Audio Research LS 25 MkII compared to actual AR production ?

Maybe look for a Ref 3 if you want to stay with ARC. I had a LS25 MKII for a week in my system on loan and it didn't do it for me. I preferred my former ModWright SWL 9.0 SE over it. Ended up with a ModWright LS36.5 which is a big step up over the LS25 MKII, at least in my system. In my system I felt the LS25 MKII was a little lean in the bass as well, not horrible but it didn't pull me into the music like the ModWright preamps do. Good luck.

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As adg101 says,  get a Ref3.
It will rock your world. 
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 I had an LS25 mk2 for many years and finally upgraded to a Ref 5se. This was a huge jump and improved on every aspect of the LS25 mk2 in a big way. I have tried a lot of ARC preamps over the years but the Ref 5se convinced me I needed to upgrade.

 What power cord are you using? Shunyata (not Venom) or Sane work really well with ARC. You will be amazed at the difference it makes. I went from Kabala to Shunyata  and the bass improved while the system became far more focused.

Hi all and thanls for your forst suggestion.

Today I tried a Shunyata Black Mamba HC on the LS25. Difference between Furutech FP-3TS20 and Shunyata is great, but I'm puzzled about the bass that became maybe too dry (remember with spectral and wilson I *need* body ;-) ). Tomorrow I will continue my listening session.
Shunyata effort on sound is, generally speaking, invasive, and its contribution has to be evaluated carefully, IMHO (I love Shunyata but I'm conscious they are not cables for novices) .

Ref 5se. Good suggestion , thanks @lostbears
Ref3, thaks @mitch4t  and @adg101, my only concern it's -more or less- a preamp with the same age of L25, I'm not sure I want to buy an old equipment more.

Maz
 These folks all mean well but
  your amp is 10 K input impedance.
This is what you are really up against.
 Best,
   JohnnyR
 

Johnny is right, as usual.  Just about every ARC line stage and preamp for which I've seen specs, including the LS25 MkII and the other ARC models that have been mentioned, has a **minimum** load recommendation of 20K, and the input impedance of your Spectral amp is 10K.  Although in both cases it isn't made clear if those numbers apply to the balanced or unbalanced interfaces or both.

In the case of a tube preamp a significant consequence of that sort of mismatch is likely to be perceptible rolloff of the bottom one or two octaves.  That will occur because of the output impedance rise at deep bass frequencies that will be caused by the coupling capacitor that is used at the output of most tube preamps.

Good luck.  Regards,
-- Al
 

Yes @audioconnection and @almarg , I'm aware of that.
DMA 200 S2 input impedance is 10.000 ohms and AR LS 25 MkII is 320 ohm in single-ended (now is connected in Single Ended, not the 600 ohm balanced).  The general thumb rule of 10x is respected and the most restricted Audio Research thumb rule of 30x is in the limits.
In the facts, i.e. listening, LS25mkII sounds great with no roll off or problems.
Of course, in balanced mode, the ss smp should be 20k minimum for the same rule.

I see Ref 6 (and Ref 5 se) has the same output impedance.

In the facts, i.e. listening, LS25mkII sounds great with no roll off or problems.

Good!

The general thumb rule of 10x is respected and the most restricted Audio Research thumb rule of 30x is in the limits.

To clarify, though, to assure impedance compatibility the 10x rule of thumb guideline should be applied at the frequency for which the output impedance of the component providing the signal is highest. Most impedances are specified at a mid-range frequency such as 1 kHz. It is very common for tube preamps to have output impedances at deep bass frequencies that are much higher than that specified value, often 2K or 3K or even 4K ohms. That rise at low frequencies results from the output coupling capacitor that is used in the majority of tube preamps (and also in some solid state preamps), as I mentioned in my previous post. The impedance of a capacitor increases as frequency decreases.

If as is often the case the component’s output impedance at 20 Hz is not known, and is not indicated in published measurements (such as Stereophile often provides), then to be safe a considerably higher ratio than 10x should be used, something like 50x or 75x IMO. Especially if the component is tube-based and is likely to have a coupling capacitor at its output.

Also, to clarify a common misconception I should add that failing to meet that guideline does not necessarily mean that there will be an impedance compatibility problem. It depends on how much **variation** there is in the output impedance over the frequency range. But meeting that guideline (at all audible frequencies) assures that there won’t be an impedance compatibility problem.

In this specific case, I took a look at the schematic for the LS25 MkII at arcdb.ws, and based on some quick calculations it appears that its unbalanced output impedance at 20 Hz is in the vicinity of 1.3K or so. A bit more than ideal relative to a load impedance of 10K, but reasonably consistent with your finding that it is not an issue.

Regards,
-- Al

Thanks @almarg  for your contribution, I never stop learning !

I'm also considering the new GS Pre, phono input and headphone connector are a good plus. unfortunately there are not so much comparisons between ref 5e/6 and GS Pre on the net.
GS Pre's impedance output in SE is 250 ohm.