Sennheiser HD 490 PRO on a Scarlett 2i2 4th Gen without an amp: exactly enough
110.0 dB against a 110 dB target: enough with nothing to spare. The part that actually moves is the tonal balance — 130 ohms against a 50 ohm output is a ratio of 2.6 where the rule wants 8. And the 300 mW on Sennheiser's spec sheet is not a request for a 300 mW amplifier; this chain needs 24 mW.
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)
Exactly enough: 110.0 dB against a 110 dB target, headroom 0.0 dB.
Level. Sennheiser publishes 105 dB/V at 130 Ω. Through the 2i2's 50 Ω output resistor, 2.45 V arrives as 1.77 V, and the peak level lands on 110.0 dB. At the more usual 105 dB listening peak there are 5 dB in hand. What the chain has to supply is 1.78 V, 13.7 mA and 24.3 mW — the interface publishes 22 mW at 300 Ω, so this is the edge of what it is specified to do.
Tonal balance. 130 Ω against 50 Ω is a ratio of 2.6 where the rule wants 8. For a moving-coil driver that is a real but small shift at the bass impedance peak, not a failure: the measurement most often cited for a comparable case puts it at 0.3–0.5 dB. We print the ratio and the scale rather than the words «not recommended».
The pearl: the 300 mW on the spec sheet is not a request. Sennheiser prints «Power consumption (Amp) 300 mW (100 h, noise IEC 60268)». Passive headphones consume nothing — that figure is IEC 60268-7 power handling, what the driver survives through 100 hours of noise. Read as a demand it says «bring a 300 mW amplifier», and that reading is what sells headphone amps. The actual requirement on this page is 24.3 mW for a 110 dB peak and 7.7 mW for 105 dB — twelve and thirty-nine times less. A row that describes destruction gets read as a row that describes appetite, and the difference between the two readings is the whole purchase.