Preamp Distortion Vs. Power Amp Distortion

Written by
Logan Tabor
Published on
January 21, 2021 4:48:29 PM PST January 21, 2021 4:48:29 PM PSTst, January 21, 2021 4:48:29 PM PST
Since the dawn of the tube amplifier, there have been countless heroes of tone; those who, both on stage and on record, we have all looked up to and even attempted to emulate.  They set the standard of awesome guitar tone, and for years have given us an assortment of chills, goosebumps, and weepy eyes in as little as a single note. In that never ending quest for ‘make-their-heads-spin’ sound, there are many things about our rigs which we must truly come to understand in order to achieve such immaculate tone as our idols -- one of those things is the difference between preamp distortion and power amp distortion.  Once we understand the differences therein, we can use this knowledge to master our amplifiers and adapt to many different playing environments. So...shall we?  

WHAT IS PREAMP DISTORTION?
 

The short answer is that preamp distortion derives from overloading/distorting an amplifier’s preamp section.  The preamp is where the guitar’s signal is taken from a rather weak and anemic little thing, and boosted to line level.  This is also where our sound is primarily shaped (as far as the amplifier is concerned) and where we are given typical EQ controls like Hi, Lo, and Mid.  Many modern amps, and specifically high-gain amps, are fitted with a preamp gain control as well as a master volume control. These two controls work together to create what we now know to be a nice saturated preamp distortion sound.  The preamp gain control is essentially the first volume control the signal sees in the preamp, whereas the overall signal amplitude that is sent to the power amp by the preamp, is controlled with the master volume knob. This is useful because, as many of us have no doubt experienced, a super loud amplifier is not always desirable for the given stage or studio.  Amplifiers with a preamp gain control allow the user to achieve a well-saturated and compressed distorted sound with enhanced sustain at lower volume levels (i.e., without overdriving the power amp section). Because these amps typically also offer EQ controls within the preamp section, this higher gain sound can be shaped considerably by the user. Preamp distortion is also characterized by increased compression which results in a more even or, dare I say, flat sound.  Less dramatic dynamics are typically found in preamp distortion, and preamp distortion is also where I tend to use words like ‘hairy’ or ‘fizzy.”

WHAT IS POWER AMP DISTORTION?
 

Here again, the short answer is that power amp distortion derives from overloading/distorting an amplifier’s power section.  After the preamp portion of an amplifier is ‘done’ with the signal, it is then passed on to the phase inverter and out to the amp’s power tubes.  If the signal being sent from the preamp to the power section is of too high amplitude for the power amp section, the clean ‘headroom’ of the amp will be maxed out, and the user will begin to hear what is known as power amp distortion.  At the beginning of it all, when our tone heroes were doing all the work for us, the ‘Rock n Roll’ sound was essentially defined by power amp distortion. When an amplifier is pushed to the right part of the power amp ‘brink’ we begin to hear and feel that classic BARK.  Power amp distortion typically has a more enhanced mid range from that of preamp distortion, and while power amp distortion does add compression, it tends to be less than that of preamp distortion. The result is that power amp distortion gives us that classic punchy, warm, vocal sound.  Power amp distortion is also considered much more dynamic as it responds to the human touch on the guitar and allows the player’s true dynamic abilities to shine. The downfall here is that pushing an amp to the point of power amp distortion can often put a real hurtin’ on the ear drums. Depending upon the wattage of the amp and the power tubes being used, a player may have to bring the whole venue to the ground before achieving any substantial power amp distortion.  Master volume controls are typically used to manipulate this part of the signal, and this is typically the knob we will turn when the sound guy inevitably asks us to turn down.

A NOTE ABOUT POWER AMP SAG
We’ve all heard the term ‘sag’ and we have all heard it associated with both good and bad sentiments, but what does it actually mean?  Power amp sag happens when too much of a ‘workout’ is given to an under-filtered power supply. Have you ever been sitting in your home and heard the refrigerator turn on, and then experienced an immediate and brief dimming of the lights in the house?  This is essentially the same concept as power amp sag. When the refrigerator turns on, it temporarily puts a large load on the system all at once, and the system will take a moment to pull more current to accommodate this new load. On some guitar amplifiers, if you crank the master volume and strum a big huge A chord, that big chord is just like the refrigerator coming on, which is why you may hear that smoothing squishy compressed sound right when you first hit a big note.  Some players prefer this sound, and this is why some amp designers create amps with sag in mind. This sag in the power supply will be heard when the guitarist plays harder, but will likely not be heard when lighter notes are played -- this is actually the source of those intricate touch dynamics most players experience when using a vintage amp. Therefore, more often than not, when players refer to the desired effects of “power amp distortion” they are actually referring to power supply sag.

WHAT ABOUT AMPS WITH A SINGLE ‘VOLUME’ KNOB?
 

Amps with a single volume knob -- that is, without a separate master volume and/or preamp gain control -- are typically vintage amps or are modeled after vintage style amps.  The single volume knob, in most cases, will manipulate both the preamp and the power amp sections simultaneously. One function is to attenuate the signal passing through the preamp section, and as a result of the level of said attenuation, the power amp will either be pushed more or less once the signal leaves the preamp and goes to the power section.  A benefit to this is that most vintage amps are of somewhat lower wattage, so the interaction of the preamp and power sections can allow for some desirable tonal benefits without the amp having to get overbearingly loud. In this and every other case, it is best to simply be very familiar with your amp(s) so that you can adjust, on the fly, to any given scenario. 

FINDING THE SWEET SPOT AND KNOWING YOUR AMP
With these different types of distortion, each offering their own flavors, it can be hard to know how to dial in an amp, especially if it has a preamp gain control and a master volume.  Many modern amps will have both of these controls, and more often than not, they will both have to be adjusted for any given setting.  
As an example, let’s say you are partial to power amp distortion and are therefore forced to turn up your amp’s master volume while keeping the preamp gain relatively low.  One day you arrive at a venue, you’re in the middle of soundcheck and the sound guy asks you to turn down, what do you do? If you turn down the master volume you risk losing those precious dynamics that bring tears to everyone’s eyes.  On the other hand, if you turn down the preamp side, you will not have as much of an effect on the volume level but will likely lose some of your grit and/or sustain. In all honesty, there is no perfect fix for this. One thing to do is to just spend a ton of time with your amp at home.  Try out different combinations of master volume and preamp gain settings. Find out what the lowest master volume setting is where you can still get that ethereal playability while reducing the risk of being told to turn down. Experimentation and the ability to adapt are key.
Another workaround here is having multiple amps and knowing which amp is appropriate for a given setting.  Again, lower wattage amps can still achieve that perfect level of power amp distortion while being plenty less loud than a giant 180W monster.  Here again, you’ll need to be very much accustomed to how each of these amps work, how to find their sweet spots, and which will be right for the space.  
Of course, a great deal of the preferred tonal palate (and thus the interaction of preamp and power amp gains) is based around the type of music being played, the type and amount of pedals being used, etc.  Many players will choose an amp with a high wattage power section simply for the clean headroom it provides, and will then rely on various pedals to produce a range of distorted sounds. Metal guitarists, while they do require volume, will often prefer the compression and tightness of a saturated preamp stage when picking a lot of notes.  Blues players often benefit from a low-to-medium wattage amp with the power section cranked to give them a bold warm breakup with lots of charisma and dynamics. Players who tend to use their guitar’s volume control will likely see more benefit from a pushed power amp rather than a pushed preamp, as they can use their volume knob to add and subtract distortion more easily.