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-   -   Killing The Myths of Low Volume Amp Performance! (http://www.ultimate-guitar.com/forum/showthread.php?t=1571081)

CECamps 11-02-2012 12:18 PM

Killing The Myths of Low Volume Amp Performance!
 
Time and again I see threads pop up with questions and/or assumptions pertaining to amplifier wattage and low volume performance. They often look like some variation of these:

“A 100 watt amp is too loud for home/apartment/bedroom use.”

“I’m looking for a 5 watt amp so I can get great tone at low volumes.”

“Will the 50 watt version have better tone than the 100 watt version at bedroom levels?”

“I need an amp under 5 watts so I can crank it up without it being too loud.”

“Is 30 watts too loud for home use?”

There are of course all kinds of other questions and claims that pop up about this stuff, but you get the picture. There are a lot of misconceptions and misunderstandings abound when it comes to amplifier output power and low volume performance and I wanted to share some information to help clear up some of it for whomever may be interested to learn more. Shall we?

Rule #1: An amplifier’s output power does not dictate its low volume performance or tone quality.

I just want to make sure that is clear first off. A 5 watt amp can sound great at low volumes, or it can sound like crap at low volumes. Doesn’t matter that it’s 5 watts. Likewise, a 100 watt amp can sound great at low volumes, or it can sound terrible. Having 100 watts of output power isn’t the determining factor.

Cool? OK, so let’s look at some factors that play an active role in how good an amp will sound at low volumes. Here’s a summary first, and if you care to take it further, I’ll provide some elaboration below it.

1) Output transformer design & quality
2) Power supply design
3) Signal path circuit design
4) Speaker(s)


Output transformer design & quality
The thought of the output transformer playing a significant role in low volume performance may not have ever crossed your mind. But it’s a fact. The output transformer couples the power amp section of a tube guitar amp to the speaker(s). But due to the nature of what a transformer is, this connection is not direct. The output tubes are connected to what is called the primary winding of the transformer. The speaker is connected to what is called the secondary winding. These windings are not physically connected to one another though. So how does your output signal make it to the speaker?

When the AC output signal is introduced to the primary winding by the output tubes, it is inductively transferred to the secondary winding via the magic of electromagnetism. This is where we can make or break tone quality at any volume really, because there lies the opportunity for losses and inaccuracies during that “translation” from one coil to the other.

An ideal transformer would cause no losses, degradation, distortion, or inaccuracies when performing its function. But ideal transformers only exist in theory. In the real world we have core materials and design approaches to deal with. Lower quality transformers (characterized by both material and design inadequacies) are inconsistent performers. When presented with a low amplitude signal (as when an amp’s volume is turned down low), the transfer of the output signal from primary to secondary—as a whole or only for various frequencies—may be inconsistent with that which occurs when it is presented with a high amplitude signal (as when you turn the volume up). There may be a lack of certain frequencies and too many of others. There may be distortion artifacts that present themselves in non-musical ways. This will cause unsatisfactory tone at low volumes.

Conversely, higher quality transformers (in materials and design) will be more transparent, cause less loss, degradation, distortion, and inaccuracies overall. As a result, performance will be a great deal more consistent at all volumes. This contributes to better low volume performance.


Power supply design
The power supply of any tube amp is easily one of the most overlooked and underappreciated parts of it. Essentially, when you are playing through a tube amp, you are “playing” the power supply. More specifically, your guitar signal is modulating the power supply via the tubes.

How tube amps work is beyond the scope of this post, but let it suffice to say that a well-designed power supply is critical to an amp’s performance—including how well it performs at low volumes. A shoddy power supply design will undoubtedly reduce the low volume performance potential of an amplifier.

Of course, what constitutes a “well-designed” power supply depends heavily on the rest of the circuit design. So there isn’t a black & white, one size fits all standard.


Signal path circuit design
Like the power supply design, the signal path circuit design also plays a role in low volume performance. It’s impossible for me to provide a universal explanation of what constitutes a good or bad design here, because there are an incredible number of variables that play into it—and it’s somewhat of an “application-specific” issue.

Nonetheless, design quality and component quality will work in conjunction with one another to play a role in an amp’s low volume performance.


