I thought I understood distortion for tube amps. It causes damage?

[ManisMan]

I was playing around with a 1200W amp in a local store today and I noticed that there was a red clipping LED which will occur if the input is too hot, and there's a -3dB button to reduce the signal. I looked up the manual and it says that if this light comes on, push that button.

I also have seen this clipping light on a friend's fender combo amp, which has a clean and dirty channel. I know that early guitar distortion was achieved by individuals trying to be heard and turning their gain up all the way causing a problem with faithful reproduction since the preamps are pushed further to extremes on the load line where there is more non-linearity, or the output transformer is driven to saturation which means that even if the power tubes 'request' more output, the transformer can't supply it.

To me this all seemed like the obvious reason we can get that light overdrive sound to heavy metal distortion, depending on how 'hard' past their limits each stage is driven.

But I checked out this amp head manual. It recommends not allowing your preamp tubes to distort. So I duckduckgo'd why. So many threads in so many places saying "underpowered amps compared to speakers cause speaker damage by pushing the amps too hard" or "distortion causes watt output to rise from 100rms to 200rms and that, plus high frequency spectral splatter, kills speakers".

I thought I knew how you could achieve a hard distortion sound.. push the preamp or power amps until the thing 'fails' on the eletrical engineering level (like how it was discovered before). It seemed like everyone does it and its safe. But it causing damage?

Is this why pedals for distortion are actually important to buy?

Hopefully this is the right spot to ask, if not please let me know and I can move it to somewhere else.

[92Volts]

That advice applies to hi-fi setups, not for guitar.

Distortion, for example a square wave, can be understood as a combination of sine waves... higher-frequency ones overlayed onto the base frequency. It looks like a low-frequency square-shaped wave but mathematically (and in terms of sound) there's high frequency content.

Hi-fi tweeters aren't meant to handle much power because in normal music, there's less treble power than bass. And they're protected by the crossover so even large amounts of bass power are played by the woofer and never reach the tweeter.

But distortion increases treble, which passes through the crossover and is played by the tweeter, which can damage it. It may have been designed for 10% of the overall audio power, but could be receiving nearly all of it.

Guitar speakers are meant to play distorted sound, and also... there's only one driver not a woofer/tweeter combination. So there's no reason to expect this problem to occur anyways, as long as the speaker can tolerate the wattage of the amp.

It is true that an amp will put out more power at its absolute physical limits than it can cleanly, and than it is rated for. So you would want a speaker rated for usually 50% more than the amp's advertised power, though this is sometimes violated (early Vox amps partly got their distinctive sound by over-driving their speakers) and it's not always a disaster... just a bit risky.

[pompeiisneaks]

Additionally to 92Volts points, solid state distortion can damage things, if the setup isn't designed well enough, because they get a more harsh type of clipping going on.  Tubes, when overdriven, make a more natural change to the driven signal instead of a harsh cut off.  All the other points are completely on point as well. 

Harsh solid state distortion into monitor speaker or the like can cause damage, and blow the voice coil out of them. (if too hot that is, at lower volumes it won't be a problem)

~Phil

[ManisMan]

Okay thanks. This is very interesting. Actually the reason I asked is because I have a deluge of links from guitar and bass forums saying that underpowered amps can cause speaker damage and it looked implied that causing breakup and distortion is the cause because of increased rms.

I'd like to ask a few more questions: is a class D 1200w amp equivalent to a lower watt amp that is class A? I ask because I've heard that a transistor uses more power when it turns on or off than when its in the active region. Or was I misunderstanding that? Is a class D amp more dangerous to push into distortion because of the way its pulses work?

