Modhouse specify the Tungsten as 84.7dB/V 135Ω (single-sided) or 86.1dB/V 155Ω (double-sided).
The only advantage of the Pro here would be the ability to run the headphone in balanced mode (if that headphone allows that). That mode gives full 6 dB higher voltage here and thus 6 dB more max volume. In an extreme case as with this low volume headphone balanced might be a nice feature. But it depends how you listen to music, if you need that higher volume or not.
I can share my experience with the Dan Clark Audio Stealth (a planar magnetic headphone): using the balanced output on my Pro SE provided enough power and headroom to handle music with wide dynamic swings, such as classical pieces. This ensured I could reach adequate volume levels and maintain consistent performance. If you own planars that are power-hungry, the balanced output might be worth exploring.
Is this at 155 Ohm? The manual (ADI-2 Pro FS R v3.8, DAC v3.3) says voltage at 155/2 = 77.5 Ohm - so log2(77.5/64) = 0.2761 or about a quarter of the way between 64 and 128 Ohm - is 8.8 V RMS. Balanced voltage is thus 2*8.8 V RMS = 17.6 V RMS which gives 1998.5 mW RMS. This is consistent with the power-impedance figure where 155 Ohm is about a quarter of the way between 128 and 256 Ohm. For single ended, voltage is 9.6 V RMS and the maximum level difference is thus 20*log(17.6/9.6) = 5.26 dB.
Did you try the single ended output and lacked volume? Based on the manual, at 23 Ohm, I can only see a level difference on the 2/4 of around 20*log10(2*4.1/6.9) = 1.5 dB. Compared to the Pro FS and the DAC in single ended, the difference is around 20*log10(2*4.1/5.6) = 3.31 dB and around 20*log10(2*4.1/(2*3.2)) = 2.15 dB compared to the Pro FS balanced.
This looks reasonable now. The voltage between 16 and 256 Ohm has also increased slightly by up to 0.2 V.Sherlock is back - I found out what went wrong. The Excel table includes several rows of data points, and the two current (I) graphs from the pic in the manual refer to a row of 8, 16, 32 (match) and then 42, 80, 150, 250, 600 Ohm values. That of course does not match to the X axis used here.
The pic is from 2016 and had been fixed in 2017, but for whatever reason never made it into the manuals of Pro (all versions) and DAC (same pic, because same Extreme Power output stage). Most probably it got overlooked because they are nearly identical - only @JIW is able to see the issue
None of the official tech specs etc are affected by this, they are still all valid as documented. Here is the fixed pic.
View attachment 423684
By using balanced connection each terminal of an individual driver is connected to identical amplifiers operating as a differential pair across the driver. Thus, each amplifier has half the load compared to being single ended where only one terminal is connected to an amplifier and the other terminal is connected to ground.
Thank you for explaining all that. I got the 2/4 around the same time as the Stealth and chose a 4.4 mm balanced cable for the Pentaconn connection to get as much power as possible. I was coming from an ADI 2 DAC FS, which probably would have been fine, but the 2/4 is my end game setup for headphones, so I wanted to give it every advantage. I have not tried single ended with an adapter because I am worried about damaging the headphones, so I cannot say whether single ended volume was an issue. Reading a few comments in the review thread made me cautious, and your insights here are very helpful. Greatly appreciate you for sharing your knowledge.
There is no problem using a balanced cable with a single ended amp as long as the negative terminals are connected to ground in the adapter. The other way around using a symmetrical amp with a cable with combined ground for both drivers is asking for trouble as the amps for the inverted signals are directly connected through the cable.
I casually listen to music at -44 dB to -40 dB in high gain, mostly with music from the 60s to today, and quite a bit of techno/drum n bass. This volume level is well within the capability of just about any amp. I can push it higher with classical and 192 kHz tracks, since I usually stream Apple Music at their highest resolution. For typical 44.1 kHz rock, I rarely go beyond -18 dB for more than five to ten seconds, though I have jammed at around -24 dB in high gain with a balanced connection. I’m nearly 60, so my hearing isn’t the best.
Volume -18 dB in high gain is +7 dBu at 0 dBFS. Amir measured the DCA Stealth's sensitivity as 94 dB SPL at 0.549 V RMS which is equivalent to 94 - 20*log10(.549) + 10*log10(0.6) = 97 dB SPL at 0 dBu. At -18 dB in high gain, max peak SPL is thus 97+7+3 = 107 dB SPL. At maximum voltage for 23 Ohm, 8.2 V RMS, maximum peak SPL is 94 - 20*log10(.549) + 20*log10(8.2) + 10*log10(2) = 120.49 dB SPL.