Speaker(s)
Here is an item that almost never gets discussed in low volume amp performance discussions. Speakers are an absolutely critical factor in this! One of the most important in fact!

First let’s address sensitivity ratings. Take the following scenario into account: we have two 112 cabs, each loaded with a different speaker than the other. The cabs are identical otherwise, and are being driven by a hypothetical amp at a set volume. As you may or may not know, one speaker can be louder than the other despite the two being driven by the exact same output. Why? Because of something called sensitivity.

Sensitivity most usually refers to the dB output of a speaker when fed a set frequency at 1 watt output power. It is typically measured at a set distance from the speaker. Speaker sensitivity ratings for typical guitar speakers can range from dB levels in the 80’s to over 100dB. Let’s look at an example.

A Celestion Vintage 30 has a sensitivity rating of 100dB. One can infer that when fed 1 watt of signal, the speaker will produce 100dB SPL (sound pressure level).

Let’s wait a moment here and think about something. It only takes 1 watt of output power to get the speaker to create 100dB? As a reference, you’d experience the same level of loudness standing next to a motorcycle while the engine is being revved. If you brought a motorcycle into your bedroom, fired it up, and began revving the engine, do you think anyone in the house, or outside of the house would be able to hear it? If 1 watt of output power can create this level of noise, do you think that a low wattage amp is necessarily the solution to being able to play at low volumes?

So back to sensitivity. We just learned that a Celestion Vintage 30 is rated as having a sensitivity of 100dB. If we take a look at a Celestion G12T-75, we find that it has a sensitivity rating of 97dB. 3dB less than the Vintage 30 at the same output power.

Is 3dB a big deal? Let’s have a look at how much extra power is required to get 3dB more volume out of a speaker.

To increase output by 3dB, we actually have to DOUBLE our amplifier’s output power. So for a G12T-75 to put out 100dB, it has to be fed 2 watts, versus the Vintage 30 only needing 1 watt. But let’s take a look at how this exponential relationship begins to widen the gap between the two. Below I have output SPL’s listed, and how much output power would be required to achieve it with each speaker.

100dB
V30: 1 watt
G12T-75: 2 watts

103dB
V30: 2 watts
G12T-75: 4 watts

106dB
V30: 4 watts
G12T-75: 8 watts

109dB
V30: 8 watts
G12T-75: 16 watts

112dB
V30: 16 watts
G12T-75: 32 watts

115dB
V30: 32 watts
G12T-75: 64 watts

118dB
V30: 64 watts
G12T-75: 128 watts

So to drive a V30 to 118dB only requires around 64 watts. To get the G12T-75 to that same loudness would require 128 watts! Hopefully this sheds a little light on one aspect of this in terms of speakers. This is of course a very theoretical example based on a single frequency signal in very controlled conditions, but it illustrates the point that some speakers will seem louder than others at the same given level of output power.

(Continued in next post)

CECamps 11-02-2012 12:19 PM

(Continued from above post)

The next point I want to talk about with speakers is efficiency. Efficiency has to do with how well a speaker creates varying frequencies at a given output power level. Efficiency and sensitivity go hand in hand. When we are looking at sensitivity, we are looking at loudness at a single frequency (like 1kHz for example). But our guitar signal contains a broad range of frequencies, so we are interested in looking at efficiency across that range.

If you have ever seen a frequency response plot for a speaker, you are basically looking at a panoramic view of the speaker’s efficiency at a set output power level for all intents and purposes. The output levels of each frequency vary, which gives the speaker its “voice.” Speakers could be thought of as the most significant EQ curve in your entire rig!

What’s important to note is that speaker efficiency will vary for any given frequency at any given output power. In other words, as we turn volume up or down, that speaker’s EQ curve is going to change in places. This is one of the prime reasons why tone changes when we turn amp volume up, versus turning it down. There is a dynamic relationship between output power and frequency response.

Some speakers stay truer at low volumes, and some speakers just flat-out SUCK at low volumes. That’s really what it boils down to. Have you ever experienced excessive “fizz” with high gain tones at low volumes, and then it starts to go away as the volume is turned up? In many cases, the speaker can be the prime culprit in that phenomenon. At low volumes, low frequency efficiency typically goes down the tubes and higher frequencies become more dominant. We get a skewed “EQ curve” that takes away certain frequencies and seemingly “cranks up” others. It's inaccurate.

