HD 660S2 on a Qudelix 5K: exactly 0.0 dB of headroom, which is not the same as enough
The 5K's published 2 Vrms on the 3.5 mm port puts the 300 Ω HD 660S2 at exactly 110.0 dB against a 110 dB target. It reaches the peak with the volume at maximum and nothing left over — the clearest possible picture of what "it plays" leaves out.
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)
110.0 dB achievable against a 110 dB target. Headroom: 0.0 dB. This is the cleanest example in the catalogue of why "it plays" is not an answer.
Qudelix publishes 2 Vrms as the maximum output level of the 5K's 3.5 mm unbalanced port. The HD 660S2 is 300 Ω at 104 dB/V and needs 1.995 V for a 110 dB peak. The two numbers meet almost exactly. The verdict is enough-without-headroom, and what that means in practice is: on a normally mastered track you will be at or near maximum volume when the loudest transient arrives, and there is no reserve for the next recording that happens to be quieter.
Why the target is 110 and not 85. 85 dB is a comfortable sustained listening level; 25 dB on top of it is the crest factor of an uncompressed recording — the distance between the average and the peaks. A source that reaches 110 dB has enough for both. A source that stops at 110.0 dB has enough for both provided nothing about the material is quiet, and quiet material is exactly where a 300 Ω headphone is usually pointed.
The pearl: the balanced port is the obvious answer and we are not going to compute it. The 5K has a 2.5 mm balanced output that would raise the ceiling. Qudelix publishes 2 Vrms specifically for the 3.5 mm unbalanced port, and no voltage for the balanced one on the page we cite — so there is no honest number to put in the calculator. There is also a connector problem in the way: the HD 660S2's balanced cable terminates in 4.4 mm, the 5K's balanced jack is 2.5 mm, and the stock 6.35 mm cable needs an adapter for the single-ended port. Every path from this headphone to this dongle involves a plug that did not come in either box.
What we cannot check. No output impedance is published for the 5K. The 1/8 rule wants below 37.5 Ω for a 300 Ω headphone; that is a loose requirement most outputs would meet, but it is unverified here, and any real output impedance makes the 110.0 dB above slightly lower rather than higher. Note also the host side: the 5K is a Bluetooth and USB device with its own battery, so the Galaxy S25's port is not carrying the amplifier's power — the level above does not depend on the phone at all.