap badly designed one which usually sounds better and far cheaper to make.

Which is not that much different to the old valve days and *Now* folks are trying to get old Tweed amps because they are starting to realise the basic amp designs of a past era distort in a much more user friendly way.

Even with their dismal power ratings they sounded so sweet.
you just can't get *the low power basic design magic* from a quad of 6550's running at some stupid voltage and probably running as tight as most modern SS poweramps. (great for Bass though)
My 2 cents worth. Phil

[Neosho]

Hi R.G.,

Your post is very informative.  I agree with the comment that the V-I limiter is an economy move.  Your suggestion of having everything over-built to the point that you can handle shorts / opens / transient shorts / odd impedance seems like a good approach to me.

Certainly we know from reviews of tube amps that they also have problems.  I remember reading that some of the tube amps that include circuit boards can develop permanent damage where the arcing (due to having too high load impedance or open circuit) causes the tube's PCB to get carbon burns that cannot be repaired.

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.

Hi Teemu,

Thanks for your links to the patents.  It seems like the goal in them is to sense temperature and back off the drive till danger has passed.  That seems like it might work.  It seems very complicated, though.

[Brymus]

Maybe this is a dumb idea,
But since this is for your amp,why not ditch the V-I limiters and just use a thermal shutdown device?
If your amp starts shutting down find the problem and fix it-probably right back to what RG said...
Overbuild it.Then it doesnt shut down anymore.And you still have some protection against fire from short.

[Joe]

I would recommend using the zener diodes in conjunction with the thermal switch, otherwise the transistors will roast long before the thermal switch opens. (see 1980's Fender SS amps).

[R.G.]

The really big difference in SS output devices compared to tubes is that bipolar transistors can die from second breakdown in microseconds. Second breakdown happens when some of the junction area gets hotter than the areas around it, and that region conducts more, with a lower voltage than the other areas. This makes it hotter, and that makes it conduct more. The phenomena is sometimes referred to as "current hogging". Old simple-structure power devices could go into second breakdown and burn a shorted hole through the die more quickly than one could sense the runaway. Modern analog bipolars have intricate internal base-emitter structures to keep current hogging postponed as long as possible.

The sudden-death failures was one reason protection was developed - sense the threshold of sudden-death just a few microns from the edge, and stop it before the transistor dies. Tube failures generally have the advantage of being much slower.

As for temperature sensing, I remember reading in Duncan's book on power amps about the world having quit waiting for semiconductor makers to include a temperature sensing diode inside a power transistor so things like proper bias and temperature sensing could be done. Recently, Sanken and On Semi have introduced exactly this, power transistors with built in temperature sensing diodes on the chip so that you don't have to try to sense what the chip is doing from the thermal distance of the case of the chip, which was the best one could do before.
But I have yet to see any designs use this. Maybe the whole design community just gave up and won't look back. Which is sad. This could have been done in the 1970s.

[gbono]

Not sure if the MI world is using DSP to create peak/RMS limiting protection or even a "soft clipper" (though a discrete version of a SC is also a very good solution). Is the issue that the input signal into the amp should not have any "non-linear" processing so one can overdrive the OP section? or is DSP still to costly? ??

[teemuk]

Those integrated thermal tracking diodes have some problems. For instance, they can't track the driver temperature, which makes them practically useless for CFP type outputs. Also, the diode current is not the same as transistor current, an issue which needs to be compensated. Unfortunately the compensation is usually as troublesome and uncertain as the discrete thermal tracking means.

I think the idea is definitely good but it's still in its very early stages.

Why not use lateral MOSFETs? They transition to negative tempco after the drain current grows too big. They are not totally free from secondary breakdown but are much more tolerant than BJTs.

[Neosho]

I noticed a couple of more questions in replies to my original question:

Hi Brymus:

You suggest "why not ditch the V-I limiters and just use a thermal shutdown device?  Overbuild it".

I think this is the general way I will proceed.  I need to study more about the causes for the failures, though, since the failure mechanisms might be more than just simple overheating.

Hi gbono:

You ask "Is the issue that the input signal into the amp should not have any "non-linear" processing so one can overdrive the OP section? or is DSP still to costly? ??"

I like the idea as you state above that avoiding preamp clipping would allow you to have a more flexible input section.  I think it would be nice if the preamp could handle any reasonable amount of voltage that you might find in a pedal. 

As far as DSP, I want to avoid overcomplication.  DSPs require a lot of tools and different skills than working with analog circuits and the DSP chips themselves are "here today, gone next year".  Right now the DSPs and high power processors tend to be going toward very low voltages and even require a couple of different power supplies.  And then you are working with tiny chips that require a professionally made PCB to be reliable (think of BGA packages).  So pretty soon you are not even working on an amp - you become distracted by the digital stuff. 

And anyway, there are no shortages of DSP based designs like the Line6 that have already taken things pretty far.  I have the Line6 XT Live Modeler and it is pretty good for what it does so I don't have an interest in trying to duplicate it.  It works pretty well but it does have limitations.

Hi Teemu,

The articles I have seen about the lateral MOSFETs seem to say that the parts are not very popular any more, and have become rare.  My ideal design would include very basic parts that anyone can find (from a place like Digikey or Mouser or Newark, etc.), and that you would still be able to build it 15 years from today.  No cadmium photocells, CA3080 chips, germanium, rare transistors, etc.