The Truth About True Bypass
[Rant-Mode ON]

What is true bypass? It is a switching method where, in the bypass mode, the signal goes directly from the input jack through the switch to the output jack with no connection to the circuitry in the box. In effect, it is a straight wire through the box and does not degrade the signal because there is only the micro-ohm resistance of the jacks, wires and switches inline with the signal.

In the diagram shown above, the path of the signal is along the blue arrows. The red connections represent the internal connections of the dpdt switch and the dotted lines in the switch signify that the poles move in unison. As you can see, the signal flows from the input into the switch, from center to bottom lugs via the internal poles and a jumper wire connects the sections together. It then goes to the output jack with no actual connection to the circuit board when in the bypass mode.

Compare the true bypass to the more simple method (shown below) used in many vintage pedals where the input of the circuit is left connected to the signal path at all times. This type of switching was used because SPDT switches are cheaper and easier to find than quality DPDT types required for true bypass.

As one can see, only the output jack is being switched. It can connect to the output of the circuit board for the sound of the effect, or to the input jack for bypassing the effect. However, when the input and output jacks are connected by the bypass switch as illustrated above, the input to the circuit board is still connected to the signal path and the input impedance of the circuit is loading the signal line. This type of switching can cause loss of high frequencies and overall volume, and is the reason that many wah-wah pedals "suck tone". Contrast this to the true bypass method where input and output jacks are effectively connected by only a piece of wire with absolutely no connection to the circuit board.

With true bypass switching, the input impedance of the effects pedal has ZERO influence on the discussion we are undertaking. Why? The true bypass is designed and used to eliminate the effect of a pedal's input load on the signal... the pedal is completely disconnected when the switch is bypassed so the input impedance of the circuit is not relevant! It does not matter if the circuit has a high impedance like a tube amp or is a low impedance transistor load - it is not a factor with true bypass.

Furthermore, while vintage pedals like a Big Muff can have a low input impedance, 40k for example, or even lower for a Fuzzface, this is not relevant either since the circuit is designed around that input, and tone or volume loss is compensated by the rest of the circuit design. In other designs, such as clean boosts (see mini-booster) the input impedance is 1M as with most tube amps. But again, it is not a factor in this true bypass discussion because the circuit board is completely disconnected when the switch is in the bypass position.

Let's not just theorize about the subject, we want to take some measurements of cables to see what kind of real world parameters will be encountered in working with a pedalboard and its connecting cables and then apply some basic electronics math to see what effect that might have on the signal.

I measured two 20' cables and some 1' jumpers that were in my studio.

Typical Cable Measurements
Radio Shack 1' jumper0.1 ohms0.17 nF0.003 mH
Hosa 1' jumper0.1 ohms0.16 nF0.002 mH
20' Musicians Friend cable0.4 ohms0.8 nF0.031 mH
20' Low Capacitance cable0.9 ohms0.47 nF0.019 mH

20' cables into 6 pedals w/1 ft. jumpers
6 Pedal chain13.9 ohms0.61 nF0.165 mH

You know, that's not too bad. The capacitance of all the cables does not add together to produce a substantial load on the pickups. The total capacitance of five jumpers and two 20' connecting cables is really about the same as one of the long cables alone. This is likely due to the fact that the bypass switches in the pedals are breaking up the connection of the cables and preventing the summation of capacitance as one would think would happen. The series resistance and inductance appear to be a sum of their respective values but do not present any real problem to the signal flow.

Notice that I used 1' jumpers and 20' connecting cables since those are the most commonly available lengths, although 6" jumpers are sold as well. I only used six pedals since there were only five jumpers on hand for connecting them together, but it demonstrates the principle well.

In summary, this quick experiment with a real world setup has shown that the series connection of true bypass pedals does not result in an excessive load nor does it reduce the quality of the audio signal because of the long cable runs since the capacitive load is much less than previous conjectured.

PART 2

What about a buffer? There is an article in the AMZ Lab Notebook on basic buffers that gives some background on this circuit building block. The general idea behind using a buffered pedal in the signal path is to present a high impedance, low capacitance load to the guitar pickups, which lets all of the natural sound of the guitar come through with little degradation. The buffer also has a low impedance output that is easily capable of driving long cables and capacitive loads as seen in some pedalboards.

As illustrated above, the buffer is at the front of the signal chain between the guitar and the pedalboard. If the buffer is opamp-based, the output impedance may be less than 100 ohms and can drive most any combination of true-bypass pedals with lengthy connecting cables.

If any of the pedals after the buffer is a vintage design that does not have true bypass, then its input impedance is a load on the signal path that is constantly presented to the buffer whether any of the other pedals are switched in or out. The buffer might be capable of driving the load but the sound of the other pedals will likely be altered.

Also, once you engage any pedal after the buffer, the output of that pedal is then driving everything after it and not the buffer. For example, let's say the first pedal after the buffer is a new model Big Muff with true bypass. When it is engaged, the BMP is being driven by the buffer but the audio signal line after it is getting the output of the Muff, which is around 10k instead of the 100 ohms from the buffer.

A high quality buffer can be a good thing to have at the beginning of the pedalboard chain, but the only way to have a constant low impedance signal line is to have a buffer built into each stompbox. That is exactly what Boss and Ibanez (among others) have incorporated into their pedals. Each of their pedal circuits has a buffer that drives the effect output when bypassed. This ensures that the pedal presents a high impedance load to the signal path and drives the pedals after it with a low impedance output.

A problem that can be encountered is that many of these pedals have two or even three buffers that are in the signal chain even when bypassed. The simple transistor buffers have a gain less than 1 and once you begin to cascade pedals, the noise can start to add up. If each pedal adds 3 dB of noise and you have 10 of them in series, it could be increasing the noise level by over 13 dB! It is never quite this bad in actual practice but the degradation of sound and the extra noise can indeed be audible.

Conclusion

The measurements that were made for this article have revealed that it is not a problem in practice to chain together numerous true bypass pedals. You really won't hear any difference if you use good quality guitar cables and jumpers. A buffer can be put to good use to drive long lines but combinations of pedals from different manufacturers can be a problem, especially if you have an old vintage pedal that is not true bypass. Having a buffer in each pedal circuit may be correct one problem while introducing noise and distortion.

The only good solution is to have a top quality opamp buffer in each pedal circuit that drives the output. This would mean that vintage pedals and many of the popular commercial devices should not be used... and that's not too practical for most players.

My advice: Buy true bypass when you can. Always use good quality cables and go ahead and chain your pedals together. Then, don't worry any more about it and just start jammin'!

More Reading:


Copr. 2004   Jack Orman
All Rights Reserved

This page last modified on Saturday, 16-Apr-2011 05:51:59 PDT

108342 hits since March 10, 2007