Good point, I also read that it heats up in balanced operation.
A misunderstanding on my side, I thought "it" meant the Sennheiser HD-660S gets warmer if used balanced.
You both meant ADI-2 Pro gets warmer, thanx Mr. Freud, I didn't realize as it's so unlikely!
Heating up ADI-2 Pro is only possible if one maxes out ADI-2 Pro's headphones amps by listening loud enough with headphones with very low efficiency (think HiFiman Susvara).
HD-660S is quite efficient, there would be a difference in the heat generated by ADI-2 Pro, but very, very minor, noticable only under labaratory conditions.
Let's do some math:
• Sennheiser HD-660S has a sensitivity/efficiency of 104dB at 1V input.
• Assume you won't listen too far beyond the healthy long term listening limit of 82dBA SPL (A-filtered):
http://www.sengpielaudio.com/Zulaessige … gszeit.htm
• dBA's A-filter removes the bass, with music, measured full range, you end up about 10dB higher, at ~92dB linear.
• This is 12dB or 0.25x lower than Sennheiser's 104dB at 1V.
So, to drive HD660S we need 0.25V on each cup.
0.25V into 150Ω results in a current of 0.0017A (0.25V divided by 150Ω).
• ADI-2 Pro's headphones amp has a power supply voltage of 15V.
Volt x Ampere equals Watt (Power).
15V x 0.0017A = 0.025W each channel, = 0.05W total, unbalanced.
• Now double that if you use both of ADI-2's headphone amps in balanced configuration to deliver the same current (Ampere, A), you end up with 0.1W balanced.
• ADI-2 Pro, idle state, consumes about 12W total that heats it up significantly.
Do you think additional 0.05W makes a difference?
I just did some measurement to verify my calculation:
With my power meter's resolution of 0.1W ADI-2 Pro's power consumption does jump up 0.1W if I feed 0.5V into an Sennheiser HD-58X (150Ω).
0.2W ADI-2 Pro consumption at 1V into the HD-58X.
Due to the power meters resolution this is more like an estimate than a real measurement, but still some kind of confirmation.
To do "real measuements" I'd feed 8.3V into the 2x150Ω power resistors (not to break the headphone).
Then measured the rise of ADI-2s power consumption:
This confirms my calculations above.