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CD distortion through preamp

I apologize for the long post. I’m trying to make this as brief as possible while providing all the necessary info.

The equipment: preamp is Audio Research Ls25 mkii, pass x250.5 amplifier, and bryston Bha headphone amplifier, Wilson watt puppy speakers. All connected with balanced cables. I needed the fixed output to rip records, so the headphone amplifier is connected to the 2nd variable output.

The issue: I’ve had problems with CDs distorting when the music (dynamics within the performance) gets louder. The distortion is mostly noticeable in the mid to upper range and it’s through both the speakers and headphones. No issues listening to records or dac when it is hooked up to the computer with usb. But when I tried using the CD player as a transport to the dac, the distortion returns.

here’s what I’ve tried without success:
-new tubes in ls25
-3 different CD players
-tried rca cables
-different inputs

I also tried a Bryston bp25 solid state preamp and had no problems. It seems like the ls25 is the issue. 

Any thoughts or suggestions?


tripg
twoleftears's avatar
twoleftears
5,313 posts
 
Which inputs on the preamp are you using for your experiments to isolate the problem?
mike_in_nc
1,121 posts
 
I see the preamp has a "gain" setting that is input-specific. You've tried adjusting that? (It's not clear to me from a quick view of the manual if it should go higher or lower.)
tripg OP
21 posts
 
Twoleftears, I’ve tried a few different inputs, but didn’t think to try using the dac or phono preamp’s inputs. I’ll check that out. Thanks!
tripg OP
21 posts
 
Mike_in_nyc, you are correct. there are 3 gain settings for each input. I have the CD player on the lowest setting. I’d think higher would be worse, but I’ll give it s shot to see what happens when I do. 
almarg's avatar
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almarg
9,670 posts
 
Puzzling problem, especially given that while it does sound like the preamp is responsible, whether or not the issue appears depends on whether a CD player (used as a transport) or a computer is driving the DAC.

One thing I would try as an experiment, if you already haven’t, is disconnecting the headphone amp when you are listening to speakers, and vice versa, rather than simply turning off the one that is not being used at any given time. As is the case with many ARC preamp designs the two XLR main outputs of the LS25 for each channel are simply jumpered together inside the rear panel, and the RCA main outputs (which it sounds like you are not using) are simply provided with the same signal that is provided to pin 2 of the XLR connectors. And although this is just a shot in the dark I’m thinking that a possible contributor to the issue might be driving the uncontrolled and unknown input impedance that is presented by whichever amplifier is turned off, that might even vary as a function of signal amplitude.

Also, fyi, ARC recommends a minimum load impedance of 20K for the LS25 and for most of their other line stages and preamps, although their specs don’t make clear if that applies to the balanced outputs or the unbalanced outputs or both. And it appears that your headphone amp has a balanced input impedance of only 10K, while the Pass amp has a balanced input impedance of 30K.

With both amps connected to the preamp’s XLR outputs that means the preamp is being presented with a combined load of 7.5K (30K in parallel with 10K), which is well below the recommended 20K minimum. And that is when both amps are powered up; we have no way of knowing what that figure is when one of them is turned off, and as I indicated the input impedance presented by the unpowered amp could conceivably vary as a function of signal level, with unpredictable consequences.

Good luck. Regards,
-- Al
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