Signal to Noise: Current Affairs

Fig. 1

Discover how a critical difference between a self-contained guitar amp and a component rack system affects dynamic response and feel.

Playing guitar is an exercise in discovery. The more you do it, presumably, the better you get, and the more you learn about the instrument’s possibilities coupled with your own potential to advance. People often travel repeatedly to the same places. Not out of lack of imagination, but as an extension of the discovery process. Once you’ve seen the Eiffel Tower or Grand Canyon, you can go back and discover a nice bistro on a side street you’d never noticed, or a trail that may have been hidden previously. With that in mind, here’s a question that deserves repeated exploration: How do preamps and power amps—and rack systems in general—differ in dynamic behavior from self-contained guitar amps? And why? This was a hot topic during the heyday of rack systems. Technology has come a long way since then, so it’s worth revisiting the subject.

We’ve discussed the dynamic loop created by pick attack and playing volume, and how current demand from the power amp stage affects the preamp stage [“The Big Bang” and “The Sonic Legacy of Tube Amplification”]. To recap, a self-contained amplifier, consisting of a preamp stage and a power amp stage, gets its operating current from a common power supply (Fig. 1). When you play clean and at low volume, both the preamp and power amp benefit from a healthy reserve of available current. Once the volume goes up and pick attack intensifies, the power amp’s demand on the power supply increases exponentially, leaving whatever crumbs are left to the preamp. Now, to be fair, an amplifier’s power supply is normally designed to deliver sufficient juice to the preamp even under demanding conditions. But the truth is, a lot of what differentiates amplifier personality can be directly traced to the designer’s ideas about how much preamp voltage variability is acceptable or even desirable. This gives rise to terms ranging from “spongy” and “forgiving” to “dry” and “stiff,” and—sin of all sins—“unforgiving.”

A lot of what differentiates amplifier personality can be directly traced to the designer’s ideas about how much preamp voltage variability is acceptable or even desirable.

So, let’s look at the rackmount preamp and power amp. In a system made up of separate components, the defining feature is that they each have their own dedicated power supply (Fig. 2). Right off the bat, this precludes that beast of a power amp from hogging all the juice to the preamp. The first thing we discover in playing this rig is that the dynamic feel is noticeably stiffer and absent some familiar gooeyness and bloom. In early rack systems, getting a group of components to feel like playing a normal amp usually took a back seat to the benefits of extensive signal switching and processing. My personal feeling was, and still is, that a well-executed rack system demands a power amp with a good range of control over frequency response and dynamic feel. Most of the time power output is less important than the kind of tube character you like, especially in a stereo power amp. I bring up stereo because this is a feature of rack power amps that always gets overlooked.

In a stereo power amp, the power supply is usually common to both channels, and therefore has to be sufficient to provide full output for them. It rarely occurs to players that if you only use one 50-watt channel of a stereo 100-watt power amp, you still have a 100-watt energy reservoir on tap. Naturally, that’s going to feel extra stiff compared to a single power-amp stage of a 50-watt head. If you expect to arrive at reliable conclusions about different power amps in A/B comparison tests, it’s important to run both channels to really understand what you’re hearing.


Fig. 2

Some stereo power amps use a single 12AX7 for the input stage of both channels, while others have a separate tube for each channel. The reason this is important is not immediately obvious, but it certainly bears scrutiny. Crosstalk, or signal bleeding from channel A into channel B, often occurs in a power amp with non-isolated triodes, as opposed to isolated triodes. If you use a stereo FX processor to A/B test each design type with both channels operating, the amp with non-isolated triodes will sound practically mono due to signal bleed at the first preamp stage, while the amp with isolated triodes will deliver a markedly superior stereo image. If you compare these two design styles side by side, one channel at a time, you’d totally miss the significance of isolated triodes on the input stages.

