Focusrite’s JFET Factor

When recording guitar and bass, having a dedicated instrument input on your audio interface is an invaluable way to get ideas down quickly and capture creativity when it strikes. But not all instrument inputs are made equal, and a badly designed one can suck the tone out of even the chunkiest guitar sound, and create weak-sounding recordings as a result. The good news is that there’s a type of electrical component that avoids these creativity-sapping situations, known as a Junction Field-gate Effect Transistor, or JFET for short. JFET circuitry provides particularly accurate results when recording guitars and bass, and can be found on all Clarett+ instrument inputs.

Read on to discover more about the JFET advantage, and how your Clarett+ gives you the ultimate peace of mind when recording instruments.

What problem does JFET circuitry solve?

Guitars put out a fairly low-level signal, one that needs pre-amplification before it can be recorded. There are many ways to design a preamplifier circuit that can level-up the signal, but designing a circuit that can also preserve our precious guitar tone is a bit harder. Key to capturing an authentic tone is presenting an appropriate impedance to the instrument when you plug it in. And for this, it's vital to have the right active component at the very start of the signal chain.

In modern electronics, this component will be an operational amplifier or op-amp: a small integrated circuit incorporating a dozen or so transistors that can amplify a small input signal in a stable, consistent, and precisely controllable way. The cheapest op-amps are so-called 'bipolar' designs, based around an old type of transistor known as the biplolar junction transistor. On paper, these will do the job. In practice, though, their input impedance is too low for guitars.

To capture the pure sound of an electric guitar without compromise, we need to design a no-compromise input stage. This means bipolar op-amps are out. Instead, we must turn to the higher-quality and more appropriate JFET-input op-amp, which has a much higher input impedance than other designs. Done right, in fact, a JFET design can actually be very similar to a valve-based guitar amplifier in terms of how it interacts with and preserves the sound of your guitar.

Why does input impedance matter when recording guitars?

When you're connecting one electrical thing to another, impedance can be critical. Impedance is officially defined as a measure of the opposition that a circuit or component presents to an electric current. That being the case, you might think that impedance always needs to be kept to a minimum. Surely we want our signals to flow with as little opposition as possible?

Not so. What matters isn't the impedance of an input or output measured in isolation. It's the impedance relationship between that input or output and whatever is connected to it. The ideal source has an output impedance of zero Ohms (Ω). The ideal destination presents an infinitely high input impedance. In the real world, that's not possible, but a handy rule of thumb is that the impedance of the destination device needs to be at least ten times higher than that of the source. Any less a ratio risks degrading the signal quality as the source is 'loaded down'.

Powers Of 10

A 1:10 impedance ratio is quite easy to achieve with microphone preamps, because a microphone typically has a fairly low output impedance of around 150Ω. It gets much harder with passive pickups on guitars and basses, because these don't behave very much like our ideal source. Their impedance is often quoted as being in the 5kΩ to 15kΩ range, which is already a couple of orders of magnitude higher than a mic. However, even that is an underestimate, because it ignores the way impedance varies with frequency. A pickup that presents a 10kΩ impedance at 100Hz might rise to as much as 100kΩ in the upper midrange — the region where our hearing is most sensitive, and we're most likely to notice signal loss.

What does this mean in practice? If you've ever plugged your guitar into a cheap DI box or badly designed effect pedal, you've probably encountered the phenomenon known as 'tone suck'. The sound of the instrument becomes dull and dark, and doesn't seem to respond to picking dynamics in the way it should. In short, your recordings sound uninspiring, the guitar becomes less fun to play, and impedance mismatch is usually responsible.

So if you're recording an electric guitar, a bass guitar, or even an acoustic guitar with a piezo pickup, you definitely don't want your audio interface input to have a low impedance. In fact, to ensure the sound of the instrument is going to be captured with the highest possible fidelity, and with no loss of high frequencies, it needs to 'see' an impedance of around 1MΩ — one million Ohms.

JFET in use

So hopefully you have a better idea about impedance relationships and how a JFET instrument input is a surefire way to preserve your guitar tone. But the truth is you don’t need to be thinking about the JFET circuitry every time you plug in. Just be safe in the knowledge that, when connecting your guitar, bass, electric piano or other electrified instrument to your Clarett+ interface, your tone will be captured in all its pristine glory, so you can make better recordings, stay inspired and create your best work.