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Re: Question for Atma-sphere owners

Posted by Lew (A) on February 25, 2007 at 10:35:40

In Reply to: Re: Question for Atma-sphere owners posted by Jack Williams on February 25, 2007 at 09:25:04:

No, in response to your first question. NFB has nothing directly to do with what I am talking about. I run my amps with zero NFB, so I hadn't thought about possibly creating confusion on that score. I am simply talking about the input impedance of the amplifier, which is essentially set by the grid resistors on the input stage. "Grid resistor" is a resistor that goes from grid to ground. (Input Z is probably affected by the NFB resistor too, but lets leave that aside.) Since the input of the M60 is a dual-differential amplifier (which means that two tube sections are used), there will be two grid resistors, one from each grid to the common ground. In a typical tube amplifier with balanced input (and standard in the M60) you would have two 100K resistors in these positions. Thus the R between pin 2 (which goes to the grid of one tube section) and pin 3 (which goes to the grid of the other tube section) would measure 200K ohms, if you put the leads of an ohmmeter on pins 2 and 3. Atma-sphere had heretofore suggested that when using one of their preamps with one of their amps, there could be some sonic and other advantages associated with using grid Rs of 300 ohms each, for a total R of 600 ohms. This is based on transmission line theory, which is too complex to go into here. However, there are other theories and facts which would suggest that this is not an optimal way to go, even when using a preamplifier with a circlotron output stage, of which there are only two in the world (MP1/MP3). Nevertheless, I can tell you that the results are sonically pleasing; I've just never tried my set-up (MP1/MA240) with a conventional grid resistance (i.e., 100K ohms per grid), which I am going to do today.

Parenthetically, the MP3 has a higher output Z than does the MP1, so Atma-sphere had backed off the 600-ohm recommendation and suggested that 1200 ohms was a better way to go, if you're using an MP3 (i.e., 600 ohms from each grid to ground) and wanted to take advantage of the transmission line approach. That's why I previously mentioned the 1200-ohm option, for MP3 only. Benefits of this approach are said to include a reduction in the effect on sonics of the kind, quality, and length of the interconnect between the preamp and amp. Maybe I will be able to comment on this after I have done some further experimentation, as I use 22-ft ICs between my MP1 and MA240s.

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