Those two simple subjects—power supply capacity and crosstalk—play a major role in the difference between rack gear and self-contained amps. Here, a seemingly benign question offers an opportunity to understand nuances of amplifier design that are rarely discussed.

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Signal to Noise: The Sonic Legacy of Tube Amplification

Had electric guitar music emerged from transistors, would we even recognize it today?

We’ve seen how string vibration is converted into the signal that’s sent to the guitar amplifier, and how that signal determines the response of a tube amp [“The Big Bang”]. Let’s now consider non-tube amplifiers, or what I refer to as “replacement technologies.”

When weighing the pros and cons of tube, solid-state, and modeling amps, it’s helpful to remember that tube amps came first. This means that pretty much the entire electric guitar vocabulary we build on today is informed by what the pioneers of electric guitar discovered about the behavior of tube amplifiers.

Now, imagine that solid-state amplifiers came first. It may seem like a preposterous exercise since the advent of the transistor sprang primarily from the need for improved efficiency, reliability, and conservation of space—all the things tube amplifiers weren’t good at. Yet, as a thought experiment, it’s safe to say that electric guitar music would certainly have evolved much differently had solid-state amps appeared first, and arguably not the way we recognize it today.

To sound palatable to guitarists, solid-state designs require some behavior management to minimize or eliminate the unpleasant and decidedly non-musical distortion solid-state amps can produce. In a modeling amp, most of the desirable characteristics of tube amplifiers are captured digitally, yet the modelers processing circuitry must not introduce any undesirable distortion, latency, or non-musical tonal artifacts. As important as they are to ongoing development, these methods of amplification are often trying just as hard to avoid what we don’t want as they are trying to produce what we do want. While the arrival of tube amplifiers represented a true paradigm shift from the acoustic to the amplified realm—thus opening new doors of expression for evolving players—solid-state and modeling amps simply prioritize functional evolution over inspirational involvement with the instrument.

An iconic tube amp, by its very nature, is expected to be exactly what it is, and every yard-sale find potentially offers a new path to discovery. A tube amp becomes an indispensable extension of the instrument by virtue of its unique and sometimes irascible personality. No wonder players ultimately find their way back to tube gear.

A tube amp becomes an indispensable extension of the instrument by virtue of its unique and sometimes irascible personality.

What happens in the front end (the preamp) of the amplifier is reproduced by the back end (the power amp) without necessarily influencing the latter’s performance. Preamp tube distortion as a general category includes light clipping, asymmetrical wave shape, and frequency modification, such as low-end and high-end roll-off. Power-amp distortion is highly dependent on circuit design: the amount of power this stage puts out, the type of tubes used, the construction of the power and output transformers, and, finally, how much its sound is influenced by the speaker it’s driving. Because this stage devours 90 percent of the current made available by the power supply, the power amp exerts great influence on the front end, which reacts to the ebb and flow of power-supply voltage at various playing volumes. This creates a feedback loop of sorts that causes a tug-of-war between the preamp and power amp, and is responsible for the sense of compression and touch sensitivity we experience. Only tube amps react this way in real time.

Speaking of feedback, many amps use what is known as local feedback to refine the sound of the power amp. A small amount of the output signal is fed back into the power amp input stage, which then controls whether the power amp will react more or less accurately to the input it receives. Because this feedback loop originates from the speaker output circuit, the amplifier also reacts to the speaker that it’s driving. Because different speakers have different electrical and mechanical characteristics, it’s no wonder that changing the speaker in a given amp not only changes the sound you hear from the amp, it also changes how the amp reacts to pick attack,which brings us back to square one—the Big Bang.

Picking a string sets off a chain reaction of both expected and unpredictable events that inspire the next pick attack. As a habitual string slammer who uses the blunt side of heavy Herco picks, I’ve developed a callous on the nail side of my index finger from scraping it against the strings. An evolving understanding of picking dynamics is driving me to modulate my former heavy-handed approach. Inspired by advanced pick-free explorers like David Torn and Jeff Beck, I’m slowly learning to back off and lighten my touch in an effort to develop a more nuanced, ethereal string timbre. I find the lighter my attack, the more harmonic control I have over amplifier distortion and dynamic range, and this yields an even greater appreciation for the multifaceted sonic bliss that is unique to tube amplifiers.

