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Upgrade from Pass Labs XA30.8 to X250.8. Worth it?

I am getting ready to make some upgrades to the system. I currently running a DAC into a BAT51SE, then a Pass Labs XA30.8, then finally into Decware DM947 speakers (don't worry if you've never heard of them; not the last word in speakers, but have been doing a solid job for about 5 years now.)

I am contemplating moving up the Pass Labs amplifier line, and the X250.8 looks attractive. I would like to hear more bass authority and definition, but I don't really want to sacrifice the mids and highs of the 30.8. 

I would love to hear some of your impressions of the X250.8, especially if you are able to make some direct comparisons to the XA30.8. I've spoken with both Mark at Reno Hifi, and Kent English of Pass Labs; both are wonderful and informative people. What I am interested in now is the opinions and experiences of end users of either of these amps. 

Thanks! 
Ag insider logo xs@2xwillrich47
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willrich47 OP
25 posts
 
Haha, well, I just confirmed what I heard in this review of the BAT 51se: 

"The balanced output impedance was twice that specified, at 410 ohms over most of the audioband. This is low, though it did rise to a high 4.7k ohms at 20Hz, due to the size of the output coupling capacitors. (There is only so much room within the chassis for these high-performance but physically large components.) This will not be an issue with components having high input impedances, such as BAT's own power amplifiers, but it does mean prematurely rolled-off low frequencies with low load impedances (fig.1), such as those offered by some solid-state amplifiers."

Read more at https://www.stereophile.com/content/balanced-audio-technology-vk-51se-line-preamplifier-measurements

Now the question is, how do I fix it without replacing a preamp?
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willrich47 OP
25 posts
 
@bdp24 I was thinking along the same lines. I was hoping to do it without replacing the SVS. I've been searching for a high-level to RCA input converter, but haven't found a suitable solution yet. 

But you might be right. Maybe REL is where it's at. I am a little surprised that SVS doesn't offer this type of input solution, especially on their top-of-the-line unit. I have to think that more preamps would have this issue than just mine. Could be wrong though. 
bdp24's avatar
bdp24
4,908 posts
 

If you value your SVS sub more than your BAK pre-amp, look for a pre that can drive a lower input impedance. EAR-Yoshino pre’s (tubed) are designed to drive a 600 ohm load (its designer, Tim de Paravicini, does work for clients in the pro recording field, in which 600 ohms is the standard). The EAR 868 includes two balanced/XLR pairs of outputs, and two unbalanced pairs on RCA’s, and has no trouble driving multiple low impedance inputs. Available in both line level-only (868L) and line level-plus phono (868PL) versions.

If you would prefer to keep the BAK pre, and want to keep your hookup at line level, another option is to look for a sub with a plate amp having a higher input impedance than that of the SVS. 22kHz is rather low.

almarg's avatar
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almarg
8,673 posts
 
Your calculation of the 18K is correct, and I feel pretty certain that your diagnosis of the bass issue is correct. The 18K load impedance divided by the 4.7K output impedance at 20 Hz is a ratio of 3.8, well under the rule of thumb guideline of a minimum of 10. And the wide variation from 410 ohms to 4.7K makes that situation particularly bad. (If the output impedance were 4.7K throughout the audio band the consequences would be mainly a slight gain reduction and increased sensitivity to cable effects, and perhaps a small degradation in the distortion performance of the preamp. While the large difference between the output impedance in the bass region and at higher frequencies, in conjunction with the heavy load impedance, will cause both frequency response issues and frequency-dependent phase shifts in the bass region).

To add to bdp24’s suggestions, a few years ago another member here, who uses Audio Research Reference series electronics, very successfully resolved a similar issue by having a tube-based buffer stage, providing suitably high input impedance and low output impedance, custom made for him by Tom Tutay of Transition Audio Design in Florida. It would be inserted between the preamp and the sub. I believe the cost was considerably less than $1K, although in his case balanced interfaces weren’t necessary. I don’t think Tom has a website, but a Google search should provide you with his contact information.


Good luck. Regards,

-- Al


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