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Oppo 105 and Power Amplifier Input Sensitivity

Novice here and needing some advice! Yesterday, I picked up an Oppo 105 to use as my pre/pro for a dedicated 2-channel listening setup. Until I get a set of XLRs, I am using the unbalanced RCAs. I was reading that the output on the oppo is 2.1V unbalanced and 4.2V balanced. I am running the Oppo straight to my Krell S-275 which has an input sensitivity selectable, 2.4 V or 4.8 V - stereo. Using the analog volume control, could I experience clipping that could blow tweeters or my amp? I have read about the 15x rule of thumb on input impedance, but what about the input sensitivity? The Krell is powering B&W Nautilus 804S's. I just want to be sure I don't have to worry about watching the volume when I'm tying one on! ;) Again, sorry for the noob questions. I'm usually a safety third type of guy, but not when it comes to my precious equipment, Ha! 
bconman
atmasphere's avatar
atmasphere
11,909 posts
 
You're going to need a preamp that can drive that impedance since the Oppo will be bass shy and more likely to make distortion into that load.


So then the input sensitivity of the amp will be less important.


Sounds like you want to run balanced; have you sorted out a budget and do you know if you want tube or solid state?
bconman OP
5 posts
 
Well, shoot. I thought I was being sly and budget conscious to use the Oppo as a pre/pro for the Sabre DAC... If I get a true preamp, that renders the oppo DAC useless right?  Thank you, btw, for helping me understand how this works!
oldears
386 posts
 
@atmasphere is correct if the output impedance of the Oppo 105 is greater than 2000 ohms, but it is not specified by them in the manual, and it is quite routine to specify 47 Kohms as a suggested input impedance.
It will cost you nothing to try it with the Krell input set to 4.8V. If you don't get enough volume, you can reset to 2.4V. If you are bass shy, then the output impedance of the Oppo is too high for the Krell, confirming the point by atmasphere. (Be sure to use balanced cables.) BTW, I tried the variable volume on my Oppo, but preferred the sound with a quality preamp.
bconman OP
5 posts
 
So, the input sensitivity only changes based on the cables you are running. XLR vs RCA. Still only using RCAs (2.4v) while I try to find a decent set of used XLRs, but when listening I immediately thought that it lacked the bass it had before the Oppo... I thought it was the crossover on the Oppo (not running a sub right now) so I lowered it down from 80hz to 50hz but still thought it was lacking something. From the metrics I gave in my OP, how did y'all know it was going to be bass shy? Still trying to understand the relationship of the mathematics. 

GAIN Selectable, 25.8 or 19.8 dB - stereo Selectable, 31.8 or 25.8 dB - mono INPUT SENSITIVITY Selectable, 2.4 V or 4.8 V - stereo Selectable, 1.7 V or 3.4 V - mono INPUT IMPEDANCE 20 kOhms single-ended 20 kOhms balanced
atmasphere's avatar
atmasphere
11,909 posts
 
If I get a true preamp, that renders the oppo DAC useless right?
Not at all! DACs are often inputs to a preamp.

Its common when a source is trying to drive an input that is too low for the source, that the result will be a loss of bass impact. One reason for this can be a coupling capacitor at the output of the source; there is a timing constant with relation to the input of the amp that can cause a low frequency rolloff as a result. So its an engineering thing.


You can just put in a bigger cap to sort it out either- the limit might be distortion by the source trying to drive a lower input impedance.


At any rate a preamp is a good idea- or get an amp with a higher input impedance, or do something else like a zero gain buffer that has a lower output impedance. If you ever plan to have sources other than the Oppo, a preamp is a good idea.
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