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Signal to Noise: The Big Bang

Whether you love shimmering clean or brutally overdriven electric-guitar textures, it all starts with pick attack.

What happens from the moment you pick a guitar string to that instant you’re engulfed in amplified sound? It’s all about energy transfer: When you pick a string, the energy from the velocity of a pickstroke becomes energy in the form of string vibration. How hard you pick determines how much energy the string will absorb and then transmit to the pickup, which it does by cutting through the pickup’s magnetic lines of force. How much voltage is generated by the pickup is proportional to how much skin or fingernail you’re willing to sacrifice for the glory of an astounding power chord. Even though there’s a practical limit to the voltage generated by the pickup, as determined by the magnet size, wire diameter, and number of turns, there’s still a respectable amount of potential to work with, and this gives a typical guitar pickup an insanely wide range of sensitivity. You can move from pounding the strings to delicately brushing them to producing barely measurable voltage output by literally blowing on them.

Once you connect your guitar cord to the amplifier, the electrical impulses from the pickup become transformed. Normally, the amplifier’s gain will be more than sufficient to accommodate a wide range of signal levels, and this allows us to explore a full spectrum of dynamic interplay. But it takes a special kind of amp to respond musically to such a vast range of signal input levels at any given control setting.

How hard you pick determines how much energy the string will absorb and then transmit to the pickup, which it does by cutting through the pickup’s magnetic lines of force.

Guitar amplifier gain stages are capable of high sensitivity to very small signals. When an amplifier’s preamp stages are set to reproduce small signals, a very large transient attack will readily transition into distortion and overdrive territory. And the same goes for the power amplifier stage. The beauty of this phenomenon is that we can set the amp to respond to subtleties in string attack with the guitar volume set low, and then instantly conjure up a whirlwind of sonic fury on demand.

With a tube amplifier, we usually perceive distortion as pleasant, musical, and inspiring—even essential—and we’re often barely aware that we’ve crossed the threshold from clean to distorted sound. In most tube amps, amplifier distortion is so seamlessly associated with the amp’s signature sound that it’s not really considered distortion until you’re in over-the-top territory. This is because our brain identifies certain flavors or “orders” of distortion harmonics as part of the musical structure, whereas it treats others as alien and filters them out to the extent it can. In extreme cases, this filtering process is inadequate and thus leaves unwelcome harmonics exposed. When this happens, we consider the sound to be harsh, intrusive, and unrelated to the musical content.


In Photo 2, the bottom trace shows the same chord, but slammed. This generates a very large difference in level between initial string attack (big hump in the middle of the wave) and string decay (smaller humps to the left and right). Here, the top trace shows how the amp with the same control settings as the clean example reacts to the dynamic attack: It flattens the waveform at maximum pickup level and remains in saturation mode, even though pickup output has decreased significantly.

There’s an elemental difference between a tube guitar amp and other amplifier topologies: In a tube amp, most of the ways it departs from accurate reproduction of the guitar signal are perceived to be additive—they contribute color and complexity to the sonic tapestry. In simple terms, most vintage amps stay cleaner in the preamp, which makes power-amp saturation responsible for the amp’s dominant overdriven voice. Modern amps can produce an avalanche of preamp overdrive. We tend to want to amplify this overdrive with clarity in the power-amp stage, and this results in an amp capable of infinite sustain with a high level of detail and articulation. Of course, a myriad of amp designs stake out territory in the infinite space between these two extremes.