And if we think about what we learned regarding output transformers, the variables get even thicker!

It should be mentioned that a speaker’s behavior will most definitely be influenced by the cabinet design. In fact, cab design can play a role in how well any given speaker performs at low volumes due to the physical interactions that happen in real world application.


Moral of the story
I hope this sheds some light on some of the big factors that determine how well an amp will perform at low volumes. I hope it reveals the fact that the amp’s output power does not directly correlate to how well it performs at bedroom levels.

Ultimately, because of the complex nature of everything that plays into this, the only thing anyone can really go on is personal experience. And even then we have to approach advice with caution, as we enter the territory of human subjectivity.

So go forth, take your newly acquired knowledge, and have educated discussions about low volume performance!

tubetime86 11-02-2012 12:43 PM

Epic dude. Just epic. :five:


R45VT 11-02-2012 01:05 PM

Should be stuck at the top for newb reference.

Sweet Craig!

:golfclap:

Fisheth24 11-02-2012 01:06 PM

I second that, can we get this stickied?

loki_lulamen 11-02-2012 01:08 PM

Awesome. A great read for everyone I reckon.

1 vote for sticky!

TNfootballfan62 11-02-2012 01:09 PM

Craig, write a comprehensive textbook please.

darkwolf291 11-02-2012 01:14 PM

We should get all the GG&A solder junkies to work together to put together a comprehensive book on guitar electronics, amps, and pedals.

Musicman48858 11-02-2012 01:50 PM

The only counterargument I can think of is for a blues-grit tone through a tube amp... many people believe that powertube overdrive/distortion is more desirable that preamp overdrive/distortion for that particular genre. If that is the case, then a lower wattage tube amp will break up the power tubes at lower volumes, thereby generating a "better" tone at low volume than a similar high wattage amp.

tubetime86 11-02-2012 01:56 PM

Quote:
Originally Posted by Musicman48858
The only counterargument I can think of is for a blues-grit tone through a tube amp... many people believe that powertube overdrive/distortion is more desirable that preamp overdrive/distortion for that particular genre. If that is the case, then a lower wattage tube amp will break up the power tubes at lower volumes, thereby generating a "better" tone at low volume than a similar high wattage amp.

Ever tried to get power tube distortion on a five watter? The term 'low volume' does not apply. 'Lower' sure, but that's not the argument he's addressing... Moot point is moot.

He says countless times that he is only giving a general overview of the many, many nuances of this debate. I think he covered his ass on that aspect quite well. :shrug:

GazzaCee 11-02-2012 01:57 PM

Very informative post, I learned a load of stuff here.

I also vote for sticky.

Musicman48858 11-02-2012 02:01 PM

Quote:
Originally Posted by tubetime86
Ever tried to get power tube distortion on a five watter? The term 'low volume' does not apply. 'Lower' sure, but that's not the argument he's addressing... Moot point is moot.

He says countless times that he is only giving a general overview of the many, many nuances of this debate. I think he covered his ass on that aspect quite well. :shrug:


Oh, I absolutely agree... I was mostly saying that his argument was so thorough that it removed all valid points from the opposing side of the argument except that one minor case. It was an excellent post.

tubetime86 11-02-2012 02:05 PM

Quote:
Originally Posted by Musicman48858
Oh, I absolutely agree... I was mostly saying that his argument was so thorough that it removed all valid points from the opposing side of the argument except that one minor case. It was an excellent post.

Gotcha. There's always that 'but what about X' scenario in any argument... But this thread covers all the general stuff so well that when this topic comes up in a thread we can reference this then just add a tiny bit more situational info and we should be able to cover any and all bases. :D

DisarmGoliath 11-02-2012 02:19 PM

Y'know, I saw the title on the front page and came in expecting this to be some fool ranting on about low volumes vs high volumes is a myth and there is no audible difference etc.

How wrong could I have been - hit the nail on the head analysing the various stages of the signal path and their impact on the tone at the end result. Even mentioned the speakers which is something I believe too many people exclude from the equation when talking about low-volume performance (though I admit I'm referring to the audio engineering side of things; not the stuff you are more likely to discuss here in GG&A).

