The BJT Signal Buffer for Guitar/Bass

by Adam Fet • 2026-04-12

Signal buffers are perhaps the most essential concept in audio electronics, other than knowing what AC is vs DC.

And with good reason, as in one way or another, you're very likely to find them splattered all over the place when reviewing circuit schematics.

That's cool and all but, what do these even do?

As explained in this video I made a bit back, the most basic use case for these is to get a signal and replicate it, preferably without loading our source given that, by doing so, we would effectively attenuate the desired signal and in many cases we'd just outright kill it, considerably reducing our options when processing it.

There are a couple of ways of achieving this result, and most will use some form of active electronics. In ye olde days this was achieved with vacuum tubes and a whole lotta voltage, but now we have technology and thus can achieve the same with these simpler topologies:

• Operational Amplifiers in a non-inverting unity gain configuration
• JFETs or MOSFETs in a common drain configuration
• BJTs in an emitter follower configuration

And with the latter, we get one of the most popular designs in the guitar effects world. BJTs are dirt-cheap and real easy to find nowadays (especially when compared to JFETs). Not only that, but they don't feature as many caveats as MOSFETs when trying to use them in a non-switching application.

Op-Amps do offer even more advantages over NPNs and the price difference is negligible nowadays, but knowing the basics has never hurt anyone... right?.

As such, BJT buffers will stay a part of audio for the foreseeable future, which only emphasizes the importance of knowing how to build one.

The Build

These are super simple, so let's jump straight into our design.

And a quick breakdown of what does what:

• If you've read my previous guides this will look very familiar, because we start the processing with the same band filter I use on these instrument circuits: R1 and C2 form a LPF that prevents high-frequency EMI noise from corrupting our beloved signal
• C1 acts as a decoupling cap that isolates our source from the DC bias added by R2 and R3
• These also form a HPF, but as it works on a real low frequency cutoff it shouldn't have any effect on the audible spectrum of 20-20kHz
• Q1 is our boi, a single NPN BJT receives our biased signal thru its base, gets powered by our PSU via its collector and dumps our signal back on its emitter
• We add a current limiting resistor R4 in order to keep everything from blowing up, and pick our signal back up from the emitter using a decoupling cap C3, which also removes the DC bias, effectively giving us our original signal back
• C3 and R5 once again form a HPF, that also operates well below 20Hz

As a wise man once said: "A BJT is a BJT is a BJT", so the selection of Q1 is mostly dominated by whichever NPN transistor you have available. Here I picked a super common 2N3904, but there's also the 2N2222 or the BC327, honestly just go wild here and see what you can get.

And with that we're done, we have a fully functional signal buffer which you can now use to turn your super high-impedance guitar/bass output into a clean high-impedance source for your pedals or amp to devour.

Before and after

For a quick visual demonstration, here's the captured waveforms from a DI:

Notice the HUGE difference in amplitude in the left half (no buffer) vs the right (using our BJT buffer), as well as the change in captured noise floor between the two.

These are also noticeable when capturing the signal through a guitar amplifier using a trusty old SM57, using the same amp settings of course:

Sound demos

This is one of the more boring concepts we'll ever tackle when dealing with sound demos, and the most interesting part here would be the difference in high-frequency response as well as noise floor. These should ever only be an issue when dealing with hella long cable runs, so these demos were recorded through 40m of XLR cable using pins 1 and 2 in order to try to maximize the noise picked up.

Thanks!

Thank you for reading! ♥

Hopefully you enjoyed your stay here, I'll try to write up some new guides as I release videos so keep your eyes open!

If you enjoy my work, you can always contact me via email or gift me a coffee, I do appreciate it a whole lot!

Sources

• Signal Buffers
• Small Signal Audio Design
• Transistor Amplifiers (II) - Other Amplifier Stages