[shooter]

here's a pros n cons article, and you can get as deep as your shovel will let you

http://www.audioholics.com/audio-amplifier/amplifier-classes

you might also pick up a Carver super 7 and do a side by side of the 1200w digital thing

[jjasilli]

Manis:  you're mixing apples & oranges


1.  I was playing around with a 1200W amp in a local store today and I noticed that there was a red clipping LED which will occur if the input is too hot
No doubt this a SS, not tube amp & preamp.  There are no 1200W guitar amps, so this must be a PA system with a mixer preamp.  Typically, overdriving a SS preamp, especially the ones in mixers, sounds godawful.  It is a different world from tube preamps. 


2.  . . .early guitar distortion was achieved by individuals trying to be heard and turning their gain up all the way  causing a problem with faithful reproduction
Guitar amps PROduce sound and tone.  Hi-fi and maybe PA amps REproduce sound and tone.  This is a most significant difference in function & design.

[92Volts]

I'd like to ask a few more questions: is a class D 1200w amp equivalent to a lower watt amp that is class A? I ask because I've heard that a transistor uses more power when it turns on or off than when its in the active region. Or was I misunderstanding that? Is a class D amp more dangerous to push into distortion because of the way its pulses work?

Amps are rated by their output power, not power consumption. So efficiency is complicated, but you don't need to worry about it. As long as the manufacturer is trustworthy, the label indicates how much power gets to the speaker.

Class D is more efficient, theoretically.
The transistors are either on-- dissipating little power since they pass it unrestricted...
Or off-- dissipating no power since none is flowing.
The switching time in between is indeed when most power is wasted.

Traditional amps, on the other hand, ALWAYS use the transistors (or tubes!) in a partially-on state. "Class B" is a trick to work around this, sort of (it's off half the time) but while the devices are in use, they're in a partially-on state with gradual change over time. I think for a clean sine wave you find maximum efficiency of 66%, a Class D amp could be 100% except for those switching losses and other real-world limitations.

In almost all solid-state amps, overdriving it produces nasty-sounding distortion. Tube amps are intentionally overdriven to get good-sounding distortion. So given the same physical limits, you could squeeze a bit of extra power out of a solid-state amp beyond its rating (like people do with tubes all the time), but nobody wants to use them like this.

Bass speakers and some acoustic guitar speakers have tweeters, like hi-fi stereos do. Claims that "normal" guitar rigs will be damaged by this sort of use are wrong though.

EDIT: I guess a solid-state amp "could" put DC on the speaker if it clipped unevenly which could be very bad, but this depends on the design of that specific amp.

[ManisMan]

Oh Im sorry this is the one I was using.

https://reverb.com/item/278318-eden-wt-1205-bass-amp-1200-watts-made-in-usa-2006

[jjasilli]

Ok, specifying the amp helps clear things up.  The schematic is here:  https://elektrotanya.com/eden_wt1205-1_sch.pdf/download.html


There is a tube deep in the signal chain.  I can't tell if at actually does anything; is just there for show; warms up tone; or is the source of overdrive tone. I suspect that hi signal input at the preamp input will cause unpleasant distortion in the early preamp stages.  My limited understanding of Class D power amps is that they are meant to provide a clean boost. Hence overdrive tone should occur somewhere in the preamp; and then the power amp will boost that.

[shooter]

Hi-fi and maybe PA

misunderstood myself, I wouldn't bother comparing a guitar bass amp to a carver

here's the block diagram for class D from wiki.

[Ritchie200]

This is a hybrid amp utilizing two 7025 tubes, one per channel, to give the preamp some "tube warmth".  The tube angle probably also looks good in the adverts.

Think of this particular hybrid as two different systems. 

The first system: Think of going direct.  Whether you are in a studio going direct to the board or at a show going direct to the PA.  Your preamp or line processor shapes your sound - distortion, tone, effects, etc.  In the case of the Eden amp, they are just using the 7025 tubes to add a little warmth to the signal.  In other words just squashing the signal slightly.  Sending the preamp tubes into full blown distortion may not play well with the architecture of this particular amp in how it ties into the power amp section. 

