55 Ω and 300 Ω that need exactly the same voltage: K240 Studio vs HD 660S2
Both are rated 104 dB/V, so both need 1.995 V for a 110 dB peak — a $69 semi-open and a $480 flagship, five times apart in impedance. What differs is current: 36.28 mA against 6.65 mA.
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)
Same voltage, same verdict, different bill in current. K240 Studio: 114.2 dB, 4.2 dB spare. HD 660S2: 114.3 dB, 4.3 dB spare.
AKG publishes 104 dB SPL/V for the 55 Ω K240 Studio; Sennheiser publishes 104 dB at 1 Vrms for the 300 Ω HD 660S2. Both are already per volt, so nothing needs converting and the requirement is identical: V = 10^((110 − 104)/20) = 1.995 V. iFi does not publish a voltage for the ZEN DAC 3 either, only power — >18 mW into 600 Ω single-ended — from which V = √(0.018 × 600) = 3.29 V. That covers both headphones with room to spare.
Where the five-to-one impedance difference actually shows up. At the same 1.995 V the K240 draws 36.28 mA and 72.38 mW; the HD 660S2 draws 6.65 mA and 13.27 mW. The cheap semi-open asks its source for five and a half times the power of the flagship, and every «high impedance is hard to drive» rule of thumb has it backwards on this pair. It matters when the source runs out of current rather than volts — battery dongles and phone jacks — but never for a mains-fed desktop DAC.
The pearl: the 0.1 dB gap is the output impedance, and it is the only reason the two results differ at all. iFi publishes ≤1 Ω for the headphone section. That 1 Ω forms a divider: the 55 Ω K240 keeps 3.231 V of the 3.29 V (a loss of 0.16 dB), the 300 Ω HD 660S2 keeps 3.279 V (0.03 dB). So the low-impedance headphone is very slightly the more penalised — the exact opposite of the folk rule, and the whole visible difference between 4.2 dB and 4.3 dB.
Damping: checkable, and fine. 55 Ω against 1 Ω is a ratio of 55, 300 Ω against 1 Ω is 300, both far above the 8 the 1/8 rule asks. Note that iFi's figure is «≤1 Ω», an upper bound rather than a measurement, so we record the bound and the conclusion holds at the worst end of it. Also note that the K240 name is shared by three electrically unrelated headphones — the current 55 Ω Studio, the 600 Ω Monitor and the old Sextett. Only the 55 Ω version is on this page.