Do you need a Radial J48 to record guitar into a Scarlett Solo?
If the guitar is on your desk, no: the Solo's instrument input already does the impedance conversion the DI exists for. And an active DI hands the preamp a signal 6 dB quieter than the cable did.
Signal Chain Compatibility Matrix
Select gear and parameters for live gain, impedance and SPL calculations.
Input Equipment Setup
PASS: Optimal Signal Chain
Requires minimum +60dB of clean pre-amp gain. Most budget entry interfaces max out at +50dB to +56dB and introduce hiss at max pot. An inline preamp booster (Cloudlifter CL-1 / FetHead) is strongly recommended.
The interface preamp provides sufficient headroom.
$538
$405
-25%Tonal & acoustic characteristics: Flat, wide-range frequency response for clean music and speech. Smooth, warm acoustic character with air-suspension shock isolation.
Technical advisory: With +69dB max gain, the Gen 4 Solo can drive demanding dynamics like the Shure SM7B natively without requiring an external inline booster.
Physical Interconnect Topology
Shure SM7B
Direct Cable Connection (No Intermediate Box)
1x Shielded Balanced XLR Cable (3-Pin XLR Male to XLR Female)
Focusrite Scarlett Solo (Gen 4)
Host: Apple MacBook Pro (M1/M2/M3/M4)
Short answer: for a guitar three metres from the desk, no. The Solo's instrument input is the DI box, built in.
A magnetic pickup is a high-impedance source. Feed it into a low-impedance input and it loses treble and level, which is what a DI box exists to prevent: it converts high impedance to low and unbalanced to balanced. The Scarlett Solo's second input does the first half of that job when INST mode is engaged — the input becomes high-impedance and the pickup is happy.
The numbers make the rest of the case. Straight into the instrument input, a guitar asks the preamp for roughly 20 dB. Through the J48 the requirement becomes about 26 dB, because this database lists the J48 at −6 dB of level. The Solo 4th Gen has 69 dB, so either way you are nowhere near a limit — but you have spent 239 dollars to make the preamp work harder.
The thing the word "active" hides: active means powered, not louder. The J48 draws +48 V phantom to run its input stage and still hands the preamp a quieter signal than the bare cable did. What the phantom buys is headroom and input impedance high enough not to load a hot active bass — not gain.
Where a J48 genuinely earns its price: a long run. High-impedance unbalanced cable loses treble with length because the cable's capacitance loads the pickup, and past roughly six metres it is audible. A DI at the instrument turns the rest of the journey into a balanced low-impedance line that can run thirty metres without loss, and its ground lift solves stage hum. On a desk, none of that applies.