High Current SS amps with input impedance = 200K

Foster,

Yes, it will be interesting what that device might do in your case. On paper, the VTL and McCormack appear to be a pretty good match. The DNA 500 is the only other (non Class D) 500w/ch amp that I have considered to-date.

I think there is more than just a calculated drop of output in the bass frequencies. 0.4db would be difficult to hear especially at 20Hz.

My SS amps have input imped of 67K ohms. I have used a few tube preamp that has specified output imped of 100ohm to 600ohm (may be higher in the bass freq.) Subjectively, there is tighter coupling betw the pre with the lowerest output imped and the amp. I hear better transient attack in the mids and tighter bass. In some ways, it is similar to changing the load imped on cartridges.

I suspect that there is also interaction in the time domain and/or dampening.

I am pretty confuse myself. Any help is appreciated.

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