The second system: What is the first and foremost responsibility of the second system?  Faithfull reproduction of the first system.  The last thing you want to do is overload the studio board, house board, or in this case, the power amp.  The second system's job is to give you dB.  Whether that be a signal the studio can use, or wattage to cover Yasgur's Farm.  Your tone shaping is done, you just need volume.  As has been said, solid state sounds like crap when overdriven.  However it will give you years of reliable clean volume for little $$$, if not abused, ie overdriven.

A tube amp can be considered as a complete first system.  The preamp and the power amp are part of the tone shaping.  So what is the second system?  The mic hanging in front of your stack going to the studio board or house system.  What is the first and foremost responsibility of the second system?  Faithful reproduction of the first system.

Was your friend's combo amp SS?  Again, it just might be protecting the built in power amp.  It may also just be a light to let the player know that if you keep pushing it, the signal will start sounding ratty.  My Scholz Rockman units all have that.  All these solid state power amps are DC coupled and the entire stage can go bad quick, maybe taking the speaker(s) with it if not properly protected.  However, most speaker failures are due to insufficient power ratings of the speaker for the amp driving them to achieve some long lost Van Halen tone someone read about on the interweb of lies.  Or continuous feedback which will overheat and can fry the voice coil.  Modern guitar and bass speakers are designed to absorb the transients that are present with overdriven amps if they are properly matched to the output.  If you have matched them properly, you will not hurt them.

No doubt this a SS, not tube amp & preamp.  There are no 1200W guitar amps,

You kids....   You obviously have not heard of the infamous Coral Kilowatt of the late 60's.  Although it spent more time contributing to the special effects (SMOKE!), it was a guitar amp that came with two cabinets that had 8, 12" speakers in each one.  OK, so it was a SS amp and 1000 watts was the peak power rating, but still!!!

Jim

[ManisMan]

Okay, I think I'm getting it.

A tube amp overdriven to woofers(?) or those 8 inch or 10 inch speakers, if the speaker is designed for the load, is fine and does not cause damage. Assuming that a 100w amp puts out 200w at high distortion level, a speaker that can deal with 300w should not be damaged.

The preamp stage is the one most people find pleasing for distortion. If you wanted distortion from a preamp stage that was actually tubes in a class D amp, you would push the preamp section and then turn down the volume before the output stage and/or decrease some freq response in the high end with an EQ on the unit or patched in possibly, so that the output stage is only reproducing the distorted value that is coming in, and not distorting itself.

There should be some way to roll off the highs to prevent a tweeter or horn in a bass cab from getting too much power from all the new content at high frequencies, maybe an attenuator between amp and cab or, again, messing with the EQ. Or it is possible that since the output stage has already been controlled so it isn't distorting, there is no danger for tweeters or horns.

None of these issues would matter for an all-tube amp into a cab with standard size guitar speakers capable of handling about 2x the rated watts of the amp and you could push the preamp and output to distort slightly and it is possible to have no damage.

According to this

Is this about right?

Amps are rated by their output power, not power consumption.

I got sold a used amp in guitar center once the price tag said 200 watt. I was new to the game and later on discovered the output is 75w, but the whole unit is indeed 200 watt.

[TheKT88KilledJFK]

First of all, distortion NEVER intrinsically causes damage.  If it did, then we could never listen to rock albums, because those are distorted guitars.  The reference is mis-used because distorted amps exceed their "clean" power ratings, which could potentially, maybe, destroy a speaker if it's equally matched to the amp's low distortion power rating. 

In the end, it turns out to be a moot point considering the power supplies of most SS and Tube amplifiers can't sustain the additional power from the distortion that would be there.  Even the "1200w" amp matched to an equivalent rating speaker would very much so struggle to blow the speaker.

 The power isn't on the taps from the power supply in the first place.   

Now, you could argue that there are momentary peaks above that point for impulse like samples, and there are, but they happen anyways regardless of distortion.  This goes for HI-FI, car-audio, home-theater, or guitar amps.  The notion that distortion kills speakers or any other component really comes from salesmen at best but the fact remains that distortion introduces primarily harmonics that have relatively weak power compared with the fundamental signal. 

