High Current SS amps with input impedance = 200K

I'm currently using a Burson AB-160 buffer between my McCormack DNA-500 amp and VTL preamp. I'm waiting for it and associated cables to break in and sonics to stabilize. I'll report back with developments after it gets some significant hours on it.

Emerson, as I recall it was not readily possible to determine the low frequency output impedance rise of your VTL 2.5. However, Stereophile has numbers on the 5.5, which it describes as being descended from the 2.5, and the two models have nominal output impedances which are very close to one another (195 and 200 ohms). For the 5.5 they measured 184 ohms at mid/hi frequencies, and 409 ohms at 20Hz, which is a very minimal rise that would have negligible frequency response impact into 10K.

They also have numbers on a couple of other VTL models, the 6.5 (150 ohms at 20 Hz for the unbalanced outputs) and 7.5 (69 ohms at 20 Hz for the unbalanced outputs).

So it would seem that VTL's design approaches reflect sensitivity to this issue.

Regards,
-- Al

Glai, it's only possible to offer speculation in response to your question, without having more complete information on the particular designs (which may be unobtainable). But I wouldn't be surprised if a tube preamp with a nominal output impedance spec of 600 ohms were to rise to several thousand ohms at deep bass frequencies.

And the 600 ohm impedance itself could have consequences at high frequencies as well, if your interconnect cables were long and/or had high capacitance per unit length. That could very conceivably account for the degraded transient attack you mentioned, by rolling off the upper treble.

Also, perhaps in the case of the particular designs you were using, there was some degree of correlation between lower output impedance and better quality parts in the output stage. As well as the possibility that variables elsewhere in the designs, unrelated to the output stage and impedance matching effects, might have accounted for the differences in those particular cases.

Regards,
-- Al

Al,

Thanks for responding. Th preamp in questions:

Einstein the tube, 100ohms
Allnic L400, 250ohms (supposely flat with transformer coupled design)
ARC Ref 3, 600ohms (1400ohms at 20Hz)

I use interconnects 1 meter in length Nordost and Stealth.

Many thanks for your insight

Al, thanks for the feedback as usual. I have a bit to add on the Burson AB-160 buffer. It's been in my setup for 3 days with signal going through (72 hours). What they say about it is true. It will take a budget player and lift its performance. The Burson is not connected to any of my players; it's between my amp and preamp. I have 3 different players connected to my VTL preamp, a Raysonic CD128, a Jolida player and an Onkyo. The low fi Onkyo is playing like a more expensive player now. It surpasses the other 2 players in extension, dynamics, low end, openness, you name it. And it has some inexpensive Tributaries IC's connected to it. Budget stuff in all. For the past 2 years I've only been using the Onkyo player for sending signal to facilitate break in with new gear and new cables. This is my experience with the Burson Buffer in my system 3 days. As others have said, the Burson seems not to have as significant of an affect on the performance of better players. At this point, I would say it is a detriment to the other players I have. Listening to this budget player with the Burson in place has also revealed what my speakers are capable of in the lower bass region. I had not heard these speakers go as low before.

Glai -- Wow! I looked at your system description, and I must say it is utterly magnificent. Congratulations!

As to why the Ref 3 did not give you the tight bass and the better transient response that the other two preamps do, I have no ideas to offer. What I can say is that I doubt it has anything to do with the somewhat higher output impedance of the Ref 3, considering that your interconnects are short, your power amp input impedance is reasonably high, and considering its outstanding reputation among many of our members, and the excellent reviews of it that I recall seeing.

Emerson -- Glad things seem to be shaping up. Again, there is no technical explanation that I can envision for the effects the Burson buffer stage seems to be having, and I doubt that it has any relation to the impedance issue this thread has focused on. As we all know very well lots of things in audio are both unexplainable and unpredictable.

Regards,
-- Al