And while tube amplifiers are distinguished from one another in large part by how each stage of amplification distorts or colors the sound relative to every other stage, it’s too simplistic to say that’s all there is to it. Next time, we’ll dive deeper into what differentiates tube amps from other topologies, what gives an amplifier it’s signature personality, and what gives certain guitar-and-amp combinations their timeless appeal.

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Signal to Noise: Are Handwired Amps Superior to PCB-Based Designs?

Amps with printed circuit boards often get a bad rap, yet that’s not always justified.

As long as I’ve been involved in tube amplifier design and engineering, there’s been a debate about whether amplifiers with printed circuit boards (PCB) are superior, equal to, or inherently inferior to handwired amps. Buzzwords like “point-to-point” are somehow easier for many guitarists to spout than putting actual thought into the subject. No matter how expertly executed, PCB designs always seem held in less esteem than handwired amps—even terribly executed ones. Most reasonable players probably don’t care how an amplifier is made, as long as it is reliable and sounds good. Or so I thought.

Recently, we posted before-and-after photos on Instagram of one of our amps that some field repair shop had butchered—the original wiring was horribly chopped and spliced back together. The post was intended to show a factory restore in progress, yet a follower complimented us on our cool handwiring, saying it “looks amazing.” In my mind, this attitude reflects a narrative that the mere presence of wires somehow imparts special effort.

Many elements go into amplifier development, including rigorously vetted sonic research and testing. Let’s say you want to compare the sonic properties of two different coupling capacitors. If you play an amp with capacitor A installed, and then switch to capacitor B and listen again, your results are going to be skewed by the time that passed during the swap. Perhaps there’s been a change in line voltage, or the amp isn’t positioned identically in the room. Also, if you know what part type you’re switching to, you’re going to be mentally predisposed to hear a different result. You might think that making two identical amps with these different capacitors installed would be a better solution, and you would be wrong. There are going to be more than enough tolerance differences between the two amps to make such a comparison impossible—especially if it’s a handwired amp.

Some of the best-built amps in the industry have PCB-mounted pots and tube sockets. So do the least expensive ones, but for very different reasons.

R&D on PCB amps is time-consuming and expensive. And for every PCB design, there is, or should be, a reason for that design and its execution to exist. If the engineer fails to observe essential design practice, odds are the product will be inadequate or it will fail. Copper traces that crack—a frequent criticism of PCB amps—are the result of poor mechanical design and cost cutting. When a poor design fails, PCB amps in general get a bad rap—not just the badly designed ones.

PCB layout and material selection in audio design are as much an art form as the circuit design. A well-engineered layout is informed by experience, trial and error, and an understanding of physics. The beauty of PCB design is that once it’s completed, it’s permanently locked in place. This makes for extremely consistent performance from unit to unit. However, that doesn’t guarantee reliability. The only way to lock in reliability is through careful component selection and rigorous testing. The fine print here is that consistency and reliability do not automatically guarantee a great-sounding amp. That is a function of design intent: the vision that caused the design to exist in the first place. Some of the best-built amps in the industry have PCB-mounted pots and tube sockets. So do the least expensive ones, but for very different reasons. In most cases, the price of the product will indicate what to expect in the way of build quality, consistency, and long-term performance.

The idea that point-to-point equals sonic magic derives from components, pots, and tube sockets being individually wired, which inspires trust. The truth is, solder lugs and eyelet boards require greater amounts of solder and wire than on a PCB amp. Solder, a relatively poor conductor, is meant to help secure an already valid electrical connection. Solder lugs enable unreliable solder connections, and long lead lengths open up the potential for signal loss, noise, and parasitic oscillation. PCB design minimizes or eliminates these problems by keeping signal paths short, with minimum solder. Quality board material using healthy copper width and thickness sounds as good as, or better, than copper wire and will last every bit as long as point-to-point amps. PCB design is full of potential, is infinitely variable, and allows blending tried-and-true components with the latest in new component technology. But the biggest benefit is that we don’t have to reinvent the wheel every time we make an amp.

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