[TheKT88KilledJFK]

First of all, distortion NEVER intrinsically causes damage.  If it did, then we could never listen to rock albums, because those are distorted guitars.  The reference is mis-used because distorted amps exceed their "clean" power ratings, which could potentially, maybe, destroy a speaker if it's equally matched to the amp's low distortion power rating. 

In the end, it turns out to be a moot point considering the power supplies of most SS and Tube amplifiers can't sustain the additional power from the distortion that would be there.  Even the "1200w" amp matched to an equivalent rating speaker would very much so struggle to blow the speaker.

 The power isn't on the taps from the power supply in the first place.   

Now, you could argue that there are momentary peaks above that point for impulse like samples, and there are, but they happen anyways regardless of distortion.  This goes for HI-FI, car-audio, home-theater, or guitar amps.  The notion that distortion kills speakers or any other component really comes from salesmen at best but the fact remains that distortion introduces primarily harmonics that have relatively weak power compared with the fundamental signal.

Additionally, you can see here: https://www.allaboutcircuits.com/technical-articles/average-power-calculations-of-periodic-functions/ that only a couple extra harmonics can turn a signal into a psuedo square wave.  The catch is, that the additional power these harmonics introduce is minimal compared to the original (fundamental) signal. 

[jjasilli]

First of all, distortion NEVER intrinsically causes damage. 

This is not correct. Distortion (clipping) causes signal waves to be more square-like, than sine shaped.  This significantly changes the power dynamics.  It can double the nominal power output in Watts.  This can harm power tubes, OT's, and speakers re guitar amps.  In hi-fi, woofers are usually not as robust as guitar speakers, so they are more subject to harm.  Tweeters are especially subject to harm from distortion.

This occurs when distortion is PROduced.  It does not occur when distortion is REproduced.  So, it does not harm a hi-fi system to REproduce a distorted guitar tone. 


In the TUT series Kevin O'Connor recommends producing all overdrive tone in the tube preamp, to spare harm to the tube power amp, which should provide only a clean boost to preamp tone.  I don't agree and would rather slam the power amp and damn the torpedoes.  But his advice is pertinent to this discussion. 


These points can be googled for more info.

[ManisMan]

There's one thing that confuses me on this issue. Assuming you have some type of transformer, whether tube or solid state, the output would be something like 1200w max or so, like "I'll give you 100 volts and 12 amps", or maybe its a 30w  amp who is given 300v and 100mA to play with from its parent voltage line transformer...

If your amp asks its parent for more tasty amps, voltage should drop right? The parent transformer is unwilling to give more power than what its rated for, and this is what causes clipping in output stages as well as possible preamp stages, right? So where is the extra power coming from when an amp goes into distortion from too much gain/signal?

I was certain that I'd read voltage sag due to output overdriving is what can cause some tasty distortion in the output tubes because of the interaction of a transformer along with the tube and the loss of linearity at extreme ends of the gain curves.

Its obvious to me now that class D is not a stage you'd want to intentionally overdrive, at any rate. The idea of using preamp dist and keeping output is a sound philosophy.


Okay. I found this.

(1 kHz, 120 V AC line)
76 W RMS into 8 ohms @ 1% THD
84 W RMS into 8 ohms @ 5% THD

This is the "power at clipping" numbers given by an amplifier's datasheet. I believe this amp is a complete solid state but assuming it bears a transformer the same as a tube amp, it'd have the same issues 'requesting' more power and being denied. Then it falls to the fact that there's more time spent at near-max current I guess due to the high clipping?

[jjasilli]

Clipping occurs when too much signal is in-putted to a gain stage (a/k/a voltage stage or preamp), or a power stage.  A voltage gain stage draws very little power, so damage is usually not an issue.


