Truthear HEXA on a Focusrite Scarlett 2i2: do you need a separate DAC?
Yes — but not because the interface is too weak. It is 7.1 dB too strong. The reason is 50 ohms of output resistance against a 20.5 ohm in-ear: a ratio of 0.4 where 8 is wanted, the worst in the whole catalogue. A 25-euro dongle does what the 200-euro interface cannot.
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)
Yes, buy the dongle — and then notice that the reason has nothing to do with power.
Truthear publishes the HEXA at 120 dB/V and 20.5 Ω. For a 110 dB peak it needs 0.316 V, 4.88 mW, 15.43 mA — one of the easiest loads in the catalogue. The Scarlett 2i2 4th Gen has 2.45 V on tap, seven times more than required.
Then the 50 Ω output resistance meets the 20.5 Ω load. The divider hands 71 % of the voltage to the interface's own output stage: only 0.712 V reaches the earphone, which is still 117.1 dB, 7.1 dB above target. The level was never in doubt. The impedance ratio is 20.5 / 50 = 0.4, against the 8 the rule asks for — the rule is missed by a factor of twenty, and this is the worst-matched pair in the catalogue.
Unlike a 0.3 dB wobble on a DT 770, a ratio of 0.4 on a multi-driver in-ear is not a footnote. The crossover of a hybrid earphone presents a swinging impedance across the band, and every ohm of source resistance turns that swing into a frequency response change. What you hear is a different tuning, not a quieter one.
| Source | Output | Level | Headroom | Ratio | |---|---|---|---|---| | Scarlett 2i2 4th Gen | 2.45 V, 50 Ω | 117.1 dB | +7.1 dB | 0.4 | | Audient EVO 4 | 2.48 V, 50 Ω | 117.2 dB | +7.2 dB | 0.4 | | FiiO KA13 dongle | 1.22 V, 1.7 Ω | 121.0 dB | +11.0 dB | 12.1 | | FiiO K11 | 4.24 V, 1.2 Ω | 132.1 dB | +22.1 dB | 17.1 |
The pearl: the interface loses more of its own voltage into this earphone than the dongle produces in total. The 2i2 drops 10.7 dB in its own output resistance — 2.45 V becomes 0.712 V — while the KA13, with 1.7 Ω, keeps 1.127 V of its 1.22 V and ends up louder than the interface that started with twice the voltage. Nothing about that is visible in either specification sheet, because output resistance is printed in a different row from output voltage, and neither is printed next to the load.
One limit we must state rather than calculate: with 22 dB of surplus on a K11, hiss becomes the operative question, and our schema has no field for output noise. We can give you the level and the ratio as numbers; on hiss we can only tell you where to look.