When Is True Bypass Appropriate? April 21 2014

There has been a lot of buzz about "true bypass"—what it means, why it's supposedly great, etc. Some of what you read from various sources may lead you to believe that true bypass is the only good bypassing scheme. The truth is that true bypass—while sometimes appropriate—is not the best bypassing scheme in many cases.

If you don't know what true bypass is, it is simply a hard-wire bypass around the entire circuit. When bypassed, both the input and output are removed from the circuit and connected together. This may be accomplished with either a mechanical switch or relay.

The Problem

Even I had bought into the hype and designed the first versions of the Wet Reverb with true bypass. But then I started receiving complaints from customers. In some cases, the pedal changes tone between bypass and unbypassed. In other cases, there is a pop when bypassing the pedal.

So I investigated. I knew that my pedal had a completely neutral frequency response, so that couldn't be the problem. And of course, true bypass completely bypasses the circuit; so that couldn't be the problem either, could it? It turns out that the latter assumption is false; and true bypass is, in fact, the problem.

Explanation, Simplified

You have to look at the whole signal chain—from pickup to amplifier input—to really understand what is happening. (And I'll get into the technical explanation later, for those who may be interested.) A guitar pickup is a very poor signal driver because of its high impedance. Even the relatively small capacitance of a guitar cable can have a noticeable effect on the tone of a pickup. The more cable you add, the more high-frequency attenuation you will hear.

Effect pedals are often blamed for the change in tone that may result when they are added to a signal chain. While it is true that some pedals are poorly designed and will alter the guitar’s tone, the real culprit in many cases is the additional cable that is added after the pedal.

When an effect is on, generally speaking, it is buffering your guitar's signal. This buffer assumes the burden of driving whatever cable, effects, or amplifier that follow it. When bypassed using true bypass, an effect is no longer buffering the signal; and your guitar pickup is now burdened with whatever load comes afterward. This load may include several meters of guitar cable and/or a low-impedance amplifier input. The net result will be a loss of high frequencies. So, true bypass results in switching between a buffered and unbuffered signal, which of course will result in a change in tone!

And since true bypass completely bypasses the circuit, the signal is now at the mercy of whatever comes before and after the pedal. A buffered pedal will typically ground-reference the signal: this means it removes any DC offset. However, if the driving or receiving signal has a DC offset present (even a small one), true-bypassing will result in a pop.

The Solution

The solution is quite simple: use a buffer as the first thing in your signal chain. It will not matter what comes afterward, because the buffer will drive it. This buffer could be a pedal, or active pickups, or active circuitry in your guitar. If you only run effects in a buffered effects loop, then you're already covered.

Some players go to great lengths to preserve their unbuffered tone, including expensive low-capacitance cables; but using a good clean buffer avoids all this.

Don't take my word for it: read what Pete Cornish has to say about true bypass.

And you can still use true bypass pedals after the buffer if you wish. In fact, true bypass or any form of unbuffered bypass after a buffer will result in the lowest noise and highest headroom configuration.

Now, I know there are some purists that don't want anything other than a straight wire between their pickups and amplifier. The fact is that you already have things in your signal path that influence your tone:

  • The volume pot in your guitar loads your pickup, and its value is specifically chosen to tame the resonant peak of your pickup.
  • You may also have capacitance added at your pickup to change its resonant frequency.
  • The tone circuit in your guitar adds an additional load even when turned all the way up.
  • A guitar cable adds yet more impedance (mostly capacitance), further changing the resonance frequency of your pickup.
  • Your amplifier, depending on its input impedance, can also influence your guitar’s tone.

So, what you are accustomed to hearing as the “pure” tone of your guitar is actually the confluence of these factors. It’s simply a reference point to which you have a preference. Deviations from this reference are judged to be a degradation of tone, when in fact they are simply different.

My point is: let's not fool ourselves by thinking there is something "pure" about running a guitar straight into an amp. You have a particular combination of guitar, pickups, etc. that you prefer, and the best way to preserve this sound is to buffer it.

What is the best bypass for my particular setup?

First, the type of bypass only applies to when the pedal is actually bypassed. When the pedal is active, the signal is buffered--regardless of the type of bypass. So, if you pedal will be active all the time (as a reverb pedal might be), then the type of bypass doesn't matter.

That said, I generally recommend that the first and last pedals in any pedal chain be a buffer, a buffered-bypass pedal, or an "always on" pedal. This ensures two things:

    1. a constant input impedance presented to the send (effect send or instrument), ensuring consistent tone when bypassing effects, and
    2. an uninterrupted output driver to the return (effect return or amplifier input), minimizing any true-bypass switch pops due to slight differences between local ground references.

The pedals between these two may be true bypass, if so desired. The more pedals you have, the more you may want them to be true bypass to minimize any accumulated noise. However, if you only have a few pedals, then it's less critical that they be true bypass.

Technical Explanation

I have yet to see a comprehensive technical explanation for why true bypass is flawed. So, here it is…

You can simulate the guitar pickup->cable->effect->cable->amplifier signal chain fairly simply using any Spice (circuit simulation) software. For the following examples, I used the Linear Technology LTspice freeware.

Terry Downs presented a good simulation (link is now down) of the pickups through to the amplifier input, which is what I chose as my starting point. We are more concerned with the change in the pickup’s response rather than its absolute response, so don’t be too concerned with the choice of pickup used for this simulation.

In the first example, I have simulated the effect of a typical 20-foot (6-meter) cable vs. a 40-foot (12-meter) cable. This is what happens when you use true bypass without a buffer; because when you bypass the effect, it’s like adding a longer cable to your guitar.

20-foot cable schematic:

 

40-foot cable schematic:

 

The figure below shows the resulting frequency response. The solid line is the magnitude response, while the dotted line is the phase. The green line shows the frequency response of the pickup driving 20 feet of cable, while the blue line shows the pickup driving 40 feet of cable.  So, you can clearly see how true bypass can affect your tone.

 

In the second example, I have simulated the effect of adding a buffer after the first 20-foot cable with a second 20-foot cable after the buffer:

 

The frequency response is shown below, overlayed with the 20-foot cable simulation above.  Again, the magnitude is the solid line, and the phase is the dotted line. The two traces are so close that they lie on top of one another. This is because the buffer mimics the amplifier input (by choosing an equivalent input impedance), so any length of cable after the buffer does not affect the frequency response.

 

Conclusion

True bypass has its place but is surrounded with a lot of marketing hype. You need to buffer your guitar signal to avoid the tonal effect of adding additional cable to your signal chain. As long as you do so, true bypass and unbuffered bypass are equally optimal for other effects after the buffer. However, if you have only one pedal in front of your amp, then it should use a buffered bypass.