When a power stage clips, it produces square-like waveforms.  It takes say 2x as much power to produce a square wave than a sine wave.  Where does the power come from?  From the power co. through the wall socket.  Power = Watts = Volts X Amps (current).  Since the input power voltage is limited to 120VAC, the only way to get more power is to draw more Amps (current).  But the wires in the amp, including transformer windings, and tube grids, etc., are designed to handle only so much Current.  As Current increases, they get hotter and hotter.  All wires have some resistance.  As heat increases resistance increases.  Increased resistance causes a voltage drop (sag), pursuant to Ohm's Law.  So, even more Current is drawn to maintain Watts output.  The result is a spiral effect of excess heat in the power amp.  Excessive heat may cause damage, gradually over time, thereby reducing the life of components; or suddenly and catastrophically.

[92Volts]

Let's use a solid state example and assume the power supply provides +10v and -10v.

The PEAK voltage this amp could provide to the speaker is +10v at one peak and -10v at the opposite peak.

A clean sine wave with +/-10v peaks has an average voltage near 7v. This is due to the shape of the wave which constantly rises/falls, it's not always equal to the peak voltage.
The power through an 8 ohm speaker can be calculated with (7v average)^2/8ohms=6.125watts.

Now if you overdrive it and get a square wave, you will provide an average of 10v, going from +10v to -10v with no gradual change in between. Power is now (10v average)^2/8ohms=12.5watts.

The power is always limited by the power supply and the impedance of the speaker. The limitations are no different between a clean and overdriven amp, and the power supply doesn't need to "sag" or overload to limit overdriven power. Overdriven power is fundamentally 2x greater than clean power just because of the shape of a clean vs. overdriven wave.

[jjasilli]

Rule of thumb is that a 50W amp overdriven "puts out" 100W.  A lot of that extra wattage is dissipated as heat, not volume.  That heat is destructive to the system but guitarists do it anyway cause they like the tone. 

[TheKT88KilledJFK]

Practically, I'd assert that by the time you're reproducing a perfect square wave you would have noticed the insane, absolutely hideous tone of the device, hence my comment that it only takes small amounts of harmonics to approach these levels of distortion.  This threshold is no where near twice the power output.  If you take a representative fourier series and calculate the power of the components, it's easy to see this.  I understand it's easy to say that the shape of a square wave covers more area thus expels more power than a sine wave, that logic I am not challenging. 

The internet beats on the distortion horse too much. 

[jjasilli]

No, no, no. 


1.  Shredders, e.g., love square waves  That's what "hi-gain" amps are for.  The hi-gain preamp takes a guitar signal and pretty much tuns it into a square wave.  It's not the actual gain that's needed, bu rather the effective reshaping of the waveform. 


2. Back to the point:  This is not about logic; it's about science.  Not everything that seems logical actually comports with reality.  Science takes a logical hypotheses and tests it empirically.  For the point of our discussion, the science has been done. Here's a useful though rather long article:  http://www.rocketroberts.com/techart/powerart_a.htm

[tubeswell]

Formula for area under a sine wave (above the x axis) using a definite integral function of 0 to Pii of sin(x).dx:

= (-cos Pii) - (-cos 0)

which equals (-1) - (-1) = 2.

See

This is nice and neat, but doesn't make much sense on its own. 2 things? what 2 things?




The corollary of this in audio electronics is the RMS value of a sine wave with a peak value:

X rms = X peak/(√2)

See

Here we can see how RMS is related to peak value, which is more useful for working out the absolute power of a sine wave, because Xrms x time = area




Whereas the formula for a Square wave is simply base x height (X . X peak)

[shooter]

I drew this last night but wasn't gonna post it.

a picture is work a 1,000 variables, or something like that

[ManisMan]

http://blueguitar.org/new/schem/fender/stage112se.gif

Just to be clear this is my friend's amp. I thought it was tube and so did he, so I guess we've both got egg on our face. That picture explains literally everything. Thanks.