Reactive Loads for Guitar Amps

Maybe you want a line-level or headphone output for your guitar amp, or perhaps you simply want your amp to sound like it’s being run full bore without the deafening volume level. And a reactive load sure sounds more, well, “reactive”. But what does that really mean?

This is going to get a little technical. I’ll try to keep it short and explain only what is necessary. Some details will get glossed over, because I don’t want to turn this into a dissertation.

Let’s start with some basics: a load is simply the part of an electrical circuit that consumes power. In a typical guitar amp, the loudspeaker is the load. Guitar amps that use vacuum tubes in the power section require a load for safe operation, so don’t run your tube amp without the speaker or a load attached. So, to achieve an attenuated or line-level output, you need something that can simulate the load presented by your loudspeaker but doesn’t make any sound.

You have probably encountered the term impedance, which is used to quantify a loudspeaker’s load. For example, some guitar speakers have a 16Ω (ohm) impedance. Impedance is simply the sum of resistance and reactance:

• Resistance is the load’s opposition to a constant electrical current; it statically resists the flow of current.
• Reactance is the load’s opposition to a change in electrical current; it’s opposition reacts to changes in the flow of current. Rate of change is just another name for frequency; so one can say that reactance is the frequency-dependent opposition to current.

Therefore, a “reactive” load is one that has reactance in addition to resistance, whereas a so-called “dummy load” typically has only resistance. As it turns out, the term “reactive” was not invented by some marketing guy to sell more load boxes (although it was likely co-opted by said marketing guy) and actually has a well-established definition in the lexicon of electronics.

But why does a reactive load sound different than a purely resistive one? While general-purpose audio amplifiers have a very low output impedance compared to their load, guitar amps typically do not (or they simulate this effect via damping reduction). And when the amplifier has a sufficiently high output impedance, a reactive load affects the frequency response.

So, we finally have enough background information to answer the question: a reactive load is one that affects the frequency response.

However, simply making a load reactive may not actually result in a more convincing loudspeaker simulation. There are an infinite number of ways in which frequency response can be manipulated; some may sound better, some worse. And only one will match the loudspeaker being simulated.

Here, I should point out that only a purely-resistive load is necessary for the protection of a tube power amp. The effect of reactance — being simply a filter — may be implemented after a signal has been attenuated to line level. This is less expensive, can result in a more accurate response, and is far easier to make adjustable.

In fact, other effects exhibited by a loudspeaker can be more effectively simulated via electronic means as well, and these phenomena should not be conflated with the effect of reactance:

• Loudspeakers have a significant mechanical resonance at low frequencies.
• At low frequencies, the response is affected by the loudspeaker cabinet.
• Loudspeakers exhibit a time-varying (dynamic) response due to power compression — this should not be confused with power amp compression, i.e., power supply “sag” in the amp.
• Loudspeakers exhibit nonlinear effects — essentially frequency-dependent distortion that occurs when driven too hard.
• And, of course, loudspeakers have a characteristic frequency response — independent of the load they present to an amplifier.

Knowing this, is there any situation in which a reactive load is appropriate? In fact, there is: if you want to run your amp full bore but listen through the loudspeaker at a lower level. In this case, you’ll want the attenuating load to simulate the load of your loudspeaker as closely as possible. However, reactance is just one part of the equation. The attenuated output may not sound exactly like your loudspeaker at full volume, but a reactive load should sound better than a resistive load in this case.

In summary, a reactive load is simply a load whose impedance is frequency-dependent. A reactive load is desirable when used to attenuate your amp’s output in conjunction with your normal loudspeaker. However, simply adding reactance to a load does not result in a comprehensive guitar speaker simulation. If your only goal is to achieve a line-level, speaker-simulated output from your amplifier, then you may be better served by a resistive load that feeds a comprehensive speaker simulator — one that simulates reactance as well as other loudspeaker attributes.

(Originally posted by Brian Neunaber 9/12/2018)