David Lamkins picked up his first guitar a long time ago. As best he can recall the year was 1967: the year of the Summer of Love. Four decades later David has conjured up an amalgam of folk, rock and jazz solo guitar music for the occasional intimate Portland audience.
location: Portland, OR USA

Facets: amplifiers, evaluation, loudness, physics, power, speakers, technology, @musings info

Playing guitar through a full-range amp

Here's another in a series of "everything I thought I knew was wrong" articles. I once thought that a flat, full-range amplifier was always unsuited to use with an electric guitar. I'm discovering that's not entirely true. My search for lightweight amplifiers led me to a 20-pound (that's including the carrying case) amplifier designed for acoustic guitar. This amp sounds great with my Koll guitars.

There are many common arguments against playing an electric guitar through a clean, full-range sound system, among them:

As always, such arguments (on both sides) depend upon perspective and context. If your notion of a "good" electric guitar tone depends upon distortion, you should follow the recipe that matches your expectations. If you need an articulate clean tone, give the following some consideration...

There are several options for full-range guitar amplification, including amplifiers designed for acoustic guitars, amplifiers designed for keyboards, full-range "jazz" guitar amplifiers and PA (sound reinforcement) systems. In general, these will all tend to sound like they belong in the same family of amplification, just as most Fender amps of classic design sound similar.

There will be subtle differences that may or may not be important to you. You'll probably discover that some aspect of a particular product's frequency response has been hyped. You'd think this wouldn't be the case; isn't a full-range flat-response system supposed to give an uncolored rendering of the source material? As in all things having to do with sound, these are products for sale. They're built to a price point and designed to entice a consumer to buy them in preference to other products at a comparable price point. Often this enticement is achieved by emphasizing a particular range of frequencies that will make the system seem "louder" or "deeper" or "fuller" or "crisper" - anything that will prompt a consumer to say "I like the way this sounds.".

The human auditory processing system is notoriously poor at making an objective determination of sound quality. We respond to differences, not absolutes. This observation has always driven the marketing and sales of sound reproduction and reinforcement systems. If a portion of a target audience responds positively to a "difference" in sound quality, say the presence of an emphasized midrange, this quality then becomes the focus of additional research, development and marketing.

I'm sure you've all heard of Bose. When they introduced their first product, the 901, "good" high-fidelity speakers were uniformly regarded as being necessarily large and cumbersome in order to produce an accurate rendering of the program material. The genius of Bose was the realization that listeners could infer the presence of low frequencies not reproduced by the speakers, combined with their use of room reflections to diffuse time-of-arrival information in order to convey a sense of spaciousness not present in the source material. The 901s sounded distinctive and different; consumers loved them. If you took a moment to A/B a 901 against any of the well-regarded speakers of the day, the 901 sounded like the program material was reproduced through a trumpet - all midrange. Still, consumers loved it.

Similar things have happened throughout the history of sound reproduction; I'm sure you can cite your own examples. The point is: "good" sound is purely a relative proposition, and is driven by fashion more than by science.

The traditional mid-scooped guitar sound began as an engineering tradeoff. Speakers in open-backed cabinets suffer from poor bass response due in large part to the cancellation of low frequencies caused by the out-of-phase pressure wave from the back of the speaker. High frequencies are not reproduced because the large paper cone can't move as a piston as the frequency increases. Rather than creating a closed (possibly ported) speaker cabinet with multiple speakers covering different frequency ranges, it was easier and far less expensive to change the value of a few already-present resistors and capacitors to tailor the amplifier's frequency response: compensate for weak bass and treble response in the speaker by boosting the amp's bass and treble response. This mid-scooped response became the basis for later refinement in which the scoop became even more pronounced as stage volumes increased.

Leo Fender was not seeking a distorted tone from his amplifiers. He sought increasing usable clean volume for ensemble playing with a drummer and bassist. The fact that his designs were decidedly (due to cost constraints) ill-suited to maintaining a clean, full-range sound at high volumes led guitarists to discover uses for these pushed, overdriven sounds. An accidental discovery became a fashion that remains popular over fifty years later...