Bravo, good sir! :golfclap:

darkwolf291 11-02-2012 02:21 PM

A lot of engineering talk goes down in the WTLT thread.

And Craig builds amplifiers, so he can get into all the technobabble that makes my head go 'splody.

ECistheBest 11-02-2012 02:23 PM

this is all wrong. u can tell by the guy's mustache.

gumbilicious 11-02-2012 02:28 PM

Quote:
Originally Posted by Musicman48858
The only counterargument I can think of is for a blues-grit tone through a tube amp... many people believe that powertube overdrive/distortion is more desirable that preamp overdrive/distortion for that particular genre.


that is true, many people do believe that. and many times they will cite non-MV amps like a marshall 18 watt, JTM45 or old supro, the problem with those amps is when you crank the volume you're cranking the preamp with the power amp and there is no way to 'isolate' the preference of power amp distortion.

many contemporary amps can sound 'big' and 'ballsy' at lower volumes using the techniques craig mentions (good trannie, good power supply, appropriate circuit design, proper matched speakers). and yes, those techniques can even make a good blues amp at low volumes.

but the most important part of craig's contribution: "So go forth, take your newly acquired knowledge, and have educated discussions about low volume performance!" apply the knowledge he just gave you, go out and do some double blinds and compare a bunch of amps and speakers.

if you want to find acceptable blues amps, then go out and try some stuff, don't just rely on what "many people believe".

if i am interpreting craig correctly, that was the purpose behind his Toll Free Express design, it was designed to sound like a cranked amp at lower volumes.

Quote:
Originally Posted by Musicman48858
If that is the case, then a lower wattage tube amp will break up the power tubes at lower volumes, thereby generating a "better" tone at low volume than a similar high wattage amp.


yes, the power tube will 'breakup at lower volume', but going after 'breakup at lower volume' by getting a low powered amp is generally not as effective as one may hope.

for example, a blues jr is commonly recommended over a hot rod deville (for power amp distortion anyway). unfortunately i don't know all the speaker specs on those amps, but if we just compare power outputs (15, 60 respectively) then the hot rod deville is only ~1.43 louder than the blues jr. despite the fact that the deville has 4 times the power output.

we can take to a huge extreme, lets compare something like a 5 watt amp to a 150 watt amp. mind you a 5 watt amp has 30 times less output than a 150 watt amp, but if all other factors are held constant then the 150 watt amp is only 2.7 times louder than the 5 watt amp. while i would admit, that is quite a bit of difference it should also be mentioned that most guitarists using their intuition would expect a bigger difference in perceived loudness.

that is what [i think] craig was trying to get across.

btw, here is the formula i use:
ratio louder = 2^log10(P2/P1)

Dave_Mc 11-02-2012 02:49 PM

:golfclap:

(i'll read it all in a minute :o Knowing it's you and having skimmed it slightly I know it's going to be good and I'm going to agree, though :D )

EDIT: yep, great :cheers:

CECamps 11-02-2012 03:17 PM

Thanks guys, glad you're enjoying the post!


Quote:
Originally Posted by Musicman48858
The only counterargument I can think of is for a blues-grit tone through a tube amp... many people believe that powertube overdrive/distortion is more desirable that preamp overdrive/distortion for that particular genre. If that is the case, then a lower wattage tube amp will break up the power tubes at lower volumes, thereby generating a "better" tone at low volume than a similar high wattage amp.


This is really a horse of a different color. If we're talking about power amp distortion, we are most definitely not talking about low volumes. Not by a long shot.

Unless of course we are talking about a sub-1 watt power amp. But even still, we are at the mercy of the variables I mentioned and thusly it cannot be assumed that it will sound any better than a 100 watt amp.

But overall, the quality of tone at any volume cannot be determined by the power output.


Quote:
Originally Posted by TNfootballfan62
Craig, write a comprehensive textbook please.


I've actually been in the process of writing a book that I will release on the Amazon Kindle platform. Very slow process though.

TNfootballfan62 11-02-2012 03:20 PM

Quote:
Originally Posted by CECamps
I've actually been in the process of writing a book that I will release on the Amazon Kindle platform. Very slow process though.


I'm very, very interested. A design oriented text?


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