If you’re familiar with the topic then feel free to skip ahead and watch the video to see a comparison of the two. If true bypass is new to you, it’s basically a technique of modifying a pedal so when the pedal is in the off position (bypassed) the circuitry is 100% out of the guitar’s signal path. This is essential for retaining your guitar’s original tone. Some effects, specifically vintage effects and wah pedals, are notorious tone-suckers. Some of the guitar’s signal is being fed into the pedal’s circuitry even when the pedal is bypassed. The result can be signal bleed, added noise, decreased overall output, and most likely a loss of high-end and clarity.
So with that out of the way… on to the video to hear a difference:
The video was recorded using a Fender ’69 Reissue Thinline Telecaster and a vintage Fender Super Reverb. The first video clip is simply the guitar into an unmodified Electro Harmonix Russian Big Muff then into the amp. The second half of the clip is guitar straight into the amp. If the pedal was modified for true bypass it would sound like the second clip. You can hear a massive increase in volume and see the peak around 2-4kHz when the pedal is removed from the signal chain. Click between 0:25 and 0:58 if you’re impatient. I should note that there was absolutely no post editing to the audio and everything was shot in a single take. The yellow lines show the peaks for each frequency and you’ll notice the overall output is a few dB higher.
For more reading on true bypass with basic diagrams.