But what about truly clean full-range sound? Why does that have such a bad reputation in the current canon of guitar sounds? A lot of the negative connotations come from ill-conceived experiments. For example:

  1. You plug your guitar amp into a set of hi-fi speakers; it sounds awful.
  2. You plug your guitar directly into an input on your home stereo; it sounds awful.
  3. You plug your guitar directly into the band's PA; it sounds awful.
  4. You plug your guitar directly into a keyboard amp; it sounds awful.
  5. You plug your guitar directly into an amp designed for electro-acoustic guitars; it sounds awful.

Do you see a pattern? It really looks like the only thing that works for an electric guitar is a guitar amp. But let's take a moment to look a bit closer at the reasons why these experiments fail; from that we might learn something about how to properly amplify a guitar through a full-range flat-response sound system.

Experiment 1: When you plug hi-fi speakers into the output of a guitar amp, you're violating the amp designer's assumption that the speakers will have extremely poor low- and high-frequency response. The result is a guitar sound having a thuddy and muddy bass, ear-piercing highs and no body.

Experiment 2: A home stereo has no input for an electric guitar, so you use an adapter. That solves only the physical problem of connecting the guitar. The impedance is still too low for the guitar, which sucks out all of the guitar pickups' low- and high-frequency response. Also, the home stereo doesn't have enough gain to bring the low guitar-level signal up to a usable level; you must either crank the stereo volume all the way up (thus introducing hiss) or pluck the guitar strings really hard (thus killing all subtlety and producing fatigue). If you get the bright idea to plug the guitar into the magnetic phono input you've solved the gain problem while introducing a horrendous EQ problem.

Experiments 3 and 4: Plugging your guitar into an input on a PA or a keyboard amp causes the same impedance and signal-level mismatch explained above.

Experiment 5: This one's a bit trickier, since you'd think that an acoustic guitar amp should be designed with a flat frequency response. Unfortunately, that's often not the case. Refer to my earlier comments about fashions in sound and designing products to appeal to those fashions. (Keep this in mind when you read my capsule summaries of certain acoustic guitar amps, below.)

How then does one plug a guitar into a full-range sound system without encountering the above problems? The first and best tool for such applications is an active DI box. The box should have an input impedance of 1M ohms (which is the defacto standard for guitar amp input impedance) or higher. A DI box will have a 1/4" input for your guitar and an XLR output to drive the sound reinforcement gear. If you're driving a PA desk, you simply run an XLR cable from the DI to a mic input on the desk; the mic input will have more than enough gain for your guitar - in fact you may have to use the input pad to avoid unwanted distortion.

If you intend to drive a line-level input you'll still need a DI box, but you'll also need a preamp to boost the signal level. Look for devices marketed as mic preamps. You should be able to get a preamp suitable for guitar for well under $200. Run a short XLR cable from your DI to the preamp, then run whatever kind of cable and adapter is required to reach the line input on your sound gear.

Let's turn our attention to speakers. We're not interested in the properties - such as breakup and coloration - that apply to guitar speakers. In a full-range system we must concern ourselves with tonal balance (i.e. "neutrality") and required volume.

The number and size of speakers has an impact on how much low-end volume you can get out of the system. Overall size, weight and expense is the flip side of this coin. EQ has its limitations: you can use EQ to get a lot of bass response out of a 6" speaker, but as you turn up the volume that little speaker won't be able to move enough air to keep up with the volume increase.

The flip side is that larger speakers have greater mass which makes them less capable of responding to higher frequencies. This is why you see multiple sizes and kinds of speakers in sound-reinforcement applications: different speakers handle different ranges of frequencies.

A guitar doesn't stress the low-end frequencies. The low E on a guitar in standard tuning vibrates at about 80 Hz. That's pretty easy to reproduce. To be honest, you don't really need a lot of energy in the lowest register of the instrument. Too much low-end in your sound system will invite problems with body-resonance feedback on your acoustic guitar, may add an undesirable "thud" to pick attack on your electric guitar, and will definitely encroach on the bassist's sonic territory when you play in an ensemble.

Although the fundamental frequency of a guitar only ranges up to about 1,300 Hz (24th fret on the high E string) there is important information in higher frequencies that give your notes more of a sense of articulation.

