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THX AAA789 volume problem?

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I am using my Fiio BTR5 via USB out of my computer. If I plug my HD6XX directly into it I can hear music and a comfortable volume with it set to about 40 (out of 60)...and it sounds good.  If I then use my 3.5mm to RCA cable to my new THX AAA 789 "SE Input" and connect my headphones to the 3.5mm SE jack on the front, the sound is notably lower. In fact, I have to turn the volume up all the way on the BTR5 and turn the THX amp up to full volume. Is this normal? Should I use "SE Pass"? Or do I have a defective THX amp?
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dcha12
441
May 25, 2020
Sounds like you have an issue with gain and not power, so I don't think a balanced cable for your HD 6XX is the answer. Is the volume on your computer maxed out?
dcha12
441
May 26, 2020
This is normal. I wouldn't sweat it at all. I'd actually use gain 3 and turn the pot down accordingly almost regardless of source (unless I was getting channel imbalance with sensitive stuff). Less feedback used = better sound.
Gain one on the 789 is actually a negative gain (reduction) so it absolutely makes sense that gain 1 is lower than the BTR alone, it's designed that way for easy to drive headphones/IEMS. If you have gain 2 engaged on the SE input that is 0db, aka no pre-amp gain is applied to the input voltage. If the BTR5 doesn't output more than 2 or so volts the 789 can be ran on high-gain (gain 3) with no concerns.