Is the Universal Audio Volt 1 loud enough for the Sennheiser HD 650?
Same impedance, different requirement: the HD 650 needs 2.24 V, the HD 660S2 needs 2.00 V. On the Volt 1's 2.68 V that is 1.6 dB and 2.6 dB of headroom — two «yes, but» answers, not one «yes».
Both play, neither has a reserve. HD 650: 111.6 dB, 1.6 dB over target. HD 660S2: 112.6 dB, 2.6 dB over. Below 3 dB is the answer the forums do not have a word for.
Sennheiser publishes both sensitivities per volt, so no conversion is needed: 103 dB/V for the HD 650 and 104 dB/V for the HD 660S2, both at 300 Ω. A 110 dB peak needs 2.24 V and 2.00 V respectively — 16.71 mW / 7.46 mA against 13.27 mW / 6.65 mA. Universal Audio publishes 12 mW into 600 Ω for the Volt line, which is √(0.012 × 600) = 2.68 V, and the same figure covers every Volt from the 1 to the 476.
One decibel of sensitivity is a quarter of the answer. The two headphones sit at the same impedance and differ by 1 dB/V, and that single decibel moves the HD 650 from «tight» to «tighter»: at 1.6 dB of headroom, any album mastered 3 dB below modern pop will have you at the top of the control. That is not a defect of the Volt 1 — it is what 2.68 V means against a 103 dB/V headphone.
The pearl: the model number is not the impedance. The HD 660S2 went back to 300 Ω after the HD 660S, which was 150 Ω. One digit apart in the name, a factor of two apart in current draw, and every calculator, forum post and «recommended amp» list that predates 2022 is about the other headphone. Check the sticker on the yoke before trusting any of it — including this page, which is about the 300 Ω S2.
What we cannot check. Universal Audio does not publish the output impedance of the Volt 1 headphone jack. Against 300 Ω the 1/8 rule wants 37.5 Ω or less, which most interfaces clear easily — but «most do» is not a measurement, and the HD 650 has a pronounced impedance peak near 100 Hz where a high-impedance source adds bass rather than merely removing level. With no published number, the levels above are the ceiling and the tonal balance is simply unknown.