Electric guitarists will tell you that there's no use in reproducing anything above 5,000 Hz or so. This is a good rule of thumb when you're playing with distortion as the higher-frequency components start to sound more like "fizz" than like part of the note. For fingerstyle guitar, though, an extended high frequency response is good for note articulation. But watch out: many acoustic guitar amps include a tweeter without properly designing the overall response of the speaker system, leading to overaccentuated high-frequency response which can cause listening fatigue. Some vendors attempt to "fix" this problem by making the tweeter's level adjustable.

If you're interested in playing your guitar through a full-range system, I'd recommend checking out a variety of amplifiers designed for acoustic and jazz guitar as well as a few small PA systems and powered speakers. (Remember about the active DI and preamp when evaluating PAs and powered speakers.) Your ears, rather than recommendations from other players, need to be the deciding factor.

Power, frequency response and distortion are all interrelated. It takes more power to play loud, to reproduce low notes, and to drive speaker systems that are tailored for neutral response. It's relatively easy and inexpensive for a designer to create a loud sound system. It's really difficult to create a sound system that's loud and uncolored.

Sound reinforcement amps, by nature, overload differently than guitar amplifiers. In short, you don't want to ever overload a sound reinforcement system. Its entire function is to reproduce the program source without coloration or distortion. Still, it's unavoidable that operator error or unanticipated program transients will tend to push a sound reinforcement system into distortion. For this reason, systems almost always include some kind of indicator light that flashes when the system approaches its design limits. Be aware of this indicator, and back off a bit on the volume (or your playing intensity, if you're a solo performer) when the light starts to flash. Better systems complement the flashing indicator with some kind of limiter that will attempt to reduce program peaks to avoid distortion without introducing too much undesirable coloration. But limiters do have their limits, so don't rely on them to solve all your problems. If you push a sound reinforcement system hard enough, it will sound bad.

Here are some of my own observations about a selection of clean full-range amplifiers:

I've tried most of the Fender Acoustasonic amps. To my ear, they all suffer from an overemphasized high-end response, uneven midrange response and inadequate bass response. Some of them have a tendency toward preamp overload if you attempt to use them to amplify an electric guitar.

I've sampled a few acoustic amps from several other "commodity" guitar-and-amp vendors; in general my observations match those I've noted regarding the Fender line.

The Genz Benz acoustic amp line covers a wide range of size, power, weight and features. In general they seem to be better sounding than the Fender acoustic amps.

The Fender Jazz King is a very nice amp with useful EQ and a reverb that sounds more like a hall reverb than a spring reverb. (Some complain that the reverb is hard to tame, but I love it.) This amp will cover large rooms; it's definitely overkill for intimate performances. Its flexible active EQ partially makes up for the lack of an extended-range speaker.

The Acoustic Image gear may be of interest if you can find any to try out. I was quite impressed with the one I played in a shop, but they're almost always backordered and pre-sold.

My favorite is the AER Compact 60. I use this with my custom Koll semi-hollow electric guitars played fingerstyle. At less than a cubic foot and under 20 pounds, this amp looks like competition for the smaller Roland Cube amps. But the sound quality (and price) are worlds apart.

For portable PAs, the Yamaha STAGEPAS 300 is a good choice. You'll want some kind of preamp between your guitar and the PA in order to have impedance matching and EQ appropriate to your instrument. A friend has been touring the western states with her electro-acoustic guitar, an LR Baggs floor preamp, and a STAGEPAS. The rig sounds good and has held up well to the rigors of the road.

You may also want to consider using a multi-FX processor as the interface between your guitar and a PA or keyboard amp. I have a Groove Tubes SFX100 powered speaker that I use with a multi-FX processor (Boss or Digitech). This is useful when I want both clean and effected guitar tones to complement an ensemble. The SFX100's M-S style speaker arrangement gives a convincing sense of space when used with stereo chorus or reverb. The Spacestation model in current production is almost the equivalent of the discontinued SFX100.

I've also used my STAGEPAS with a multi-FX where I need to support multiple instruments or to cover more space than the SFX100 can support.

In summary, a full-range sound reproduction system can be used very effectively with an electric guitar when properly configured. Such a setup is well-suited to the nuances and dynamic range of fingerstyle and jazz guitar.

July 07 2008 00:02:51 GMT